Devonian-Mississippian Antler Foreland Basin Carbonates in Idaho: Significant Subsidence and Eustasy...

The Mountain Geologist, Vol. 44, No. 3 (July 2007), p 119-140 119 The Rocky Mountain Association of Geologists

Devonian-Mississippian Antler Foreland Basin Carbonates in Idaho:Significant Subsidence and Eustasy Events1

PETER E. ISAACSON2

GEORGE C. GRADER3

MICHAEL C. POPE3

ISABEL P. MONTAÑEZ4

SUSAN H. BUTTS5

LISELLE S. BATT2

JASON M. ABPLANALP2

1. Manuscript received September 8, 2006; Accepted March15, 20072. Department of Geological Sciences, University of Idaho, Moscow, ID 83844-3022 Current Address (Batt): ExxonMobil, Houston, TX; Corresponding Author:[email protected]

3. Department of Geology, School of Earth and Environmental Sciences, Washington State University, Pullman, WA 991644. Department of Geology, University of California, Davis, CA, 956165. Peabody Museum, Yale University, New Haven, CT 06520-8118

ABSTRACT

Idaho’s mid- and late Paleozoic sedimentary successions are carbonate and mixed carbonate-silici-clastic units showing >5 km subsidence combined with several organic buildups in downslope andeustasy-influenced foreland basin settings. Overall, Silurian through Late Carboniferous sediments aver-age 6-10 km thickness. Idaho’s margin was passive in Early to Middle Devonian time, and transitionedinto a foreland basin in the Late Devonian with additional large tectonic and sedimentary loading accom-modation events in the Carboniferous. The Middle and Late Devonian Jefferson Formation (> 2 km thick)is composed of downslope marine units that pass upward into downslope buildups comprised of stro-matoporoids and disphyllid corals. A drowning event stopped buildup growth, followed by eustatic fallthat subaerially exposed the buildups. Abrupt high amplitude sea level changes in the late Frasnian toFamennian were a result of coeval initiation of glaciation in Gondwana. Coincident with the first LateDevonian to Carboniferous tectonic loading event, the resulting foreland basin received flysch derivedfrom western uplands. To the east, a >3 km thick succession of Mississippian carbonate was deposited inthe distal foreland basin. The first Carboniferous foreland subsidence event is in Kinderhookian time,and a second is during early Chesterian time, with eustatic overprinting in the mid- to late Chesterian.The carbonates are subdivided into two packages: 1) uppermost Devonian - lower Mississippian shaleand middle Mississippian progradational carbonates containing abundant pelletal and ooid grainstones;and 2) upper Mississippian carbonates dominated by muddy skeletal banks that grade upward intomixed carbonates and siliciclastics. Although the general history of these units is similar to that of adja-cent, on-strike regions, accommodation appears to be more significant than what occurred in Nevada,where crustal thickening through Late Devonian thrust faulting is clearly demonstrated; in Idaho coevalthrusting has yet to be defined.

INTRODUCTION

Idaho’s mid-Paleozoic succession has only recently beenstudied in enough detail to provide a reasonable assembly ofits stratigraphic successions, depositional settings, and tec-tonic versus eustatic imprints. A pattern is emerging in theoverall depositional history, which parallels that of Nevadaand adjacent Canada, with an initial passive margin transition-ing to an active compressional and possibly transcurrent mar-gin during the Late Devonian. We describe stratigraphicfeatures herein that suggest at least three episodes of tectonicand sediment loading in Carboniferous time.Overprinting thesubsidence events are shorter term eustatic cycles duringLate Devonian, late Mississippian and Pennsylvanian times.

Compared to its adjacent states and provinces, notablyUtah, Nevada, British Columbia, and Alberta, understandingmuch of the Paleozoic stratigraphy of Idaho is at an earlystage of knowledge.Not until the late 1970s and 1980s werelitho- and biostratigraphic assessments of the Devonian andCarboniferous strata completed. Based on literature availableat the time, as well as thesis work completed by Universityof Idaho students, Isaacson et al. (1983) assembled twenty-six stratigraphic columns for Paleozoic (and Mesozoic) unitsin south-central, and southern Idaho.

Idaho’s thick mid- to late Paleozoic sedimentary succes-sion records several significant accommodation events.These events range from subsidence in marginal, exten-sional basins during the passive margin phase (Early to Mid-dle Devonian rocks), and tectonic loading during thecompressional margin phase (Late Devonian through lateMississippian rocks). The succession has few widespreadregional unconformities, thereby providing a more completerecord of kilometer-scale progradational or aggradationalevents not seen in the U.S. Midcontinent.This paper reviewsthe current understanding of Idaho’s mid-Paleozoic sedi-mentary succession focusing on its biostratigraphic frame-work, a foundation for eventual sequence stratigraphicinterpretations, and carbonate buildups.

Structural Setting

Mountain ranges between the Beaverhead Mountains andthe White Knob Mountains in east-central Idaho (Fig. 1) con-tain a Proterozoic and Cambrian-to-Permian succession ofsedimentary rocks measuring approximately 6 and 10 kmrespectively (Skipp and Hait, 1977; Ruppel and Lopez, 1988;Link et al., 1988; 1995). Generally, Paleozoic interbeddedsandstone and carbonate of the eastern areas grade west-ward into more pure carbonates that, in turn grade intothick, fine-grained deep-water siliciclastics (Wood River andPioneer Mountains area).

