R. Stephen Fisher

Diagenetic history of Eocene Wilcox sandstones, South-Central Texas☆

Abstract Sandstones and shales of the lower Eocene Wilcox Group extend from outcrop to depths greater than 5 km where temperatures exceed 200°C. Eight authigenic minerals have been identified. Quartz, kaolinite, calcite, ankerite and albitized plagioclase are volumetrically significant, whereas chlorite, illite, albite overgrowths and dolomite are present only in trace amounts. Quartz, kaolinite, and ankerite cements are present over the entire depth range sampled (1528 to 4446 m). Calcite cement occurs only in sandstones shallower than 3300 m. Albitized plagioclase is the predominant feldspar in samples deeper than 2600 m whereas detrital potassium feldspar has been completely dissolved out of sandstones deeper than about 3200 m. No systematic variations in the chemical composition of calcite with depth, no zoning within calcite or ankerite, and no compositional differences between pore-filling and grain-replacing calcite or ankerite were found. Calcite cements have 87 Sr 86 Sr ratios generally less than 0.708, whereas ankerite cements are significantly enriched in 87Sr. The oxygen isotopic composition of carbonate cement ranges between −9 and −14%. (PDB) for calcite and between −8 and −12%. for ankerite with no systematic variation with depth. Calculated δ18O values for authigenic quartz and kaolinite range from +22 to +31%. (SMOW) and from +14 to +17%., respectively. Volumetrically important amounts of authigenic quartz and kaolinite precipitated during early burial before calcite cementation, secondary porosity generation, or smectite to illite transformation in encasing shales. The isotopic composition of strontium in calcite cement shows most of it to be of Eocene or older marine carbonate origin. Ankerite cementation followed the generation of secondary porosity and occurred after the products of organic maturation and shale diagenesis, plagioclase albitization or potassium feldspar dissolution significantly affected the sandstone-pore water system.

Wilcox sandstone diagenesis, Texas Gulf Coast: a regional isotopic comparison with the Frio Formation

Summary The burial diagenesis of Eocene Wilcox sandstones differs significantly from the burial diagenesis of other Gulf Coast terrigenous clastic formations. Differences between the onshore Wilcox and Oligocene-Miocene Frio Formations are larger than regional variations within either formation. Younger, offshore units have undergone less diagenetic alteration than either of the older, onshore units, whereas Mesozoic sandstones are generally more extensively altered. Cementation by quartz and calcite was the first diagenetic modification of volumetric significance to affect either the Wilcox or Frio Formations. The average δ18O of Wilcox quartz cement is approximately +25‰ SMOW, in contrast to + 31‰ for the Frio. In both formations, quartz cement is more abundant in the overpressured zone. Calcite has an average δ18O of −10.8‰ PDB in the Wilcox Formation (compared to −7.2‰ for the Frio). 87Sr/86Sr values suggest that calcite was derived from coeval nannofossils, carbonate rock fragments of Mesozoic age, or by local input of fluids from underlying Mesozoic carbonates. Most calcite cement (and therefore quartz which pre-dates calcite cement) was apparently emplaced prior to extensive alteration of detrital silicates which would have released strontium with a high 87Sr/86Sr ratio. The most common carbonate cement in the Wilcox Formation is ankerite which replaces calcite with increasing depth. The δ13C of ankerite is essentially identical to that of calcite at the same depth, and is also similar to Frio and younger carbonate cements. In contrast, δ18O values of Wilcox ankerite (average −9.8‰) indicate emplacement at higher temperature or from more depleted water than was true of Wilcox calcite. 87Sr/86Sr values for Wilcox ankerite are very radiogenic (>0.7100), indicating that ankerite emplacement occurred during or after active silicate diagenesis. Both ankerite and dolomite are very uncommon phases in Frio and younger sandstones despite massive smectite stabilization, suggesting that the conversion of smectite to illite was not the source of iron and magnesium for the late cements. The volume of secondary porosity is similar in both Wilcox and Frio sandstones. Albitization and K-feldspar removal from both formations are essentially complete below 3000 m and essentially no unaltered detrital feldspar occurs below that depth. Differences between the Gulf Coast formations are attributed to different geothermal gradients, differences in the basinal sediments over which the units prograded, and to the changing nature of connate fluids in the units themselves. Diagenesis of the Wilcox Formation, like other Gulf Coast terrigenous wedges, is understandable only in terms of interaction with underlying units during the large-scale evolution, in both time and in space, of the Gulf Coast diagenetic system.

