Miocene Gypcretes from the Calama Basin, northern Chile

Miocene Gypcretes from the Calama Basin, northern Chile

ADRIAN J. HARTLEY and GEOFFREY MAYDepartment of Geology and Petroleum Geology, King's College, University of Aberdeen, Aberdeen, AB243UE, UK (E-mail: [email protected])

ABSTRACT

Gypcretes of Miocene age are preserved beneath a 9á53 � 0á36 Ma ignimbrite along the

eastern margin of the Oligo-Pleistocene Calama Basin, northern Chile. They are

restricted to a single stratigraphic horizon developed within laterally extensive

(>35 km) coalesced alluvial fan deposits, developed along the margin of an endorheic

basin. Two types of gypcrete are recognized. Type 1 comprises almost completely

gypsum-cemented sandstones containing alabastrine nodules and columns, sub-

vertical and horizontal veins of ®brous gypsum and `v-shaped' cracks in®lled by

clastic material, and are interpreted as surface weathered gypsic crusts. Type 2

gypcretes are composed of massive, reddened poikilitic and mesocrystalline gypsum

(up to 80% of the rock) with isolated bedding-parallel, clast-rich lenses (200 ´ 30 cm)

and sub-vertical veins of ®brous gypsum. The massive texture resembles that of well

developed B horizons in Quaternary alluvial desert soils. The crystal forms suggest an

origin as a subsurface gypsic crust formed by a combination of hydromorphic

(poikilitic) and illuvial (mesocrystalline) processes with the ®brous gypsum veins

suggestive of periodic surface exposure.

Gypcrete horizons are up to 25 m thick and composed of both gypcrete types. They

represent superimposed phases of surface and subsurface gypcrete development.

Quaternary gypcretes are developed in arid climatic regimes, but are not considered to

develop under hyper-arid climates. An arid climate is considered to have prevailed in

the study area up to 9á5 Ma after which a change to hyper-aridity favoured gypcrete

preservation.

INTRODUCTION

Indurated gypsic soils (gypcretes) are well knownfrom Quaternary hot and arid regions across theworld (Watson, 1985). However, in contrast toother arid zone pedogenic horizons such ascalcretes or silcretes, gypcretes are virtuallyunknown from the rock record due to: (1) theirtendency for dissolution following burial beneaththe water table; (2) dissolution related to in-creased run-off due to climate change; (3) aeoliande¯ation following surface exposure; and/or (4)calcite replacement of gypsic soils to formcalcretes (see Watson, 1989; Cooke et al., 1993for reviews). While it is likely that long-termgypsum crust preservation is uncommon, Watson

(1983) noted that because of the dif®culty ofpreservation, few workers have considered thepossibility of a terrestrial rather than marine,littoral or lacustrine origin for gypsum deposits inthe rock record.

Gypcretes have been reported from desertregions, including the present day Atacama,where annual rainfall is generally less than250 mm year)1 (Watson, 1985). Due to therestriction of gypcretes to arid climates and theirtendency for dissolution, their occurrence can beused to infer periods of aridity in Quaternarypalaeoclimatic reconstructions (e.g. Watson,1988).

Here we provide one of the ®rst descriptions ofpre-Quaternary gypcretes, from the Miocene

Sedimentology (1998) 45, 351±364

Ó 1998 International Association of Sedimentologists 351

section of the Calama Basin, northern Chile. Weidentify two forms of gypcrete, one of whichshows close similarities to descriptions andmodels established for Quaternary examples(e.g. Tucker, 1978; Watson, 1985, 1988, 1989;Cooke et al., 1993) and a second which differsfrom established models. The palaeoclimaticimplications of the development and preservationof these Miocene gypcretes is considered.

