Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
West Texas Geological Society Fall Symposium, Midland, Texas, October 25-26, 2001
Evaporite Deposition in the Midland BasinEnd of an Era
Susan D. Hovorka
Salado evaporite deposition during the Ochoan was the final major event in the Paleozoic evolution of the Midland Basin. Basin-scale cross sections and isopach maps based on analysis of 555 wireline logs document two significant influences on sedimentary patterns: an early westward tilting toward the Delaware Basin followed by a phase of deposition focused near the current structural axis of the Midland Basin.
The lower part of the Salado Formation is characterized by thin, laterally continuous, high-frequency upward-shoaling cycles. These cycles are composed of (base to top) a thin insoluble residue, a sulfate bed (anhydrite or polyhalite) of variable thickness, and a thick interbedded halite and siliciclastic unit. Alternating halite and siliciclastics are interpreted as higher frequency flooding/desiccation cycles. This pattern is similar to marine-sourced high-frequency evaporite cycles throughout the Permian Basin. Gradual westward thickening of each cycle and subtle facies changes across the Midland Basin and Central Basin Platform document regional tilting toward the Delaware Basin. Similar cycles in much of the Salado of the Delaware Basin suggest that the entire area was an accommodation-limited shallow-water to exposed platform.
The cycle pattern in the upper part of the Salado Formation is disrupted and lithologic correlation more complex. The upper Salado depocenter is near the structural axis of the Midland Basin. Depositional patterns have been somewhat obscured by postdepositional salt dissolution; however, it is clear that basin geometry shifted dramatically from its earlier westward tilt. Sparse anhydrite and complex facies patterns suggest, by analogy with other Permian units, that evaporites may be partly nonmarine, having been recycled by intrabasinal dissolution.
D. Hovorka, Research Scientist