Geochemistry and hydrodynamics of deep-basin brines, Palo Duro Basin, Texas, U.S.A.

Abstract Permeable strata of the Deep-Basin Brine aquifer system underlie bedded Permian evaporites in the Palo Duro Basin, Texas Panhandle. Formation water samples collected from four U.S. Department f of Energy test wells and two independent oil and gas company wells in the basin were analyzed for chemical and isotopic compositions to characterize the geochemical environment and to determine the origin and compositional evolution of the water. Formation waters are Na Cl brines that contain 140 to 290 g/l total dissolved solids (TDS). Chemical and isotopic compositions suggest grouping the brine samples into one of two types. Samples from western Palo Duro Basin have high Cl:Br ratios, Sr isotopic ratios that are significantly more radiogenic than Permian seawater or precipitates from Permian seawater, and oxygen isotopic compositions that are depleted with respect to values predicted for equilibrium with calcite under in situ conditions. Samples from central and eastern Palo Duro Basin have low Cl:Br ratios, Sr isotopic ratios only slightly radiogenic relative to Permian marine Sr, and oxygen isotopic compositions that suggest equilibration with calcite. The concentrations of major and minor ions appear to be controlled by equilibrium with calcite, dolomite, anhydrite, celestite, low albite, microcline, and Na-smectite. High Na activities drive ion exchange reactions, which, coupled with maintenance of chemical equilibrium, elevate the concentrations of divalent cations and lower the concentrations of dissolved SO 4 and carbonate. The origin and evolution of deep-basin brines are interpreted from chemical and isotopic compositions integrated with results of previous geological and hydrogeological investigations. Brines from western Palo Duro Basin acquired salinity by dissolving halite along a lateral flow path from the recharge zone; radiogenic Sr was derived from detrital silicate minerals and oxygen isotopic equilibrium with aquifer minerals has not been attained. Brines from central and eastern Palo Duro Basin originated as either recharge that achieved chemical and isotopic equilibrium with the host rock during long residence times or as Permian seawater concentrated by evaporation. Regional chemical and isotopic variations augment the results of hydrogeological modeling: brines in the western part of the basin flow from the west and southwest where siliciclastic sedimentary rocks predominate; brines in the central and eastern parts of the basin flow from the south and southwest where carbonate strata predominate.

Clay Minerals in Evaporite Host Rocks, Palo Duro Basin, Texas Panhandle

ABSTRACT The mineralogy and chemical composition of clays in evaporite strata of the Palo Duro Basin, Texas Panhandle, were investigated to determine the type and extent of alteration and to explore the preservation of diagenetic clays in changing geochemical environments. The coarse clay-size fraction (2-8 µm equivalent settling diameter (ESD)) contains predominantly illite and irregularly interlayered chlorite/vermiculite, whereas the fine-clay-size fraction (< 0.5 µm) contains predominantly illite and irregularly interlayered chlorite/smectite. Detrital illite, biotite, chlorite, and smectite are the probable parent clays. Electron microprobe analysis of clay-size flakes shows that the diagenetic clays are enriched in magnesium relative to detrital clays. Chemical composition , the geologic history of the Palo Duro Basin, and comparison to clays from other Paleozoic marine evaporites suggest that alteration occurred syndepositionally in the presence of hypersaline marine evaporite brine. Mineral assemblages of clays encased in primary marine halite, clays from regions where halite has been dissolved, and clays from strata approximately 300 m beneath the salt section are structurally and chemically similar, suggesting that these clay minerals record paleohydrologic and paleoenvironmental conditions.