Calama Basin

The Oligocene to Pleistocene Calama Basin issituated within the Andean forearc of northernChile between 22 and 23°S at an elevation of2000±3500 m (Fig. 1). It comprises one of a seriesof pre-Andean basins developed between the Pre-Cordillera (Cordillera de Domeyko), a fault-bounded mountain range up to 4500 m highcomprising Palaeozoic to Eocene sediments andigneous rocks, and the 4500±6000 m high Mio-cene to Recent volcanic arc which forms theAndean Cordillera. The basin is bounded to thesouth and west by the Cordillera de Domeyko, tothe south-east by overthrusted sediments of theCretaceous-Eocene Purilactis Group, and by thepresent day volcanic arc and associated ignim-brites to the north and east (Fig. 2). Previous workon the Calama Basin includes sedimentary andstructural descriptions in regional mapping re-

ports (Marinovic & Lahsen, 1984; Boric et al.,1990) and stratigraphic syntheses (Naranjo &Paskoff, 1981, 1982). Recent work including radio-metric dating of extensive ash and ignimbritedeposits has resulted in substantial revision of thebasin stratigraphy (May et al., 1996; May, 1997), abrief summary of which is presented here.

The basin-®ll comprises over 700 m (G. Chong,personal communication, 1995) of Oligocene toPleistocene continental sediments. The succes-sion can be divided into four, conformable tolocally angular (2±10°), unconformity-boundedunits (Fig. 3).

1 An Oligocene unit (the Calama Formation)which crops out locally to the east and north ofCalama (Fig. 2) where it rests unconformablyupon Palaeozoic metamorphic and Mesozoicsedimentary rocks. The unit is characterized bypoorly strati®ed, matrix- and clast-supportedconglomerates interpreted as alluvial fan deposits.2 A Lower to Mid-Miocene unit (equivalent to thelower part of the El Loa Formation of Marinovic &Lahsen, 1984) which crops out over much of thebasin and unconformably overlies the CalamaFormation near Calama, and Mesozoic andPalaeozoic basement elsewhere. In the basincentre it comprises interbedded mudstones andthin sandstones, which pass laterally into con-glomerates and sandstones along the basinmargins. Gypcretes are developed along the

Fig. 1. Location map of the Calama Basin within the forearc of northern Chile (after Boric et al., 1990). The boxed areacorresponds to Fig. 2.

352 A. J. Hartley and G. May

Ó 1998 International Association of Sedimentologists, Sedimentology, 45, 351±364

eastern basin margin towards the top of thisstratigraphic unit. They are stratigraphicallyequivalent to the `Hollingworth Gravels' de-scribed by Naranjo et al. (1994) from the north-west margin of the Salar de Atacama.3 An Upper Miocene to Upper Pliocene unit(equivalent to the upper part of the El LoaFormation of Marinovic & Lahsen, 1984), whichcrops out over much of the basin where itunconformably overlies older strata. It comprises

lacustrine limestones, diatomites, arenaceouslimestones and calcretes in the basin centrewhich pass laterally into alluvial conglomeratesand sandstones along the basin margins. Region-ally extensive ignimbrites and ashes of the SanBartolo Group (de Silva, 1989; Fig. 3) weredeposited along the eastern margin of the basinin the late Miocene to Pliocene.4 An Upper Pliocene to Lower Pleistocene unit(the Chiuchiu Formation of Marinovic & Lahsen,

Fig. 2. Geological map of the Cala-ma Basin (after Marinovic & Lahsen,1984). The numbered sections are:(1) Quebrada Yalqui; (2) QuebradaTuina; (3) Quebrada San Martin;(4) Quebrada Agua de la Teca;(5) Barros Arana; and (6) Angostura.Arrows correspond to general pa-laeocurrent directions derived fromclast imbrication in the pebblysandstone facies.

Miocene Gypcretes from Chile 353


1984) is exposed in the central and eastern partsof the basin where it unconformably overlies theEl Loa Formation. It comprises thinly beddedmudstones, diatomites and evaporites in thebasin centre which pass laterally into interbeddedconglomerates, sandstones and mudstones. Sedi-mentation ceased across the basin in the EarlyPleistocene when incision of the basin-®ll due todowncutting of the Rio Loa and Rio San Salvadortook place (Mortimer, 1980), resulting in by-passing of the depositional surface.

