Report of Investigations No. 156

Cyclicity in the Middle PermianSan Andres Formation,Palo Duro Basin, Texas Panhandle

Michael A. Fracasso Susan D. Hovorka

The University of Texas at AustinAustin, Texas 7A713

lower unii 2

Glorielo Formotioo

1e86ffi@Btrreau of Economic GeologyW. L. Fisher, Director

Report of Investigations No. 156

Cyclicity in the Middle PermianSan Andres Formation,Palo Duro Basin, Texas Panhandle

Michael A,. FracassoSusan D. Hovorka

Prepared for the U.S. Department of Energy, Salt Repository Project Office,under contract no. DE-AC97-83WM46651

ffi@1e86Bureau of Economic GeologyW. L. Fisher, DirectorThe UniversiW of Texas at Austin.A,ustin, Texas 78713

The cover figure illustrates the regional continuity of cyclic units in.a roughlydip-oriented.cross section ofthe (en Andicc Fnrmation eytendlns from the Amarillo Uolift (north) to the Northern Shelf of the MidlandThe cover figure illustrates the regional continuity of cyclic units in.a routhly clip-oriented-cross sectlon otthe San Andies Formation extendlng from the Amarillo Uplift (north) to the Northern Shelf of the Midlandthe San Andres Formation extendlng lrom tne ABasi n (south). Leaders schematical ly locate.selected pho

sequences. Texture photographs appear asBasin (south). Leaders schematicallylocate.selected photographs of typical San Andres textures with respect

to their chaiacteristic positions in San Andres cyclic lithofacies sequences. Texture photographs appear as

i6r."i 15,22,and 28 in the text; the cross seciion is a simplified'and shortened version of plate 1.

CONTENTS

ABSTRACT .. ..INTRODUCTION

Geologic SettingData and Methods

CYCTICITY IN THE SAN ANDRES FORMATIONldeal Vertical Cyclic Facies Sequence

Ease of Cycle-Dark MudstoneCarbonateAnhydriteHalite.Terrigenous Red BedsVariation from the ldeal Vertical Cyclic Seguence

Correlation of San Andres Formation CyclesStyles of Cyclicity in the San Andres Formation ....

Lower San Andres Genetic SeguenceMiddle San Andres Genetic SeguenceUpper San Andres Genetic Seguence

Lateral Facies Relationships, Depositional Systems, and Structural lnfluence onSedimentation in the San Andres FormationLateral Facies Relationships

Cyclic and Noncyclic Controls on Red-Bed DistributionTime Lines

Depositional Systems.Structural lnfluence on Sedimentation .

SUMMARY ....ACKNOWLEDGMENTS ...REFERENCES ...

1

2

23

7

77B

1622252627

28

31

3537

39

3940404242

43

45

45

101112

1213131414

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10.

11.

12.

13.

14.15.

16.

Stone and Webster Detten No. 1 coreDepositional environments in lower San Andres unit 4 in cored wellsBrachiopod biomicrudite having abundant, large, mostly intact brachiopod shellsSyntaxial overgrowth on an echinoderm plate, showing an early generation ofcloudy cement with possible relict bladed fabric after aragonite, followed by sparry calciteRipple-laminated dolomitized carbonate grainstone with carbonate mudstone drapesMicritic ooliths, slightly compressed at the top and strongly compressed at the bottomLaminae in rippled dolomicrite are defined by concentrations of ooids .

Ooid replaced by halite, anhydrite, and dolomite

Figures1. Stratigraphic column of Permian rocks in the Palo Duro and Dalhart Basins, Texas Panhandle2. Permian formations interpreted as transgressive-regressive,

carbonate-evaporite and clastic genetic couplets 33. Stratigraphic divisions of the San Andres Formation 44. Location of San Andres Formation cross sections and cored U.S. Department of

Energy wells in relation to basement structures, Texas Panhandle . . . 55. Permian shelf-margin progradation,Texas Panhandle ....... 65. Vertical facies sequence and gamma-ray log pattern of an idealized cycle,

San Andres Formation, Palo Duro Basin . 87. Anhydritic black mudstone, base of unit 5, lower San Andres Formation,

8. Contorted and brecciated texture in insoluble residue fromthe DOE-Gruy Federal Grabbe No. 1 core 9

9. lnsoluble residue at the base of a middle San Andres cycle,

17. Dolomitized carbonate mudstone from the Stone and Webster Mansfield No. 1 core18. Nodular anhydrite in laminated dolomite mosaic19. Typical nodular anhydrite mosaic from a thin upper San Andres cycle,

Stone and Webster Detten No. 1 well20. Cradation between nodular anhydrite and anhydrite having some nodular texture and

some poorly preserved halite pseudomorphs after gypsum21. Unusual subhedral anhydrite nodules in dolomite matrix .

22. Halite pseudomorphs after gypsum in halite, in anhydrite, and in dolomite matrix23. Tall pseudomorphs after gypsum, now halite with relict growth bands defined by

anhydrite, in anhydrite matrix .

24. Anhydrite pseudomorphs after gypsum crystals in dolomite matrix .

25. Disrupted fabrics at the contact between halite (dark) and anhydrite (light)26. Model for the origin of the sequence of fabrics observed in the anhydrite parts of cycles27. Vertically oriented crystals in halite outlined by diagenetic anhydrite28. Photomicrograph of chevron structure in halite, showing fluid inclusions trapped along

relict growth surfaces of bottom-nucleated halite crystals29. Karst pit in light-colored halite with preserved brine-pool fabrics, filled with coarser halite .

30. Karst pit in banded halite filled with a first generation of coarse halite, then with mudstone,finally with displacive halite .

31. Chaotic mudstone-halite from top of the San Andres Formation, Stone and Webster Harman No. 1 well32. Very fine sandstone containing small-scale scours, clay rip-up clasts,

clay drapes, and ripple crossbedding33. Typical mudstone bed within lower San Andres unit 5, showing disruption of fabric by

displacive growth and subsequent removal of halite crystals34. Soft-sediment microfaults in ripple-laminated, very fine grained sandstone with

clay drapes and sand-sized clay rip-up clasts. .

35. Lower San Andres unit 5, a generally northwest-southeast cross section through coredU.S. Department of Energy wells .

35. Relationship between the halite at the top of San Andres unit 4 and the basal cycle of unit 5 . . . . .

37. Anhydrite at the top of the lowest incomplete cycle in unit 5, overlain by dark mudstone38. Facies variation in the third cycle of unit 5, showing a similar pattern of

salinity fluctuation in all the cores .

39. Northwest-southeast cross section of middle San Andres cycles in cored U.S. Department of Energy wells. . . . . .

40. Northwest-southeast cross section of the lower three upper San Andres cycles incored U.S. Department of Energy wells . .

Plates (in pocket)1. North-south, roughly dip-oriented correlation section A-A', San Andres Formation, Texas Panhandle2. North-south, roughly dip-oriented correlation section B-B', San Andres Formation, Texas Panhandle3. East-west, roughly strike-oriented correlation section C-C', San Andres Formation, Texas Panhandle4. East-west, roughly strike-oriented correlation section D-D', San Andres Formation, Texas Panhandle

and eastern New Mexico

1516

17

181819

1920202123

2324

26

2425

26

27

303233

3436

38

tv

ABSTRACTThe San Andres Formation in the Palo Duro

Basin is a middle Permian carbonate-evaporitesequence situated between two red-bed units,the underlying Clorieta and the overlyingundifferentiated Queen-Grayburg sequences.The San Andres Formation, deposited duringrelative structural quiescence in the region, iscomposed of cyclic sequences of dark anhydriticmudstone, skeletal li mestone, dolomite, nodularanhydrite, bedded anhydrite, and halite. Thesecyclic vertical lithofacies sequences reflectsediment deposition by regressive, upward-increasing salinity cycles in a shallow platformsetting. Thin interbeds of red siliciclasticmudstones and siltstones are present throughoutthe San Andres but become more abundant andthicker near the top of the formation. Texturalevidence provided by core shows that the entiresuite of cyclic facies reflects predominantlyshallow-water deposition. Evidence of subaerialexposure is sparse within carbonate rocks, absentin anhydrite, but intermittently abundant inhalite.

Cycles can be traced across the entire PaloDuro Basin study area using geophysical log data.Much thinner sequences, on the order of tens ofcentimeters thick, also can be recognized usingcore data and correlated over distances of 118 kmbetween cored wells. The extraordinary regionalcontinuity of thin cyclic units, combined withtextural evidence of their subaqueous origin,suggests that sedimentation occurred on anextremely broad, featureless shallow-water shelfcharacterized by nearly uniform depositionalenvironments over large areas. The great widthof the shelf combined with a shallowwater depth

Keywords: basin subsidence, core, cyclicity, depositional systems, eustatic sea-level change, evaporites, geneticsequences, geophysical logs, insoluble residue, Palo Duro Basin, Permian, San Andres Formation, sedimentaggradation, sedimentary petrography, Texas Panhandle

restricted circulation, which caused develop-ment of a lateral salinity gradient soon after theinitial, near-normal-marine transgressive phaseof each cycle. This salinity gradient is ref lected bythe lateral equivalence of halite, anhydrite, andcarbonate facies along a north-south trend. Thedominant source of normal-marine water ap-pears to have been the Midland Basin to thesouth, although a western normal-marine influ-ence is indicated in some cycles. Vertical andlateral patterns of sedimentation in the SanAndres Formation reflect changes in watercirculation and resulting brine salinity, which arefunctions of the dynamic interplay between ratesof eustatic sea-level change, regional basinsubsidence, and sediment aggradation on a

broad, low-slope, low-relief platform.Systematic vertical changes in the thickness

and completeness of cycles allow division of theSan Andres Formation in the Palo Duro Basin intothree informal genetic sequences. Cycles of thelower San Andres sequence exhibit the completeideal vertical facies sequence and have a

relatively thick carbonate member. Cycles of themiddle San Andres sequence contain relativelythin carbonate members and are incompletebecause they lack halite members. Cycles of theupper San Andres sequence also have relativelythin carbonate members, but halite is present.The f undamental mode of cyclicity is the same ineach of the genetic units, but the cyclic tempo is

different. The temporal evolution of cyclic stylereflects changes in regional basin subsidencerate or frequency of eustatic sea-level change orboth.

INTRODUCTION

Geologic SettingThe San Andres Formation (Cuadalupian) is

composed of cyclic sequences of carbonate,anhydrite, and halite in the Palo Duro Basin. Theformation contacts are defined by two red-bedunits-the underlying Clorieta and the overlyingQueen-Grayburg sequences (fig. 1). (The Queenand Crayburg Formations are distinct unitsthroughout much of the Permian Basin, but arenot differentiated in the Palo Duro Basin.)Carbonate-evaporite and red-bed lithologicunits in the Palo Duro Basin have been informallygrouped as genetic couplets (Presley, 1979b,1980; Handford and Bassett, 1982; fig. 2) thatrepresent large-scale transgressive-regressivecycles superimposed on an overall regionalsouthward regressive trend that spanned most of

FIGURE 1. Stratigraphic column of Permian rocks in the Palo Duro and Dalhart Basins,Texas Panhandle. After Budnik and Smith (1982).

Permian time (King, 1942; Hills, 1972). Smallerscale cyclicity within the San Andres Formationin the Palo Duro Basin has been documented(Presley, 1979a,1979b,1980,1981b) and has led toan informal two-fold division of the San Andresinto a lower unit in which cycles are pronounced(informally designated as units 1 through 5) andan upper unit in which cycles are less marked(Presley, 1979a; fig. 3).

The Palo Duro Basin and surrounding struc-tures (fig. a) appear to be wrench tectonicfeatures. Basin-margin structures are rhombgrabens and horsts that developed maximumrelief in middle Carboniferous time because ofcompression during plate convergence (Kellerand Cebull ,1973; Kluth and Coney,1981; Duttonand others ,1982; McCookey and Budnik, 1983;R. T. Budnik, personal communication, 1984).

Polo Duro Bosin Dolhort Bosin

SYSTEM SERIES GROUP FORM ATION FORMATION

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WICHITA

WOLFCAMPIAN

Relief on the bordering Amarillo-Wichita Upliftto the north and Matador Arch to the southdecreased rapidly after Missourian-Virgilian(Late Pennsylvanian) time, as shown by rates ofgranite-wash accumulation (Dutton and others,1982, their fig. 5). Both of these positive trendswere completely buried by the end of the Wolf-campian (Early Permian) (Dutton and others,1982). The rest of the Permian has, until recently,been considered a time of structural quiescence.lnherited basin relief was filled by a series oftransgressive-regressive cycles (tig.2), docu-mented in part by the southerly progradation ofsuccessive shelf-margin positions (fig. 5). By thetime of deposition of the San Andres Formation,the shelf margin was probably nearly 200 kmsouth of the Matador Arch (Ramondetta, 1982).The area of the Palo Duro Basin was part of anextensive shallow-water shelf environmentextending from New Mexico to the Texas Pan-

handle and into Oklahoma and Kansas. Recentresearch (Fracasso, 1983, 1984) suggests episodesof subtle yet active intrabasin and basin-marginstructural influence on deposition of the SanAndres Formation.

Data and MethodsFour regional cross sections were prepared

through the San Andres Formation (fig. 4; pls. 1

through 4). lndividual wells were projected ontoeach transect line along perpendiculars to thetransect that pass through the wells. Wells aredistributed unevenly throughout the TexasPanhandle; most are concentrated along theAmarillo Uplift and Matador Arch. Well density islower in the basin (average well spacing alongtransect lines is 10.1 km) but adequate toestablish precise correlation within the SanAndres across the entire basin. Camma-ray logs

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PALO DUROBASIN

FIGURE 2. Permian formations interpreted a.s transgressive-regressive, carbonate-evaporite and clastic genetic couplets. Notevertical trend of facies tract regression to the south. Adapted from Presley (1981a).

