Abstract Over the past fifteen years a great deal of information has been gained concerning evaporites, such that it is now possible to examine core and outcrop material in order to accurately work out the environ-ment(s) of deposition and some of the subsequent diagenetic history. It must be admitted that because of postdepositional alteration many cores or outcrops convey little information save the presence or absence of one or another mineral phase. In these cases, inferences as to the condition of origin must be made through a veil of obfuscation. Simple processes, such as burial with attendant compaction and loss of water of crystallization due to modest geothermal heating, can result in rocks which look superficially similar to those produced as the primary facies of a different origin. Subtleties within the rock record may reveal clues important in unraveling features of primary deposition, as opposed to later diagenesis.

Abstract Recent sediments from the Abu Dhabi coast provide one of the best Holocene analogs for many ancient carbonate and evaporite sequences. These Recent deposits (locally up to 10 m thick) occur as a seaward-prograding, shoaling-upward veneer, several hundred miles long and up to 25 mi wide, over Pleistocene and Tertiary rocks. Shallow-water carbonates associated with the coastal flats include bioclastic, grapestone, and oolitic sands, pelleted aragonitic muds, and algal stromatolites. Tidal and supratidal evaporites include gypsum, anhydrite, halite, and dolomite. Landward the sediments grade into windblown clastics. Sediment cores and trenches show that changes in evaporite mineralogy and texture occur landward across the coastal flats. The most significant changes include: 1) A bedded mush of gypsum crystals becomes more contorted and diapiric in a landward direction as it is progressively replaced by chicken-wire anhydrite. 2) Simultaneously this bed is overlain by storm washover sediments in which anhydrite occurs as nodules and blebs. 3) Further landward the anhydrite nodules coalesce to create contorted thin layers, ptygmatic in structure, that form polygonal saucers at the sediment surface. 4) At the landward margin of the coastal flats the diagenetic sequence is reversed and anhydrite is totally replaced by gypsum. These occurrences of evaporites have structures common to shoaling-upward cycles that typify many ancient sequences of carbonate and evaporite.

Abstract Few examples of ancient continental-sabkha evaporites have been recognized in the geologic record, despite the fact that numerous other continental-type deposits have been described. Furthermore, detailed sedimentological descriptions of Recent continental evaporites are also lacking. Work that has been done (Hardie et al., 1978) indicates that depositional processes and facies characteristics of Recent continental sabkhas are often similar to those of coastal sabkhas. Thus, it is quite possible that ancient continental-sabkha deposits do exist in the geologic record, but have been misidentified as marine in origin. In light of our limited knowledge of continental sabkha deposits, this report presents some sedimentological data and interpretations of Bristol Dry Lake, California (Fig. 1), a 155 km 2 continental-sabkha playa basin filled with at least 300 m of interbedded terrigenous clastics, gypsum, anhydrite, and halite (Bassett et al., 1959). Approximately 1-1/2 weeks of reconnaissance field work, supported by the Texas Bureau of Economic Geology and the U.S. Department of Energy, was conducted during 1979-80. Bristol Dry Lake is well suited for sedimentological studies because there are numerous trenches and pits dug by salt companies across the playa. Many of these are up to 6 m deep and expose intricate details of playa shallow-stratigraphy.

Abstract A TexasGulf composite core representing deposition during Wilkins Peak time of the Green River Formation (Eocene) is described. Two major facies, a mudstone facies and a trona facies, represent deposition within ephemeral alkaline saline Lake Gosiute. A laminated mud-stone subfacies indicates a distal saline mudflat subenvironment, whereas a crumbly, homogeneous mudstone formed within a proximal saline mudflat subenvironment. Oil shale signifies flooding and expansion of the lake with associated organic blooms. Drying of the lake permitted precipitation of trona in two forms; a) a displacive nodular type consisting of radiating crystals, and b) a granular or sugary type of detrital origin. A third, postdepositional trona type is described as a columnar crystalline vein type associated with hydraulic fracturing.

