Water-Level Controls on Halite Sedimentation: Permian Cyclic Evaporites of the Palo Duro Basin

Extensively cored cyclic evaporites of the San Andres Formation (Guadalupian) of the Palo Duro Basin, Texas Panhandle, provided fundamental information for interpreting evaporite depositional processes. These evaporites preserve exceptionally sensitive indicators of water-level fluctuation in their depositional environment: (1) sedimentary fabrics that formed during evaporite deposition; (2) sedimentary features, surfaces, and insoluble residues that formed during evaporite dissolution; and (3) diagenetic overprints that formed during evaporite brine evolution. Lateral and vertical facies relationships among evaporite facies defined on the basis of sedimentary fabrics were examined to document the evolution of the evaporite basin and the sedimentological response to water-level fluctuations.San Andres evaporite sediments that were deposited in a variety of brine pool environments preserve evidence of water depth and brine pool character. Chevron structures defined by fluid inclusions trapped during crystal growth in halite indicate that the halite formed in very shallow, agitated and oxygenated water. Laminated, darker halite was interpreted to form in slightly deeper or more density-stratified brine pools by a combination of cumulate (surface-formed rafts) sedimentation and bottom growth. Facies relationships and fabrics indicate that bottom-grown fabrics in gypsum can form in slightly deeper water than bottom-grown fabrics in halite because density stratification in gypsum-saturated water is less.This study documents using evaporite-dissolution features as paleoenvironmental indicators. Evaporite dissolution occurred (1) when water level rose and the evaporite basin became flooded by undersaturated marine water and (2) during emergence when water level fell and evaporites became exposed to undersaturated meteoric waters. The distinctive sedimentary fabrics and facies relationships created by both of these processes define the history of water-level fluctuation in the basin. High-frequency water-level fluctuation produced interbedded meter-thick zones of anhydritic halite (brine pool) and muddy halite (frequently exposed). Siliciclastic silt and clay concentration and structures that formed by karstic dissolution are recognized as evidence of exposure. Lower order cyclicity (parasequence scale) formed upward-shallowing cycles 2 to 30 m thick within the San Andres Formation. Insoluble residues that formed during the transgression that initiated cycles have a distinctive wavy "upside-down-accreted" fabric that may aid in identifying residues in other settings. Residue thickness was found to be inversely proportional to the amount of preserved halite in the underlying cycle. The relative timing of dissolution is documented by the facies and geometry of overlying sediments and by diagenetic phases incorporated in the residue.Diagenetic overprints obscure primary textures in San Andres evaporites, but because the diagenesis is dominantly synsedimentary, it also provides information about the depositional environment. When water level fell, paleowater tables within the porous evaporites formed a vadose zone of halite dissolution and a phreatic zone of halite precipitation. The amount of brine pool fabric preserved is therefore a measure of water-level stability, in which the most altered fabrics are found to correspond to the highest frequency of fluctuation in updip or ephemeral environments.The water-depth criteria developed by this study show that San Andres halite has preserved much depositional information and that cycle patterns in halite can be analyzed using the same techniques as are commonly used on carbonates. The techniques used in this study may be of use in other evaporite basins where water depth and basin evolution are unknown.