Marine to nonmarine facies transition in Permian evaporites of the Palo Duro Basin, Texas: Geochemical response

The late Paleozoic Palo Duro Basin evolved from a marine to nonmarine environment as it was infilled. The evaporitic part of the sequence (Leonardian through Ochoan) is composed of regressive carbonate-anhydrite-halite cycles that displaced open-marine environments toward the south during basin filling. Systematic vertical changes in evaporite geochemistry through the evaporite section include increasing 87Sr/86Sr ratios and decreasing (altered from marine) δ34S in anhydrite, decreasing (recycled) bromide content of the halite, and increasing meteoric influence on the stable isotopic composition of fluid inclusions in halite. Geochemical changes correspond to changes in evaporite sedimentology and document the evolution from marine-dominated to partly nonmarine evaporites. Geochemical tracers, when used with sedimentological and facies analysis, provide reliable indicators of the sources of water and solutes in an anhydrite- halite facies tract. Perennial, intermittently stratified, and ephemeral brine- pool conditions affect the amount of synsedimentary recycling. Variable rates of reflux and marine recharge control residence times of brines in the depositional environment. These are two key processes in relating evaporite geochemistry to depositional environment.

A sulfur and strontium isotopic investigation of Lower Permian anhydrite, Palo Duro Basin, Texas, U.S.A.

Abstract Lower Permian Wolfcamp and Wichita carbonates and anhydrites, Palo Duro Basin, Texas Panhandle, record a change from a normal marine to marine evaporite depositional environment. Isotopic compositions of S and Sr in anhydrite were determined to investigate the age of Wolfcamp and Wichita strata and the paragenesis of eight anhydrite forms. Bedded nodular mosaic (Wichita) and replacive nodular (Wolfcamp) anhydrites have S and Sr isotopic compositions that record precipitation from Early Permian (Wolfcampian) to Leonardian) seawater. Silicified nodular, coarsely crystalline nodular, fossil- filling, and vein-filling anhydrites have enriched S isotopic compositions relative to Permian seawater, whereas euhedral and anhedral replacive anhydrites have depleted S. The Sr isotopic composition of most anhydrite forms indicates an Early Permian marine origin, however, Sr in anhydrite veins is slightly radiogenic compared to Permian seawater. The Sr isotopic compoition of bedded nodular mosaic anhydrite indicates as Leonardian seawater source of material. The γ 34 S values are slightly enriched. relative to values predicted from the S age curve, suggesting an excursion of the S isotopic evolution trend of Early Permian seawater. Replacive nodular anhydrite 87 Sr/ 86 Sr ratios are slightly higher than those of bedded nodular mosaic anhydrite reflecting incorporation of Sr from older marine pore waters or replaced sediments or from dissolution of detrital siliciclastic minerals. Anhydrite paragenesis is interpreted from stratigraphic, mineralogic, petrographic, and isotopic data. Bedded nodular mosaic and replacive nodular anhydrite precipitated as marine evaporite strata prograded across underlying normal marine carbonate sediments. Siliified and coarsely crystalline nodules and anhydrite fossil-filling precipitated at essentially the same time as bedded and replacive nodular anhydrite but in environments where sulfate reduction was more extensive. Euhedral anhedral replacive anhydrite formed in shallower environments where S redox cycling occurred prior to CaSO 4 precipitation. Anhydrite vein-fills formed last and incorporated radiogenic Sr released during alteration of detrital siliciclastic grains.

Rb-Sr Ages of Diagenesis of Mg-rich Clay in Permian Sediments, Palo Duro Basin, Texas Panhandle, U.S.A.