The Cordilleran thrust belt meets the Rocky Mountainforeland approximately in the Beaverhead Mountains (Fig. 1)on the border of east-central Idaho with Montana (Kulik andSchmidt, 1988; Hamilton 1988). A thickened crust thererecords episodes of thrusting that include tear faults, out ofsequence thrusting, reactivated basement uplifts, and refold-ing of thrusts (Perry et al. 1988; Skipp 1988). Paleozoic stratain the study area are broken by imbricate thrusts and foldsof the Sevier and Laramide deformation events, but occur ona single tectonic block – the Hawley Mountain thrust plate(Link et al., 1988). Regional structural deformation and tele-scoping of strata resulted in thrust dislocations of distinctivePaleozoic successions (Skipp and Hait, 1977; Rodgers andJanecke, 1992). Other depositional (and paleo-structural)factors associated with the Neoproterozoic - PaleozoicCordilleran “miogeocline” and Late Paleozoic Antler orogenymay also have influenced late structural kinematics (Skipp,1988; Winston, 1986; Ruppel, 1986; Skipp and Link, 1992;Dorobek et al. 1991). Probable deformation via imbricatethrust, uplift and solution interaction with Upper Devonianevaporite deposits occurred above and locally within theDevonian units (Hait, 1965; Beutner, 1968; Sandberg andMapel, 1967; M’Gonigle, 1982; Dorobek, 1991; Grader, 1998).Rhyolite dikes, jasperoid silicification of Paleozoic carbon-ates, and hydrothermally emplaced sulphides occur locally.Igneous rocks were intruded into this area during the

Peter E. Isaacson, George W. Grader, Michael C. Pope, Isabel P. Montañez, Susan H. Butts, Liselle S. Batt, Jason M. Abplanalp

The Rocky Mountain Association of Geologists 120

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . 120

Structural Setting . . . . . . . . . . . . . . . . . . . . . . . 120

Stratigraphic Overview . . . . . . . . . . . . . . . . . . . 121

DEPOSITIONAL HISTORY . . . . . . . . . . . . . . . . . . 122

PASSIVE MARGIN SUCCESSION . . . . . . . . . . . . . 122

Devonian Passive to Active Margin Transision . 123

Early and Middle Devonian . . . . . . . . . . . . . . . 126

Early Late Devonian (Frasnian) . . . . . . . . . . . . . 126

Late Devonian (Famennian) . . . . . . . . . . . . . . . 126

ACTIVE MARGIN SUCCESSION . . . . . . . . . . . . . . 128

Early Mississippian . . . . . . . . . . . . . . . . . . . . . . 128

Late Mississippian (Chesterian) . . . . . . . . . . . . . 128

Late Mississippian Biostratigraphy . . . . . . . . . . 129

Tectonic Controls on Structure and

Accommodation . . . . . . . . . . . . . . . . . . . . . 130

Chesterian Paleogeography . . . . . . . . . . . . . . . 130

Cyclicity in Chesterian Rocks . . . . . . . . . . . . . . 130

DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . 133

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . 133

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

Ordovician (Beaverhead Mountains), Late Cretaceous (IdahoBatholith to the west), and Eocene (Evans and Zartman,1988).

Idaho’s Paleozoic rocks are exposed in northwest-south-east trending Tertiary extensional fault-block (basin andrange style) mountain ranges in central Idaho (Fig. 1).Most ifnot all Paleozoic units are allochthonous, having been trans-ported a considerable distance to the east and possiblynortheast by Late Paleozoic to Tertiary thrust and possibletranscurrent faults (Skipp and Hait, 1977; Ruppel and Lopez,1984; Link and Janecke, 1999; Tysdal, 2002). Eocene andyounger low-angle normal faults extend former Sevierthrusts (Hait, 1988). Janecke, (1992; 1994), Janecke etal.(1991), and VanDenburg et al., (1998) documented signifi-cant Eocene to present, variously oriented extensionalevents. Because of identification of Devonian Lemhi Archflank lithofacies in the transitional area between the south-ern and central Lemhi Range (Grader, 1998; Grader andDehler, 1999), it is possible that Paleozoic paleogeographymay reflect its approximately original distribution (sensu

Levy and Christie-Blick, 1989). That is, Paleozoic compres-sion may have been restored to original positions by Ceno-zoic extension.This needs to be further tested.

Stratigraphic Overview

Recently Idaho’s mid- to Late Paleozoic succession hasbeen re-interpreted with regards to biostratigraphy, deposi-tional sequences, parasequences, paleogeography and paleo-tectonic settings (Fig. 2). A synoptic overview indicates theIdaho succession developed in a similar sequential develop-ment as in Nevada.The main points are as follows:

1) The Ordovician-Devonian shelfbreak lies within Idaho,and both shallower and deeper water Ordovician - Sil-urian rocks were deposited here.

2) A passive margin characterized this area during theOrdovician to Middle Devonian, but it transitions into aconvergent (or transpressional) margin in the Late

DEVONIAN-MISSISSIPPIAN ANTLER FORELAND BASIN CARBONATES IN IDAHO: SIGNIFICANT SUBSIDENCE AND EUSTASY EVENTS

121 The Rocky Mountain Association of Geologists

Figure 1. Generalized geologic map of central Idaho(Wilson and Skipp; 1994; and Link, 2002), showingmountain ranges and relative positions of Paleozoicshelf, carbonate banks, younger igneous plutons andvolcanics, and Antler orogenic sediments (Nilson, 1977).AC - Antelope Canyon; AH - Arco Hills; CM - CopperMountain; DP - Doublespring Pass; HB - Howebuildups; GC - Grandview Canyon; SC - Snaky Canyon.

Devonian. The timing of this transition is quite clearlyseen in the stratigraphy, although a definitive thrust sys-tem (i.e., the Roberts Mountains Allochthon of Nevada)has not been clearly demonstrated in Idaho.

3) A foreland basin with significant subsidence ensues inthe Mississippian and later.A thick Carboniferous carbon-ate and mixed carbonate-siliciclastic succession (at leastthree times greater than that of Nevada) oversteps flyschin the foreland. Tectonic subsidence and eustatic cycles(Batt et al., 2006a,b in review) of differing durations con-trolled accumulation of this sedimentary succession.