Slone ond Webster Engineering Corp.Zceck No. I, Swisher Counryr Teros

Gommo I Borehole-compensoiedcoliper sonic

Bose Oueen /

gommo - roy

Bose unil u-3

Bose unil u-2

ott

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FIGURE 3. Stratigraphic divisions of the San Andres Formation, illustrated with a reference geophysical log from SwisherCounty, approximate center of Palo Duro Basin, Texas Panhandle. The informal unit subdivisions of genetic sequences areworking siratigraphic intervals, recognizable on geophysical logs and mappable. Cenetic cycles are smaller scale intervalsrecognized on thb basis of core analyiis and cannot generally be distinguished individually on geophysical logs. Unit bases inall caies correspond to genetic cycle bases. We find that Presley's cycle 1 cannot be traced with certainty as far north as SwisherCounty; thus, we suggdst that the base of the San Andres Formation in this area and further north corresponds to the base ofunit 2. Location of well is shown on figure 4.

see fig.4O)

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see fig. 35)

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FIGURE 4. Location of San Andres Formation cross sections and U.S. Department of Energy wells in relation to basementstructures, Texas Panhandle. Well name abbreviations: Man - Stone and Webster Mansfield No. 1; J. Fr. - Stone.and Webster

J. Friemel No. 1; Det - Stone and Webster Detten No. 1; C. Fr. - Stone and Webster C. Friemel No. 1; Hol - Stone andWebster Holtzclaw No. 1; Har - Stone and Webster Harman No. 1; Zee - Stone and Webster Zeeck No. 1; Cra - DOE-GruyFederal Crabbe No. 1; R. Wh. - DOE-Gruy Federal Rex White No. 1; Saw - Stone and Webster Sawyer No. 1.

are particularly useful for regional correlationin this predominantly carbonate-evaporite se-quence because relatively thin siliciclastic anddolomite beds at the base of each cycle producecharacteristic sharply defined, high API Sammapeaks well above the low API gamma evaporitebaseline. Sample logs, borehole-compensatedsonic density logs, other density logs, and corehave also been examined to help distinguishbetween lithologies.

An intraformational horizon-the pi-marker(Dunlap, 1967; Pitt and Scott,1981; Ramondetta,1982, his f ig. 39)-is used as a horizontal datum inthe correlation sections. The pi-marker is a 3-m-thick terrigenous clastic red bed and associatedoverlying carbonate unit that produces an easilyrecognized high APl, sharp gamma peak. lt de-fines the base of the upper San Andres (newinformal division, discussed in the followingsection) and can be traced throughout the Palo

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FIGURE 5. Permian shelf-margin progradation, Texas Panhandle. From Ramondetta ltSeZ;.

Duro Basin and some distance beyond (north-western Midland Basin, northern NorthwestShelf of the Delaware Basin, Dalhart Basin, andnorthern Eastern Platform of the Midland Basin).

The primary advantage of using an intraforma-tional horizon as a datum in constructingregional cross sections is that it compensates forthe effects of postdepositional structuraldeformation on the geometry of the preservedsequence and thus approximates the originaldepositional stratigraphy and facilitates well-to-well correlation.

Sequences of continuous core representingmost or all of the San Andres Formation used inthis study have been recovered f rom 10

stratigraphic test wells drilled by the U.S.

Department of Energy (DOE) in the Palo DuroBasin (Hovorka, 1986; Hovorka and Cranger,1986). Textural, petrographic, and geochemicalanalyses of core allow interpretation of thesequence of depositional and diageneticenvironments in the San Andres Formation. Atleast one cored well was incorporated in eachcross section.

CYCLICITY IN THE SAN ANDRES FORMATION

ldeal Vertical CyclicFacies Sequence

An idealized San Andres cycle, formulatedfrom core examination, is composed of (1) basal,anhydritic dark mudstone, (2) skeletal Iimestone,(3) dolomite, (4) nodular anhydrite, (5) beddedanhydrite, and (6) halite (fig. 6). This sequencereflects the increasing salinity of the depositingwater body. Halite members are interrupted bythin red-bed units that become more frequentand thicker in the upper part of the San AndresFormation.

Base of Cycle-Dark MudstoneTransgressive sediments at the base of San

Andres cycles are thin (0.1 to 3 m thick)anhydritic dark mudstone. These mudstones areregionally extensive throughout the Palo DuroBasin and consistently occur between the haliteat the top of each cycle and the carbonate oranhydrite of the next cycle (fig. 7). Most beddingin the dark mudstone is intensely deformed bysoft-sediment microfaults and folds, boudinage,and disruption of indeterminate origin (fig. B).

The upper part of some mudstone beds lacks

anhydrite and exhibits primary structures,including ripple lamination and fissility (fig. 9).

The fissile mudstone appears to be organic rich.Total organic carbon (TOC) in two samples of

dark mudstone from the base of lower San

Andres unit 4 in the DOE-Cruy Federal RexWhite No. 1 core was 0.58 and 5.01 weightpercent (S. P. Dutton, personal communication,1eB4).

The dark mudstones are at least partly a

residue of the insoluble components of theunderlying halite, which was partially dissolvedby interaction with transgressing normal- ornear-normal-marine waters. Collapse fabrics inthe mudstone formed as the underlying halitedissolved. The position of the dark mudstonewithin the cycle indicates that mudstones weredeposited as salinity decreased f rom halitesaturated to normal or nearly normal marine,probably in response to a transgressive increasein water depth that provided better circulation.Anhydrite beds were not precipitated at thistime, suggesting that the salinity decreaseoccurred rapidly.

The ripple-laminated upper part of themudstones may have f ormed either byreworking of the residue during transgression orby introduction of additional fine terrigenousclastics by dust storms or other mechanisms. Thedark color of the mudstone is probably related toreducing (pyritic) conditions within the over-lying limestone, dolomite, and anhydrite andmay reflect either the depositional or thediagenetic environment of the mudstone.

GAMMA.RAY CURVE

Bos of cycle

LITHOLOGY STRUCTURES

EXPLANATIONLITHOLOGY

anhvdrtre ffi no,,r"' FfTrTrrm

SEDIMENTARY STRUCTURES

Dork-colored onhydrilic mudstone, bose of nexl cycle

Holile with subcycles

gypsum crystols

Doloslone-ooid groinstone wilh dolo-mudstone dropes

ItttI

m f-7--vH=+ Limestone W Dolostone

[Z n'rl ,*",",o, s,oin. J7-l crossbeds

l|]ll s*"r",or.o,a l-l r-o,ino

oC'6oooC

N@E

F""ll oo,o, F7l ,,,,o*. lE-l Di.ruot"o foor," ll .7 Mudstone-hotrtewilh choolic fobric

------Tronsgression

I ow ----------------i highAPI unils

Nodules

Ri pples

fiiill y.,1;.o1 ".y.ror, fV-l ei,

oA 5288

IDEAL SAN ANDRES CYCLE

Felted, disrupled onhydrite ol onhydrite-holite contocl

Lominoted onhydrile wilh pseudomorphs ofler

Nodulor mosoic onhydrite

Burrowed skelelol groinstone, wockestone, mudstone

Dork-colored onhydrilic siliciclostic mudslone

. *.0 muds'tone ! aro"r mudstone

Red mudstoneinlerbed

Fol cn.,,on. @ :#:Hxl' l"-*-l annvo'it'intu'u'a

cryslo ls

FIGURE 6. Vertical facies sequence and gamma-ray log pattern of an idealized cycle, San Andres Formation, Palo Duro Basin.

Carbonate

ln most cycles, carbonate overlies the darkmudstone at the base of the cycle. Carbonatefabrics occur repetitively throughout the sectionbut do not exhibit well-developed vertical

sequences or cyclicity (fig. 10). Examination ofcore, supplemented by reconnaissance petrog-raphy, allowed identification of six recurringcarbonate facies: (1) coarse, burrowed skeletalpackstone (mostly limestone) with diverse fauna;(2) dark, wispy-laminated packstone and wacke-

lo-

CM-

0-

FIGURE 8. Contorted and brecciated texture in insolubleresidue from the DOE-Gruy Federal Grabbe No. 1 core,2,342.8 ft. The halite (H) of the top of unit 5 was corrodedand pitted, and the mudstone (dark) and anhydrite (A)insoluble components in the salt were concentrated and letdown as the underlying salt dissolved. Thin section,transmitted light. Photograph width is 3.5 cm.

FIGURE 7. Anhydritic black mudstone, 12 cm thick, base ofunit 5, lower San Andres Formation, Stone and WebsterMansfield No. 1core,1,542ft. Bedded halite (H)from thetop of unit 4 at the base of the slab, black mudstone (M)containing characteristic disrupted residue fabric in themiddle, and anhydrite (A) at the base of unit 5 at the top.

ments and spines, echinoid plates, foraminifers"fenestrate bryozoan fragments, coated grains-and abundant algal material including oncolitesand blue-green algal fragments. Cirvanella-likealgal filaments were identified in San Andreslimestones by Bein and Land (1982). Althougtrmost of the skeletal material is f ragmental, mold:of large gastropods and cephalopods and Iarseproductid brachiopods having spines intact aremoderately abundant, indicating that not all theskeletal material is detrital (fig. 11). Most of thepackstone is extensively burrowed, but cross-bedded fabrics are preserved in some of thecoarse-grained intervals.

Diagenesis of skeletal packstones includesearly cementation with calcite and possiblebladed aragonite (fig.12l; local neomorphicreplacement of aragonite skeletal material b1calcite; local dolomitization of grains and matrix:local cementation and replacement of allochernsand matrix by anhydrite blades and nodule(fig. 11); replacement of allochems by halite"either by leaching and cementation or b,.neomorphic replacement of aragonite by halite:and pervasive halite cementation of all pores-

Dark, wispy-laminated fine-grained pack-stone and wackestone (mostly limestonecontaining diverse fauna and large burrousinterfinger with coarse, skeletal packstone aniexhibit a similar stratigraphic distribution-Allochems are the same as those in the packstonebut are sparser and smaller. Burrowing aniprobable microstylolitization obliterateiprimary sedimentary structures and yieldecwispy lamination. These finer sediments har.ebeen variably cemented by calcite and halite.producing a mottled or nodular appearance-Bedding in halite-cemented areas appears rohave been compacted around calcite-cementedareas, suggesting that cementation by calcitepreceded cementation by halite. The limestonehas locally been partially dolomitized.

Ri pple-lam i nated coated-grai n,/ool ite grain-stone (mostly dolomite) is found throughout theSan Andres Formation and is one of thedominant carbonate fabrics in the middle ancupper San Andres (fig. 13). lt is stratigraphicaltrrassociated with silty, ripple-laminated dolo-

FIGURE 9. lnsoluble residue at the base of a middle SanAndres cycle, Stone and Webster Detten No. 1 core,2,288 tt. The upper part of the insoluble residue is ripplelaminated, but the characteristic disrupted texture' ispresent in the lower part. The upper part may be reworkedresidue or may have formed by influx of fine clastics notassociated with dissolution of underlying halite.

stone (mostly limestone) with diverse fauna andlarge burrows; (3) ripple-laminated coated-grain/oolite grainstone (mostly dolomite);(4) packstone and wackestone (mostly aphano-crystalline dolomite) with low faunal diversityand small, sparse burrows; (5) silty, ripple-laminated dolomicrite; and (6) anhydriticdolomicrite.

Coarse, burrowed skeletal packstone (mostlylimestone) containing a diverse fauna is presentonly in the thick (as much as 30 m) carbonateunits in the cycles of the lower San AndresFormation (units 2, 3, and 4), where this pack-stone makes up more than half of the carbonate.Skeletal packstones are dominantly composed ofsand-sized skeletal debris. ldentifiable grainsinclude mollusk fragments, brachiopod frag-

10

Stone ond WebsterMonsf ield No.l

Stone ond WebslerDetten No.l

Stone ond WebsterG. Friemel No.l

oOf,oocl

JA

Anhydrite

Normol-mcrinecorbonote

ooo

oof

oooE

O

c ycle

o0o

o

'oooaE

o

Anhydri te

Hypersol inecorbon o te

s9oxooE-oo12i!o

lnsoluble residue

EXPLANATION

LITHOLOGY

ffi@f^^ ^TLIJJ

l--+-l-|l.-l

Holite W annyartu Mudstone Limestone ffi oorc^ir"ffi

SEDIMENTARY STRUCTURES

Cross strotificotion Displocive holite Skeletol sond la?i ,,,,o*.

[4El oo,o.Chevron structures in holite

Eedding

lETl sru,u,or sroins l%l *oou,..

QA 5330

--:c6...+

:o;-;:

FIGURE 10. Depositional environments in lower San Andres unit 4 in cored wells. The left column of each well shows percentlithology; the center column shows fabrics. The limestone intervals show no pattern of salinity increase, and the f luctuation offacies is best interpreted as being due to local migration of higher and lower energy facies in a carbonate-shelf facies mosaic.

11

rimmed by a layer of microspar a few crystalsthick. The microspar layer remains a constantthickness between grains even where one grainhas been compressed into another. The originalcomposition of these grains, which could haveallowed the concentrically laminated part to beplastically deformed while the microspar rimretained its thickness, has not been determined.Bein and Land (1982) speculated that the genesisof these grains involved formation and diagene-sis in highly saline water. Support for thisconclusion is found in the occurrence of similarcompressed ooids in the Leonardian WichitaGroup of the Palo Duro Basin, which also exhibitsevidence of hypersaline deposition anddiagenesis (Hovorka and Budnik, 1983).

Packstone and wackestone (mostly aphano-crystalline dolomite) with low faunal diversityand small, sparse burrows occur in intervals a fewmeters thick. Most allochems are silt-sizedgrains, which were replaced either by aphano-crystalline dolomite or by halite and aretherefore of indeterminate origin. Accumula-tions of intact skeletal grains, including molds ofgastropods, ostracods, and pelecypods, are

FIGURE 12. Syntaxial overgrowth on an echinoderm plate-showing an early generation of cloudy cement with possiblerelict bladed fabric after aragonite, followed by sparry'calcite. Thin section from DOE-Gruy Federal Crabbe No. 1

core, 2,850.4 ft, lower San Andres Formation unit 2. Thinsection width is 0.85 mm; crossed nicols.

FIGURE 11. Brachiopod biomicrudite having abundant,large, mostly intact brachiopod shells. Thin section from theDOE-Gruy Federal Grabbe No. 1 core, 2,919.6 ft, lower SanAndres Formation, unit 2. Thin section width is 7.7 mm;crossed nicols.

micrite and dolomitized packstone and wacke-stone having small, sparse burrows. ln the lowerSan Andres cycles where dolomitized coated-grain /oolite grainstone and associatedlithologies occur in the same cycle as doesskeletal limestone, the dolomite generallyoverlies the limestone and underlies anhydrite.Grains in the coated-grain/oolite grainstoneinclude micrite-coated skeletal grains, micriticooids having concentric lamination defined bysubtle changes in micrite grain size and in traceamounts of very fine crystalline pyrite andorganic material, and structureless micriteintraclasts or pellets. ln some laminae, ooidsexhibit a peculiar fabric, as noted by Bein andLand (1982). Concentrically laminated ooidswere compressed against each other anddeformed without being fractured, forminghorizontally elongated ovals or paisley-shapedgrains (fig. 1a). ln many compressed ooids,concentric laminae are preserved. Some grainsare compressed to plate-shaped grains only a fewtens of microns thick. Compressed and normalround ooids are interlaminated, suggesting thatthey may have a common origin but a differentdiagenetic history. The deformed grains are

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FIGURE 13. Ripple-laminated dolomitized carbonategrainstone with carbonate mudstone drapes. Slab fromStone and Webster Mansf ield No. 1 core, 1,357 tt, base of anincomplete cycle in the middle San Andres. Core width isapproximately 9 cm.

grainstone. This indicates that the physicalenvironment remained stable while the geo-chemical environment became carbonateprecipitating.

