Depth indicators in Permian Basin evaporites

Abstract

AbstractThe Permian Basin of West Texas and New Mexico contains one of the world's best-preserved and most extensively studied evaporite basin-to-platform sequences. From analysis of fabrics and small-scale cycle patterns, reconstruction of the position of these elements in the basin-filling sequence and comparison to laboratory-grown and modern evaporite fabrics, we created a table of fabrics that serve as water-depth indicators. Evaporites formed in deeper water (more than a few to hundreds of metres) in both halite- and gypsum-precipitating settings in the Permian Basin are characterized by cumulate fabrics. Cumulates are fine crystals or rafts of fine crystals formed at the air – brine interface that fall though the water body and accumulate on the basin floor with fine lamination, draping relationships, dark colours and minimal early diagenesis. Intervals of coarser crystals precipitated on the basin floor are interpreted as evidence for episodic transport of saturated surface water to the basin floor during perturbation of stratified conditions. Shallow water evaporites in the Permian Basin are dominated by bottom-growth fabrics such as halite chevrons and near-vertically oriented gypsum crystals. Bands of fluid and other inclusions record high frequency changes in depositional rate. Truncated crystals document flooding by undersaturated fresh or marine water under shallow conditions where mixing was adequate to cause undersaturated low-density water to contact the basin floor. Formation of base-of-cycle insoluble residues is a strong indicator of shallow water during the flooding event that initiated each sedimentary cycle. In the Permian Basin, exposure is documented by formation of synsedimentary evaporite karst pits and pipes, truncation, dissolution and recrystallization of earlier fabrics, and precipitation of cements. Red siliciclastic mudstones are associated with the late stages of depositional cycles when the surface was subaerially exposed. Repetition of alternately exposed and saline water table conditions created an array of distinctive fabrics including chaotic mudstone–salt mixtures, karst fills, replacement and recrystallization fabrics, and cracks and saltpolygons.