DEPOSITIONAL HISTORY

The Early Devonian through late Mississippian strati-graphic interval (Fig. 3) merits consideration for three rea-sons. First, it contains the most recently studied Paleozoicrocks in Idaho. Second, this interval records significantdepositional and paleo-tectonic changes that generally paral-lel the record in adjacent Nevada and Alberta, Canada, butthey were not previously delineated in Idaho. The funda-

mental tectonic change is a Late Precambrian throughDevonian passive margin (Stewart, 1972) that changes intoan active, convergent margin in the Late Devonian. Third,there are remarkable depositional facies differencesbetween Nevada and Idaho. For example, Idaho buildups inthis interval have dysphyllid corals and stromatoporoid(Devonian) biostromes. Whereas similar age buildups inNevada are less diverse biologically, containing only stromat-poroids. Also, the Devonian-Mississippian carbonate banksuccessions in Idaho are considerably thicker than those inNevada, and show continuous deposition through key timeintervals that record sea level and paleoclimatic changes andproxies for Gondwana glaciation.

PASSIVE MARGIN SUCCESSION

Within the Tippecanoe and early Kaskaskia (Late Ordovi-cian through Frasnian, early Late Devonian) 2nd-order trans-gressive sequences (Sloss, 1962), there are several platformcarbonate units in east-central Idaho (Isaacson et al., 1983).Most of these units thinned toward the craton into SW Mon-



Figure 2. Summary of stratigraphy, paleotectonics, sedimentary accommodation and sedimentary accumulation events, central Idaho. Wesuggest at least five significant subsidence events, discussed herein. Eustatic overprinting by Gondwana glacio-eustasy is evident in stratacorrelating to time of glaciation.

tana or were subsequently eroded before Frasnian time.Early Paleozoic units exhibit deepening to the west towardthe continental margin (Fig. 4).

Devonian Passive to Active Margin Transition

The late Tippecanoe regression (Early Devonian) andearly Kaskaskia transgression (Middle Devonian) produced astratigraphic framework of thin (< 100m) to very thick (>1600m) shelf strata in east-central Idaho (Fig. 3). Easternshelf strata include the Beartooth Butte, Carey, Jefferson andThree Forks formations (Isaacson, et al., 1983), which pri-marily consist of carbonate ramp deposits with multiplechanneled sandstones and peritidal units. Strata are divided

into lower and upper lithostratigraphic successions sepa-rated by the late Frasnian sequence boundary “SB lFr0” (Figs.4, 5).The Lower Devonian succession represents depositionon a passive margin and the upper Late Frasnian-Famenniansuccession represents deposition on a subsident transitionalmargin (Fig. 4).

Regionally, peritidal strata of the Jefferson Formationoccur below the Three Forks Formation and have low shalecontent. Meter-scale cycles consist of shallow subtidal tointertidal and local supratidal facies, that are typicallydivided by sharp bedding contacts. Shallowing-upwardcycles consists of a dark black bioturbated, partly fossilifer-ous and faintly laminated dolomudstone, followed bymedium gray traction-laminated dolomudstone with rip-upclasts, and low-angle lamination truncations with soft sedi-



Figure 3. Summary of significant Silurian-Pennsylvanian stratigraphic units of central Idaho. Mountains ranges located on Figure 1. In gen-eral, “western” units discussed herein refer to those of the Lost River Range, and “eastern” refer to units in the Beaverhead Range. Units inthe Lemhi Range are transitional: shelf edge during Silurian and Early Devonian, and foreland basin during Carbnoniferous time.



Figure 4. Five step evolution of the Devonian carbonate ramp reflects passive to transitional, and active depositional and subsidence pat-terns over established Paleozoic hinge zones. Reference points are sequence boundaries L0, lFr0, Fm0, and flooding surfaces fsTf andfsM0. FdM0 is a Mississippian (McGowan Creek Fm., duplicata zone) drowning surface that is associated with initiation of significant tec-tonic loading and subsidence with siliciclastic flysch from multiple sources.

➤

Figure 5. Left: Regional chronostratigraphy of Devonian rocks in Idaho. Arranged and modified after Skipp and Sandberg (1975), Dehler(1995), Hobbs et al. (1991); Johnson and Sandberg (1977), Wiler (1992), Johnson et al. (1985); Hait (1965); Grader (1998); Elliott, Grader,Dehler (unpublished fieldwork), Sandberg et al. (1989). Euramerican Sea level curve depocycles from Johnson et al. (1985). Absolute stageboundaries after Gradstein and Ogg (1996). Center: Late Devonian sea level curve showing 3 levels of Belgian mudmounds and Canadianbuildup levels, modified after Sandberg et al. (2002). Note 2nd-order Devonian Sea level turnaround occurs after the Frasnian and beforethe Mississippian (during the early part of Sloss’s Kaskaskia 1st-order “Supersequence”) Right: Late Devonian relative sea level curve,sequence boundaries and lithologic successions for the Idaho shelf (after Johnson et al., 1985). Flooding over the Grandview buildup wasfollowed by subaerial exposure and deposition of late Frasnian sequences.Analogue for lower through middle Famennian rocks in Idaho isfound in the eastern Pilot Basin (Nevada; Giles et al., 1999).

ment deformation or ripple laminations, followed by verylight gray, burrowed or stromatolitic, finely crystalline dolo-mudstone. Subtidal facies also contain very thin fining-upward turbidite-like beds that indicate down-slope eventspossibly within local depressions.

Early and Middle Devonian

Early, Middle and early Late Devonian rocks (the lowersuccession) consist of approximately 1000 m of Lochkovianto late Frasnian (rhenana Zone) sandstones and dolostonesof the local Beartooth Butte Formation, distal shelf CareyDolomite (Isaacson et al., 1983), and lower Jefferson Forma-tion (Fig. 5). Deeply incised channels filled by conglomerateand sandy carbonate (up to 170 m thick in the LemhiRange) and thick, mainly carbonate deposits of the JeffersonFormation (1000m+ in the central Lemhi Range and LostRiver Range) jointly thin to the east to less than 300 m, withabrupt facies changes (conglomerates and sands) at theLemhi Arch flank. These were formed during the EarlyDevonian 2nd-order lowstand.To the west, accommodationduring the Late Silurian and Early Devonian preserved anadditional 600 m of similar marine carbonates. Early throughearly Late Devonian units are composed of meter-scale shal-lowing-upward, high frequency cycles within decameterdeepening-upward successions that are bound by Type 1sequence boundaries with erosional and karst surfaces onthe middle to distal shelf (Sarg, 1998; Lehrmann and Gold-hammer, 1999). Fish-bearing channel sandstones occurringat the 3rd-order scale are typically incised into dark subtidalbiostromal units (e.g., lower two Jefferson members, Fig. 5).This incision and fill pattern is itself repeated at SB lFr0 onthe 2nd-order scale, where intraclastic and sandy peloidalcross-bedded units of the upper Jefferson Formation cutacross the subaerially exposed Devonian buildup (see below).

The most consistent depositional cycles of the JeffersonFormation occur in the Dark Dolomite units of the thirdMember (Fig. 4), which has widespread coral biostromesand buildups.Above gradually back-stepping Frasnian rocksand deposition of the Nisku time-equivalent Grandviewbuildup (Isaacson and Dorobek, 1989) on the outer Jeffer-son shelf ramp, formation of the regionally identified lFr0sequence boundary was followed by units reflecting 2nd-order global sea level turn-around and switch from green-house to transitional icehouse conditions, as well as theregional switch from passive to active margin.

Early Late Devonian (Frasnian)

Dark fossiliferous units dominate much of the Frasnianmid-shelf section (Member D3, Fig. 5). Stratiform and latecross-cutting breccias (M’Gonigle, 1982; Grader, 1998) also

occur throughout the section, and are thick (>10 m) in theLemhi Range. Three latest Frasnian 50 m thick shelfsequences of 3rd- to 4th-order scale (rhenana-linguiformis

zones) are well developed west of the Lemhi Arch (Fig. 4).These units are characterized by: 1) basal quartz arenitesand reworked carbonate conglomerates, sandy stromatoliticcarbonate and collapse breccia (LST), lower IIc of Sandberget al. (2002) (Fig. 5); 2) laminated black subtidal to fossilifer-ous units (TST, IIc); overlain by, 3) unconformity and LST, ora fossiliferous sandy carbonate (progradational HST). Thisupper sequence (Fig. 5) and its wide lateral extent matchesmiddle IIc of the global sea level curve of Sandberg et al.(2002).To the west, in the Grandview area, these sequencesare more cyclic internally and occur with a higher percent-age of grainstones than further to the east (Figs. 4, 5).

The last Frasnian transgressive deposits include Thamno-

pora and Amphipora coral-stromatoporoid biostromes (Fig.5). Latest Frasnian carbonate ramps prograded within ashort period of time, as a global response to maximumglobal eustatic transgressive-regressive pulses (Fig. 5). Also,early Antler foreland loading provided accommodation forthese sequences.

An Upper Devonian (upper Frasnian), coral-dominatedcarbonate buildup (40 m) occurs in the upper part of themember D3 (IId, 5) of the Jefferson Formation at GrandviewCanyon (Custer County) in east-central Idaho (Isaacson andDorobek, 1989; DeSantis, 1986). The complex has variable,turbulence and rate of sedimentation-influenced growth(e.g., bulbous and encrusting) morphologies of stromato-poroids (including Amphipora). Other fossils include tabu-late corals (“Thamnopora”and Syringopora).The uppermoststage in buildup development reflects a deepening or trans-gressive event, which is abruptly terminated by an erosionalsurface of up to 2 m relief (upper IId, SB Fm0, Fig.s 5, 8).Dorobek and Filby (1988, Fig. 3) described local brecciasand dissolution vugs filled with cross-laminated silt formedat this subaerial exposure surface.

Late Devonian (Famennian)

Famennian quartz sand beach-barrier lithofacies werederived from the eastern craton and accumulated in anintra-shelf basin on the Lemhi Arch flank. Trapping andpreservation of sandstone occurred due to significant localaccommodation. Breccias of the upper Jefferson in theLemhi Range and in parts of the Lost River Range are inter-preted as collapse breccias after evaporates (M’Gonigle,1982). Stratifrom breccias are less common to the west.

During the Famennian, upper Jefferson lithofacies accu-mulated locally in the Lemhi Range. They are fossil-poor,sandy, and stromatolitic, and where not destroyed by karstand evaporite effects, meter-scale shallowing-upward periti-dal (mudstone to burrowed packstone) cycles remain a





Figure 6. Carboniferous stratigraphy (Skipp et al., 1979a,b) of central and southeast Idaho, with Mamet et al., (1971) foraminifera zonesand updates (Groves, 1984; Ross and Ross, 1987; Poole and Sandberg, 1991).

characteristic feature. Partly time-equivalent facies to thewest (Lost River Range) are similar but measure 200 m lessthan the Lemhi Range deposits. Evaporites occur in the sub-surface of Montana, so their artifacts in the form of verythick solution breccias suggest a restricted basin on theIdaho shelf.

On the shelf and onto the Montana craton, deposition ofthe Jefferson Formation was followed by deposition of<100m thick of fossiliferous marine limestone and yellow toblack shales of theThree Forks Formation (Figs. 4, 5).TheTri-dent Member occurs across much of the shelf with themudstone to fossiliferous packstone stacking patterns and iswell dated (Sandberg, 1979; Fox, 1985). The SappingtonMember reported by Sandberg et al. (1989) mainly occurs inMontana and are missing in Idaho, thus hiatus, erosion, andmissing conodont zones occur below Mississippian down-slope turbidites (Fig. 6).

Rapid middle Famennian sea level changes of the TridentMember occur within an otherwise overall backsteppingdepositional system (carbonate and shale replace restrictedperitidals, evaporites and shoreline quartz arenite of theupper Jefferson Formation).This has been shown as a deep-ening-upward pattern (Giles et al., 1999) on a similar transi-tional shelf in Nevada, although we suggest that depositionalenvironments remained quite shallow overall. Sandberg etal. (1989) argued that the Three Forks Formation wasdeposited over the Jefferson Formation following a hiatus.There is some physical evidence for an unconformity, andmarginifera Zone conodonts occur both in the basal ThreeForks Formation at Grandview Hill and in the Lost RiverRange respectively (Sartenaer and Sandberg, 1974; Wiler,1992), and in the Jefferson “False Birdbear” in the Beaver-head Range to the east (Sandberg and Poole, 1977).