Diagenesis of the silty ripple-laminateddolomicrite and of the ripple-laminated coated-grainloolite grainstone, packstone, and wacke-stone having low faunal diversity includespervasive dolomitization. Crains and matrix arereplaced by aphanocrystalline to very finecrystalline (3 to 10 g) dolomite that has a dustyappearance; many of the dolomite grains arerounded rather than euhedral. Halite cementand replacement of allochems by halite is

common (fig. 16). Anhydrite is present as

isolated laths, as cement, and as replacementnodules.

Anhydritic dolomicrite is found only in thelower three San Andres cycles (units 2,3, and 4),where it occurs in sequences as thick as severalmeters and forms a transition between thecarbonate and halite parts of the cycle. Thedolomite in the anhydritic dolomicrite is tan and

FIGURE 14. Micritic ooids, slightly compressed at the topand strongly compressed at the bottom. Note concentriclam ination visible in larger grains, dark orga n iclpyritic sta inon some grains, and microspar rims separating even thecompressed ooliths. Thin section from incomplete cycle atthe top of the middle San Andres, DOE-Cruy Federal RexWhite No. 1well, 1,487 ft. Thin section width is 3.25 mm;crossed nicols.

typically sorted by size and shape and ref lect lowfaunal diversity. Burrows are distinctly smallerand more uniform than those characteristic oflimestone intervals; many burrows arehorizontal. Compressed flecks of organicmaterial are present in some intervals, andorganic and clay mineral impurities wereconcentrated, probably by microstylolitization,to form wispy lamination. Many burrows werealso flattened by compaction.

Silty, ripple-laminated dolomicrite is com-monly associated with ripple-laminated coated-grain/oolite grainstone where the micrite is

draped over grainstone ripples (fig. 13). Thickersequences of finely laminated and ripple-cross-laminated dolomicrite may be transitional toburrowed packstone and wackestone. Ripplesare thin, discontinuous, and defined by laminaeof siliciclastic silt or fine carbonate allochems,including pellets, ooids, and skeletal fragments(fig. 15). The crossbedding suggests that themicrite may initially have been pelleted, but noevidence of pellets has been identified. ln manySan Andres cycles, the ripple-laminated top ofthe dark terrigenous mudstone is transitionalinto ripple-laminated dolomicrite and rippled

13

firfli

ili.[,.,;:

ii

i1

ir

prolonged episodes of normal- and nea:-normal-marine salinity. Although the predorn -nant allochems are transported grains, thepresence of some large brachiopod shells hal irEintact spines attests to local production c'skeletal carbonate in the northern Palo Durr:Basin. The diverse faunal assemblage, abundanceof large burrows, and early diagenetic histon c'the carbonate, including neomorphism o'aragonitic shells and possible bladed aragoniti:cements, are further indications of near-norma -marine shelf conditions during limestonedeposition.

Facies variations within the limestone areinterpreted as having resulted from migration o'high-energy bars that deposited coarse pac[<.-

stones across lower energy interbar areas, wherefiner sediments accumulated in a shallow-shetrrenvironment. Unlike other parts of the SarAndres cycle, the limestone here shows noevidence of increasing salinity during deposi-tion. On the basis of the petrographic relation-ships and the apparent normal-marine characterof the carbonate, halite and anhydrite withir:carbonates are interpreted as having formedduring burial diagenesis. The diagenetic fluids

FIGURE 16. Ooid replaced by halite (H), anhydrite (A), anddolomite (D). The matrix has been dolomitized. Thin sectionf rom DOE-Cruy Federal Grabbe No. 1 core,2,859.3 ft, lowerSan Andres unit 3. Thin section width is 0.85 mm; crossednicols.

*

FIGURE'15. Laminae in rippled dolomicrite are defined byconcentrations of ooids. Thin section f rom the basal cycle ofthe middle San Andres, Stone and Webster Mansf ield No. 1

core. Thin section width is 3.25 mm; crossed nicols.

well bedded (beds 1 to 5 cm thick), in contrast tothe Bray, massive character of the othercarbonate rocks (tig.17\. Some intervals in theanhydritic dolomicrite are laminated on a

millimeter scale by brown organic-rich partings.Anhydrite in beds and nodules is a majorcomponent in the anhydritic dolomicrite.Anhydrite also occurs as cement and as a

diagenetic replacement of carbonate minerals.Sedimentary structures typical of this intervalinclude blebs of anhydrite, similar to bird's-eyefabric, along dolomite bedding planes andnarrow anhydrite-f illed vertical fractures that cutand deform the dolomite bedding. Contortedbedding occurs in some intervals of finelyinterlaminated anhydrite and dolomite.

San Andres carbonate rocks can be dividedinto those showing evidence of deposition innormal- or near-normal-marine environmentsand those reflecting deposition in hypersalineenvironments. Thick limestone sequences in thelower San Andres cycles, including coarse,burrowed skeletal packstone containing diversefauna and dark, wispy-laminated packstone andwackestone having diverse fauna and Iargeburrows, are interpreted as representing

14

that caused dolomitization and precipitation ofanhydrite and halite cement were probablygenerated when marine carbonate environ-ments were replaced by sulfate- and halite-saturated brine pools, which allowed highersalinity waters to invade the permeablecarbonates. This pattern of diagenetic alterationof preexisting sediments by fluids derived fromoverlying parts of the cycle was repeated in otherlithologies within the San Andres cycles.

The San Andres rocks that contain featuresinterpreted to indicate hypersaline depositionalconditions include (1) ripple-laminated coated-grain/oolite grainstone, (2) packstone andwackestone having low faunal diversity andsmall, sparse burrows, (3) silty ripple-laminateddolomicrite, and (4) anhydritic dolomicrite. Thestratigraphic position of these carbonatesbetween normal-marine carbonate and anhy-drite suggests conditions of increasing salinity.The low faunal diversity and the dominance ofsmall, sparse burrows indicate physiologicallystressful environmental conditions. Dolomitiza-tion is pervasive, and dolomite is f inelycrystalline, as is typical of sediments deposited inhypersaline environments. The compressedooids may also indicate high-salinity conditions.The relationship between grainstones andcarbonate mudstones indicates an inter-fingering, rather than a vertical cyclic pattern,and therefore may represent the migration oflocal high-energy and adjacent lower energyenvironments.

The position of the anhydritic dolomicrite inthe facies tract between carbonate rocks andevaporite rocks might suggest that this facies was

the supratidal barrier commonly envisionedbetween the normal-marine and evaporiteenvironments (for example, Schreiber, 1978\.

Sedimentary structures within the anhydriticdolomite would seem to support this interpre-tation. Anhydrite bird's-eye structures resemblefenestral cavities common in supratidalcarbonate environments (Shinn, 1983). Verticalfractures may represent desiccation cracks, andfinely laminated dolomite may have formed as

supratidal algal laminites. San Andres nodularanhydrite and contorted bedding in interbedded

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FIGURE 17. Dolomitized carbonate mudstone from theStoneandWebsterMansfield No. 1core, 1,799 ft,topof thelower carbonate interval of unit 4. Carbonate mudstones atthe top of the carbonate intervals exhibit thin, irregularbeds, disrupted fabric, and replacement of carbonate byanhydrite in nodules and along fractures. However,diagnostic supratidal fabrics are sparse.

anhydrite and dolomite resemble fabricsidentified in the Trucial Coast sabkhas of SaudiArabia (Shearman ,1978) and may have formed ina comparable environment. An analogous suiteof fabrics in a similar position within cyclicevaporites of the Clear Fork Formation in thePalo Duro Basin has been interpreted byHandford (1981) to be a marginal intertidal tosupratidal sabkha.

Reconnaissance petrography of these facies,however, has failed to confirm evidence ofsupratidal exposure. The centers of anhydritebird's-eyes have ghosts of small allochems; thebird's-eyes have a rim of anhydrite with abun-dant dolomite remnants. These features,therefore, formed when anhydrite cementprecipitated in an allochem mold that served as a

nucleus for replacement of dolomite by anhy-drite. No evidence of primary fenestral voidspace exists, and these features thus have nosignificance in the interpretation of depositionalenvironment. ln thin section, the morphology ofvertical fractures also appears to be caused bydiagenetic replacement of dolomite byanhydrite along a hairline crack. When thediagenetic increase in width of the fracture is

recognized, the f ractures look more like joints orother f ractures typical of lithif ied rock than thosetypical of soft-sediment mud cracks. Well-formed features typical of subaerially exposedcarbonates, such as sediment-filled mud cracks,imbricate carbonate rip-up clasts, caliche andpisolite horizons, upward-coarsening shorefacesequences, truncation surfaces, and scouredchannels, are absent from all but a few intervalsof anhydritic dolomicrite. The anhydriticdolomicrite clearly formed in an environment ofat least intermittently increased salinity, as shownby the anhydrite beds and by the laminae withinit. The relationships between the sedimentarystructures produced in the depositional environ-ment and the structures produced by diageneticmodif ication of anhydritic dolomicrite, however,are unclear.

AnhydriteBedded anhydrite in the San Andres

Formation contains abundant primary deposi-

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FIGURE 18. Nodular anhydrite in laminated dolomitemosaic. Laminae in dolomite beds are not deformedenough to account for the volume of the nodules;therefore, a replacement origin is interpreted for theanhydrite nodules. Slabbed core from an incompletemiddle San Andres cycle, Stone and Webster Zeeck No. 1

well,2,520ft.

tional structures. The anhydrite part of the cycledisplays a well-developed vertical sequence:(1) anhydrite nodules in dolomite matrix;(2) anhydrite mosaic; (3) laminated anhydritewith or without pseudomorphs after bottom-nucleated gypsum crystals; and ( ) anhydrite-halite transition. The contact between anhydriteand dolomite is gradational through a few tens ofcentimeters and consists of anhydrite nodules indolomite mudstone. The relationship betweenthe dolomite and anhydrite is most apparentwhere the dolomite is well laminated (fig. 1B).

Dolomite mud appears to have been replaced byanhydrite or by a gypsum precursor, followed bycompaction of the dolomite mud around thesulfate nodules. Displacive growth of anhydritenodules may have occurred but has not beendocumented for San Andres anhydrite.

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FIGURE 19. Typical nodular anhydrite mosaic from a thinupper San Andres cycle, Stone and Webster Detten No. 1

well, 2,176 ft. Slabbed core, width 10 cm.

Anhydrite mosaic (terminology of Maiklemand others,1969) makes up the lower part of theanhydrite unit (fig. 19). Nodules average 'l to3 cm in diameter and are equant to horizontallyelongated. Variation in size and orientation ofnodules defines crude bedding. Noduleboundaries are defined by dark material, whichwas petrographically identified as minoramounts of aphanocrystalline dolomite and traceamounts of clay minerals and pyrite. lntervals ofanhydrite having as much as 25 percent dolomiteas interbeds or as a dolomite matrix in ananhydrite nodular mosaic occur at the base of orwithin the anhydrite mosaic of some cycles.Nodular anhydrite and nodular mosaic anhydritein the Palo Duro evaporites have beeninterpreted as sabkha sediments on the basis ofcomparison with modern Persian Gulf examples

(Handford, 1981). ln the Persian Culf, sulfatenodules are formed within subaerially exposedsediment when marine and terrestrially derivedwaters are concentrated by evaporation toanhydrite saturation. However, nodularanhydrite has been observed in numerous otherenvironments, such as in laminated deep-watersediments (Dean and others,1975) and in areasshowing diagenetic alteration of primary-bedded, subaqueously deposited gypsum tonodular fabrics. Loucks and Longman (1982)identified relict fabrics indicative of shallowsubtidal deposition in anhydrite mosaic in theFerry Lake Anhydrite of East Texas. Schreiber andothers (1982) documented the formation ofanhydrite mosaic during dehydration of beddedgypsum caused by heating along faults.Apparently, the nodular fabric in anhydrite is aresult of diagenetic alteration of many sulfatesand cannot be used alone as an indication of thetype of depositional environment. Some SanAndres nodular anhydrite-carbonate mixturesmay have formed in sabkha-like subaerialenvironments, especially those within possiblesupratidal anhydritic carbonates. However, mostof the anhydrite mosaic is interpreted to be theresult of diagenetic alteration of subaqueouslydeposited gypsum. Evidence of a subaqueousbrine-pool origin for most of the San Andresanhydrite mosaic includes (1) absence of thetypical sabkha cycle (Shearman, 1971), (2) typicalposition of the anhydrite mosaic overlyingsubaqueously deposited carbonate and under-lying subaqueously deposited laminated anhy-drite, (3) common gradation between nodularand laminated fabrics manifested by occurrenceof deformed and altered laminae and presenceof pseudomorphs after gypsum crystals withinanhydrite mosaic (fig. 20), and (a) occurrence ofrelict anhydrite pseudomorphs after bottom-nucleated gypsum crystals in dolomite (fig. 21).

Laminated anhydrite having pseudomorphsafter vertically oriented gypsum crystals overliesthe anhydrite mosaic. The pseudomorphs aftergypsum crystals are commonly poikilotopichalite crystals in an anhydrite matrix (figs.22and 23), but anhydrite pseudomorphs inanhydrite matrix are also found (fig.2a\. Pseu-

FIGURE 21. Unusual subhedral anhydrite nodules l:dolomite matrix. Possibly originated as gypsum crystals :::carbonate matrix. Compare with figure 20. Slabbed corefrom Stone and Webster J. Friemel No. 1 well, 2,329.5 ti-incomplete middle San Andres cycle.

domorphs vary from a few millimeters to 40 cmtall and exhibit a variety of morphologies. Smallpseudomorphs along bedding surfaces appear tohave replaced prisms of gypsum, many of whichwere twinned. Larger crystals may occur in beds(tig. 221or in intervals lacking apparent bedding(fig. 23). Large pseudomorphs commonly showcomplicated forms, including crystals that arewider at the top, and contain relict crystalgrowth surfaces defined by anhydrite in halite

FIGURE 20. Gradation between nodular anhydrite andanhydrite having some nodular texture and some poorlypreserved halite pseudomorphs after gypsum. Thegradation between fabrics suggests that the nodularanhydrite formed by alteration of brine-pool gypsum.Slabbed core from DOE-Gruy Federal Crabbe No. 1 well,2,487 It,lowest cycle of lower San Andres unit 5.

