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Abstract: Reconstruction of Dissolved Evaporite Facies

HOVORKA, SUSAN D., Bureau of Economic Geology, The University of Texas at Austin

Evaporites are key elements in basin evolution; however, in many settings, their potential for preservation is poor. Concepts developed in the Permian Basin of Texas by comparison of areas of preserved halite, gypsum, and anhydrite beds with areas from which evaporites have been partly or wholly dissolved allow recognition of the former distribution of evaporites and permit reconstruction of depositional facies.

Evaporites are commonly extensively dissolved in the shallow subsurface where fresh water intersects evaporite-bearing strata. Evaporites are also dissolved in the depositional environment during exposure or marine flooding, along paleoexposure surfaces by recharge water, and in the deep subsurface by undersaturated brines. Features that can be recognized as indicators of the former presence of evaporite include pseudomorphs after diagnostic evaporite crystals and sedimentary textures; insoluble residues; distinctive diagenetic phases associated with halite or gypsum that remain after the evaporite mineral has been dissolved; diagenetic phases that replace evaporite minerals as they dissolve; diagnostic thickness changes; and breccia and karst structures formed where evaporite has been irregularly dissolved. Relationships among the different types of indicators are used to separate synsedimentary dissolution from dissolution that occurred under burial conditions.

Reconstruction of dissolved evaporite units is needed to interpret depositional environments, to understand the diagenetic evolution of associated rocks, and to interpret brine evolution within the basin. Reconstructions of the extent, processes, and timing of halite and gypsum dissolution are used in both environmental and hydrocarbon production applications to predict unit thickness, flow conditions, fracture distribution, and permeability structure.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah