ABSTRACT: New Interpretation of Vertically Aligned Gypsum Fabrics: Implications for Gypsum Depositional Environments and Diagenesis

Michael R. Rosen, John K. Warren

Gypsum and anhydrite fabrics in trenches and deep 500-m cores from Bristol Dry Lake, California, exhibit a vertical alignment of crystals similar to the fabric seen in bottom-nucleated brine pond gypsum. However, geochemical and sedimentologic evidence indicates that the gypsum formed in Bristol Dry Lake precipitated as a diagenetic

displacive mineral within the sediment where groundwater saturated with respect to gypsum recharges around the playa margin (groundwater seepage gypsum). Evidence for displacive growth of gypsum comes from (1) the geometry of the deposit, (2) stable isotopic data and the water chemistry of the brine, and (3) inclusions of matrix that follow twin planes and completely surround crystals as they grow.

Because the fabrics and textures of this diagenetic gypsum formed by groundwater in playa settings are similar to those of primary gypsum formed in a brine pond, it is necessary to refine the criteria for the distinction between subaerial, groundwater, and subaqueous gypsum. When compared to the futures observed in a Holocene subaqueous gypsum deposit (Marion Lake, Australia) and a Holocene subaerial deposit (Abu Dhabi sabkha), the following criteria can be used to distinguish between the three different types of gypsum deposits: (1) amount of matrix, (2) geometry of the deposit, (3) gypsum crystal size and orientation, (4) distribution of matrix and fluid inclusions within individual crystals, and (5) fabric of the matrix around the gypsum. Distinguishing between subaqueous and groun water gypsum may be difficult. However, sufficient mesoscale differences exist such that, in most cases, the different gypsum types can be resolved from core slabs and thin sections. These criteria may be applied in ancient examples even after gypsum has been converted to anhydrite. For example, within 100 m of the surface, the gypsum in Bristol Dry Lake dehydrates to anhydrite, although the same vertically aligned fabric is retained through the diagenetic process.

Chemically, it is unlikely that all three types of gypsum could precipitate in the same basin at the same time. But if the saturation state of the water changes, it is possible to spatially overprint the different gypsum types with time. Therefore, it is important to determine detailed facies and diagenetic relationships in order to sort out the different gypsum types.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990