Replacement of evaporites within the Permian Park City Formation, Bighorn Basin, Wyoming, USA

Sedirnentology (1994) 41, 1203-1222

Replacement of evaporites within the Permian Park City Formation, Bighorn Basin, Wyoming, USA

DANA S . ULMER-SCHOLLE and PETER A . SCHOLLE

Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, USA

ABSTRACT

The Permian Park City Formation consists of cyclically bedded subtidal to supratidal carbonates, cherts and siltstones. Early diagenesis of Park City Formation carbonates occurred under the influence of waters ranging from evaporative brines to dilute meteoric solutions and resulted in evaporite emplacement (syndepositional nodules and cements), as well as dolomitization, silicification and leaching of carbonate grains.

Major differences are seen, however, in the diagenetic patterns of subsurface and surface sections of Park City Formation rocks. Subsurface samples are characterized by extensively preserved evaporite crystals and nodules, and preserve evidence of significant silicification (chert, chalcedony and megaquartz) and minor calcitization of evaporites. In outcrop sections, the evaporites are more poorly preserved, and have been replaced by silica and calcite and also leached. The resultant mouldic porosity is filled with widespread, very coarse, blocky calcite spar.

These replacements appear to be multistage phenomena. Field and petrographic evidence indicates that silicification involved direct replacement of evaporites and occurred during the early stages of burial prior to hydrocarbon migration. Siliceous sponge spicules provided a major source of silica, and the fluids involved in replacement were probably a mixture of marine and meteoric waters. A second period of replacement and minor calcitization is inferred to have occurred during deep burial (under the influence of thermochemical sulphate reduction), although the presence of hydrocarbons probably retarded most other diagenetic reactions during this time interval. The major period of evaporite diagenesis, however, occurred during late stage uplift. The late stage replacement and pore-filling calcites have FL3C values ranging from 0.5 to - 25.3%0 and 6’*0 values of - 16.1 to - 24.3%0 (PDB), reflecting extensive modification by meteoric water. Vigorous groundwater flow, associated with mid-Tertiary block faulting, led to migration of meteoric fluids through the porous carbonates to depths of several kilometres. These waters reacted with the in situ hydrocarbon-rich pore fluids and evaporite minerals, and precipitated calcite cements.

The Tosi Chert appears to have been an even more open system to fluid migration during its burial and has undergone a much more complex diagenetic history, as evidenced by multiple episodes of silicification, calcitization (ferroan and non-ferroan), and hydrocarbon emplacement.

The multistage replacement processes described here do not appear to be restricted to the Permian of Wyoming. Similarly complex patterns of alteration have been noted in the Permian of west Texas, New Mexico, Greenland and other areas, as well as in strata of other ages. Thus, multistage evaporite dissolution and replacement may well be the norm rather than the exception in the geological record.

INTRODUCTION

Over the last two decades, numerous studies have 1980; Harwood, 1980; Ulmer & Laury, 1984; Scholle documented the occurrence of evaporite replace- & Melim, 1988; Lee & Harwood, 1989; Scholle et al., ments and their associated fabrics within carbonate 1992). These often spectacular fabrics commonly strata (Pittman & Folk, 1970; Folk & Pittman, 1971; include both calcitized and silicified evaporite Siedlecka, 1972; West, 1973; Clark & Shearman, minerals, calcitized dolomites (‘dedolomites’), and

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I204 D. S. Ulmer-Scholle and P. A. Scholle

dissolution and solution enlargement fabrics (including solution collapse breccias). However, there have been relatively few detailed studies of these fabrics and the processes involved in their formation, despite the fact that replacement of evaporites is of economic as well as of scientific interest. The dissolution/precipitation processes involved in evaporite replacement can potentially significantly change the porosity and permeability of a unit. The study of such replacements, more- over, may provide valuable information on the history of pore fluid movements and compositions within the basin. Consequently, predictions of reservoir quality, where based on surface data, may be highly erroneous if late stage (outcrop- or uplift-related) diagenesis was important.

Evaporite-related diagenetic fabrics from outcrops and cores of the Permian Park City Formation within the Bighorn Basin, Wyoming, show that there was extensive dissolution and replacement of gypsum, anhydrite and possibly also halite within these marine to supratidal carbonate and chert units. This study was undertaken to describe the fabrics associated with evaporite diagenesis, to determine the possible timing of those reactions and to constrain the geochemical conditions of replacement within these strata. The complexity of the dissolution and replace- ment processes required a multidisciplinary approach to understand better the timing and conditions under which evaporite alteration took place. For this study, integrated standard petrographical techniques (including staining and cathodoluminescence) and stable isotopic analyses were used. Petrography establishes a basis for the timing of diagenetic events, especially in relationship to hydrocarbon migration; stable isotopic analyses aid in the understanding of the processes and conditions existing at the time of evaporite replacement.

REGIONAL SETTING

The Permian units within the Bighorn Basin were deposited in part of a north-south trending graben system that had been active since the Late Precambrian (Carr & Paull, 1983). Throughout Palaeozoic and early Mesozoic time, the Wyoming Shelf remained a comparatively stable area, receiving one of the more complete Palaeozoic sections in North America. Permian strata, in particular, were widely distributed throughout this region (Fig. I), although present day outcrops are limited to the

flanks of the Tertiary basins. During Permian sedi- mentation, a period of extensional deformation resulted in the development of the proto-Bighorn Basin (Tonnsen, 1986). Many of the structural fea- tures present in the area today are probably a result of reactivation of these earlier structures (Simmons & Scholle, 1990a).

Mid-Permian strata are divided into four time- equivalent, intertonguing units (Fig. 2): the Shedhorn, Goose Egg, Phosphoria and Park City formations (McKelvey et al., 1959; Peterson, 1984). The Shedhorn Formation is a massive sandstone unit found primarily north-west of the study area and is probably largely aeolian in origin (Thornburg, 1990). Thin sandstone and siltstone wedges, equivalent to the Shedhorn Formation, are correlable across most of the Bighorn Basin and are key marker horizons (Peterson, 1980). These siliciclastic horizons are associated with sea level falls, and are inferred to have resulted from progradation of dunes and sabkhas across the exposed shelf (Peterson, 1980; Thornburg, 1990). Western outcrops of the Park City Formation intertongue with the Phosphoria Forma- tion (Tosi Chert and Retort Phosphatic Shale). Bedded cherts and phosphorites are the prominent lithologies in the Phosphoria Formation, and repre- sent the deepest water facies (Sheldon, 1980). Carbonate units of the Park City Formation grade eastward into redbeds, evaporites and discontinuous, intertidal to supratidal dolomites of the Goose Egg Formation (Boyd, 1975).

The redbeds comprise 215 m of the Triassic Dinwoody and Chugwater formations and uncon- formably overlie the Park City Formation through- out the field area. Renewed tectonism during Late Triassic time created widespread intracratonic basins which received a thick sediment package (nearly 3050 m of siliciclastics, evaporites and carbonates) during Jurassic and Cretaceous sedimentation (Oriel & Armstrong, 1986). The Idaho-Wyoming thrust belt and Laramide orogeny was initiated during the Early Cretaceous and continued to the end of the Eocene. The Fort Union Formation (Palaeocene) is a thick section (over 765 m) of siliciclastics which were derived from the ancestral Rocky Mountains and Idaho thrust belt. Basin and Range block faulting started as early as the late Eocene (Oriel & Armstrong, 1986) in the study area, and resulted in the present day structural configuration of the Bighorn Basin. In the centre of the Bighorn Basin, Permian strata are buried beneath more than 4.5 km of younger sediments (Fig. 3; Orr, 1974).

Evaporite replacement, Bighorn Basin 1205

1x1 Permian Absent

Clastics

Evaporites

Impure Carbonates

Rl Clean Carbonates

Phosphorites

- - ’ Area of Figure 7 l - -J Fig. 1. Lithofacies map of Guadalupian strata in Wyoming and adjacent states. Modified from Rascoe & Baars (1972).

DEPOSITIONAL ENVIRONMENTS

The Phosphoria-equivalent strata are subdivided into three major cycles (Fig. 4) based on lowstand clastic wedges and highstand chert marker horizons: the Grandeur, Franson and Ervay (Peterson, 1984). These Leonardian and Guadalupian cycles may be correlable to major glacio-eustatic sea level fluctu- ations (Sheldon, 1984; Inden & Dean, 1986; Ross & Ross, 1988) and associated pluvial to interpluvial climate changes.

The strata of the Grandeur cycle unconform- ably overlie aeolian deposits of the Pennsylvanian Tensleep Formation. Grandeur cycle dolomites are, in places, very sandy and contain a fauna indicative of normal marine conditions (McKelvey et al., 1959).

The Grandeur cycle is 1.5 to 13 m thick, is extensively recrystallized, partially silicified and brecciated. The highly irregular top of the Grandeur cycle results from post-depositional dissolution and erosion, caused by a subaerial exposure event that terminated that cycle of deposition.

The overlying Franson cycle is divided into the Meade Peak cherts and phosphorites, and the lower, middle and upper Franson carbonates (Fig. 4). The Meade Peak cherts and phosphorites are exposed in the Wind River Range, but do not crop out within the Bighorn Basin. The Franson carbonates represent a wide range of depositional environments, from normal marine biostromal buildups to intertidal algal mats and supratidal evaporite flats, and have a widespread aerial distribution. Interbedded with

1206 D. S. Ulmer-Scholle and P. A. Scholle

I IDAHO 1 WYOMING I

Fig. 2. Stratigraphical nomenclature for Pennsylvanian to Triassic strata in north-western Wyoming and eastern Idaho. Modified from Peterson (1984).

0

2 h

E

5 4

Y v

Mean Surface Temperature = 7" C 6 (Bredelloft et id., 1992)

Geothermal Gradient = 29" C/km

8 300 250 200 150 100 50

Time (millions of years B.P.) Fig. 3. Burial history diagram for the Permian Park City Formation, Bighorn Basin.

Franson carbonates are siltstone and sandstone units which increase in thickness from west to east. Terri- genous clastic units may have formed as a result of

minor sea levelklimatic fluctuations resulting in the progradation of aeolian sands across the shelf. The Franson member has a combined thickness (both

Evaporite replacement, Bighorn Basin 1207

Dolomite

Chert Phosphatic Shale Sandatone Cross-bedded Sandstone

Fig. 4. Idealized stratigraphical section of the Permian Park City Formation.

carbonates and siliciclastic interbeds) ranging from 15 to 75m. Most of the Franson carbonates are extensively dolomitized and contain numerous evaporites (now calcitized), ranging from individual crystals of former gypsum to large anhydrite nodules formed along well defined horizons.

The final period of Permian marine sedimentation in the Bighorn Basin, the Ervay cycle, consists, in ascending order, of the Retort Phosphatic Shale, the Tosi Chert and the Ervay Carbonate (Fig. 4). The Retort Phosphatic Shale crops out at only a few localities in the study area (Anchor Dam and the South Fork of Owl Creek) and is typically less than

1 m thick. The abundant brachiopods, phosphatic clasts and ooids, high organic carbon content and the lack of fauna typical of shelf areas suggest the Retort Phosphatic Shale represents the deepest incursion of Permian seas into the study area.

The Tosi Chert is a highly burrowed, very silty, bioclastic, dolomitic chert with abundant evaporite nodules that have been both silicified and calcitized (Figs 5 and 6). The thickness of the Tosi Chert varies greatly over the field area. The thickest deposits (over 15 m) are in the south-western part of the study area, but these thin north- and eastward to less than 2 m. Based on the fauna, trace fossils and associated sedimentary structures, the Tosi Chert was deposited in waters that were significantly deeper than those involved in deposition of the carbonate units (McKelvey et al., 1959; Peterson, 1984). The evapor- ites in these deep water deposits were formed inter- stitially by density driven brines that formed on the shelf and migrated through the siltstones of the Tosi Chert. At permeability barriers, evaporites precipi- tated out as nodules which displaced the siltstones and shales.