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FIGURE 22. Halite pseudomorphs after gypsum in halite, inanhydrite, and in dolomite matrix. Slabbed core from Stoneand Webster Harman No. 1 well, 2,483 lt, top of anincomplete middle San Andres cycle.

pseudomorphs and dolomite in anhydritepseudomorphs. Even tall crystals did not projectfar above the surface of the host sediment, as

shown by tracing relict growth surfaces of thepseudomorphs into the bedding of the hostanhydrite (tig.2q. The height of the crystalreflects repeated re-nucleation in continuitywith the existing crystals after sediment influxrather than the formation of new crystals. Thebedded and locally ripple cross-laminated

FIGURE 23. Tall pseudomorphs after gypsum, now halitewith relict growth bands defined by anhydrite, in anhydritematrix. Slabbed core from Stone and Webster C. FriemelNo. 1 well, 1,902 tt, anhydrite bed in the upper San Andres.

19

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exposure. Large gypsum crystals in the San

Andres have been replaced by halite pseudo-morphs, and fine gypsum sediment betweercrystals has been replaced by anhydrite.

At the top of the anhydrite unit is a transitionto halite. The transition interval is a mixture oihalite and anhydrite and, in some places, a minoramount of red-brown siliciclastic mudstone. Thetextures in this interval are complex and variableand include contorted beds, interbedded anhr-drite and halite, randomly oriented coarse blades

of anhydrite in halite cement, and partly de'stroyed anhydrite pseudomorphs after gypsurn(fig. 25). The distinctive textures in the zone oitransition from bedded anhydrite to beddedhalite are characterized by fine interbedsand diagenetic replacement relationshipsbetween halite and anhydrite. These texturesare interpreted as being the result of dia-genetic alteration when conditions within thesediment and within the precipitating water

FIGURE 24. Anhydrite pseudomorphs after gypsum crystalsin dolomite matrix. Note Iaminae in larger dolomite masses

that show successive positions of the brine-pool floorduring sediment accumulation. Stone and Webster DettenNo. 1 well, 2,550 ft, lowest incomplete cycle of unit 5.

structure of the host anhydrite suggests that hostanhydrite accumulated as detrital grains, but thegrain size and initial mineralogy (gypsum oranhydrite) have been obscured by diagenesis.Hardie and Eugster (1971) and Schreiber andKinsman (1975) described similar verticallyoriented gypsum crystals and interpreted themas indicating growth in a shallow pond environ-ment. The absence of truncation surfaces acrossthe crystals suggests that pond depth was

sufficient to preclude lengthy episodes of

FIGURE 25. Disrupted fabrics at the contact between halite(dark) and anhydrite (light). Slabbed core f rom the top of ananhydrite bed in the upper San Andres, Stone and WebsterHarman No. 1 well, 2,205 {t. Photograph width is 9 cm.

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body alternated between gypsum saturation andhalite saturation.

The main cause of differences in texture andmineralogy among anhydrite mosaic, beddedanhydrite, and anhydrite-halite transition isinterpreted to be the relative timing ofintroduction of more saline diagenetic fluids(fig. 26). Anhydrite mosaic underlies beddedanhydrite and therefore underwent initialdiagenesis in f Iuids derived from gypsum-precipitating ponds. Mosaic textures also havebeen observed in the upper few centimeters ofbedded anhydrite beneath dolomite beds ordark mudstone residue at the base of cycles,indicating that formation of mosaic textures canoccur under the inf luence of carbonate-precipitating normal-marine waters. Thesuppressed development of mosaic textures andcorresponding good preservation of primaryfabrics appears to be associated with a stage ofdiagenesis in halite-saturated waters. ln most

intervals where primary fabrics are well pre-served, large gypsum crystals were replaced byhalite pseudomorphs. However, the uppermostanhydrite bed in the cycle, which was subjectedto halite-saturated waters shortly afterdeposition, has poorly preserved fabrics. On thebasis of observed location in the cycle, optimumconditions for preservation of primary fabricsappear to be an initial episode of diagenesisbeneath the gypsum-precipitating brine pool,subsequent burial of 1 to 3 m, and, finally,introduction of waters derived from a halite-saturated brine pool.

Dehydration of gypsum to anhydrite is acommon process, occurring both in Recentsediments and under laboratory conditions inresponse to increasing salinity or increasingtemperature (Deer and others,1962, p. 209-211;Shearman, 1978). Pseudomorphous replacementof gypsum by halite is commonly recorded inancient sediments (Schaller and Henderson,

filJ As solinily increcses, gypsum isdeposited obove corbonote.

2 Orrru^ deposition conrinues.6\Q) lncreosing restrtclion rncreosessolinity to holite solurotion.

Sulfole-soluroled brines penelroteunconsolidoted cobonoles ond depositreplocive/displocive nodules ofgypsum or onhydrite.

Sulfole-soturoted brines penelrotingpreviously deposited gypsum ot shollowburiol depths dehydrote gypsum toonhydrite. During olterotion, nodulorfobric develops.

Holite-soluroted brines penetrotingpreviously deposited gypsum ot shollowburiol depths reploce remoining gypsumwilh holite but do not offect onhydrite.The holile replocement therefore offectslhe uppermost beds ond lorge gypsumcrystols lhol were slow to dehydrole.

FIGURE 26. Model for the originof the sequence of fabrics observed in the anhydrite parts of cycles. The salinity of the water inthe brine pool influences the diagenesis'of previously deposited sediments.

21

1 932; Stew art, 1949 ; J o n es, 1 965; H old ow ay,197 8)

but has not been reported in Recent sediments.Phase diagrams of the anhydrite-gypsum systemshow that the solubilities of both gypsum andanhydrite are enhanced in a sodium chloridesolution and that at higher salinities and highertemperatures gypsum is more soluble than isanhydrite (Bock, 1961). Textural evidence in SanAndres bedded anhydrite suggests that the firstphase of diagenesis dehydrated the initialgypsum sand or mud matrix around the largegypsum crystals and produced anhydrite. Thecenters of the large crystals had no time to reactbefore the concentration of sodium chloride inthe overlying water body increased pore watersalinity to favor replacement of gypsum by halite.The preservation of the f ine laminae that formedalong gypsum-crystal growth surfaces within thepoikilotopic halite is evidence that halitereplaced gypsum without formation of voidspace (figs.22 and 23). The uppermost anhydritewas immersed early in concentrated sodiumchloride brine, probably before much dehydra-tion of gypsum occurred. Therefore, dehy-dration of gypsum and replacement of gypsumby halite occurred nearly simultaneously. Someprimary fabrics are preserved by anhydrite rims,whereas others were too completely altered byhalite replacement to be recognizable. Someprimary anhydrite also may have formed as thebrine approached the halite-saturation level.However, some small bladed anhydrite pseudo-morphs after gypsum extend to the contact of theanhydrite and hdlite sequences, suggesting that a

phase of dominantly primary anhydriteprecipitation did not occur.

Red-brown mudstone that occurs within or atthe top of the anhydrite-halite transition is

interpreted to be genetically similar to red-brown mudstone beds within the halite section.ln a few examples, mudstone appears to havecollapsed into voids in the anhydrite. Some ofthese voids appear to be pseudomorphs aftergypsum and are partly f illed with halite,suggesting that the cause of collapse may havebeen dissolution of some halite from theanhydrite-halite mixture. Evidence of similar

processes are associated with mudstone bee:throughout the halite section.

ln some bedded anhydrite no halite ::r

present, and the anhydrite forms well-preserre*:pseudomorphs after small gypsum blades. Tredifference between the conditions that forrne:this fabric and the conditions that forrne:nodular mosaic fabric is not understood.

HaliteBedded halite rock having interbeds ar"r.

disseminated impurities of anhydrite ancmudstone forms the upper part of each c.vcle"

Complex textures in halite result from its higi'solubility and reactivity during early diagenesis,During deposition, many preserved primarrtextures in halite formed as crusts of crystals onthe floors of extensive, shallow brine pootrs.

Fabrics in halite that are interpreted to hareresulted from brine-pool precipitation inclucie(1) vertically elongated halite crystals havinerelict growth surfaces defined by variations inabundance of minute fluid inclusions (chevronzoning), (2) vertically elongated halite crystalslacking zoned fluid inclusions, and (3) halitebedding defined by color changes or b',interbeds of mudstone or anhydrite.

Vertically oriented halite crystals and crystalshaving upward-pointing chevron-shaped fluid-inclusion zones are abundant (figs. 27 and 2Bi.Similar features have been observed inexperimental and modern sediments forming bvbottom-nucleated growth in shallow brine pools(Arthurton, 1973; Lowenstein and Hardie, 1985).The regular zonation of fluid inclusions in thechevron halite of the San Andres Formation is

interpreted by Roedder (1982) to be the result ofdiurnalvariation in the brine-evaporation rate inextremely shallow (less than a meter deep) brinepools. Halite having abundant chevrons istypically white because of the fluid inclusionsand the presence of anhydrite as the dominantimpurity. Halite composed of vertical crystals butlacking abundant chevron structures is typicallydark colored, resulting from the presence ofminor amounts of reduced gray clay or traces oforganic materials, or both, in addition toanhydrite. The absence of well-formed chevron

tt

FIGURE 27. Vertically oriented crystals in halite outlined bydiagenetic anhydrite. The vertical orientation of crystals,color banding, and white cloudy areas due to fluidinclusions trapped along growth faces (chevrons) arediagnostic of halite deposited on the floor of a brine pool.Slabbed corefromStoneand WebsterJ. Friemel No. 1 well,2,673 ft, lower San Andres unit 4. Photograph width isapproximately I cm.

fabric may reflect slower halite precipitation inslightly deeper water.

Horizontal red, white, and black bands inhalite formed as a result of introduction ofimpurities such as organic material, airborne orwaterborne dust, or anhydrite into the brinepool. Thin seams of anhydrite formed duringinterruptions of halite precipitation that werecaused by periodic dilution of the water withinthe brine pool by the introduction of marine-

FIGURE 28. Photomicrograph of chevron structure in halite,showing fluid inclusions trapped along relict growthsurfaces of bottom-nucleated halite crystals. Stone andWebster Mansfield No. 1 core, 1,377 ft,top of unit 5 halite.Photograph width is 7.7 mm, plane light.

derived water or meteoric water or both. Thepreviously deposited halite was corroded andpitted on a scale of centimeters, and chevronfabrics were truncated (fig. 27). Cypsum, formedin the initial phase, was precipitated from thediluted brine. As evaporation continued, halitesaturation was reached and halite precipitationresumed.

Other halite brine-pool fabrics, such asaccumulations of hoppers and rafts that formed

23

at the water/air interface (Arthurton ,1973)' may

have been present. All trace of them, however,has been lost by recrystallization or dissolution'The presence of detrital halite ooids and mud has

been documented in modern environments(Weiler and others,1974), but few examples ofpossible detrital halite have been observed in San

Andres halite.The brine pools intermittently dried up and

became exposed halite f lats during interruptionsof the marine-derived brine influx' Microenvi-ronments on the isolated dry f lats were subjected

either to further evaporative concentration of

the standing brine or to partial dissolution a-:recrystallization of halite in contact rt "-meteoric water. Low-salinity water penetra: -;the halite along grain boundaries and ver- -;f ractures dissolved karstic pits and solution pi:t:(figs. 29 and 30) and induced recrystallization :-halite, destroying primary fabrics. Cessation --halite growth permitted accumulations of :::siliciclastic mudstone. Mudstone was trar-:-ported onto the halite surface by eolian c-ritorrns and sheetwash processes. Vertic:f ractures in mudstone beds are f illed with fibro-:red halite and appear to form polygonal patte!-:i

FTCURE 29. Karst pit in liSht-colored halite with.preservedbrine-oool fabrics', f illed with coarser halite. Slab f rom Stone

and Webster Mansfield No. 1 core, 1,424 It, top of unit 5

halite; width 10 cm.

FIGURE 30. Karst pit in banded halite filled with a firstgeneration of coarse halite, then with mudsto.ne, f inally withSisolacive halite. Repeated episodes of dissolution and

recrystallization *ouid produce chaotic mudstone-halitet"*t,1t". Slab from Stonb and Webster Zeeck No' 1 well,2,039.4 ft, upper San Andres.

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in plan view. Repeated episodes of wetting andhalite dissolution, followed by drying andprecipitation of halite cement and halitedisplacive cubic or skeletal crystals, destroyedbedding in both primary halite and terrigenousmudstone. The resulting fabric retains little

FIGURE 31. Chaotic mudstone-halite from top of the SanAndres Formation, Stone and Webster Harman No. 1 well,1,955 fr.

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9-

primary structure and is described as chaoticmudstone-halite (fig. 31).

The halite member of the cycle does not showa well-developed textural or geochemicalvertical profile but is composed of 1- to 4-m-thicksequences of alternating zones of anhydritichalite having preserved primary fabrics andzones of abundant mudstone interbeds andchaotic mudstone-halite fabrics (Hovorka andothers, 1985).

Terrigenous Red BedsMudstone, siltstone, and very fine grained

sandstone red beds, commonly0.5 to 2.5 m thick,are present within the halite members of cycles,at the tops of cycles, and between incompletecycles in the San Andres Formation. The thickerred beds represent more significant pulses ofclastics into the evaporite environment, and theirgeophysical log signatures can be correlatedregionally. These beds display ripple lamination,claystone drapes, and, in some intervals, well-sorted, coarse sand-sized clay rip-up clasts(f ig. 32). Fine sandstone or siltstone-filled casts ofhalite crystals are locally abundant. Anhydritenodules, displacive halite cubes, and skeletalhalite crystals are common (fig. 33); soft-sediment microfaulting is abundant in someunits (fig. 34). The typical red-brown color is dueto oxidized iron stains in clay and clay coats onsand grains. Some of the sandier red beds, such asthe pi-marker bed, are partly reduced beneathanhydrite or carbonate beds. The resulting light-gray colors are easily distinguished from the darkcolors of the genetically different anhydriticmudstones at the base of cycles. The compositionand sedimentary structures of the red beds in theSan Andres Formation are similar to the adjacentClorieta and Queen-Crayburg sequences. TheseSan Andres red beds represent minor episodes ofclastic progradation across the shelf thatprobably prefaced the major progradationalevent represented by the overlying Queen-Crayburg red-bed sequence. The occurrence ofclastic units appears to be extrinsically controlledand is partly independent of the typical SanAndres salinity-controlled cyclicity. Red beds are

25

CM

0-

tICURE 32. Very fine sandstone containing small-scalescours, clay rip-up clasts, clay drapes, and ripplecrossbedding. These structures are typicalof the pi-markerbed, a 3-m-thick red bed that marks an episode of clasticinflux across evaporite environments. Stone and WebsterC. Friemel No. 1 well, 2,125.9 {1.

most common within and at the top of the halitepart of the typical San Andres cycle, althoughthey may occur elsewhere in the cycle. Wherered beds were deposited at or near the top of acycle, the character of the black mudstone at thebase of the next cycle is partly or completelyimprinted on the red bed. In such configura-tions, the boundary between the underlying redbed and the overlying insoluble residue is

blurred and difficult to establish precisely.