Study area: eastern basin margin

Conglomerates, sandstones and mudstones fromthe lower part the El Loa Formation are exposed

in dry valleys (quebradas) along the easternmargin of the Calama Basin (Fig. 2). The upperage of these exposed sediments is constrained asearliest Late Miocene, as they are overlain by theregionally extensive Artolla and Sifon ignimbrites(Fig. 4). At Barros Arana (Fig. 2) the Artollaignimbrite has been radiometrically dated at9á53 � 0á36 Ma (K/Ar technique, de Silva, 1989)and the Sifon ignimbrite at 8á27 � 0á13 Ma (Ar/Artechnique, May, 1997).

Gypcretes are developed in the top 5±25 m ofstrata which directly underlie the ignimbrites inall the studied sections. In all instances, sedi-ments overlying the ignimbrites show no evi-dence of pedogenesis, constraining gypcretedevelopment as pre-emplacement of the Artollaand Sifon ignimbrites.

SEDIMENTOLOGY

Gypcrete horizons are developed within thelower part of the El Loa Formation. Thisstratigraphic interval ranges in thickness from25 to 50 m along the eastern basin margin to200 m in the basin centre near Calama andcomprises three, laterally equivalent facies asso-ciations (Fig. 4).

Alluvial fan facies association

Two different, but interbedded facies can berecognized developed along the basin margin:strati®ed pebbly sandstone which comprises over80% of the studied sections, and massiveconglomerate which forms the remainder. In bothfacies, clasts are subangular to subrounded,although occasional more rounded, faceted andpolished (wind-abraded) types are present. Clastsare composed primarily of dark, ®ne grained andporphyritic andesites and occasional ®ne grainedred metasediments which can be matched withlithologies from the Triassic Tuina Formation(Fig. 2).

Strati®ed pebbly sandstone facies

This facies comprises a gradation from clast-supported conglomerates through to strati®edpebbly sandstone and coarse grained sandstonebeds. Bed thickness varies from 1 to 105 cm.Pebble and ®ne cobble grade clasts with acoarse sandstone matrix are the most commonconstituent. Beds are planar-strati®ed, oftendisplay imbrication and form 10±80 cm thick

Fig. 3. Stratigraphy of the Calama Basin based uponMay et al. (1996) and May (1997). Note that the Artollaand Sifon ignimbrites compose part of the San BartoloGroup (see de Silva, 1989; for further details).



Fig. 4. Correlation of stratigraphic sections exposed along the eastern margin of the Calama Basin and the basin centre (see Fig. 2 for section location).For details of radiometric age determinations see May (1997). Palaeocurrent data are derived from cross-strati®ed sandstones. A small angular un-conformity (6±8°) is present between the gypcrete horizons and the overlying strata at Quebrada Tuina. At Barros Arana the gypcrete horizon isdeveloped in sediments directly overlying the Purilactis Formation. The diatomite and palustrine carbonate facies comprise part of the upper El LoaFormation and are described in May (1997).

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planar-strati®ed sheets and elongate lenses 2±30 m long. Lenses and sheets often show adivision into a lower pebbly unit and an uppercoarse (pebbly) sandstone unit.

The absence of large-scale channelizationsuggests deposition from uncon®ned to semi-con®ned ¯ow. The imbrication and beddingparallel alignment of clasts indicate transport asbedload material, with the poor sorting andlimited bed thicknesses suggestive of rapid,shallow, ephemeral ¯ow. Transport and deposi-tion by sheet¯oods on an alluvial fan is proposed.These deposits are texturally and compositionallysimilar to those of recent sheet¯oods in theAtacama, and are comparable with those de-scribed from modern and ancient alluvial fans(e.g. Hogg, 1982; Ballance, 1983).

Massive conglomerate facies

Massive, 0á2±2 m thick, laterally continuous bedsof conglomerate have planar to very gentlyundulose bases and top surfaces. They arecomposed of very poorly sorted, sub-angular,pebble to cobble grade clasts supported by a siltto coarse sand matrix. A continuum is presentbetween matrix-supported and clast-supportedconglomerates. Clasts account for 20±50% of bedvolume within matrix-supported beds. Normal orreverse grading is absent, although the largestclasts often `¯oat' and protrude from upper bed-ding surfaces. In contrast, clast-supported unitsmay ®ne upwards and show weak imbrication.

The unsorted, non-erosive nature of these bedsare typical of debris ¯ow deposits in whichsedimentary particles and entrained ¯uids moveas a single viscoplastic body (Johnson, 1970). Theabsence of clay-sized sediment within the matrixsuggests that ¯ow was largely non-cohesive.Virtually cohesionless ¯ow has been detailed byPierson (1981) who has shown that in slowmoving ¯ows, buoyancy and grain to graincontact suspends clasts within the matrix. Themassive clast-supported conglomerates resemblethe hyperconcentrated ¯ood ¯ow deposits ofSmith (1986) and ¯uidal sediment ¯ows of Nemecand Steel (1984), where turbulent ¯ow and clastdispersive pressure provided support during ¯ow(Todd, 1989).

Ephemeral ¯uvial facies association

The ephemeral ¯uvial facies association is inter-bedded with, and grades basinwards from, allu-vial fan deposits (Fig. 4). It comprises 10±15 m

thick, erosive-based, ®ning upward cycles ofconglomerate, interbedded with sandstone andmudstone units. The base of each cycle isrepresented by horizontally strati®ed and troughcross-strati®ed conglomerate facies. Individualconglomerate bodies are 2 m thick, channel-shaped, up to 30 m wide and may be stacked toform composite units up to 4 m thick. Clasts arewell rounded and sizes are typically pebble gradealthough small cobbles are occasionally present.Imbrication is common. Conglomerate bodies areoverlain by up to 12 m thick successions ofsandstones and mudstones with sheet-like geo-metries. Beds within the sandstone facies arelaterally continuous for over 100 m and rangefrom 5 to 50 cm in thickness. They have planar,non-erosive bases and are massive, planar strati-®ed and/or trough cross-strati®ed. Planar strati®-cation at the base of beds often passes up intotrough cross-strati®cation. The sandstones arecoarse grained and locally pebbly with mudstoneintraclasts. Normal grading at bed tops to ®nesandstone and mudstone is common. Symmetri-cal, straight-crested, bifurcating ripples are ob-served on the tops of some sandstone beddingplane surfaces. Individual mudstone beds rangefrom 1 to 30 cm in thickness, may form packagesup to 8 m thick and extend laterally for over100 m. They are structureless or ®nely parallellaminated and often contain desiccation cracks.

The channelized geometry, erosional base,sedimentary structures, presence of well roundedclasts and ®ning upwards nature of the conglom-erates indicate deposition in ¯uvial channels.The ®ning upwards nature at the top of thechannel-®lls indicates decreasing energy suggest-ing waning ¯ow conditions. The sheet-likegeometry of the sandstone beds indicates deposi-tion from poorly con®ned sheet¯ow. Planarlaminae develop during upper-phase ¯ow condi-tions (Tunbridge, 1981) and grade to trough cross-strata representing lower-phase ¯ow probablyduring waning ¯ow conditions. Ripples devel-oped on bed tops are interpreted as the product ofwave oscillation in standing bodies of waterfollowing sheet¯ood events. The ®nely laminatedmudstone sheets are considered to record deposi-tion from suspension in standing bodies of water,desiccation cracks indicate drying of water bodies(Hardie et al., 1978).

Playa-sand¯at facies association

The playa-sand¯at facies association is developedwithin the basin centre, and is laterally equivalent



to the alluvial fan and ephemeral ¯uvial faciesassociations (Fig. 4). It comprises interbeddedevaporitic mudstones and sandstones withsheet-like geometries. Mudstone beds form themajority of the studied sections (80%). They areidentical to those of the ephemeral ¯uvial faciesassociation but in addition, contain moulds andnodules of displacive gypsum and/or halitecrystals. The sandstones are 2±30 cm thick, ®negrained and are trough cross-strati®ed or planarstrati®ed. Sandstone beds often ®ne upwards intothe mudstone facies. Gypsum crystals are presentwithin the sandstones.

The sandstone beds are interpreted to representuncon®ned ephemeral sheet¯oods. Mudstonebeds represent deposition from suspension instanding bodies of water. The presence ofevaporitic crystals is typical of playa sand¯atand mud¯ats found within closed basins distal toalluvial fan or ¯uvial systems (Hardie et al., 1978;Cooke et al., 1993).