The Tosi Chert is traceable throughout the study area, and at several localities on the eastern boundary of the Bighorn Basin the unit consists of multiple cycles of cherts, siltstones and carbonates. These multiple horizons probably result from the fact that the eastern edge of the basin had the shallowest water depths and, therefore, the overall depositional pat- terns were more sensitive to subtle changes in relative sea level.

Within the Bighorn Basin, the Ervay Carbonate subcycle (including the Retort Shale and Tosi Chert members) is the thickest package of carbonate sedi- ments, with some sections exceeding 25 m in thick- ness. The south-western sections are dominated by bryozoan, crinoidal and brachiopod biotherms. Northern and eastern sections consist of shallow marine packstones and grainstones which grade into inter- to supratidal, dolomitized, algal laminites and bedded anhydrite/gypsum (Lane, 1973; Boyd, 1975). Field evidence suggests that along the eastern margin of the basin, at the Sheep Mountain and Little Sheep Mountain anticlines, Permian tectonism resulted in the formation of linear, low relief island chains (Inden & Anderson, 1986; Simmons, 1990a,b). Associated with these islands are beach facies, con- sisting of coarse grained packstones and grainstones, and barrier-capping facie$ of pisolitic dolomites and algal laminites with teepee structures. These units also contain abundant examples of calcitized

1208 D. S. Uimer-Schoile and P. A. Scholle

Fig. 5. Polished slab from the Tosi Chert at the South Fork of Owl Creek. Note the multiple generations of silica. The darker cherts (b) are burrow replacements and the white replacements (e) are after evaporite nodules. Scale bar=2 cm.

evaporites, dedolomites, and solution collapse breccias andrauhwackes (Brucker, 1941) produced by the removal of evaporites. The end of Ervay deposi- tion is marked by a regional unconformity which involved widespread subaerial exposure. Palaeosil- cretes, solution collapse breccias and selective dolo- mitization of the upper parts of the bryozoan bioherms are associated with this event in western sections.

METHODS

Extensive sample suites from 12 outcrop sections of the Permian Park City Formation in the Bighorn Basin were collected during the summers of 1987 and 1988. In addition, samples from 13 slabbed cores from the Ervay member and the Tosi Chert (Fig. 7) were obtained from the U S Geological Survey core repository in Denver. Thin sections from these samples were studied in detail utilizing standard petrographical techniques, fluorescence and cathodo- luminescence microscopy, and stable isotopic geochemistry.

Four hundred standard petrographical thin sec- tions were stained with a combined Alizarin Red-S and potassium ferricyanide solution for dolomite and iron-rich calcite (Dickson, 1965). Thin Sections were examined under both reflected and transmitted light. Slabs were etched in a 10% hydrochloric acid

Fig. 6. The South Fork of owl Creek outcrop, The base of the section is the top of the Retort phosphatic Shale (arrowheads). The dark nodular unit is the Tosi Chert (c). and the light, bedded unit is the Ervay Member (1). Total thickness of this outcrop is 31 m.

Evaporite replacement, Bighorn Basin 1209

0 25 50 km I

1. Karogen Co. #1 Two Dot Ranch 2. Two Dot Ranch 3. Cody 4. Husky 011 #82 Pitchfork 5 . Altus #21-1 Federal 6. South Fork Owl Creek 7. Anchor Dam 8. Buffalo Canyon 9. Ditch Creek

10. Lear Oil #12-2 Federal 1 1. Nowood Creek 12. Bass Ent. #28-22 Meyers Gulch 13. Cedar Mountain 14. Shell Canyon 15. Sheep Mountain 16. Little Sheep Mountain 17. Horseshoe Bend

Fig. 7. Sample localities within the Bighorn Basin.

solution for 40 s to enhance the primary sedimentary fabrics. Slabs were also stained with a combined Alizarin Red-S and potassium ferricyanide solution in 5% hydrochloric acid to aid in the determination of various diagenetic minerals. Cathodoluminescence petrography was done with a Technosyn cold cathode luminoscope model 8200 Mk I1 mounted on a Leitz petrographical microscope.

For isotopic analyses, 650 samples were drilled on a Jensen micro-milling machine. Stable carbon and oxygen isotopic analyses were performed at the Texaco Research Laboratory in Houston, Texas. The approximately 25 mg samples were roasted at 380°C for 1 h, reacted with anhydrous phosphoric acid on-line, and the CO, was extracted at 90°C (Dawson, pers. comm.). Evolved gases were analysed on a Finnigan-Delta E mass spectrometer. The intra- laboratory standard used was an NBS-20 calcite calibrated relative to PDB (Craig, 1957). Ana- lytical precision was & 0.1%0 for both oxygen and carbon.

Chert samples for oxygen isotopic analyses were isolated from the carbonate matrix by chipping off pieces from cut nodules. Samples were crushed and soaked in dilute (10%) HCI to remove any carbonate inclusions, and were rinsed in distilled water several times to dissolve salts. Samples were reacted with hydrogen peroxide to remove organic compounds (hydrocarbons) and were also inspected under a

binocular microscope to remove physically any insolubles. Oxygen isotopic analyses were run at Case Western Reserve University in Cleveland, Ohio, by Dr Samuel Savin and Linda Abel.

EVAPORITE REPLACEMENT FABRICS

Replacement silica

Chert, megaquartz and length-slow chalcedony are found replacing anhydrite andlor gypsum within both outcrop and core specimens of the Park City Formation throughout the Bighorn Basin (Ulmer & Scholle, 1990). Silica replacement occurs mainly within evaporite nodules of the ‘deeper’ water Tosi Chert member. Silica replacement of evaporites within the carbonate members is less frequent, com- monly occurring only in the basal sections of the carbonate units, which are transitional from deep to shallower water environments.