ACTIVE MARGIN SUCCESSION

Early Mississippian

During the Carboniferous, east-central Idaho was locatedon the western margin of Laurentia between 10° and 20°north latitude (Scotese, 2002). Earlier Paleozoic tectonicevents greatly influenced subsequent Mississippian andPennsylvanian deposition. The Late Devonian to Mississip-pian Antler Orogeny produced an asymmetric, north-southtrending foreland basin bounded on the west by the AntlerHighlands in present-day central Idaho (Poole and Sandberg,1991; Skipp et al., 1979b). Dover (1981, 1982) asserted thereis no field evidence for Antler-age Roberts Mountainsallochthons in Idaho.Along this western basin margin (eastof the Antler Highlands) flysch deposits of the Copper Basinand McGowan Creek formations accumulated (Bissell, 1974;Nilsen, 1977; Link et al., 1996).

Huh (1968), reassigned Ross’s (1934, 1962) Brazer Lime-stone of the Lost River, Lemhi, and Beaverhead ranges (cen-tral Idaho) into four stratigraphic units in ascending order, asfollows: the Osagean-Meramecian Middle Canyon, Merame-cian-Chesterian Scott Peak, and the Chesterian South Creekand Surrett Canyon limestones (Fig. 6).These units are over-lain by Chesterian mixed carbonate and siliciclastics nowdefined as the Arco Hills and Bluebird Mountain formations,respectively (Skipp et al., 1979a). Isaacson (1985) describedtwo principal tectonic subsidence events in the Mississip-pian record of central Idaho.The first accommodation event(Huh, 1968; Rose, 1976; Nilsen, 1977) occurred during earli-est Mississippian time, with appearance of the dysoxicMcGowan Creek flysch.The flysch is very fine grained, wasderived from the west (Nilsen, 1977), and reflects accommo-dation presumably from tectonic loading by the AntlerOrogeny. Mississippian shallow-water carbonate depositsprograded westward from the cratonic platform located tothe east of the flysch trough (Nilsen, 1977; Skipp et al.,1979b).

Meramecian-Chesterian rocks of central Idaho and south-western Idaho were recently interpreted as a single 2nd-order tectonically-influenced depositional sequence (Link etal., 1996).A transect across east central Idaho and southwestMontana reveals that two tectonically produced accommo-dation cycles provided accommodation for the thick IdahoMississippian carbonate succession in Idaho (Isaacson,1985). The lower tectonic accommodation cycle includesthe Kinderhookian-Meramecian Middle Canyon and lowerScott Peak formations. The upper tectonic cycle includesaccommodation of the Chesterian South Creek, SurrettCanyon and Arco Hills formations. .

Overstepping the McGowan Creek flysch are anoxic anddysoxic carbonate mudstones (Fig. 3) of the Middle CanyonFormation (Lofland, 1986). Gradual basin infilling is repre-sented by the next unit, the Meramecian-Chesterian ScottPeak Formation (610 to 650 m thick; Stamm, 1985; Evans,1989), which contains oolitic shoals and some coral baffle-and framestones.The base of the Scott Peak is composed of125 m thick-bedded skeletal grainstone; the middle of theformation consists of very-thick-bedded dark fine-grainedcherty limestone; and the upper part (Chesterian) 200 mconsists of medium- to thick-bedded alternating lightpeloidal and oolitic grainstone and dark fine-grained chertylimestone (Hait, 1987).All units are fossiliferous with corals,brachiopods, bryozoans, and pelmatozoan debris (Hait,1987).

Late Mississippian (Chesterian)

The latest Mississippian (Chesterian) succession in east-central Idaho formed on an equatorial carbonate ramp alongthe distal edge of Laurentia. To the west, the episodically



subsiding Antler foredeep and associated highlandsrestricted open circulation with the adjacent Panthalassanocean (Dorobek et al., 1991). A second-order sea level fallcontrolled the vertical facies distribution in east-centralIdaho (Batt et al., 2006a).

The South Creek Formation (0-120 m, Fig. 6) is a thin-bedded to fissile cherty sparsely fossiliferous clayey carbon-ate (Hait, 1987). The Surrett Canyon Formation is 402 mthick at Arco Hills (Batt, 2006), and an average of 200 m(Galvin, 1981) of dark to medium gray, massive, fossiliferouslimestone (brachiopods, bryozoans, and abundant crinoiddebris) with locally abundant large rugose corals and possi-ble constructional mud-mounds (Galvin, 1981). It is over-lain by the Arco Hills Formation, in which theMississippian-Pennsylvanian boundary occurs (Abplanalp,2006.). The Arco Hills Formation has less than 100 m of

cyclically bedded carbonate and siliciclastic rocks (Butts,2003). The next unit is the Bluebird Mountain Formation,which was assigned to the latest Mississippian (Skipp et al.,1979a); but now it is interpreted as early Pennsylvanian age(Abplanalp, 2006). It is composed of primarily quartz aren-ites with minor limestone and dolostone interbeds. TheBluebird Mountain Sandstone is 105 m thick in the east andthins drastically to 7.6 m thick in the White Knob Moun-tains (Skipp et al., 1981).

Late Mississippian Biostratigraphy

The Mississippian biostratigraphy of Idaho was initiallyestablished with foraminifera (Mamet et al., 1971; Groves,1984, 1986). Stamm (1985) found that the conodont



Figure 7. Revised conodont biostratigraphy of central Idaho (Abplanalp, 2006), showing five conodont zones, with zones 1 through 4defining Chesterian strata. Zone 5 is Pennsylvanian (Morrowan). See text for discussion.