Lower El Loa Formation: depositional setting

The strati®ed pebbly sandstones and massiveconglomerates represent alluvial fan deposits.The lateral extent, clast composition, limitedpalaeocurrent data (from imbrication), locationand amalgamated nature of these deposits suggestthat they represent coalesced fan apron deposits(bajada) developed along the ¯anks of the TuinaFormation outcrop (May, 1997); a similar scenarioto the present day where extensive (up to 15 kmradius) bajadas are developed along the edge ofmountain fronts ¯anking the Calama Basin.Alluvial fan deposits pass basinwards intoephemeral ¯uvial and playa sand¯at and mud¯atenvironments. These facies association relation-ships indicate an arid to semi-arid climate withdeposition in an endorheic basin.

Gypcretes

Along the eastern margin of the Calama Basin(Fig. 2), two types of gypcrete are developed inalluvial fan conglomerates and pebbly sandstonesof the lower El Loa Formation. They can be tracedcontinuously for over 35 km along the easternbasin margin (Figs 2 and 4). In some areas,gypcrete development is insigni®cant, with origi-nal sedimentary structures retained. In otherareas, often only a few metres away, gypcretedevelopment has completely modi®ed the origi-nal depositional fabric. Gypcretes are developed,at least partially, in �90% of the exposed sections

beneath the Artolla and Sifon ignimbrites alongthe eastern margin of the Calama Basin (Figs 5and 6a). Two types of gypcrete have beenidenti®ed (Figs 5 and 6a).

Type 1

Type 1 gypcretes comprise white gypsum nodules1±10 cm in diameter, vertical to sub-verticalalabastrine columns up to 60 cm long, vertical(`v'-shaped) veins 1±25 cm long and discontin-uous horizontal veins 1±3 cm thick (Fig. 6a,b).They are developed within a light brown to redgypsum matrix containing scattered ®ne to coarsegrained sand, pebbles and mudstone clasts. Anyoriginal depositional features have been comple-tely destroyed by gypsum cementation. At bedtops (where recognizable), vertically orientated`v'-shaped cracks up to 20 cm long and 5 cm wideare in®lled with pebbles and coarse sand from thebed above. Petrography reveals the predominantforms of gypsum to be ®brous in veins, and ®negrained (alabastrine) and mesocrystalline withincolumns and nodules (Fig. 6c). The larger meso-crystalline horizons frequently exhibit dissolu-tion textures along crystal boundaries. Thisgypcrete type comprises up to one third of thestudied sections and is commonly associatedwith type 2 gypcretes.

Type 1 gypcretes are interpreted as surfacegypsic crusts. Sediment ®lled cracks are consid-ered to record desiccation of an alluvial surface(cf. Kocurek & Hunter, 1986). The discontinuoushorizontal and vertical, `v'-shaped, ®brous gyp-sum-cemented cracks are akin to those describedfrom polygonal patterned ground in northernIraq, and may result from tensional stressescaused by desiccation (Tucker, 1978) or thermalcontraction (Kocurek & Hunter, 1986). Thealabastrine nodules and columns are similar tothose described from gypsic crusts in southernTunisia (Watson, 1985). In the Tunisian exam-ples, dissolution textures and the presence ofalabastrine is interpreted to indicate dissolutionof mesocrystalline gypsum and rapid reprecipita-tion (as alabastrine) due to surface weathering ofsub-surface gypsic material, a process whichresults in the nodular texture (Watson, 1985,1988).

