Stratigraphy and Conodonts of Simpson Group (Middle Ordovician), Beach and Baylor Mountains, West Texas

Abstract

The Simpson Group is a well-known petroleum-bearing unit that ranges up to 2,300 ft in thickness in the subsurface of West Texas. No published accounts prior to this study definitely acknowledge the presence of outcrops of Simpson rocks in that area. However, a close inspection of the interval between the outcropping thick carbonate rocks of the El Paso Group (Lower Ordovician) and those of the Montoya Group (Middle-Upper Ordovician), from two measured sections in the Beach and Baylor Mountains, revealed the presence of part of the Simpson Group. Conodonts were collected systematically from these beds and found to be correlative, in part, with the lower Simpson in the Arbuckle Mountains of Oklahoma. Multioistodus compressus, the most diagnostic conodont, with Chosonodina lunata, Acodus campanula, Acontiodus curvatus, and Scandus sinuosus, among others, indicate a Joins to Oil Creek assignment. The persistent argillaceous dolomites and quartz sandstones of the lower Simpson reflect deposition in a shallow-marine depression called the Tobosa basin. This elliptical depression was separated from the Oklahoma basin by the partly emergent Texas peninsula. Warping of the Tobosa basin, concurrent with regional emergence on the Mid-Continent at the end of Simpson deposition, resulted in erosion of the Simpson and the unconformable overlap of Montoya strata; in any traverse from the basin center to the basin margin, progressively older Simpson strata are overstepped by the Montoya. This additionally verifies that the Simpson in Beach and Baylor Mountains (at the margin of the Tobosa basin) is lower Simpson.