Taphrognathus varians occurred in the lower beds of theScott Peak Formation suggesting they correlated to Mamet’sForaminiferal Zones 10, 11, and 12 (Meramecian). Not onlydid he confirm that the base of the unit was younger on thewest, but he proposed a new conodont zone for the north-ern Rocky Mountains.This is the phylogenetic change fromHindeodus penescitulus to H. cristulus, and the two taxawould replace the longer-ranging lower Cavusgnathus

zone.More recent biostratigraphic work on the South Creek,

Surrett Canyon,Arco Hills and Bluebird Mountain (Chester-ian) strata in east-central Idaho has involved conodonts.Measured sections representing a deep to shallow shelftransect reveal five applicable conodont zones for theupper Mississippian (4 zones) and early Pennsylvanian (1zone) of the northern Rocky Mountains (Fig. 7). Two ofthese five zones are newly proposed, whereas the otherthree are modifications of previous work from the U.S. Mid-continent and western interior (Dunn, 1970; Lane andStraka, 1975; Sando et al., 1981; Baesemann and Lane, 1985).The northern Cordilleran (Idaho) conodont assemblagesare similar, but some of the existing zones are modified.Our collections across a continuous, uninterrupted sedi-mentation record in Idaho now allow a clear conodont-based zonation of the Chesterian (Abplanalp, 2006). Ourzone 1 is characterized by the first appearance of Hindeo-

dus cristula and/or Vogelgnathus campbelli and spans theuppermost Meramecian to the lower Chesterian. This zoneis newly proposed and was first observed by Stamm (1985;see above), within the Scott Peak Formation and we extendit into the South Creek Formation. Zone 2 is based on thefirst occurrence of Cavusgnathus naviculus and overlapswith the Mamet’s zones 17, 18, and the base of 19 (Fig. 7).This zone was widely applied in previous studies and isobserved for the South Creek and lower Surrett Canyon for-mations. Zone 3 is based on the appearance of Adetog-

nathus unicornis and correlates with the middle offoraminiferal zone 19.This zone is based on previous stud-ies, although the range was extended for the northernRocky Mountains due to the rare occurrence of Rhachistog-

nathus muricatus. Zone 3 occurs within the Surrett Canyonand Arco Hills formations. Zone 4 is characterized by thefirst appearance of Adetognathus lautus and is a newly pro-posed biostratigraphic division based on the upper subdivi-sion of the R. muricatus zone of Baesemann and Lane(1985). This stratigraphically thin zone correlates with theuppermost Mamet zone 19 and occurs within the SurrettCanyon,Arco Hills, and Bluebird Mountain formations. Zone5 (Pennsylvanian) is based on the first occurrence of Penn-sylvanian forms and is limited by the full range of R. primus.Chronostratigraphic correlation of Chesterian strata acrosseast-central Idaho indicates stratal thickening to the west asthe foci of sedimentation prograded into the foreland basin(Rose, 1976).

Tectonic Controls on Structure and Accommodation

During the Meramecian the carbonate ramp of Idaho andsouthwestern Montana was tectonically segregated into aseries of depositional highs and lows (Rose, 1976) (Fig. 8).High-energy grainstones having little siliciclastic inputformed on the highs, whereas deeper water or restrictedsediments with abundant siliciclastics were deposited in thelows (Dorobek et al., 1991; Chen and Webster, 1994).To theeast, mixed carbonate and siliciclastic rocks were depositedin the Big Snowy Trough, a structurally controlled low area(Sando et al., 1975; Brewster, 1984, Dutro et al., 1984).Thisstructure was likely produced by foreland basin reactivationof Precambrian basement structures, as was shown for Mer-amecian units (Dorobek et al., 1991).

Chesterian Paleogeography

The Antler foreland basin, inherited from the Late Devon-ian (Craig and Varnes, 1979), was filled with Antler High-land-derived clastics from the west and with carbonates thatprograded from the eastern (craton) side of the basin(Nilsen, 1977). The western North American margin waswithin fifteen degrees south of the equator, situated so thatthe equatorial tradewinds swept westward across the cratonover the Antler foreland basin and highlands. Two majortopographic features were present, the Antler highlands andassociated foreland basin (Fig. 8) and the Big Snowy Trough,oriented nearly east-west across Montana, which may haveaffected Mississippian sedimentation patterns (Dorobek,1991). An intermittent highland-lowland, referred to as theBannock Highland or the Beaverhead Arch, persisted parallelto the Antler Highlands (Sando et al., 1975).

Cyclicity in Chesterian Rocks

A recently measured section (Batt, 2006) of Chesterianstrata at Antelope Creek (Fig. 1), in the southern WhiteKnob Mountains, is a thick succession of cyclic carbonates(mudstone to packstone successions) punctuated by lami-nated to burrowed siltstones or fine sandstones in theupper half of the section. There the siliciclastics make asharp contact with underlying carbonate and grade upwardinto skeletal wackestone/packstone that are in turn over-lain by cherty lime mudstone that grades upward intoskeletal wackestone and packstone.At the top of the skele-tal wackestone/packstone is another sharp surface overlainby siliciclastics.These successions are similar to the “cycles”described by Stamm (1985) and Galvin (1981) that are 18-50 m thick and record long-term relative changes in sealevel in this area, owing to the overall length of the Chester-ian (>10 my, Fig. 6).



Siliciclastics within these predominantly carbonate sec-tions and mixed carbonate-siliciclastic sections to the eastare not known in detail but could provide clues about localtectonism in this basin. A westward decrease in grain sizeand concomitant increase in carbonate content of the Blue-bird Mountain sandstone suggests this unit had an easterlysource (Skipp et al., 1979a). However, a recycled passivemargin origin for sandstone/conglomerate clasts in lower

Mississippian coarse-grained units in the foredeep (Link etal., 1996; Preacher et al., 1995) indicates that these siltstonesand sandstones were likely derived from the orogenic high-lands to the west.

Parasequences have been defined as cycles with geneti-cally related sequences of rock bounded by flooding sur-faces or their correlative surfaces (Van Wagoner et al., 1987).They are usually scale independent; in the Lost River and



Figure 8. Mississippian cross-section through central Idaho andMontana, showing foreland basin to west and Montana platform toeast (adapted from Rose, 1976). Significant accommodation occurswith accumulation of deep ramp carbonates, muddy skeletal bankand ultimately mixed carbonate - siliciclastic shoals in centralIdaho. Dashed lines are time lines (from Fig. 7).