Type 2

This type of gypcrete comprises approximatelytwo thirds of the studied sections and in manylocalities is the only form present. It ranges in



thickness from 0á5±10 m, but is commonly 2±3 mthick. It is characterized by an apparentlystructureless mixture of isolated clast-rich lenses(pebbles 1±4 cm in diameter) within a largelyclast-free, red-coloured, gypsiferous matrix. Clast-rich lenses occupy »20% of the unit. Lenses areup to 2 m long, 30 cm thick and aligned parallelto bedding (Fig. 7a). The upper and lowerboundaries between the lenses and the rest ofthe unit are sharp, lateral boundaries are moregradational and poorly de®ned. Original imbrica-tion and crude horizontal strati®cation canoccasionally be observed (Fig. 7a). Clasts oftenexhibit pitted, faceted and polished/smoothed

surfaces and in places are seen to be split. Largerclasts (up to 7 cm in diameter) occasionallydisplay well de®ned faceted faces. The matrixcomprises silt to coarse grained sand. Sand grainsare angular to well rounded (Fig. 7b). The sandgrains comprise andesitic material with a similarcomposition to the clasts. Clast-rich lenses aresupported and encompassed by massive gypsumwhich constitutes 80% of the unit. Isolated clastsand sand grains occur throughout the massivegypsum cement.

The gypsum cement comprises mixed poikili-tic, mesocrystalline and ®brous crystals. Poiki-litic crystals average 1 mm in length, but may be

Fig. 5. Graphic logs of typical gypcrete sections at (a) Quebrada Tuina and (b) Barros Arana (see Fig. 2 for location).Type 1 gypcretes are characterized by alabastrine nodule development, in contrast, type 2 gypcretes contain scatteredpebbly patches. C � claystone, Slt � siltstone, Fs � ®ne sandstone, Ms � medium sandstone, Cs � coarsesandstone, G � granulestone, Pc � pebble conglomerate.



Fig. 6. (a) General view of the gyp-crete horizon at Barros Arana; type1 gypcrete (lower part of photo-graph) comprising alabastrine no-dules, and vertical and horizontalgypsum veins, overlain by massivetype 2 gypcrete (upper part of pho-tograph), hammer for scale. (b) De-tail of type 1 gypcrete from BarrosArana, illustrating discontinuoushorizontal veins of ®brous gypsum(the hammer head is alligned par-allel to bedding), alabastrine no-dules and, to right of hammer head,a small vertical gypsum vein. Thematrix is composed of reddenedmesocrystalline gypsum with scat-tered sand grains and pebbles.(c) Photomicrograph (cross-polars)of type 1 gypcrete (Barros Arana)illustrating elongate, ®brous gyp-sum, very ®ne grained alabastrinegypsum and coarse grained meso-crystalline gypsum (®eld of view is2á5 by 1á75 mm).



up to 4 mm long (Fig. 7b,c,d) and are sometimesvisible in the ®eld as `desert roses'. Individualcrystals are equi-dimensional with both uniformand undulose extinction patterns. Crystal bound-aries are highly undulatory with a smoothedstylolitic pattern. Fibrous gypsum in®lls fractures(Fig. 7c) and mesocrystalline gypsum (0á1±0á5 mm) forms well de®ned lozenge-shaped andgranular crystals. A later granular micro-sparitecarbonate cement has locally replaced some ofthe gypsum.

The origin of these gypsic matrix-supportedconglomerates is problematic. The petrographicand ®eld evidence is equivocal as they containfeatures considered characteristic of illuvial,hydromorphic and surface gypcretes. Poikiliticgypsum crystals are considered to form asprecipitates from evaporating groundwater within1±2 m of the land surface and are characteristic of

hydromorphic subsurface crusts (Coque, 1962;Watson, 1985, 1988, 1989). The lenticular andgranular mesocrystalline gypsum is consideredcharacteristic of illuvial subsurface crusts (Cas-tens-Seidell & Hardie, 1984; Watson, 1985, 1988).In subsurface crusts, gypsum deposited at thesurface is dissolved and leached into the soil zonewhere it precipitates during subsequent desicca-tion (Watson, 1989). This process results in thedevelopment of a massive, clast-poor interval,similar to the B horizons described from matureilluvial saline desert soils of Quaternary age (e.g.Amit & Gerson, 1986; Reheis, 1987; Birkeland &Gerson, 1991). However, there is little evidence ofhorizonation within type 2 gypcretes, such that apurely illuvial origin can be discounted. The®brous gypsum fracture-®lls, as discussed above,are considered to be common features of surfacegypsic crusts. Field and petrographic observations

