FOUR LAYER WATER MASS STRATIFICATION OF THE PERMIAN BASIN

Ocean water masses are vertically stratified with abrupt boundaries at pycnoclines, generally with little mixing between masses. The nature of the rock record within basins frequently gives the impression of gradual lateral changes because boundaries may only be apparent where they impinge upon a slope at a low angle. The characteristics of these transitions have been poorly studied in the modern oceans because of the slow, subtle processes, the spread-out nature of the intersection with a gently dipping substrate, and the depth. Recent developments in the understanding of these processes reveal the critical impact they have on nutrient, heat, and energy cycling within the oceans as well as the movement of sediments. Awareness of the stratified nature of ancient basins would help to form scaffolding and context for the basinal data set. In light of recent work within strata of the Permian Basin, data from the Permian shallow slope indicating the presence of a pycnocline and from the deeper basin indicating bottom currents enable a model for a four-layer stratification of the water column. From the surface down, the strata consist of 1) a well-mixed surface layer roughly corresponding with the photic zone, 2) a thick, poorly mixed, intermediate layer, 3) a thin, warm, saline layer produced by sinking of salty water originating by evaporation on the shelf, and 4) a basal layer of cold saline water.

The boundary between the well-mixed surface layer and poorly mixed layer is indicated at the base of the Capitan Reef facies. Evidence is documented there of sigmoidal wedges of coated grain peloidal grainstone, glauconite, and platform directed cross bedded sand waves. These data indicate that a pycnocline impinged upon the slope and internal waves moved sediments and provided mixing for upwelling of OMZ nutrients (e.g. Jeffery 2022). The thin, warm saline layer is interpreted as being related to bottom currents formed by sinking saline water from the northwest shelf that resulted in contourite drifts such as those mapped in the Wolfcampian and Leonardian (e.g. Price et al., 2022). The depositional trend of traction transport following around the basin slope margin indicates that this layer was likely above a deeper layer in the middle of the basin that was denser, perhaps colder and or higher salinity.