FIGURE 33. Typical mudstone bed within lower San Anc-=.unit 5, showing disruption of fabric by displacive gror.:-and subsequent removal of halite crystals. ln this exarrc =relict clay drapes, clay rip-up clasts, and ripple cros;-lamination can be identified between vaguely euhei::areas of disturbed fabric, which are collapsed molds :'halite crystals. A second generation of displacive zone:halite crystals has been preserved. Light areas are anhydr-.;enodules. Slabbed core f rom Stone and Webster Mansf ielcNo. 'l well, 1,494 ft. Core width is approximately B cm.

Variation from the ldealVertical Cyclic Sequence

Variations f rom the ideal cyclic facies pattern,observed in individual cycles in the Palo DuroBasin, offer two kinds of information.Comparison of successive cycles in one areaprovides data on the temporal evolution of cyclicstyle. Tracing vertical variations within a singlecycle across the basin helps to delineatedepositional environments and controls onsedimentation.

Vertical facies sequences within cycles of theSan Andres Formation may differ from theidealized cycle in four ways: (1) The initialregional transgression may not have introducednormal-marine water into a given local area, so

the carbonate units in the lower part of the idealcycle may not have been deposited. Therefore,the cycle may begin with dolomite, nodularanhydrite, or bedded anhydrite above a thin dark

26

mudstone residue. (2) The cycle may have beeninterrupted by a transgression before halitedeposition, causing omission of the upper unitsof the ideal cycle. (3) The halite at the top of thecycle may have been completely removedduring the transgression initiating the next cycle.(a)The cycle may be punctuated by introductionof clastics.

Recognition of nonideal, incomplete cycles is

crucial to understanding the cyclic nature of theSan Andres Formation. ln later sections of thisreport, the lateral changes from incompletecycles to complete cycles will be examined. Fewcycles within the San Andres Formation exhibitthe complete, ideal facies sequence.Recognition of incomplete cycles permits

division of the sequence into geneticallysignificant units. For example, the San Andres"cycle 5" of earlier workers (fig.3) wasidentified, on the basis of log reconnaissancestudies, as a sequence composed of a lower,domi nantly anhydrite-carbonate u nit overlai n bya dominantly halite unit (Presley, 1980).However, in some locations, the loweranhydrite-carbonate unit contains thin halitebeds. The upper halite unit contains variablythick anhydrite beds. The apparent variabilitywithin this interval initially created confusionabout whether the contact between cycle 5 andcycle 4 of earlier workers (fig. 3), and Iikewisebetween the top of cycle 5 and the base of theoverlying middle San Andres sequence, shouldbe placed at the lowest anhydrite or at theuppermost halite bed. Detailed examination ofthe previously described cycle 5 in core andregional cross sections permits identification offive complete and incomplete genetic cycles thatcan be traced Iaterally. Recognition of theseincomplete cycles thus clarifies the stratigraphyof the interval and eliminates correlationdifficulties. These genetic cycles are discussedlater in this report under the heading Styles ofCyclicity in the San Andres Formation; unit 5 isdiscussed in further detail under the headingLower San Andres Cenetic Sequence.

Correlation of San AndresFormation Cycles

Cycles in the San Andres Formation consti-tute a diverse suite of lithologies includingsiliciclastic, carbonate, and evaporite facies, as

described in the previous section. These faciesexhibit distinctive geophysical log characters(Handford, 1980; McCillis, 1980; Presley, 1981c;Presley and Ramondetta,lg8l; Ramondetta andMerritt, 1982; Ruppel and Ramondetta, 1982),and individual cycles can be correlated withconfidence on gamma-ray logs. Other geo-physical, caliper, and sample logs were used tohelp discriminate lithologies and augmentcorrelation.

The key to the recognition of cyclicity andcorrelation within the San Andres on gamma-ray

FTGURE 34. Soft-sediment microfaults in ripple-laminated,very f ine grained sandstone with clay drapes and sand-sizedclay rip-up clasts. Pi-marker bed, Stone and Webster ZeeckNo. 1 well, 2,291 tt. Slabbed core width is approximately7 cm.

o

"l, ,'ri,'!"s"'*

w{i ..:a'{:\. 'i ' .'\l..'^- '1-ai "- r;.1ry '- rsq

27

logs is the identification of the dark mudstonebed that def ines the base of each cycle. lnsolubleresidues and carbonates that define the bases ofthe complete, relatively thick cycles of the lowerSan Andres (units 2,3, and 4; pls. 1 through 4)produce characteristic asymmetric signatures ongamma-ray logs (fig. 6). The log pattern displays a

high API-unit basal peak (equivalent to darkmudstone) that is sharply set off from the lowAPI-unit evaporite baseline below and is toppedby a gradual shift upward toward the evaporitebaseline.

Mudstone and carbonate at the base of thethin cycles characteristic of the middle and upperSan Andres (combined mudstone-carbonateunit thickness ranges approximately 0.6 to 6 m)produce a single, sharp gamma-ray peak that is

difficult to differentiate from the sharp gamma-ray peaks caused by siliciclastic red beds that arenot associated with the bases of cycles. Thedistinction on a regional scale between mud-stone cycle bases and other siliciclastic beds is

facilitated by their identification in core andextrapolation to correlative gamma-ray peaks inother wells.

Four cross sections of the San AndresFormation in the Palo Duro Basin and adjacentareas show the vertical repetition and lateralextent of cycles ({ig. a; pls. 1 through 4). Thesharp peaks on gamma-ray Iogs that correspondto the bases of cycles can be traced on geophysi-cal logs over the entire present structural limits ofthe Palo Duro Basin and beyond, throughoutareas of approximately 16,000 to 26,000 km2. lnsome locations, these base-of-cycle facies areless than 1 m thick, rendering them at or belowthe lower limit of consistent geophysical logresolution. Subtle variations in the sequence offacies that are below the level of resolution ofgeophysical logs can also be correlated betweencores across the northern Palo Duro Basin(figs. 35, 39, and 40).

Extension of San Andres Formation crosssections from the Palo Duro Basin north into theDalhart Basin and south onto the Northern Shelfof the Midland Basin (pls. 1 and 2) suggestsseveral extrabasinal correlations. The Yellow-house Dolomite on the Northern Shelf of the

Midland Basin is the southern equivalent to :;:lower San Andres unit 4 carbonate (Ramonde:--.1gB2). To the north, a siliciclastic unit situaie':between the San Andres unit 4 carbonate ani :;:lowest San Andres unit 5 anhydrite correlatir e -the Dalhart and Anadarko Basins was describe:by some workers as being Flowerpot SX-ra,e

(Jordan and Vosbu r9,1963, their f igs. 10 and l-n .

Because the Flowerpot Formation north of theAmarillo Uplift is directly overlain by the BlaineFormation, the Blaine appears to be correlatirewith the middle San Andres and the lower Sa:r

Andres unit 5 and possibly with the upper 5arAndres, as well, in the Palo Duro Basin. TheFlowerpot Formation is therefore equivalent iounit 4 of the lower San Andres in the Palo DurgBasin and also to the underlying units 2 and 3 as

far north as they can be recognized.The transgression that initiated depositiori

of lower San Andres unit 4 is unquestionabl,v theevent of greatest magnitude that occurredduring development of the San Andressequence. Unit 4 is the thickest individual cvclein the central area of the Palo Duro Basin ancextends f urther laterally than cycles 2 and 3 of theIower San Andres. The distribution of the lorrenunits 2 and 3 is largely confined to the existingstructurally defined limits of the Palo Duro Basin.The thickness and facies distribution of uniLc land 3 were strongly influenced by the AmarilloUplift belt to the north. Both units lose theingeophysical log definition eastward-approaching the outcrop belt of the BlaineFormation (pls. 3 and 4). The basal unit -1

mudstone-carbonate-anhydrite sequence-however, continues north across the AmarilloUplift and extends f urther east toward the BlaineFormation outcrop belt.

Styles of Cyclicity in theSan Andres Formation

Cyclic sedimentation is characteristic of manlPermo-Carboniferous stratigraphic sequences(Merriam, 1 964; Wi lso n, 197 5 ; Crowel l, 1 978). lt isa particularly prominent feature of NorthAmerican Permian Basin stratigraphy at severalscales (Jacka and others, 1969; Meissner, 1969;

28

Silver and Todd,1969; Dean and Anderson,1982;Handford and Bassett, 1982) and has also beenrecognized in the Late Permian EuropeanZechstein evaporite basin (Smith, 1981). Cyclicityin the San Andres Formation has been docu-mented in east-central New Mexico (Pitt andScott, 1981) and the Northern Shelf of theMidland Basin (Chuber and Pusey, 1969) inaddition to the Palo Duro Basin.

Potential allocyclic controls on cyclicity innon.clastic depositional systems includechanging lithospheric plate configuration, eu-static sea-level fluctuation, climatic oscillation,and variation in regional basin subsidence rate.The primary autocyclic control is sedimentaggradation.

Sediment aggradation may well have influ-enced the within-cycle vertical facies successionsevident in the San Andres Formation. Becausecarbonate-evaporite sedimentation rates areusually greater than basin subsidence rates,continual deposition on a shallow shelf mightgradually decrease the water depth duringmarine stillstands, leading to restrictedcirculation and brine concentration that wouldresult in a vertical succession of increasinglyevaporitic facies within each cycle (Wilson,1975).Allocyclic controls, however, must still beconsidered to explain the repetition of cycles.The short-period cyclicity of the San AndresFormation cannot be explained by long-periodchanges in lithospheric plate configurations;moreover, the paleogeographic position of theTexas Panhandle area was nearly constant duringthe Late Paleozoic (Bambach and others, 1980).Local climatic oscillation also seems an unlikelycontrol because both marine and nonmarinedeposits in the Texas Panhandle area appear torespectively record parallel trends of increas-ingly restricted evaporitic and arid continentaldepositional environments throughout thePermian. Therefore, the two most likelyallocyclic controls on San Andres cyclicity areeustatic sea-level fluctuation and changing basinsubsidence rate.

Tectonic (Valentine and Moores, 1972;Pitman, 1978) and glacial controls (Jacka andothers, 1969; Silver and Todd, 1969; Crowell,

1978; Pitman, 1978) have both been invoked toexplain inferred eustatic sea-level changes in thelate Paleozoic. Volumetric changes of mid-oceanic ridge systems related to tectonic eventswere too long in duration and too high inamplitude to account for San Andres cyclicity(Pitman, 1978). However, Permo-CarboniferousCondwana glacial deposits are known fromlower Guadalupian Series equivalents (Crowell,1978),thus encompassing the time intervalof SanAndres deposition.

Clacially controlled eustatic sea-levelchanges appear to possess the appropriatemagnitude and periodicity to produce many ofthe upward-shoaling cycles evident in thegeologic column (Cuidish and others,1984) andpossibly the San Andres cycles as well. Episodicchange in the regional basin subsidence rate alsoremains a plausible control on San Andrescyclicity. Unfortunately, the effects of changingsubsidence rate and eustatic sea-level changecannot be differentiated at this time. Cuidish andothers (1984) also suggested that changes in basinsubsidence rate are linked to eustatic sea-levelchanges, which would render the combinedeffects on cyclicity particularly diff icult todiscriminate.

Three discrete genetic packages of cycles canbe recognized in the San Andres Formationwithin the present structural limits of the PaloDuro Basin. These packages are differentiated onthe basis of regional continuity of cycle bases andsystematic differences in the vertical distributionof cycle completeness and thickness. The SanAndres Formation in the Palo Duro Basin isaccordingly divided into informal lower, middle,and upper genetic sequences (fig. 3; pls. 1

through 4). Differences in the styles of cyclicityamong the genetic units probably reflect eitherchanges in regional basin subsidence rate orchanges in frequency and amplitude of glacio-eustatic sea-level fluctuation or changes in both.ln the following discussion, the three observedstyles of cyclicity in the San Andres Formation arerelated to possible scenarios of changingregional basin subsidence rate and changingfrequency of eustatic sea-level rise and fall,which are treated as both independent and

)

29

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No horizoniol sco e EXPLANATIONLITHOLOGY

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SEDIMENTARY STRUCTURES

lwlx:fl;l:1,. [7ll p;ii3;" ffi 'n"'on no"n llBl[i,ifu:* l3rl [i;r,r",,"", E ;;g;ii3'

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oa 5292

tIGURE 35. Lower San Andres unit 5, a generally northwest-southeast cross section through cored U.S. Department of Energywells showing that what appears to be a fairly simple carbonate-anhydrite un.it overlain by.a halite sequence in Deaf SmithCounty is actually a composite of four or f ive thinner cycles. Each cycle has a carbonate-anhydrite or anhydrite lower unit and ahalite upper unit. The halite has been preserved in allthe cycles in Swisher County but has been removed from the lower cyclesin Deaf Smith County.

30

interdependent. Further study is required,particularly in correlative extra-basinal areas, todiscriminate between these factors.

Lower San Andres Genetic SeguenceThe lower San Andres genetic sequence

reported here corresponds to the lower SanAndres Formation reported by previous workersand includes the previously designated units 1

through 5 (fig. 3; pls. 1 through 4). These cyclespossess a complete verticalfacies sequence fromthe basal insoluble residue to the upper halite.Cycles of the lower San Andres genetic sequenceare relatively thick; units 2 through 5 togetherinclude slightly more than half of the totalthickness of the San Andres Formation in thebasin center. The existence of thick carbonates atthe bases of these cycles implies prolongedepisodes of open circulation and near-normal-marine salinity at moderate water depths. Suchconditions might have best been obtainedduring times of relatively rapid basin subsidencethat kept pace with high carbonate productionduring transgressive events.