Within the Tosi Chert, the evaporite nodules were silicified from the exterior toward the centre. These rinds of silica replacement include a mixture of length-slow chalcedony and chert that preserves both the detailed external (Fig. 8) and the partial internal (Fig. 9) morphology of the evaporite nodules. This earliest stage of silicification is characterized by the replacement of individual anhydrite laths and matted

1210 D. S. Wlmer-Scholle and P. A. Scholle

Fig. 8. External morphology of silicified evaporite nodules from the Tosi Chert of the South Fork along Owl Creek. Scale har=4 cm.

Fig. 9. Photomicrograph of a partially silicified and calcitized evaporite nodule from the Tosi Chert at Owl Creek. Note the well preserved evaporite (anhydrite) pseudomorphs replaced by length-slow chalcedony and megaquartz, and the late zoned calcite fills. The thin section has been stained with a combination of potassium ferricyanide and Alizarin red-S stain. Scale har=2 nun.

masses of replaced crystals (Fig. 9). In hand speci- anhydrite (Fig. lo). Some of these anhydrite inclu- men, this phase of silica is milky white, while petro- sions, especially along fractures and edges of mega- graphically it appears dark brown due to bitumen quartz crystals, were leached during a later stage of and hydrocarbons filling intercrystalline porosity. diagenesis, and now are filled by non-ferroan to

The next stage of silicification is represented ferroan, blocky calcite spar. In hand specimen, this by large, clear, euhedral megaquartz crystals which generation of replacement silica is dark brown to contain inclusions of evaporite minerals, usually black due to the filling of leached anhydrite crystal

Evaporite replacement, Bighorn Basin 121 1

Fig. 10. Detailed photomicrograph of megaquartz replacement of anhydrite with both preserved and leached pseudomorphs of anhydrite crystals from the Tosi Chert at the South Fork of the Owl Creek. Scale bar=20 pm.

moulds by hydrocarbons. In thin section, the mega- quartz crystals are clear, because unlike the chert and chalcedony phases, they have few inclusions. Two similar stages of silicification were also recognized by Chowns & Elkins (1974) in the Mississippian Warsaw and Fort Payne Limestones of Tennessee, and by Hayes (1964) in the Mississippian Warsaw Formation of Iowa.

Other fabrics characteristic of silica replacement of evaporites (Folk & Pittman, 1971) found within the Park City Formation include fortification zoning and pseudo-cubic terminations. Many of the evaporite replacement nodules contain spherical masses of silica, typically free-floating in the core of the nodule or cemented by calcite. Presumably, these masses are recrystallized opaline-cristobalite spherules that were associated with evaporite replacement as well as opal to opal-cristobalite transformations (Maliva, 1987).

Primary hydrocarbon inclusions were not found within the megaquartz, but hydrocarbons fill the porosity formed by the dissolution of anhydrite, intercrystalline pores and remnant vugs. Many of the nodules, especially at Anchor Dam and the South Fork of Owl Creek, contain liquid hydrocarbons, asphalt or bitumen (Fig. 11). Nodules filled with hydrocarbons also tend not to have calcite cements within them.

Hydrocarbons within the Tosi Chert and Park City carbonates were derived predominately from the organic-rich shales and phosphorites of the

Phosphoria Formation (Claypool et al., 1978). Hydrocarbon generation and migration from the basin to the shelf carbonates of the Park City For- mation probably occurred by Early Jurassic time (Orr, 1974).

Silicification of anhydrite nodules clearly pre-dates hydrocarbon migration. In addition, the evaporite nodules were slightly flattened by compaction prior to silicification. Post-silicification compaction was more effective and resulted in the shattering of many of the nodules. Silicification of evaporites occurred between the end of Permian deposition and Early Jurassic hydrocarbon migration (Orr, 1974).

Replacement calcite

Replacement calcite and calcite spar filling of leached evaporite nodules are found both in the carbonate members of the Park City Formation and the Tosi Chert. Within the Tosi Chert, calcite fillings of par- tially silicified evaporite nodules are found both in outcrop and in some subsurface cores, although most core specimens still retain the original evaporite minerals or their silicified equivalents (Fig. 12). Replacement calcites within the Tosi Chert are typically coarsely crystalline, complexly zoned (non-ferroan to ferroan), blocky spars (Fig. 13). Calcitization within the Tos’i Chert evaporite nodules clearly post-dates silicification and subsequent leaching, since the silica phases contain abundant

1212 D. S. Ulmer-Scholle and P. A . Scholle

Fig. 11. Hydrocarbon staining of replaced evaporite nodule with a central asphalt fill from the South Fork of Owl Creek. The arrow points to a large (5 mm) hydrocarbon inclusion within later calcite fills. Scale bar=2 cm.

evaporite inclusions and the calcites are normally inclusion-free and fill evaporite moulds within the silica replacements. Many of the calcite spars contain primary hydrocarbon inclusions (occurring as trains of inclusions in the spar cements), indicating that calcite precipitation probably occurred during or after hydrocarbon migration. The fluids responsible for calcite precipitation may have flushed many of the nodules, removing the hydrocarbons and rem- nant evaporites.

Calcite spars within the Franson and Ervay mem- bers of the Park City Formation commonly do not preserve the original internal morphology of the evaporites. External features include gypsum pseudo- morphs, anhydrite nodules and enterolithic struc- tures (analogous to those forming in modern evaporitic carbonate environments). These replace- ments normally are coarsely to very coarsely crystal- line, non-ferroan, blocky calcite spars. Evaporite inclusions are not found in the spars, but in subsur- face cores and outcrop sections, calcitized evaporites are sometimes associated with euhedral crystals of pyrite and fluorite.

Samples from the Franson member, more com- monly than those from the Ervay member, have calcites that preserve original evaporite fabrics. These replacements contain evaporite inclusions (anhy- drite), and the calcite crystals are normally ferroan, Fig. 12. Partially silicified anhydrite nodule (arrow- slender and elongate. The calcite replacements are heads) from the Bass no. 28-22 Myers Gulch core. Scale matted, as with their siliceous counterparts, but do bar=2 cm.

