About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 59 (1975)

Issue: 7. (July)

First Page: 1126

Last Page: 1135

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

Author(s): R. W. Suhm (2), R. L. Ethington (3)


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.

Pay-Per-View Purchase Options

The article is available through a document delivery service. Explain these Purchase Options.

Protected Document: $10
Internal PDF Document: $14
Open PDF Document: $24

AAPG Member?

Please login with your Member username and password.

Members of AAPG receive access to the full AAPG Bulletin Archives as part of their membership. For more information, contact the AAPG Membership Department at [email protected].