Units 2,3, and 4 are the most complete cycles,based on examination of core from Deaf Smith,Swisher, and Randall Counties. These units arecharacterized by a thick, burrowed normal-marine limestone in the lower part and a well-formed halite section in the upper part. The grosssequence of Iithologies observed in these unitscould represent the deposits formed in a singlegenetic cycle. Examination of facies in core,however, shows that the sequences are notalways simple salinity-increasing cycles. Forexample, in the carbonate part of unit 4, a thickinterval of burrowed limestone with abundant,diverse skeletal material overlies several metersof sparsely burrowed, unfossiliferous anhydriticdolomicrite (fig. 10). This sequence appears toreflect deposition of carbonate at the beginningof the cycle under hypersaline conditions,yielding dolomite, followed by a decrease insalinity to near-normal-marine values, yieldinglimestone, which would thus appear to representa transgressive sequence deposited underconditions of decreasing salinity. However, in

most cores, the lower dolomite sequence is moreanhydritic at its top, directly beneath theapparently normal-marine sequence. At theStone and Webster Detten No. 1 well, beddedanhydrite on top of the basal dolomite indicatesthat the hypersaline water body locally reachedgypsum saturation. The lower dolomite of theSan Andres unit 4 carbonate, therefore,represents an incomplete, salinity-increasingcycle truncated by transgression of the near-normal-marine waters that deposited theoverlying carbonates. The lower dolomitesequence of the unit 4 carbonate examined incore from the farthest updip well (Stone andWebster Mansfield No. 1) is overlain by beddedanhydrite and bedded halite, forming a

complete cycle (fig. 10). The lateralfacies changebetween a complete cycle and an incompletecycle confirms the interpretation that the lowerdolomite of the unit 4 carbonate could representa regressive cycle that is genetically separatefrom the overlying near-normal-marine carbon-ate. A similar composite character can beobserved in unit 3. Unit 2 has not yet beenstudied in core but appears to be a single geneticcycle.

Unit 1 of the lower San Andres Formation,identified by Presley (1979b, his figs. 27 and 29;1980, his fig. 9; 1981b, his fig. 21; Presley andRamondetta, 1981, their fig. 40), is best expressedon geophysical logs near the southern margin ofthe Palo Duro Basin. lt cannot be differentiatedfrom the alternating red beds and halite of theunderlying Clorieta sequence in the northernpart of the basin. Unit 1, therefore, represents aminor transgression having a limited influence,not extending far enough north in the basin toproduce a recognizable facies change inavailable cores.

Throughout the northern Palo Duro Basin,the composite nature of unit 5, consisting of fiveincomplete cycles, thus being the most complexunit of the lower San Andres, is apparent (fig. 35).The character of this unit is transitional betweenthe cycles of the lower and middle San Andresgenetic sequences. The thick halite bed at thetop of the unit (fig. 35, cycles 5-c and 5-d) is likethose of units 2,3,and 4, whereas the composite

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FIGURE 36. Relationship between the halite at the top ofSan Andres unit 4 and the basal cycle of unit 5. The datumfor the section is hung on the base of a thin anhydritemarker within unit 4, which is interpreted as being a brieftransgressive episode that can be used as an approximatetime line. The thin halite section at the top of unit 4 in theDeaf Smith County cores corresponds to the area where a

thick dark mudstone occurs at the base of unit 5. ln thissame area, the cycle at the base of unit 5 is thick anddolomitic.

nature of the lower nonsalt facies of the unit(fig. 35, cycles 5-a and 5-b) is more similar to thatof the middle San Andres interval. Recognitionof the composite character of unit 5 permits com-parison of subtle facies changes within cycles.Details of the relationships within cycles are at ornear the limits of geophysical log resolution, sothe best interpretation can be made from theareas having core control. The basal cycle ofunit 5 is dominantly anhydrite. Carbonate isfound in the basal cycle only in the Stone andWebster C. Friemel No. 1 core in Deaf SmithCounty, where it occurs as a minor componentinterbedded with anhydrite. The basal cycle ofunit 5 is also thickest in the Deaf Smith Countyarea and becomes thinner to the north and west

(fig. 36). These relationships are interpreted as

indicating that salinity was closer to normalmarine in the Deaf Smith County area duringdeposition of this cycle. The distribution of thedark anhydritic mudstone at the base of unit 5

follows a pattern similar to that of the anhydriteof the basal cycle because it attains its maximumthickness of '15 cm in the Deaf Smith Countyarea. The dark anhydritic mudstone at the base ofthe cycle is interpreted to be an insoluble residuederived from dissolution of underlying halite.Therefore, a thicker residue may indicate thatmore halite has been dissolved. Examination ofthe stratigraphy of the underlying unit 4 haliteindicates that dissolution probably caused thedistribution of this residue. Areas where theuppermost regionally traceable halite beds ofunit 4 are absent correspond to the occurrenceof thicker residue mudstone in unit 5 (Hovorkaand others,1985).

These stratigraphic relationships within theIowest genetic cycle of San Andres unit 5 implythat the transgressing waters that initiated thecycle were of the lowest salinity in Deaf SmithCounty and produced the combined dissolutionof more of the underlying halite and depositionof carbonate. This locally lower salinity mayreflect either proximity to a source of normal-marine water or to an area of slightly deeperwater where circulation was slightly moreeffective in maintaining closer to normal-marinesalinities. The thickness of the cycle in Deaf SmithCounty indicates either (1) longer residence ofan hyd rite-precipitati n g water (transgressedearlier or remained longer or both) or (2) morerapid subsidence rates, resulting in accumulationof a thicker sequence. Halite is preserved at thetop of this cycle only in the Stone and WebsterHarman No. 1 and the DOE-Gruy FederalCrabbe No. 'l cores (fig. 35), but evidence of theformer presence of halite is found in all cores.Evidence of the former presence of haliteincludes (1) the presence of fabric at thean hyd rite-to-overlying-mudstone transition thatis similar to fabrics at the transition betweenanhydrite and overlying halite in completecycles, (2) the inf luence of halite-saturated brineson the diagenesis of the anhydrite, notably the

32

halite pseudomorphs after gypsum, and (3) thepresence of dark anhydritic mudstone withsedimentary structures that characterizeinsoluble residues (tig.37). The absence of halitef rom cycles that exhibit these features isinterpreted to reflect removal by dissolution,probably during transgression of the overlyingcycle. Conf irmation of the textural evidence thatdissolution, rather than nondeposition of halite,caused the discontinuous distribution of thishalite member can be found by examining therelationship between the preserved halite andthe facies in the overlying unit. The areas wherehalite is preserved underlie areas whereanhydrite, rather than carbonate, is the basaldeposit of the succeeding genetic cycle (fig. 35).The presence of halite in the lowest cycle ofunit 5 (5-a) corresponds to the facies distributionin the overlying cycle (5-b), not to the faciesdistribution within that cycle.

The second cycle of unit 5 (5-b) is almost even-ly thick, 6 to 10 m, throughout the core controlarea (f ig. 35). lt is composed of a few centimetersof dark anhydritic mudstone insoluble residue, afew tens of centimeters of discontinuous, ripple-cross-laminated, dolomite-oolite-coated grain-pellet grainstone and ripple-laminated dolomi-crite, a thick interval of variable proportions ofnodular anhydrite mosaic and laminated anhy-drite having pseudomorphs after gypsum, and athin halite interval. Halite is absent where thecycle is thin and thickens to 4 m where the cycleis thick, causing most of the thickness variation ofthis cycle. Textures at the top of the anhydritethat are typical of the anhydrite-halite transitionand abundant diagenetic halite within thelaminated anhydrite exist in allcores even wherehalite is absent. These features are interpreted, asin the underlying cycle, as evidence that halitewas originally present at the top of the cyclethroughout the area of core control. The areas ofpreserved halite, however, do not fullycorrespond to a high-salinity facies in theoverlying cycle. Halite is absent from the top ofthe cycle in Deaf Smith County beneath a thin,dark, anhydritic mudstone residue and a thincarbonate at the base of the overlying cycle, as istypical of syndepositional transgressive dissolu-

to-

CM_

0-

FIGURE 37. Anhydrite (light) at the top of the lowestincomplete cycle in unit 5, overlain by dark mudstoneinterpreted to be a residue from dissolution of halite at thetop of the lowest cycle (fig. 35, cycle 5-a) during thetransgression initiating the next cycle. The disrupted fabricat the top of the anhydrite is similar to that shown infigure 25, which is the typical fabric at the top of theanhydrite where the halite is preserved. Slabbed core fromStone and Webster J. Friemel No. 1 well, 2,536 tt.

tion. However, halite is preserved at the top ofthe cycle in Swisher County, beneath a thickerinsoluble residue and a thick, burrowed dolo-mite. Other factors in this cycle, such as thedepositional thickness and purity of the halite,may have controlled preservation of the halite.Several thin beds of anhydrite in the overlyingresidue in Swisher County indicate that the halitethat was initially deposited at the top of theunderlying cycle contained anhydrite beds,which may have acted as barriers to halite-

Stone ond WebsterMonsfield No. /Oldhom Co

Stone ond WebslDellen No. /Deof Smith Co

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FIGURE 38. Facies variation in the third cycle of unit 5, showing a similar pattern of salinity fluctuation in all the cores.Explanation of lithology and sedimentary structures is shown on figure 35.

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dissolving water. These anhydrite beds thickensouthward and are visible on geophysical logsfrom Hale County (pl. 2).

The third cycle of unit 5 (5-c) contains, abovethe dark mudstone at the base of the cycle, acomplex alternation of nodular anhydritemosaic, dolomitized burrowed packstones,wackestones, and mudstones, and laminatedanhydrite (fig. 35). However, examination of thedetails of facies distribution in this cycle showsthat the overall pattern of upward-increasingsalinity in units can be traced throughout thearea of core control (fig. 38). A thin, basal,ripple-laminated carbonate is overlain bynodular anhydrite except in Swisher County,where carbonate depositional conditions weremaintained. This reflects a different regionalsalinity pattern than that in the lowest unit 5

cycle; the lower salinity facies in the third cycleare situated toward the south rather than thewest. Complex interfingering of anhydrite anddolomite are observed within this unit inregional cross sections, but geophysical logresolution is not sufficient to map these thin,incomplete cycles individually. This sequencewas terminated without reaching halite satura-tion in the area of core control, and transgressingwaters lowered salinity to levels of carbonatedeposition. lncreasing salinity replacedcarbonate deposition with sulfate deposition(now nodular mosaic and laminated anhydrite)and culminated in halite deposition. Within thelower meter of the halite, a thin bed of anhydriterepresents a brief return to sulfate precipitationthat occurred throughout the area of corecontrol. The thickest halite sequence of unit 5,

34

ranging f rom 10 to 14 m thick, is contained in thiscycle.

The fourth, incomplete cycle in unit 5 (5-d)lacks evidence of a major salinity drop. Manycores contain no insoluble residue beneath thebasal anhydrite member, and no dolomite is iden-tified in this cycle north of the Matador Arch(fig. 35). The unit is composed of two thin anhy-drite beds separated by a halite bed, repre-senting salinity fluctuation rather than initialtransgression followed by increasing salinity. Theunit is generally even in thickness over the corecontrol area and reaches a maximum of 4 m inthe Stone and Webster Mansfield No. 1 core. It isnot thick enough to be consistently identif ied ongeophysical logs alone until the next latitudinaltier of counties to the south (pls. 1 and 2).

The uppermost unit 5 cycle (5-e) is 5 to 11 mthick and contains dolomite only in SwisherCounty, indicating that the lowest salinitiesduring this transgression were confined to theeastern part of the area examined. ln the area ofthe Stone and Webster Detten No. 1 core fromDeaf Smith County, halite deposition was inter-rupted by only two episodes of sulfate precipita-tion (fig.35). Regionally, the relationshipbetween the upper cycle of unit 5 and the orui-lying base of the middle San Andres depends onthe character of the basal middle San Andresfacies. ln Swisher County where carbonate is welldeveloped at the base of the lowest middle SanAndres cycle, the underlying halite is thin.Where the carbonate is thin elsewhere in theIowest middle San Andres cycle, the underlyinghalite in the unit 5 cycle is present.

Middle San AndresGenetic Sequence

The middle San Andres genetic sequence(new informal division) corresponds approxi-rnately to the largely anhydritic lower half of theupper San Andres division recognized byprevious workers (fig. 3; pls. 1 through 4).Cyclesof the middle San Andres are relatively thin andincomplete compared with those of the lowerSan Andres. Eight incomplete cycles can beidentif ied from core study (fig. 39). The cycles are

thin and halite is not preserved, so consistentcorrelation of individual cycles on geophysicallogs is difficult. Therefore, the genetic sequencewas divided into two informal operational units(fig. 3)for regionalcorrelation and mapping. Themiddle San Andres cycles show a minimum oflateral variation in facies distribution andthickness. Mudstones at the base of cycles areanhydritic and as much as several tens ofcentimeters thick. Carbonate units are thin andinclude coated-grain,/oolite grainstone, siltyripple-laminated dolomicrite, and packstoneand wackestone having low faunal diversity andsmall, sparse burrows. AII carbonate isdolomitized, and the faunal diversity is low in allstudied intervals. The transition betweencarbonate and anhydrite throughout the middleSan Andres consists of a few centimeters ofnodular anhydrite in burrowed or rippleddolomite. The anhydritic dolomicrite facies islacking, and no evidence of any other possiblesupratidal facies exists in this interval. Anhydrite,approximately half mosaic and half bedded, isthe dominant lithology in these cycles. Beddedhalite was preserved only at the top of the sixthmiddle San Andres cycle in the Stone andWebster Mansfield No.'l core (fig.39, cycle m-f).The former existence of halite is inferred in mostother cycles according to the same criteria asthose of unit 5, such as the presence of theinsoluble residues defining bases of incompletecycles and the presence of halite-influenceddiagenetic fabrics in the upper part of theanhydrite in many cycles.

lncomplete cycles of the middle San Andresgenetic sequence exhibit the same fundamentalcyclic pattern of vertical facies sequences as thelower San Andres. The thin, incomplete cycles ofthe middle San Andres, therefore, suggest achange in the tempo rather than in the mode ofcyclic depositional style. This change in tempomay have been caused by a decreased rate ofbasin subsidence or an increased frequency ofeustatic sea-level change or by both, as discussedin the following text.

Thin carbonates of the middle San Andresmust have been deposited during shorter timespans than those of the thick carbonates of the

35

Oldhom County Deof Smilh County Swisher County

, 11hi , R4ml . ar2-l . t^e^r .

| 5.3 lm | 15.5 km | 5O.3 km I t74 kh I

I

E

Slone ond WebsterMonsf ield No.l

No horizonlol scole

Sione ond WebsterDelten No I

Slone ond WebsterG. Friemel No.l

EXPLANATIONLITHOLOGY

SEDIMENTARY STRUCTURES

Stono ond WebslorHormon No,l

Slone ond WebslerZeeck No.l

ffico,oonot, 77 onn on," I ro,n. lT.lrono.,on. Elr,,,,,on. I

l#l};fl;fi," llTl;;;,l1is;. ffi .n",,on no,,," @lliiJiru*' l3l [HF,r",,"". E :if,r]r:,r:,

l'--ffil ;,"":t:i"". lT{-'1 oo,o" lul ggg,"". lFlBsur.. ll+l y;;i;1' lrTl .u,,o*.