Abstract Petrographic and geochemical studies of cap-rock core of two salt domes, Gyp Hill in South Texas and Oakwood in East Texas, reveal significantly different diagenetic histories for each dome. Cap rock on Gyp Hill is now forming within shallow meteoric aquifers. In contrast, cap rock on Oakwood Dome formed principally during the geologic past within deeper saline aquifers in the East Texas Basin. Gyp Hill cap rock, which is 271 m (890 ft) thick, is composed of 149 m (490 ft) of anhydrite overlain by 122 m (400 ft) of gypsum. An uncemented anhydrite sand marks the salt/cap-rock interface. From 4 m (13 ft) above the interface to the top of the anhydrite, porosity is occluded by poikilotopic gypsum cement. Occurrence of gypsum cement indicates low-temperature and low-salinity conditions during cap-rock formation, that is, dome dissolution in a shallow meteoric aquifer. The overlying gypsum results from hydration of anhydrite by meteoric ground water. Oakwood cap rock, which is 137 m (450 ft) thick, is composed of 78 m (256 ft) of anhydrite overlain by 59 m (194 ft) of calcite. In contrast to Gyp Hill anhydrite, the Oakwood anhydrite is entirely devoid of gypsum cements except that the interface between anhydrite and calcite. The anhydrite has been deformed and recrystallized into a moderately well-developed granoblastic texture that is indicative of high-temperature and high-pressure conditions. The anhydrite section is thought to have formed by salt dissolution at deep, high-temperature, saline conditions. Petrographic, geochemical, and isotopic evidence from the dark calcite indicates that it is the product of calcium sulfate reduction by hydrocarbons in a saline, deep-basinal fluid. Another deep-basinal fluid more enriched in Sr, Ba, Mg, and Mn, dissolved dark calcite, which reprecipitated as coarsely-crystalline light calcite. The only effect of meteoric water on the Oakwood cap rock is the presence of gypsum in the calcite/anhydrite transition zone.

Abstract The Lower Cretaceous Ferry Lake Anhydrite, as indicated by a core study of the Cities Service Co. No. 1 J. B. Kitchens well in Henderson County, East Texas, is composed of alternating carbonate and eva-porite units that were deposited in a broad, shallow-subtidal hypersa-line, lagoonal environment. The evaporitic lagoon was up to 260 km (160 mi) wide in East Texas and was barred from the sea by shelf-margin rudist bank buildups and possibly by a tectonic sill. Vertically to randomly aligned (elongated) mosaic anhydrite was deposited as subaqueous gypsum palmate structures that were transformed to anhydrite during diagenesis. Some bedded gypsum was also deposited subaqueously in the lagoon. The thick [up to 6.7 m (22 ft)] evaporite units contain thin carbonate mudstone layers with anhydrite rip-up clasts which represent either exposure or storm flooding of the lagoon. Most of the carbonates, predominantly mudstones and wackestones, also were deposited in a restricted, shallow-subtidal to low-intertidal environment. The fauna consists primarily of ostracods and mollusks. Several units contain Orbitolina , and/or echinoids indicating near normal salinity prevailed periodically. Evaporites and carbonates deposited in a sabkha environment are rare. Evaporites deposited subaqueously in a broad-shelf lagoonal setting have good lateral continuity and form excellent seals for interbedded carbonate reservoirs, and for reservoirs in the underlying stratigraphic section.

Abstract Shallow-water carbonates of the Upper Jurassic Smackover Formation contain proven exploration targets throughout the U.S. Gulf Coast. Evaporites have played a role in Smackover reservoir development. Evaporites are found in the underlying Louann Salt, the overlying Buckner Formation, and within the Smackover itself. The Buckner has been important in three ways: 1) Buckner anhydrites and shales are important seals for Smackover reservoirs; commonly the Buckner inter-fingers with the reservoir at the updip limit of a field and as a result of basinward progradation, forms an impermeable cap; 2) during Buckner time early diagenesis of the uppermost Smackover occurred as brines were introduced into the lime sands of the Smackover from laterally adjacent tidal flats; and 3) thickness maps of the Buckner serve as a valuable exploration tool for the Smackover because porous facies are associated with Smackover highs underlying the Buckner thins. The stratigraphic relationship between the Smackover and Buckner is illustrated with conventional cores from a well in Columbia County, Arkansas.