ABSTRACT Mid- to upper Permian sedimentary rocks in the shallow (< 3 km), intracratonic Palo Duro Basin, Texas Panhandle, occur in multicyclic carbonate-evaporite-redbed sequences, interpreted to be hypersaline shelf depositional assemblages. Certain marker horizons can be traced continuously over the entire basin. Clay minerals include distinctive varieties of Mg-rich smectite, chlorite, or mixed-layer species resulting from diagenesis in an evaporitic environment. Discrete particle-size fractions of clay were separated from core recovered from the Tubb (Leonardian), San Andres Unit #4 (lower Guadalupian), upper Seven Rivers (upper Guadalupian), and Salado-Tansill (Guadalupian-Ochoan) Formations to determine the times of mineral genesis. Rb-Sr isotopic data, for aliquots of a clay sample that has been leached by diverse ion exchange reagents, plot as a linear array in a Rb-Sr isochron diagram. For ultra-fine clay (< 0.05 µm), interpreted to be diagenetic, the slope of the data array corresponds to the time of diagenesis. Ages of clay diagenesis in the four sampled stratigraphic units agree generally with ages of deposition according to the geologic time scale, signifying that diagenesis was penecontemporaneous with deposition. Analyses of coarser clay fractions (0.2-0.5 µm and 2-4 µm) indicate the presence of a detrital component. In most instances, initial 87Sr/86Sr in diagenetic clay is higher than the Sr isotope ratio measured directly in associated anhydrite. Introduction of radiogenic Sr from occasional stream influx or from concurrent diagenetic dissolution of silicate detritus could have elevated 87Sr/86Sr in the ambient brine that mediated clay diagenesis.

Successive pore fluid generations in a Lower Permian brine aquifer, Palo Duro Basin, Texas Panhandle, U.S.A.

Abstract The successive presence of four compositionally distinct pore fluids in Lower Permian carbonate strata of the Palo Duro Basin, Texas Panhandle, is interpreted from (1) lithological, minerlogical, and petrographic evidence for depositional conditions; (2) isotopic compositions of C, O, and Sr in limestone and dolomite; (3) limited data on fluid inclusions in sphalerite; and (4) chemical and isotopic analyses of formation water. Wolfcamp carbonate mudstone containing normal marine fauna was initially deposited on an open, shallow shelf. The C and O isotopic compositions indicate precipitation from Permian seawater with minor terrestrial and meteoric influence; Sr isotopic compositions document a Leonardian seawatersource of material. Conditions gradually became more restricted and seawater was concentrated by evaporation, resulting in deposition of penecontemporaneous dolomite and anhydrite in overlying Wichita strata. Reflux of Wichita brine dolomitized underlying Wolfcamp carbonate mudstone and generated the upper Wolfcamp aquifer. Isotopic compositions of Sr in Wolfcamp dolomite suggest that the reflux brine was evaporatively concentrated Leonardian seawater. Expulsion of warm, saline formation water from deeper in the basin through the upper Wolfcamp aquifer is recorded in fluid inclusions in sphalerite. This change in pore fluid composition occured before and during regional Tertiary uplift and tilting of the Texas Panhandle. More recently meteoric water has recharged at least the western one-third of the upper Wolfcamp aquifer. This modern flow regime was established within the past 10–15 Ma.

Relations between bromide content and depositional processes in bedded halite, Permian San Andres Formation, Palo Duro Basin, Texas

Geochemical data, stratigraphic mapping, and textural relations within bedded halite in the Permian San Andres Formation, Palo Duro Basin, Texas Panhandle, were integrated to determine environments of deposition and alteration and to evaluate influences on the amount of bromide in halite.Four factors control the bromide concentration of bedded halite. First, samples that contain large amounts of mudstone have anomalously high bromide concentrations, reflecting bromide in cements or intergranular fluids. Second, highly irregular bromide-versus-depth profiles record continual alteration of parent brine and interruptions of halite precipitation in a broad, shallow brine pool. General vertical trends in bromide content record large-scale lateral migrations of depositional environments over time that result in varying proportions of first-cycle marine, second-cycle marine, and second-cycle meteoric evaporite brine. Third, lateral variations in the bromide content of correlative samples from different geographic positions in the basin reflect the amount of primary marine halite, recycled marine halite, and recycled meteoric halite in the evaporite environment. Finally, similar bromide concentrations in texturally distinct samples indicate that most halite precipitated in a marine-dominated evaporite environment in which alteration occurred syndepositionally. Patterns in the bromide content of halife samples augment interpretations based on detailed stratigraphic mapping and provide sedimentologic information that is not available from other data sources.