Lemhi ranges they are 1-10 m thick. Parasequences of thesethicknesses have been reported by Read (1985). Parase-quences consisting of a shallowing-upward complete or par-tial facies succession within the Arco Hills Formation aregenerally less than 10 m thick. Parasequences in the ArcoHills Formation would have durations of approximately375,000 years each (owing to the number of parasequenceswithin Chesterian time), consistent with 4th-order glacio-eustatic sea level changes (Weber et al., 1995). Cyclicity inthe latest Mississippian Arco Hills Formation has been estab-lished by facies stacking patterns (alternations from highenergy grainstone through undisturbed mudstone, then anew parasequence with grainstone) and brachiopod paleo-communities (Butts, 2005).

Transition from the Surrett Canyon Formation to the ArcoHills Formation shows distinct changes in scales of cyclicity,lithology, and facies stacking patterns (Batt et al., 2006a,b, inpress), which indicate a global climate shift at this time(Butts, 2005). However, cycles identified in the SurrettCanyon Formation (Galvin, 1981) are generally tens ofmeters thick (averaging around 35 m thickness), although insome sections they are 5–15 m thick. Basal thick beddedsubtidal facies coarsen upward to open marine skeletalshoals and lagoonal carbonates, which are overlain by thininterbedded siliciclastic and skeletal grainstone depositsacross the mid-Carboniferous boundary (Batt et al., 2005;Butts, 2005).

Parasequences in the latest Mississippian Arco Hills andBluebird Mountain formations indicate shallowing-upwardsuccessions (Butts, 2005). Two considerations play impor-tant roles in determination of relative water depths in theseparasequences: the paleoecological constraints of organ-isms within a biofacies, and the sedimentary structures asso-ciated with each biofacies. Low oxygen conditions of thedeepest (and lowest) biofacies are indicated by the pres-ence of an inarticulate brachiopod (Orbiculoidea) and thelack of any other fauna (Fig. 9).Dysoxic conditions persist indeep stratified basins or shallow restricted waters makingthis not a strictly depth-controlled issue; however, the associ-ated dark mudstone and accessory minerals (phosphatesand chert) suggest that this is the deepest biofacies.This bio-facies (Biofacies A, Fig. 9) shallows upward into a packstoneand wackestone (Biofacies B) with a productid/orthotetidbrachiopod association.The functional morphology of thesebrachiopods includes adaptations for life on soft muds.Spines, for anchoring in muds, trails (geniculated ventralvalve) for deflection of muds from the mantle cavity, andlarge surface area to body ratio, all indicate muddy-bottomsediment, suggesting below storm wave base deposition.The packstone facies (Biofacies C) grades to grainstone withpara-autochthonous to allocthonous fossil content, thatoften occur in couplets with fine-grained bioclastic calcare-ous siltstone. The strong taphonomic signature of the spir-iferid/athyridid brachiopod association in this biofacies



Figure 9. Typical parasequence (left), and biofacies of Arco Hills Formation and associated lithologies. A = Orbiculoidea; B = Produc-toidea-Orthotetoidea; C = tempestites or shell lags with high degree of disarticulation and current orientation; D = encrinite shoals; E =behind shoal sandy lithologies with rare mollusks and brachiopods (Butts, 2005).

suggests that they may not have lived as a community. How-ever, the condition of the fossils suggests the degree of bios-tratinomic processes vary considerably, with some bedsdisplaying very little evidence of transportation (distal stormdeposits) and others showing extensive reworking (stormdeposits).Above this is grainstone (Biofacies D, Fig. 9) com-posed almost entirely of size sorted and planar cross-beddedpelmatozoan ossicles and broken and abraded non-echino-derm bioclasts.This facies was deposited in high- to moder-ate-energy wave-influenced bars or shoals that formedwithin fair weather wave base.The uppermost Mississippianbiofacies was also within fair weather wave base, but wasprotected by bioclastic shoals. It consists of carbonate withconsiderable craton-derived silt and abundant transportedbrachiopods, bivalves, and gastropods in a nearshore rela-tively shallow environment.

DISCUSSION

Late Devonian and Carboniferous tectonic controls ofdepositional patterns are apparent both in Nevada andIdaho. Significant changes in depositional patterns and inter-mittent high-amplitude relative sea level changes are indi-cated by both deep-water palmatolepid conodont-bearingThree Forks shale facies, differential erosion, and conodonthiati suggest shorter-term glacio-climatic controls (Sandberget al., 1988; 2002). Other complications involve brittle defor-mation and reactivation of basement faults with passage of aflexural foreland forebulge (after the Nevada model ofGoebel, 1991; Giles and Dickinson, 1995; Sandberg et al.,2002). Various models for the style of Antler orogeny inIdaho are summarized in Link et al. (1995), who show thatsignificant subsidence was limited to the Mississippian.Glacio-eustatic overprinting is supported by the combina-tion of punctuated stratigraphic stacking patterns on globalpaleo-equatorial shorelines and glacial diamictites widelydeposited at the end of the Famennian in Gondwana (Díaz-Martínez et al., 1999; Isaacson et al., 1999).

CONCLUSIONS

The Early Devonian through late Mississippian strata ofIdaho record an almost continuous succession of rocksabove Proterozoic and early Paleozoic rocks. Passive marginextension and then active, tectonically loaded events accom-modated a continuous record of Cordilleran tectonic eventsoverprinted by eustatic sea level changes.A late Tippecanoethrough early Kaskaskia (Early to Middle Devonian) superse-quence is characterized by mainly shallow water and tidalflat carbonates on the shelf margin.