Evaporite replacement, Bighorn Basin 1213

Fig. 13. Stained photomicrograph of zoned calcite fills in silicified evaporite nodules from the Tosi Chert at Anchor Dam. Scale bar = 20 vm

not preserve the intricate details seen in the siliceous replacements.

Subsurface cores generally contain more preserved evaporites than do outcrop sections. Although gyp- sum is found in some cores, anhydrite is the most common evaporite mineral, forming nodules and cements within the carbonate strata, and filling mouldic pores as well as fractures. Substantial re- mobilization of evaporites was found within these Permian strata. Shallow cores (less than 1 km deep) from the uplifted margins on the eastern side of the Bighorn Basin exhibit extensive leaching of evapor- ites. Well preserved evaporite moulds can be found within the carbonates from shallow cores; deeper cores (from the central and western edges of the basin) still have significant evaporite contents. Based on this, it is evident that Permian carbonates have undergone considerable late stage leaching of evapor- ites, resulting in the development of significant sec- ondary porosity. In addition, anhydrite has been hydrated to gypsum in some shallow cores and in outcrop specimens (Fig. 14). In outcrop, most of the evaporites have been completely removed, and only the thicker bedded evaporites remain, largely associ- ated with the Goose Egg Formation on the eastern edges of the basin.

In outcrop samples, calcite spars that have demonstrably filled evaporite moulds are texturally similar to calcites that fill early formed biomouldic porosity and fractures. In thin section, samples from

subsurface cores have anhydrite cements filling bio- mouldic porosity while outcrop samples from litho- logically similar units contain coarsely crystalline calcite spars. Based on a comparison of the fabrics preserved within the cores and outcrop sections, many calcite cements are interpreted to have formed after evaporites (Fig. 15).

ISOTOPIC RESULTS

Siliceous minerals

Four silicified evaporite nodules (both chert and megaquartz) were analysed for their oxygen isotopic composition. The values ranged from +26.79 to +28.85%0 (SMOW); an average value for the quartz crystals is +27.85%0 with an approximate I%o vari- ation. The only noticeable variation within the data set is that the oxygen isotopic values increase slightly from the western to the eastern side of the Bighorn Basin.

Carbonate minerals

Stable isotopic data for calcite cements and spars within evaporite moulds from the Park City Forma- tion are summarized in. Table l . Calcite cements and spars are further subdivided on the basis of the carbonate members from which they were sampled in

1214 D. S. Ulmer-Scholle and P. A. Scholle

Fig. 14. Photomicrograph of quartz and gypsum after anhydrite. Within the megaquartz are inclusions of anhydrite (bright specks). Gypsum (g) after anhydrite encases the quartz (9) replacements and dolomite groundmass (d). Sample is from the Ditch Creek section. Scale bar= 1.3 mm.

Tables 2 and 3, respectively. A cross plot of oxygen versus carbon stable isotopic data from Table 1 is presented in Fig. 16.

A comparison of data from both calcite cements and calcite spar filling of evaporite moulds indicates that there is no significant difference between the two data sets. Taking into consideration previously men- tioned field and petrographical criteria, the isotopic data reinforce the conclusion that calcite cements formed after the leaching of evaporite minerals and at the same time as calcite spars filled evaporite moulds. Both groups of calcites formed under com- parable hydrological and environmental conditions. From Fig. 16, it can be noted that stable isotopic values for calcite spars and calcite cements in the carbonate members form a large data field. The carbon and oxygen isotopic compositions for both data sets have a spread of more than 31%0 for carbon and 21%0 (PDB) for oxygen. This large scatter for calcite cements and spars indicates that there may be more than one process responsible for evaporite replacement and calcite precipitation, or that the processes responsible for calcitization operated over a broad span of time.

The ferroan calcites (both ‘cements’ and calcitized evaporites) from the Franson and the Tosi Chert members and the non-ferroan calcites from the Tosi Chert have significantly more enriched carbon iso- topic values (+3.5 to - 12.0%0) than other calcites.

The heavier carbon values indicate that hydro- carbons were not undergoing degradation during precipitation. Calcite spars formed under reducing conditions are normally more enriched in iron and manganese, if these elements are present in the sys- tem. Therefore, the higher iron contents of the zoned calcite spars indicate that fluids were less oxygenated, and therefore fluid circulation may have been more restricted.

DIAGENETIC MODELS

Silicification The Tosi Chert has had both evaporites and carbon- ate burrow-fills (Bromley et al., 1975 ) replaced by silica. Burrow-fill replacements are typically aphano- crystalline to very finely crystalline cherts with minor chalcedony, and three dimensional burrow geom- etries are preserved. The burrow replacements rarely exhibit signs of early compactional effects. Bioclasts within the silicified burrows are normally intact, whereas bioclasts in unsilicified parts of the Tosi Chert are crushed, indicating that silicification occurred prior to significant consolidation of the surrounding Tosi sediments. Silicificatioh of carbon- ates prior to extensive lithification is common in many other carbonate platform deposits (Harris, 1958; Namy, 1974; Maliva & Siever, 1989) and

Evaporite replacement, Bighorn Basin 1215

Fig. 15. (a) Photomicrograph of anhydrite filling mouldic porosity in the Husky no. 82 Pitchfork core (polarized light). Scale bar=50 pm. (b) Photomicrograph of coarsely crystalline calcite spar filling biomouldic porosity from the Two Dot Ranch section. Scale bar=50 pm.

probably starts as early as a few metres of burial has been reached and may continue down to 1OOOm. Based on petrographical and field relationships, the silicification of burrows and the coarser crystalline evaporite silicification occurred after Permian time but prior to hydrocarbon migration during Early Jurassic time (Orr, 1974).

A probable source of silica for both evaporite and burrow replacements in the Tosi Chert was amor- phous silica from sponge spicules and, possibly,

radiolarians. Spiculitic chert nodules, silicified sponges and spicule moulds are very common within these cherts. In addition, the high organic carbon content of the Tosi Chert (Maughan, 1975; Claypool et al., 1978) may have been partially responsible for some of the silicification. Organic acids complexing with silicic acid (Bennett & Siegel, 1987) may have decreased silica saturation within the pore fluids, resulting in the dissolution of associated terrigenous clastics within the Tosi Chert. Silica precipitation

1216 D. S. Ulmer-Scholle and P. A. Scholle

Table 1. Summary of stable isotopic data (%o) for calcite spars from evaporite nodules (CE) and cements (CC) from the Park City Formation.