Fig. 7. (a) Type 2 gypcrete fromQuebrada Agua de la Teca. Note thetextural variations between theclast-supported fabric, the matrix-supported (gypsum-cemented) fab-ric and the massive clast-free, gyp-sum fabric. Original strati®cation ispartly picked out by the texturalvariations and in places a crudebedding-parallel clast orientationand imbrication can be recognized(palaeo¯ow to right of photograph).(b) Photomicrograph of type 2 gyp-crete from Barros Arana, illustratingthe development of poikilitic gyp-sum crystals enclosing detrital sandgrains. Note the variation in shapeof sand grains from angular to sub-rounded (®eld of view 3 mm by2 mm). (c) Photomicrograph of type2 gypcrete showing adjacent ®brousand poikilitic gypsum enclosingangular to rounded detrital sandgrains (®eld of view 2 mm by1á5 mm; Quebrada San Martin).(d) Photomicrograph of mesocrys-talline gypsum from a massive type2 gypsum horizon. Note the irre-gular crystal boundaries possiblyindicative of dissolution (®eld ofview 1á75 mm by 1á35 mm; BarrosArana).



therefore suggest that this type of gypcreterepresents a subsurface gypsic crust formed by acombination of hydromorphic and illuvial pro-cesses and subject to periodic exhumation andweathering.

DISCUSSION

Controls on gypcrete thicknessand depositional model

Quaternary subsurface gypsic crusts of eitherhydromorphic or illuvial origin generally rangebetween 1 and 2 m in thickness, although 5 mthick hydromorphic crusts are known (Watson,1985, 1989; Cooke et al., 1993). The greaterthickness, lack of horizonation and interbeddednature of the surface and subsurface crusts

suggests that these Miocene examples representa composite gypcrete pro®le. It is postulated thatthe periodic accumulation of alluvial materialand aggradation of the zone of illuvial-hydro-morphic gypsum precipitation resulted in incor-poration of alluvial deposits into the crusts. Thishypothesis is supported by the presence ofshattered clasts within each clast-rich lens,because the process of mechanical clast splitting(by salts) is generally restricted to the top 0á5 m ofalluvial desert soils (Amit et al., 1993).

The Calama Basin gypcretes are developedwithin and on the margins of coalesced alluvialfan (bajada) deposits. Watson (1983) in a study ofQuaternary gypcretes from the Sahara noted thatthey were generally better developed on lowerrather than upper slopes of alluvial fans andsimilar variations have been noted by Berger(1993) on different segments of present day

Fig. 7c,d.



alluvial fans in the Atacama Desert. The presenceof poikilitic gypsum crystals also suggests a lowerslope location as their precipitation normallyrequires the water table to lie within 1±2 m ofthe ground surface (Watson, 1989). A deposi-tional model is envisaged (Fig. 8) in which mixedorigin hydromorphic and illuvial gypcretes de-veloped on the lower slopes of coalesced alluvialfans, and periodic deposition of sheet¯ood anddebris ¯ow deposits allowed gypcrete aggrada-tion.

Controls on gypcrete development

Studies of Quaternary gypcretes (e.g. Tucker,1978; Arakel & McConchie, 1982; Watson, 1983,1985; Jacobson et al., 1988; Arakel, 1991; Cookeet al., 1993) have shown that their developmentand preservation is a complex function of rainfall,dustfall, topographic position, groundwater evo-lution, nature of parent material, rate of sedimen-tation, age and climatic history. A complexcombination of these variables in¯uences thedevelopment of speci®c gypcretes. However,some general observations applicable to manygypcretes, including those from northern Chile,can be made.

One of the principal sources of gypsum for theformation of gypcretes in most arid regionsappears to be airborne dust (e.g. Coque, 1962;Reheis, 1987) or in arid coastal regions assulphate-rich aerosols (e.g. Martin, 1963; Erick-sen, 1981). A recent study of Quaternary gypsum-rich pedogenic horizons in northern Chile(Berger, 1993) concluded that the primary sourceof gypsum in the Pre-Andean ranges (including

the Calama Basin) was through aeolian transportof material eroded from saline playas in thevolcanic arc. The in¯uence of aeolian processesin the development of Miocene alluvial fansurfaces in the Calama Basin is revealed by theoccurrence of millet seed grains in the matrix ofthe gypsum-cemented conglomerates and wind-abraded pebbles. The products of the volcanic arcare well documented throughout the Miocenestratigraphy of northern Chile (e.g. Rutland et al.,1965; Lahsen, 1982; de Silva, 1989), indicatingthat then as now, the principal source of gypsumfor gypcrete formation may well have been viaaeolian transport of gypsic dust eroded fromsaline playas in the volcanic arc.