FlGURE39. Northwest-southeastcrosssectionof middleSanAndrescyclesincoredU.S.Departmentof Energywells.Thelogof.each well shows, left to right,.visually estimated percent lithology, sedimentary structures, and a graph-of interpretedsalinity. Note the consistent thickness and sequence in cycles across the basin and the intervals wheie textural evidencesuggests the former presence of halite at the top of cycles.

0a 5294

36

lower San Andres, assuming that the carbonatesaccumulated at similar rates. These thinnersections, therefore, imply that conditions ofopen circulation and near-normal-marinesalinity were significantly shorter during themiddle San Andres sequence. An increasedfrequency of eustatic sea-level change in themiddle San Andres would have shortenedtransgressive intervals of near-normal-marinesalinity without a change in regional basinsubsidence rate from that of the lower SanAndres. Alternatively, a regional decrease inbasin subsidence rate during the middle SanAndres interval could have produced theabbreviated cycles without a change in thef requency of eustatic sea-level f luctuation fromthat of the lower San Andres. lf the basinsubsidence rate was lower than the rate ofcarbonate production during normal-marinetransgressive events, the seafloor would haverapidly aggraded, effectively restricting circula-tion and promoting evapgrite deposition.lnitially high carbonate production in a regime oflow subsidence rate is thus a self-limitingprocess, producing a relatively thin carbonateunit before the onset of evaporite deposition. lfthe basin subsidence rate remained less than therate of subsequent evaporite sedimentation, theevaporites presumably would have also rapidlyaggraded to the upper limit, at sea level. A thinhalite upper unit would have been particularlysusceptible to complete dissolution during thetransgressive, near-normal-marine phaseinitiating the next cycle. ln summary, the changein tempo of cyclicity that produced the thin,incomplete cycles of the middle San Andrescould have been caused by either an increasedfrequency of eustatic sea-level change or, a

regional decrease in basin subsidence raterelative to conditions that produced the lowerSan Andres sequence, or by both.

Upper San AndresGenetic Sequence

The upper San Andres genetic sequencecorresponds to the halite-bearing top half of theupper San Andres division (recognized by

previous workers), situated above and includingthe pi-marker bed (figs. 3 and 40. pls. 1

through 4). Six to twelve complete and incom-plete cycles in the lower part of the upper SanAndres are characterized by thin basalcarbonate-anhydrite units similar to those of themiddle San Andres sequence, yet halite uppermembers are also preserved. Mudstones at thebase of cycles are thick, and some are silty andretain some red color inherited from their originas thick red beds within or at the top ofunderlying halite sequences. Carbonate units areall dolomitized and are less than 1 m thick.Coated-grain,/oolite grainstone, silty ripple-Iaminated dolomicrite, and packstone andwackestone with low faunal diversity are thedominant carbonate facies and indicategenerally hypersaline conditions. Carbonateinterfingers with anhydrite mosaic and beddedanhydrite. Halite members of the cycles rangefrom locally absent to a maximum of 15 m thick.The dominant impurity in halite is mudstone, andchaotic mudstone-halite is more abundant herethan in the lower San Andres. The upper 50 m ofthe upper San Andres contains no dark mud-stone, carbonate, or anhydrite within the halite,so cycles are not identif iable. Prominent,relatively thick red beds are abundant through-out the upper San Andres interval and, inaddition to the disseminated mudstone withinthe halite and the thick dark mudstone beds atthe base of cycles, contribute to making theupper San Andres more clastic rich than theother San Andres genetic sequences. Three tofive regionally correlative red beds occur withinthe noncyclic upper 50 m of the upper SanAndres. The pi-marker, a 1-m-thick siltstone andfine sandstone bed used as a datum for regionalcorrelation because of its great areal extent,sharply overlies an incomplete cycle at the top ofthe middle San Andres.

Again, the fundamental mode of cyclicityappears to be the same in the upper San Andresas that of the lower and middle San Andressequences, but the tempo in the upper sequencediffers from the others. A pronounced asym-metry in the transgression versus regression ratesduring individual eustatic sea-level fluctuations

Oldhom County Deof Smith County

8.4 mi3.3 mikm

Stone ond WebsterDetten No.l

3l2mikm 50

Stone ond WebsterG. Friemel No. I

EXPLANATION

LITHOLOGY

SEDIMENTARY STRUCTURES

Stone ond WebsterHormon No.l

Stone ond We'cs:3.Zeeck Nc..

z

_z=-z=

Stone ondMonsf ield

WebsierNo. I

No horizontol scole

Nodu loronhyd rile

Coo rsegroinslone

r=:i;-l Holite wrlh I===lI r,-,rl equont mosoic l:- - | BeddingF'r' I fobric I---l

QA 5323

'daSE2r

=so&UFaa

z-2LuZE_,

23

tYaf,Ez)ul:=5oeUFaa

o22'ruZE',

2A

ooJ

-ij

02.<:uZE-12-fi<_.

-*aflEz)UF

=gOEUFao

IzaUE

FZ)a

cI

U:.

=5OEUF@a

w@ffi

corbonore W onnrorrr. f] ,0,,r. lf.,----,lil ,ono.,on. E.,n.,on. ! r,o,,on"

l77l;g;3ig;" li4;l chevron ho te M [i,tiil;lr]' E [l;f.t,,.", E yry5;i3'

FI;loo,o. lffil [fl0,,1:,"". ly^,] mi;1", lrrT y;;g1 1r7,,,,o*,

FIGURE 40. Northwest-southeast cross section of the lower three upper San Andres cycles in cored U.S. Department of Energywells. The log of each well shows, f rom left to right, visually estimated percent lithology, sedimentary structures, and a graph ofinterpreted salinity.

38

relative to the lower and middle San Andres,superimposed on a relatively constant regionalbasin subsidence rate, might produce suchcycles. A rapid transgressive pulse followed by aprolonged regressive phase would produce a

cycle having thin basal nonsalt units and a

relatively thick halite upper unit, provided thatthe basin subsidence rate was high enough topreserve them. Alternatively, f luctuating re-gional basin subsidence rates relative to those ofthe.lower and middle San Andres, superimposedon symmetric rates of transgression andregression during eustatic sea-level changes,could produce the same effect. A basin subsi-dence rate that is slow relative to the rate ofcarbonate production during a eustatic transgres-sion would result in rapid vertical aggradation ofthin basal carbonates, causing restrictedcirculation and promoting evaporite deposition.A rise in basin subsidence rate during the phaseof regression and evaporite deposition couldpreserve relatively thick halite facies overlyingrelatively thin carbonate facies. However, f luctu-ating basin subsidence rates alone or even incombination with asymmetric eustatic sea-levelchanges are unlikely to have produced the upperSan Andres sequence. Both circumstancesrequire the coupling of a low basin subsidencerate with transgression and a high basinsubsidence rate with regression. lt seemsunlikely that this association could produce a

repetitive sequence because of the presumedindependence of basin subsidence rates and trueeustatic sea-level changes.

The marked increase in thickness andfrequency of red beds in the upper San Andresinterval appears to be dependent on changes inthe clastic source area. These clastic interbeds arelithologically similar to the overlying Queen-Crayburg red-bed sequence and were probablycreated by minor pulses that preceded the majorQueen-Crayburg progradation. lnterfingeringand lateral transition from upper San Andresevaporites into sandstone can be seen onregional cross sections (pls. 1 through 4). Thishigher rate of clastic influx during deposition ofthe upper San Andres may represent a reactiva-tion of tectonism in the source area or a regionalchange of continental clastic dispersal patterns.

Lateral Facies Relationships,Depositional Systems, andStructural Influence onSedimentation in theSan Andres Formation

Lateral Facies RelationshipsLateral changes in the San Andres Formation

can be studied both within and between faciesand at both regional and local scales. Eachperspective contributes to our understanding ofdifferent facets of the San Andres depositionalsystem-an environmental complex withapparently no close modern analog.

Regional-scale lateral facies changes withincycles of the San Andres Formation were largelycontrolled by lateral salinity gradients that devel-oped across the broad depositional shelf duringthe regressive phase of each cycle. Lateral facieschanges from halite to anhydrite and from anhy-drite to carbonate are evident in the lower andmiddle San Andres along the southern margin ofthe Palo Duro Basin, just north of the MatadorArch (pls. 1 and2; Presley and Ramondetta,1981,their fig. 40). This large-scale facies change, seenin nearly all cycles, presumably reflectsdecreasing brine salinity southward in the PaloDuro Basin caused by the influx of normal-marine water f rom the Midland Basin. Analysis ofthese facies changes is primarily made on thebasis of geophysical logs. Bein and Land (1982)studied textural and geochemical details of thenature of these changes along one transect.

Updip facies equivalents of the evaporiteenvironments are terrigenous clastic red beds.The carbonate and anhydrite of lower SanAndres units 2 and 3 interfinger with clastic redbeds toward the north and west and pinch outinto sandstone along the southern border of theAmarillo Uplift in Potter County (pls. 1 and 2).The updip pinch-out of the evaporite units of theIower San Andres unit 4 and the middle SanAndres have not been identified. Carbonate andclastic parts of these cycles continue across theAmarillo Uplift. Near outcrop of the units, thedepositional extent of bedded halite across theAmarillo Uplift has been obscured by dissolution

(Custavson and others,19B0; Boyd and Murphy,1sB4).

Cyclic and Non cyclic Controls onRed-Bed Distribution

The distribution of clastic red beds through-out the San Andres section shows both a strongpositional relationship in carbonate-evaporite,increasing-salinity cycles and yet a partial inde-pendence from typical San Andres cyclicity. Redbeds are most abundant in the halite members ofcycles, and textural evidence indicates that manyof them were deposited during episodes of sub-aerial exposure during the most restricted part ofthe cycle (Hovorka and others,19B5). Clastic bedsare commonly found at or near the top of thehalite of some cycles in the lower part of theupper San Andres. The stratigraphic position ofthe clastics at the top of the cycle indicatesconditions of near-maximum regression duringdeposition. Similarly, a strong correlation existsbetween the absence of carbonate and anhydritebeds and the presence of abundant mudstoneand siltstone beds in the top 50 m of the upperSan Andres, indicating that this interval repre-sents an updip part of the facies tract. Duringdeposition of the uppermost San Andres, regres-sion of the normal-marine-source water body toa location south of the Matador Arch limited itsability to episodically flood the northern halite-precipitating environment and resulted in morefrequent and/or longer episodes of subaerialexposure that favored the progradation of clas-tics. Correspondingly, the clastic sediments inthe Dalhart Basin that are equivalent to much ofthe San Andres Formation have prograded totheir southernmost position during latest SanAndres deposition, where they are visible oncross sections within the northern part of thePalo Duro Basin. The distribution of clasticswithin the halite section of the sequence in theupper San Andres is therefore partly influencedby the same cyclic control (relative sea-levelchange) as is the salinity change. However, redbeds are also found interbedded with cyclicevaporites throughout the San Andres section.The pi-marker clastic bed is especially significantbecause it overlies a thin interval of dolomitic

anhydrite and anhydrite mosaic and shons noevidence that halite was deposited before clasticinflux. The relatively thick and regionally exten-sive pi-marker clastic unit prograded to the southat an apparently random interval with respect todeposition of the underlying cycle and, there-fore, is interpreted to represent an episode ofefficient clastic input into the carbonate-anhydrite-halite environment that was con-trolled by events in the clastic source area, inde-pendent of the marine-controlled cyclicity.

Closely spaced cores in the northern part ofthe Palo Duro Basin permit examination ofdetails of red-bed and carbonate-evaporitefacies relationships. The dark mudstones at thebase of cycles and the clastic red beds, such as thepi-marker, facilitate correlation on a fine scaleboth between cores and on regional crosssections made from geophysical logs. The car-bonate, anhydrite, and halite members in eachcycle can be correlated in great detail betweenwells, so that areal variations in depositionalenvironment can be identified.

Time linesTime lines must be inferred to interpret the

lateral facies relationships within cycles at finelevels of resolution. Three lines of evidence areused to infer time lines: (1) the facies relation-ships at the base 'of cycles, (2) the faciesrelationships between the pi-marker clastic bedand its overlying and underlying evaporites, and(3) the facies relationships within cycles.

Facies relationships at the base of cyclessuggest that transgression occurred too rapidlr'for the accumulation of transgressive deposits.The thin, dark mudstones at the base of cycles,representing insoluble residues of underlyinghalite, are the only transgressive depositsidentified in any of the more than 25 cyclesexamined. lf transgression had taken placeslowly relative to the rate of sedimentation, thefinal hypersaline deposits in the cycle would beoverlain by sediments precipitated in increas-ingly lower salinity environments. This type ofsequence has not been identified in the stud;*area. Rather, the basal sediments of each cyclewere always deposited in lower salinity environ-

ments (that is, more normal marine) than theimmediately overlying sediments. Some cyclesexhibit a basal anhydrite overlain by carbonate,thus appearing at first glance to exhibittransgressive, upward-sal i n ity-decreasin gtrends. However, close examination of thesewell-developed sequences reveals a texturalpattern of upward increase in salinity in theanhydrite and a sharp contact between theanhydrite and the overlying carbonate. Theserelationships indicate that the apparent cycle isactually a composite of at least two discretetransgressive pulses, each showing an abruptbase and a salinity increase as depositionproceeded. Successive pulses may representdeposition from lower salinity brines, but theyare each discrete transgressive events and do notform a single transgressive continuum. The lackof basal transgressive primary sediments in anycycle indicates that the transgression rate wasvery rapid relative to the sedimentation rate;therefore, the transgressive bases of cycles areapproximate time Iines.

The upper and lower boundaries of the pi-marker red bed within the Palo Duro Basin mayalso be interpreted as being approximate timelines, as inferred from the foregoing discussionthat this bed was deposited as a sudden clasticinflux and is independent of marine-controlledcyclicity. lf the base of the pi-marker is assumedto be an isochronous surface, then the evaporitefacies beneath the pi-marker must be time-equivalent deposits. A thin anhydrite sequenceconsistently underlies the pi-marker and isvisible in all cored wells and recognized on allexamined geophysical logs. This relationindicates that similar geochemical conditionsexisted across the entire basin just before thedeposition of the pi-marker bed (fig. 39,cycle m-h). Within our limits of resolution, singleevaporite facies units in the palo Duro Basininterior, therefore, appear to be virtuallyisochronous and to replace each other in verticalrather than lateral succession. This implies thatduring the regressive phase of each cycle,environmental changes, mainly brine salinity,occurred nearly synchronously over wide areasof the basin. Bedded anhydrite underlies the pi-

marker clastics in the Stone and Webster DettenNo. 1 core; the equivalent interval is nodularanhydrite in the Stone and Webster C. FriemelNo. 1 and Harman No. 1 cores, and in the Stoneand Webster Zeeck No. 1 core it is dolomiticlaminated anhydrite (fig. 39). These subtledifferences in anhydrite facies between coresrecord local variations in the basinwideanhydrite depositional environment.