Average Range

Sample type n 6I3C 6180 613C 6 1 8 0

CE 54 - 7.92 - 17.89 3.12 to - 28.25 - 6.04 to - 27.29 Ferroan CE 12 - 2.95 - 16.92 3.48 to - 12.01 - 6.37 to - 23.17 cc 33 - 12.13 - 17.30 4.43 to - 24.59 - 2.61 to - 24'68 Ferroan CC 8 - 3.52 - 16.86 0.20 to - 10.12 - 9.22 to -21.52

Table 2. Summary of stable isotopic data (%o) for calcite cements subdivided on the basis of carbonate members from Permian Park City Formation.

Average Range

Formation n 6I3C 6''O 6I3C 6 ' 8 0

Grandeur Mbr. 1 - 17.01 - 18.12 Franson Mbr. 2 - 5.31 - 16.86 - 12.41 to - 21.61 - 13.38 to -20.34 Tosi Chert 1 - 18.16 -21.91 Ervay Mbr. 29 - 11.37 - 17.14 4.43 to -24.59 -2.61 to -24.68 Franson Mbr. (ferroan) 4 - 5.11 - 16.23 - 1.84 to - 10.12 - 9.22 to - 19.99 Ervay Mbr. (ferroan) 3 - 0.88 - 17.91 0.20 to - 1.64 - 15.67 to - 21.52

Table 3. Summary of stable isotopic data for calcite spars after evaporites subdivided on the basis of carbonate members from Permian Park City Formation.

Average Range

Formation n 6I3C 6180 613C 6 1 8 0

Franson Mbr. 19 -4.67 - 17.23 3.12 to -24.21 - 11.33 to -23.17 Tosi Chert 6 -2.88 - 16.08 3.11 to - 11.71 -6.04 to -20.00 Ervay Mbr. 27 - 11.69 - 18.89 0.49 to - 28.25 - 8.06 to - 27.29

Tosi Chert (ferroan) 4 -2.38 - 14.50 0.44 to - 8.24 -6.37 to - 17.88 Franson Mbr. (ferroan) 7 - 3.48 - 19.33 3.48 to - 12.01 - 16.17 to - 23.17

Ervay Mbr. (ferroan) 1 - 1.52 - 9.69

would have resulted from the oxidation and break- down of these organic acids, releasing silica into solution.

Replaced evaporites contain abundant solid in- clusions, but very few fluid inclusions. Therefore, determination of the compositions and temperatures of the pore waters responsible for the replacements is difficult. A few assumptions can be made about the pore fluids, however. The waters must have been undersaturated with respect to anhydrite and saturated to supersaturated with respect to silica. Evaporite replacement occurred prior to significant burial (depths of probably less than 500m) since

nodules show little deformation. Using a mean annual surface temperature of 2G25"C at the time of Permian deposition and a geothermal gradient, based on present day values, of between 23°C km - (Header & Hinckley, 1985) and 29°C km- ' (Orr, 1974), a temperature of 3240°C was the approximate temperature at the time of evaporite silicification.

Milliken (1979) determined that waters with a composition of sea water to mixed meteoric - sea water were responsible for silicification of evaporites within Mississippian strata of Kentucky and Tennessee. These Mississippian silicified evaporites exhibited a progressive change in replacement

Evaporite replacement, Bighorn Basin 1217

-30 -25 -20 -15 -10 -5 0

6l80 (%o PDB) Fig. 16. Crossplot of stable isotopic data of calcitized evaporites, calcite spars filling evaporite moulds and calcite ccment with fields marked for calcitization after hydrocar- bon migration within the Tosi Chert (I), during thermal sulphate reduction (HI), and uplift-related calcitization (11).

morphology and fluid isotopic composition with increasing burial, as inferred from oxygen isotopic analyses. Microcrystalline cherts formed during shallowest burial (25-32°C) from waters with a composition of nearly pure sea water; mega- quartz replacements formed at greater burial depths (3040°C) from mixed meteoric and marine waters.

Within the Tosi Chert, silicification of evaporites appears to have proceeded in a similar manner, with cherts forming earlier than megaquartz replacements. To determine the possible range of water oxygen isotopic compositions and formation temperatures, the equation of Knauth & Epstein (1976) for iso- topic fractionation between quartz and water at low temperatures,

lo3 In a = 3.09(1O6TP2) - 3.29,

was used to generate Fig. 17. Figure 17 shows that oxygen isotopic data, uncon-

strained by other evidence, could be used to argue for evaporite silicification under a wide variety of con- ditions from syndepositional to telogenetic. The isotopic data are compatible, for example, with low temperature (synsedimentary) replacement in a setting which had waters similar to ones found in modern, highly evaporative carbonate basins (+4.5%0 SMOW). It is equally compatible with extremely late stage replacement from modern groundwaters in the Bighorn Basin, where meteoric

-12 -10 -8 -6 -4 -2 0 2 4 6

6'80 (SM0W)-Water Fig. 17. Crossplot of calculated possible temperatures of precipitation versus formation water oxygen isotopic geochemistry for silicified evaporites in the Tosi Chert. Curves were calculated using the equation of Knauth & Epstein (1976) for isotopic fractionation between quartz and water at low temperatures. Shaded band indicates formation temperatures inferred from field and petrographi- cal data coupled with assumptions of Permian palaeogeo- thermal gradients and mean annual surface temperatures (see text).

fluids average - 12.0%0 SMOW (Yurtsever & Gatt, 1981), as well as with a full spectrum of intermediate temperature -water chemistry conditions. The use of other geological evidence is therefore critical to the interpretation of the isotopic data. Using the tem- perature conditions inferred from the field and petrographical evidence on the timing of the silicifi- cation (32-4OoC), coupled with the curves on Fig. 17, we can deduce that the waters responsible for silicification had oxygen isotopic compositions between - 3.1%0 (mixed meteoriclmarine) and + O . ~ % O SMOW (slightly evaporative sea water). Both the temperatures and the isotopic compositions are very similar to those determined by Milliken (1979) for megaquartz replacements within her study area and are compatible with geological evidence for non-marine conditions in the Bighorn Basin during latest Permian and Early Triassic time.