Timing of gypcrete development

Studies of Quaternary gypsic soils suggest thatthey can develop quickly, often reaching fullthickness within a few thousand years althoughthe time taken to reach maturity (i.e. full B-horizon development) takes longer (Amit &Gerson, 1986; Reheis, 1987; Birkeland & Gerson,1991). Fully mature soils are often reported asbeing much older. For example, Amit et al. (1993)have reported mature alluvial saline soils fromthe Negev desert that are 500 000 years old andBerger (1993) has used thermoluminescencemethods to date a gypcrete from an alluvial fansurface in northern Chile that is over 230 000years old. Due to the long time period required toform mature gypcretes, it seems likely from thecomposite nature of the Calama Basin gypcretesthat they have taken at least 500 000 years andprobably longer to develop.

Fig. 8. Summary of main factors considered to have in¯uenced gypcrete formation, see text for details.



Palaeoclimatic implications

It is well established that gypcretes are developedin arid climates where annual precipitation isbetween 250 and 25 mm, and that precipitationlevels below 25 mm year)1 favour the formationof pedogenic halite crusts (Watson, 1989). Thepresent day hyper-arid climate of northern Chile(precipitation < 10 mm per annum) does notfavour gypcrete development, but does favourgypcrete preservation. This would suggest thatthe climate in northern Chile was arid duringgypcrete formation in the late Middle Miocenebecoming hyper-arid in the early Upper Miocene(prior to emplacement of the Artolla Ignimbrite at9á5 Ma).

SUMMARY AND CONCLUSIONS

Two types of gypcrete have been identi®ed withinlate Middle to early Upper Miocene alluvial fandeposits along the eastern margin of the CalamaBasin, northern Chile. Type 1 gypcretes comprisegypsum nodules, vertical to sub-vertical alabas-trine columns, horizontal and `v'-shaped cracksdeveloped within a red, gypsum-cemented ma-trix. These features are characteristic of weath-ered surface gypsum crusts and show strongsimilarities to those described from the Quatern-ary of Tunisia, Namibia and Iraq. Type 2gypcretes comprise a structureless mixture ofisolated pebble-rich lenses within a largelyclast-free, red-coloured, gypsiferous matrix andappear similar to well developed B-horizons ofQuaternary horizonated illuvial gypsic soils.Fibrous sub-vertical gypsum veins are alsocommonly present. The massive gypsum com-prises both poikilitic and mesocrystalline crys-tals, forms which studies of Quaternary gypcretessuggest are indicative of a hydromorphic andilluvial origin, respectively.

Gypcretes are developed beneath regionallyextensive ignimbrites of early Late Miocene age,are up to 25 m thick and are considered torepresent a composite pedogenic event developedduring a regional hiatus. The presence of gypcretein rocks of this age suggest that the present hyper-arid climate of northern Chile has prevailed forthe last 9á5 Ma, as gypcretes do not develop underhyper-arid climates, but are only likely to bepreserved under this climatic regime.

ACKNOWLEDGMENTS

We thank the Natural Environmental ResearchCouncil (GM), the Nuf®eld Foundation and theUniversity of Aberdeen (AJH) for funding ®eld-work in northern Chile. G. May was supported bya NERC studentship (GT4/93/2/G). We would liketo thank Peter Turner (University of Birmingham)for extensive discussion and B. Valero-GarceÂs andB. SchuÈ tt for comments on the manuscript. Wegratefully acknowledge Guillermo Chong Diaz(Universidad CatoÂlica del Norte, Antofagasta) forinvaluable logistical and scienti®c support.

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Miocene Gypcretes from the Calama Basin, northern Chile

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