The Kaskaskia transgression produced thick late Middleand Late Devonian successions with Late Devonian (Frasn-

ian) organic buildups of the disphyllid coral-stromatoporoidtype as described in nearby western Canada. Some of theserocks have been placed into a sequence stratigraphicframework, but the Famennian rock record remains poorlyunderstood. The Grandview buildup and overlying smallerbiostromes, like others globally in similar stratigraphic posi-tions were terminated by the environmental effects ofGondwana ice sheet expansion. Characteristic 4th-ordersequences during the late Frasnian were masked by accel-eration in tectonic induced subsidence (of 3rd-order scale).Late Frasnian subaerial exposures and Famennian sequenceboundaries occur on the Devonian margin at the same timethat it evolved into an active, probably transtensional ortranscompressional restricted margin. Both increased andsuddenly decreased subsidence rates and relative sea leveleffects were felt both in intrashelf basins and intermittentlyon the distal shelf in response to foreland lithospheric load-ing. This pattern matches other locations along the LateDevonian Cordilleran margin with major accommodationfollowing. The buildup was terminated by Gondwana icesheet expansion producing rare Late Devonian subaerialexposure at the newly active margin.As with other localitiesalong the Late Devonian Cordilleran margin, major accom-modation follows, produced by tectonic loading as the pas-sive margin yielded to compression. Although the RobertsMountains Thrust has not been recognized in Idaho, tec-tonic induced accommodation produced a Carboniferousflysch trough, over which a very thick succession of carbon-ates prograded.

Two tectonic loading events punctuate the very thickCarboniferous carbonate accumulation, one in lower Missis-sippian and a second in late Mississippian event. Chesterianevents are the most completely documented part of this his-tory. Carbonate banks are drowned and overlain by dysoxicmudstones are succeeded by wackestones and packstones.Capping the Chesterian succession are 4th-order cyclic shal-lower shelf mixed carbonates and siliciclastics that recordan additional set of glacioeustatic sea level fluctuations.

ACKNOWLEDGMENTS

Support for much of this work was provided by a grantfrom the National Science Foundation (EAR 0229369). S.L.Bachtel, K.L. Canter, W.J. Clark, G.P.Wahlman, and an anony-mous reviewer offered superb comments and editing sug-gestions on an earlier version of the manuscript.

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THE AUTHORS

PETER E. ISAACSON

After his bachelor’s degree at the University of Colorado, Boulder (1968), PETER ISAACSON receivedhis Ph.D. degree at Oregon State University (1974). He has been an adjunct professor at Univer-sity of Massachusetts, Amherst and Research Associate at Amherst College. He is now Professorof Geological Sciences at the University of Idaho, Moscow. Research interests are Silurianthrough Upper Carboniferous stratigraphic and biostratigraphic systems across the platform andinto the western assemblages and foreland basins of the Cordillera of Idaho, Nevada and Bolivia.He is also expanding knowledge of the Late Devonian glaciation (Andes and eastern Laurentia),as well as its global effects.

GEORGE W. GRADER

GEORGE GRADER received his M.S. and Ph.D. in geology at the University of Idaho in 1998 and2003. His M.S. thesis was on Devonian stratigraphy and carbonate depositional systems acrossthe margin in Idaho. His dissertation was on carbonate facies and sequences of the Pennsyl-vanian-Permian Copacabana Formation of Bolivia. He is an instructor at Washington State Uni-versity and has been active in Miocene hydrogeology of Northern Idaho. He continues tocollaborate with authors investigating Late Paleozoic stratigraphy in east-central Idaho andBolivia, with special emphasis on carbonates as proxies for glacially-induced sealevel changes.

MICHAEL C. POPE graduated with a B.S. in Earth and Space Science from UCLA in 1985, earned aM.S. in Geology from the University of Montana in 1989 and a Ph.D. from Virginia Tech in1995. He was a post-doctoral research associate at MIT from 1995-1998 then worked for MobilExploration Company from 1998-1999. Mike has been teaching at Washington State Universitysince 1999. His research interests are in understanding the sedimentary record of carbonatesdeposited during icehouse, greenhouse and transitional climate periods.

MICHAEL C. POPE



THE AUTHORS

ISABEL P. MONTAÑEZ

ISABEL P. MONTAÑEZ graduated with an A.B. in Geology in 1981 from Bryn Mawr College, and sub-sequently received a Ph.D. in Geology from Virginia Polytechnic Institute in 1990. Isabel beganher university career in 1990 as an Assistant Professor at the University of California, Riverside andhas been a Full Professor of Geology at the University of California, Davis since 2000. Herresearch focus is in deep-time paleoclimatology, in particular in reconstructing marine- terrestrialrecords of greenhouse gas-climate linkages during past major climate transitions.

SUSAN H. BUTTS

SUSAN BUTTS received a B.A. from Hobart and William Smith Colleges in New York, and her PhDdegree at the University of Idaho. She was a postdoctorate researcher at Yale University. Sheis now the Collections Manager in the Division of Invertebrate Paleontology at the PeabodyMuseum of Natural History, Yale University. She works on primarily on brachiopods and isinterested in Paleozoic paleoecology and the process and controls on silicifiication of fossils.

LISELLE BATT received her B.S. at James Madison University in 1999 and her PhD at the Univer-sity of Idaho, 2006. recently completed her dissertation on the sequence stratigraphy and car-bon isotope record of Late Mississippian carbonates and siliciclastics in Idaho and southwestMontana. She is interested in proxies that record long term climate change, and in the interplayof tectonic, eustatic and autocyclic processes on the deposition of carbonate sedimentary rocks.Presently she is employed as a petroleum geologist with ExxonMobil, where she is gainingexperience with carbonate and siliciclastic stratigraphy in the Middle East.

JASON ABPLANALP received his B.S. in Geology from Juniata College in Pennsylvania in 2004. Hereceived his M.S. at the University of Idaho in 2006. His thesis title is “Late Mississippian (Ches-terian) Conodont Biostratigraphy of East-Central Idaho and Southwest Montana”. Currently,Jason is pursuing a PhD at the University of Idaho, and his dissertation involves biostratigraphicinvestigation of the Pennsylvanian Snaky Canyon Formation of east-central Idaho.

LISELLE S. BATT

JASON M. ABPLANALP

Devonian-Mississippian Antler Foreland Basin Carbonates in Idaho: Significant Subsidence and Eustasy...

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