Calcite precipitation and. calcitization

Based on petrographical and geochemical analyses, calcitization of evaporites within the Park City

1218 D. S. Ulmer-Scholle and P. A . Scholle

Formation of the Bighorn Basin probably resulted from at least three separate, but sometimes over- lapping processes that all left unique signatures.

Process 1. The first period of widespread calcitiz- ation within the Permian Park City Formation post-dates silicification and was associated with hydrocarbon migration through the Tosi Chert and carbonate members. There is evidence, as discussed previously, that the siltstones and silty dolomites of the Tosi Chert were major hydrological conduits for fluid migration. The lack of evaporite inclusions and pseudomorphs after evaporites within the Tosi Chert indicate that they had probably been removed prior to calcite precipitation, but after silicification. Evaporite dissolution may have been caused by meteoric fluids flowing from the eastern side of the study area or the initial stages of fluid expulsion from the basinal sediments of the time-equivalent Phosphoria Formation.

The first stage of calcite precipitation within evaporite nodules of the Tosi Chert is a thin zone of ferroan calcite spar encasing the silica replacements. Following this period of calcite filling, hydrocarbons migrated through the Tosi Chert and into the avail- able pore space within the overlying Ervay member. The hydrocarbons filled many of the nodules and, in some cases, effectively sealed them off from further diagenetic reactions until Tertiary deformation. It is not uncommon to find chert nodules in outcrop which contain liquid hydrocarbons or asphalt. In nodules which were later breached, due to compac- tional or structural failure associated with the forma- tion of the Bighorn Basin, the hydrocarbons have been partially to completely flushed, and a later, spectacularly zoned, non-ferroan to ferroan calcite was precipitated. Due to the Ervay members lower permeability and early oil emplacement, this later stage of calcitization had very little effect.

Within parts of the Franson member, as previously mentioned, the calcites do show indications that they were a result of direct replacement of evaporites. These ferroan calcites, as well as the calcites from the Tosi Chert, are isotopically distinct from other cal- cites in the Park City Formation (Fig. 16). Their carbon isotopic values are less depleted; the heavier carbon values indicate that the fluid responsible for their precipitation was not significantly influenced by the breakdown of hydrocarbons, and the system was probably rock dominated. The ferroan nature of the replacements indicates that they formed under reduc- ing conditions. The oxygen isotopic values provide

no characteristic signature for these calcites, except that they formed at elevated temperatures or from isotopically depleted pore waters.

Process 2. Thermochemical sulphate reduction (TSR; Orr, 1974; Machel, 1987) is a late diagenetic process whereby hydrocarbons and sulphates react at depth and elevated temperatures with no bacterial intervention. Generally, TSR requires temperatures greater than lOO"C, anoxic conditions and the pres- ence of hydrocarbons. Under these conditions, hydrocarbons and dissolved sulphates react to produce altered hydrocarbons, bitumen, carbonate ions, hydrogen sulphide, miscellaneous metal ions (depending on the host rock and fluid compositions) and heat (Machel, 1987).

Since liquid hydrocarbons in the Bighorn Basin are considered to be a single source reservoir from within the Permian strata (mainly the organic-rich Phosphoria Formation), any variation within the hydrocarbons should be a reflection of the varying tectonic and subsidence histories found within dif- ferent parts of the basin (Orr, 1974). Permian strata within the Bighorn Basin today occur from the surface to deeper than 5 km. An average present day geothermal gradient within the Bighorn Basin is 23.3"C km- (Header & Hinckley, 1985), with an average surface temperature of approximately 7°C (Bredehoeft et al., 1992). Orr (1974), however, using well data, calculated a higher geothermal gradient (29.2"C km - I ) for the Bighorn Basin. Local varia- tions in the geothermal gradient can be significant in the Bighorn Basin due to the region being an active geothermal area (Header & Hinckley, 1985). Furthermore, it is probable that in the past, during formation of the Bighorn Basin and the Yellowstone flood basalts, the geothermal gradient might have been significantly higher.

Based on the following observations, Orr (1974) concluded that TSR was a major process in the degradation of hydrocarbons within the Bighorn Basin. The depth of burial (over 5 km) would be more than sufficient to obtain the temperatures (>lOO"C) needed for TSR to start. Hydrogen sul- phide is a byproduct of these processes, and concen- trations in wells of the Bighorn Basin have increasing H,S concentrations with increasing depths. The 613C and 634S of the oils are more enriched ( - 27.1%0) than would have been predicted for mature oils.

Diagenetically, TSR within a carbonate host could result in the following changes: (a) dissolution of sulphate minerals; (b) calcite precipitation in

Evaporite replacement, Bighorn Basin 1219

evaporite moulds (one of the products of TSR reac- tions is carbonate ions); (c) isotopic compositions with very light or depleted oxygen and carbon values, owing to the elevated temperatures and the break- down of the hydrocarbons; (d) the potential to form minerals typical of Mississippi Valley Type deposits, such as fluorite, sulphur, barite, pyrite, galena and baroque dolomite, if the appropriate ions are present in the system; and (e) bitumen or asphalt fillings within pore spaces (Machel, 1987).

The extremely depleted and variable carbon and oxygen isotopic values (Fig. 16) of some of the coarsely crystalline, blocky, calcite spars are due to the processes associated with thermal sulphate reduction. In addition, these spars are associated with fluorite, pyrite, baroque dolomite and barite. Bitumen pore fillings are also present within these units.