Facies within individual cycles can becorrelated f rom core to core, but correlations arecomplicated by local lateral facies variations. Forexample, the third genetic cycle in lower SanAndres unit 5 exhibits a vertical sequence that isrepeated in all cores (fig. 38). The sequence is(1) a basal, thin, ripple-laminated dolomite,(2) nodular anhydrite, (3) thick, burroweddolomite, (4) variably thick nodular anhydrite,(5) laminated anhydrite, (6) thin halite, (7) thin,laminated anhydrite, and (B) thick halite. Theimportance of facies correlation within the thirdcycle of unit 5 with respect to the interpretationof time lines is that the identical pattern ofcomplex fluctuations of salinity, both increasingand decreasing, is noted among the cores. Thesalinity f luctuations are therefore episodicchanges that affected the entire area of corecontrol, rather than simply local shifts of a faciesmosaic or migration of a facies tract across thearea. Therefore, time Iines appear to runapproximately parallel to rather than across thefacies boundaries. The failure of time lines in anyother orientation to produce reasonable lateralfacies relationships corroborates the validity ofthe inference that time lines parallel faciesboundaries. Local aberrations from the patternof regionally continuous facies have been notedin cycle 5-c, such as the absence of the thicknodular anhydrite in the Zeeck No. 1 core, thepresence of one extra dolomite bed in theanhydrite in the G. Friemel No. 1 core, andtwo extra dolomite beds in the anhydrite inthe Detten No. 1 core (fig. 38). Dolomite islaterally equivalent to anhydrite because of locallateral variations in the depositional environ-ment. However, a pattern of areally extensiveenvironments occupied by the same facies isdominant within San Andres cycles. Correlations

41

of 1- to 4-m-thick alternating zones of anhydriticand muddy halite within the halite of lower SanAndres unit 4 show a similar pattern of lateralcontinuity (Hovorka and others, 1985).

Although such detailed studies of subtlevariation within lithofacies units cannot betraced outside the area of core control, thelateral continuity of the nearly isochronous,dominant lithofacies packages is apparentthroughout the Palo Duro Basin.

Depositional SystemsBasi nwide lateral continuity of individual thin

cyclic facies has been demonstrated bygeophysical log correlation (pls. 1 through 4).Textural evidence of corresponding facies,provided by core, documents broadly uniform,predominantly subaqueous deposition. Boththese complementary lines of evidence pertainto the depositional systems complex of the SanAndres Formation. The depositional surface atthe start of each cycle must have been anextremely broad shelf having negligibletopographic relief. The cyclic lithofacies arelargely subaqueous sediments, deposited in avery shallow yet regionally contiguous waterbody. With few exceptions, most of the evidenceof intermittent subaerial exposure occurs in thehalite facies deposited in the late regressivephase of each cycle. The apparent isochroneityof laterally extensive cyclic facies units suggestsrapid sedimentary responses to sequentialsalinity changes that affected nearly the entiredepositional basin. The San Andres depositionalplatform appears to have been both broadenough and f lat enough so that minor changes inwater depth exerted profound effects oncirculation patterns. These, in turn, controlledwater salinity and facies development over theentire platform.

Structural lnfluenceon Sedim entation

On a regional scale, patterns of sedimenta-tion in the San Andres Formation of the PaloDuro Basin can be readily conceptualized asfunctions of a dynamic interplay between rates of

eustatic sea-level change, regional basin subsi-dence, and sediment aggradation on a broad,low-slope, low-relief depositional platform. lnaddition to these primary factors, increasing evi-dence suggests subtle, local structural controlson San Andres deposits both within the PaloDuro Basin and along its margins (Fracasso,1983,1984). Several indications of such structural influ-ence are outlined in the following text.

As noted previously, the distribution of rela-tive thicknesses of cycles in the three San Andresgenetic sequences suggests the possibility of epi-sodic changes in overall rate of regional basinsubsidence. lsopach maps of the basal, nonhalitemembers of cycles (mudstone + carbonate +

anhydrite) show an increasing geometric com-plexity north of the Palo Duro Basin, as indicatedby increases in the overall density of contourlines and the number of closed-contour areas(Fracasso, 1984). These northern zones ofcomplex geometry coincide with the underlyingAmarillo-Wichita Uplift belt and differ markedlyfrom the broader, more regular trends of thick-ness change evident in the basin interior. lnaddition, many loci of vertically recurrentclosed-contour thickness anomalies of thesenonhalite members have been identified(Fracasso, 1984). The highest density of these locioccurs over the Amarillo-Wichita Uplift belt,although many are also present to the south inthe basin interior. These loci largely correspondto structures defined by the top of the underlyingTubb sandstone, which implies an active struc-tural influence during their deposition.

The northern depositional edges of the non-salt facies of lower San Andres units 2 and 3terminate against the southern margin of theAmarillo Uplift, whereas units 4 and 5 and cyclesof the middle and upper San Andres geneticsequences continue north across the uplift. Thechanging depositional limits of these individualcyclic facies may represent either the relativemagnitudes of eustatic sea-level changes, theintermittent structural activity and changingtopographic relief along the northern basinmargin, or a combination of both.

The effect of structure on sedimentation inindividual cycles is demonstrated by comparing

42

the Stone and Webster Mansfield No. 1 core,f rom a more structurally complex area relative toother wells, with other cores (fig. 10). lndivid-ual cycles traced f rom other cores to theMansfield No. '1 core change markedly in thick-ness and lithology. For example, the lower SanAndres unit 4 carbonate is recognized as a

composite of two cycles in most cores. An incom-plete lower cycle usually culminates in less than1 m of either nodular or bedded anhydrite,indicating that salinity reached the gypsumsaturation level. A complete upper cycle is alsopresent, and normal-marine limestone is atits base. ln contrast, the lower cycle in theMansfield No. 1 core is thicker than elsewhereand is complete, culminating in several meters ofbedded anhydrite and halite. Apparently, duringthe latest, most hypersaline part of this cycle, thearea of the Mansfield No. 1 site was morerestricted and accumulated thicker sequences

of sediment. Differential movement of isolatedfault blocks might explain this apparent paradox.Relatively uplifted blocks may have acted as localbarriers that restricted circulation, whileadjacent, relatively downdropped blocks sub-sided more rapidly to allow the accumulation of athicker evaporite section. Alternatively, gypsumand halite may have been deposited in the lowerunit4cycle in the Mansfield No. 1 core,whilethetransgression that initiated the overlying unit 4cycle had begun elsewhere. Variations in thethickness of correlative halite facies are moredifficult to interpret than are similar variations innonsalt facies. This is because it is not clear towhat extent these differences in halite thicknessare due to variations in the rate of halitedeposition, dissolution of halite during subaerialexposure at the end of the cycle, or dissolutionduring the transgression initiating the next cycle.

SUMMARY

Predominantly carbonate-evaporite cyclicsequences in the San Andres Formation of thePalo Duro Basin are recognized in core and ongeophysical logs. The ideal vertical faciessequence displays a regressive pattern, succes-sive facies representing increasingly restrictedand more saline depositional environments. Thebase of each cycle in the basin interior is definedby a black anhydritic mudstone, interpreted as

being an insoluble residue, which produces a

sharp, high-APl gamma-ray peak on geophysicallogs. The basal insoluble residue is ideally over-lain in sequence by normal-marine limestone,hypersaline dolomite, anhydrite, and halite.Fabrics in these facies indicate predominantlyshallow-water subaqueous deposition. lndivid-ual cycles can be correlated regionally through-out areas Breater than 16,000 to 26,000 km2;much thinner sequences within cycles, on theorder of tens of centimeters thick, can be tracedfor distances up to 118 km where core controlexists. The observed fabrics and remarkable lat-

eral continuity of these thin sequences implysimilar depositional environments over a vastshallow-water subaqueous shelf having negli-gible topographic relief. No exact analog inmodern evaporite environments has beendescribed. Evidence of subaerial exposure, suchas halite karst and mudstone accumulation, is

common only in the halite facies and alternateswith other features indicative of subaqueoushalite deposition. Thin siliciclastic red beds arepresent throughout the San Andres Formationbut become more common and thicker near thetop of the sequence where San Andres halitefacies interfinger with Queen-Grayburg terrige-nous clastic red-bed facies. Each of these redbeds in the halite represents a minor pulse ofclastic progradation across the San Andres shelfand appears, as a whole, to have prefaced themajor clastic progradation represented by theQueen-Grayburg sequence.

Variations from the ideal cyclic vertical faciessequence are observed on several scales in core

and on geophysical log cross sections. Thisvariation appears to be a function of complexinteractions between changing rates of sedimentaccumulation, local and regional basin subsi-dence, and eustatic sea-level fluctuation.Complete cycles with thick carbonate facies char-acterize the lower San Andres genetic sequencein the interior of the Palo Duro Basin. Completecycles, but with much thinner carbonate facies,typify the upper San Andres genetic sequence.The middle San Andres genetic sequence, how-ever, displays incomplete cycles, in whichcarbonate facies are thin or lacking above thebasal mudstone insoluble residue. Halite facies,now absent, were originally deposited in eachcycle, as inferred from the insoluble residues atthe base of cycles. However, each halite memberwas probably originally thin, proportional to thepreserved nonsalt members, and was completelyremoved by dissolution before or during deposi-tion at the start of the succeeding cycle. Thefundamental mode of cyclicity is the same in allthree Benetic sequences. The main differencesamong them are the relative thickness of thecarbonate facies and the presence or absence ofhalite. These differences appear to be caused bychanges in the tempo of cyclic deposition. Theevolution of cyclic style in the San Andres Forma-tion probably ref lects systematic change in ratesof regional basin subsidence or eustatic sea-levelchange or both.

Large-scale facies changes in each of thegenetic sequences are evident in the areas of thenorthern and southern structural margins of thePalo Duro Basin. To the south, halite in both thelower and upper genetic units is laterallyreplaced in sequence by anhydrite and thencarbonate in the area of the Matador Arch andNorthern Shelf of the Midland Basin. This lateralfacies equivalence reflects the lateral salinitygradients that persisted across the broad SanAndres depositional platform.

The carbonate and anhydrite cyclic facies ofthe lower San Andres units 2 and 3 thin and pinchout to the north adjacent to the Amarillo Uplift,whereas the equivalent nonsalt facies of lowerSan Andres units 4 and 5, the middle San Andres,and the upper San Andres genetic sequences

thin but persist over the Amarillo Uplift. Theseand smaller scale facies changes observed in coreappear to have been caused by local variations inbasin subsidence rate.

The San Andres Formation cycles are largelyshallow-water subaqueous evaporite deposits,Descriptions of shallow-water evaporites arescarce (Schreiber and others, 1982). Modernexamples of shallow-water subaqueous evap-orites occur in small water bodies physicallyrestricted f rom a normal-marine water source bysupratidal barriers. Examples include the supra-tidal halite flats at Salina Ometepec in BajaCalifornia (Shearman, 1970), salinas in SouthAustralia (Warren,19B3), the Culf of Karabogaz-Col on the Caspian Sea (Dzens-Litovskiy andVasil'yev, 1962), brine pans marginal to the RedSea, Sinai (Cavish,19B0), and artificially dammedpools, such as those along the Dead Sea (Weilerand others, 1974). ln contrast, the San Andresevaporites of the Palo Duro Basin were depositedover a far more extensive area, and no regionalsupratidal barrier system to the southerlynormal-marine water source has yet been identi-fied. Brine concentration and the developmentof lateral salinity gradients appear to have beencaused by continuous evaporation of a thin sheetof water flowing over a broad, low-relief, low-slope shelf . The change in slope at the shelf edgeand the shallow depth of the water on the shelfappear to have effectively restricted free circula-tion with the deeper Midland Basin, the domi-nant southerly source of normal-marine water.

The San Andres Formation in the Palo DuroBasin thus appears to represent an exemplarycase of shallow-water shelf, subaqueous evapo-rite deposition. Modeling of such shallow-watersubaqueous evaporite depositional systems hasreceived little attention since the pioneering an-alyses of Shaw (19641and Irwin (1965) that relatedevaporite deposition and lateral salinity gradi-ents to the dynamics of circulation in epeiric seas.This neglect seems largely due to the morerecent detailed studies and actualistic modelingof sabkha evaporite depositional systems. Epeiricsea evaporite models are necessarily deductiveand non-uniformitarian, as comparable recentdepositional environments are unknown at the

44

scale of the Palo Duro Basin. Sabkhas, however,are demonstrably real, and the correspondinglyactualistic sabkha evaporite depositional modelhas thus been considered more attractive and hasbeen applied uncritically to most ancient evapo-rite deposits that are not obviously of deep-waterorigin. Documentation of evaporite textures incore and the regional correlation of cyclic evap-orite facies permits the discrimination of shallow

subaqueous evaporite facies from sabkha ordeep-water evaporites in the San Andres Forma-tion. More detailed studies of the vertical, lateral,and temporal distribution of evaporite facies inthe San Andres Formation of the palo Duro Basinmay aid in refining a general model of shallow-shelf evaporite deposition, which may proveapplicable to similar evaporite sequenceselsewhere.

ACKNOWLEDGMENTS

This study was f unded by the U.S. Department of Energy,Salt Repository Project Off ice, under contract no. DE-AC9Z-83WM466s1.

Barbara Luneau, Sterling Thomas, David Noe, DavidPurguson, Ed Cazier, Jeffrey Thurwachter, and MadisonWoodward prepared the lithologic core logs used in thisstudy. Ceorge Donaldson, Daniel Ortui'o, and others at theCore Research Center of the Bureau of Economic Geology,The University of Texas at Austin, slabbed and transportedthe core. Thin sections were prepared by Ethel Butler. CecilyCrebs and Patti Cranger photographed slabbed core. KeithThompson and Patti Cranger assisted with figurepreparation.

The manuscript was critically reviewed by Don C.Bebout, Jules R. DuBar, Thomas C. Custavson, Stephen C.Ruppel, and Noel Tyler of the Bureau of Economic Ceology.

Word processing was by Dorothy C. Johnson andtypesetting was by Lisa L. Farnam, under the supervision ofLucille C. Harrell. Figures were drafted by Marty Thompsonunder the direction of Richard L. Dillon. Jamie S. Haynesdesigned this publication. Text illustration camerawork wasby lames A. Morgan. Diane Callis Hall and R. Marie Jones-Littleton edited this report.

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