Process 3. The final period of evaporite dissolution and calcite cementation within the Park City Forma- tion is uplift-related calcitization. Uplift-related cal- citization (Harwood, 1980; Lee, 1989; Scholle et al., 1992) occurs after deeply buried units have been uplifted into the region of active meteoric fluid circu- lation which can extend to great depths due to head-driven circulation from high elevation recharge zones. These surface-derived sulphate-poor pore fluids cause evaporite hydration, dissolution and replacement. Such processes, which represent ongo- ing diagenetic alteration of the units, are controlled by the rate of influx of meteoric fluids. Calcite replacement or cementation represents the most near surface of these processes whereas anhydrite hydra- tion and dissolution characterize deeper subsurface settings located further from meteoric water sources.

Within the Bighorn Basin, mid-Tertiary block faulting resulted in progressive uplift and deforma- tion of the Permian strata to produce the present day configuration of the basin. This deformation increased the permeability of the Permian strata due to fracturing within the carbonate units (Simmons, 1990a). Prior to mid-Tertiary deformation, deep ground water circulation was probably minimal throughout the region because evaporites and hydro- carbons within the carbonate units effectively sealed the Park City Formation. Fracturing of the carbon- ate members resulted in renewed flow of meteoric fluids, migrating from uplifted recharge areas at the edges of the basin (Bredehoeft et al., 1992). These fluids were able partially to degrade and flush the hydrocarbons, to dissolve evaporite minerals within

the carbonates and to develop and enlarge porosity zones. By either mixing meteoric waters with hydrocarbon-rich pore fluids derived from basinal sources or supersaturating meteoric waters with respect to calcium and carbonate ions, non-ferroan calcite precipitated in the evaporite crystal moulds.

Uplift-related ‘calcitization’ in the Bighorn Basin has several unique characteristics. Calcite crystals should be found in outcrop filling moulds of eva- porites whereas in the deeper parts of the basin, where ‘modern’ meteoric waters do not circulate (Bredehoeft et al., 1992), anhydrite should still be preserved. In addition, calcites forming from mete- oric waters should be more enriched with respect to 6”O than those produced by TSR (Fig. 16), even though the meteoric surface waters currently found in the Bighorn Basin area have a 6 ’ *0 of - 12.0%0 SMOW (Yurtsever & Gatt, 1981). The

carbon isotopic composition could vary depending on the degree of hydrocarbon interaction with mete- oric waters. Because many of the most productive oil wells are along the eastern flank of the Bighorn Basin and are very shallow (€300 m), these type of interac- tions are potentially important. Finally, calcites forming under surficial conditions should be pre- dominantly non-ferroan replacements due to limited sources of iron and oxidizing conditions.

CONCLUSIONS

Silicification of evaporites within the Permian Park City Formation, especially the Tosi Chert member, is pervasive in both core and outcrop samples. This implies that it is not a local diagenetic effect related to recent tectonic activity or to uplift cycle hydrological events in the Bighorn Basin. Based on petrographical and field relationships, silicification occurred during the early stages of burial, prior to hydrocarbon migration. Diagenetic fluids were mixtures of waters derived from dewatering of the basin and meteoric fluids migrating within the Tosi Chert.

Three stages of evaporite removal and calcite pre- cipitation can be recognized within the carbonates and cherts of the Park City Formation. The majority of calcite spars formed during late diagenesis. How- ever, within the Tosi Chert and some evaporite nodules in the Franson members, calcitization started prior to hydrocarbon migration and continued after the hydrocarbons had been emplaced. This is sup- ported by the presence of hydrocarbon inclusions in the calcites, higher iron contents and more positive carbon isotopic values.

1220 D. S. Ulmer-Scholle and P. A. Scholle

Within most of the carbonates, evaporite dissolu- tion and calcite precipitation occurred over a longer period of time, commencing during deepest burial (Eocene) and continuing through later Tertiary hydrological events. If a geothermal gradient of 29°C km (Orr, 1974) is assumed to be an average regional value, then TSR would have started when Permian strata were buried to about 3.4 km (Jurassic). Because the maximum burial within the centre of the Bighorn Basin is over 5 km, a large part of the basin remains at temperatures greater than 100°C. Byproducts of TSR in the study area are calcite, fluorite, pyrite and barite.

Finally, as a result of Tertiary tectonics, the Park City Formation carbonates were brought back up into the regime of active meteoric water flow. Sulphate-poor meteoric fluids caused evaporite hydration, dissolution and replacement. Uplift- related ‘calcitization’ represents the most near surface of the evaporite replacement processes and is con- trolled by the influx of meteoric fluids into the basin. Calcite crystals associated with this stage of calcitiz- ation are large, scalenohedral, non-ferroan spars. TSR and uplift-related calcitization both represent ongoing processes within the Bighorn Basin.

Significant porosity changes have taken place in the Park City Formation carbonates during both ‘early’ and ‘late’ diagenesis, and the isotopic vari- ations in the calcite spars provide a record of water and hydrocarbon migration through these units. Studies of porosity evolution and cement history of evaporitic carbonate rocks must take into account uplift-related alteration, which is commonly of great importance, as well as diagenetic effects produced by the thermal breakdown of hydrocarbons during deep burial. Reservoir predictions in such units based only on outcrop samples should be viewed with caution.

ACKNOWLEDGMENTS

We would like to thank Drs Richard Koepnick, Robert Laury, James Brooks and Patrick Brady for their helpful comments on an early version of this manuscript. We would also like to thank Drs Gill Harwood, Hans Dronkert and Stuart Burley whose comments greatly improved this manuscript, and Drs Richard Inden, Philip Choquette and Edward Maughn for their helpful suggestions during initial stages of this study. Dr William Dawson, Texaco Research Laboratories, provided carbonate isotopic

analyses, and Dr Samuel Savin and Linda Abel of Case Western Reserve University ran silicate oxy- gen isotopic analyses for this study. Financial sup- port was obtained from Texaco Exploration Company, the United States Geological Survey, the Institute for the Study of Earth and Man at South- ern Methodist University, the Dallas Geological Society Scholarship Fund and Marathon Research Laboratories.

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(Manuscript received 28 July 1993; revision accepted 3 May 1994)