Lower and Middle Pennsylvanian strata in the Orogrande and Pedregosa Basins, New Mexico

Abstract

Continued field work from 1955 to the present shows that Lower and Middle Pennsylvanian limestones crop out in various mountain ranges over most of southwestern New Mexico and northern Chihuahua, representing a once continuous sheet from 100 to 600 m thick. A regional NE-SW cross section from the Sacramento to the Big Hatchet Mountains shows detailed microfacies and fusulinid control and demonstrates that cyclic Lower Pennsylvanian (Morrowan) oolitic strata pinch out northward across southern New Mexico. The cross section also shows that Middle Pennsylvanian (Derryan and Desmoinesian) strata are widely represented by limestones with upward-shoaling cyclothems (generally upward-coarsening sequences) that number about a dozen at each location. Although the cyclothems are not correlative from section to section, they are generally present in all localities.Petrologic studies show that the limestones are exclusively open marine and dominantly bioclastic lime wackestones and packstones. Tectonic stability during Early Pennsylvanian time caused the deposition of shallow-marine carbonates over wide shelves east and west of the Pedernal Uplift. In the Delaware Basin and on the west flank of the Pedernal Uplift terrigenous clastics are present. Small channels on the carbonate shelves, far west of the Pedernal Uplift, contain polymictic limestone-chert conglomerates. Quartzose sandstones at the base of the Pennsylvanian contain variable amounts of matrix, are often channeled, and may contain abundant terrestrial-plant fossils.An isopach map of Lower and Middle Pennsylvanian strata shows that the Orogrande Basin is asymmetric in shape with marked subsidence along its eastern margin adjacent to the Pedernal Uplift. An anomalously thin area in the west-central part of the basin makes the basin narrower during Early and Middle Pennsylvanian time than in Late Pennsylvanian time. The Florida Uplift appears to transect thickness contours of the Lower and Middle Pennsylvanian, suggesting that uplift postdated development of the extensive carbonate sheet. The extent of carbonate deposition on the positive area, and the timing of the uplift, is difficult to prove since all Pennsylvanian and lowermost Permian (lower Wolfcampian) strata have been removed. The great thickness of Lower and Middle Pennsylvanian carbonates present in southwestern New Mexico and Arizona diminishes southward into Mexico (Chihuahua and Sonora). Two widely separated outcrops near Minas Plomosas, Chihuahua, and Bavispe, Sonora, contain only about 150 m of fusulinid-bearing shelf carbonates. Presumably these areas are located on opposite sides of the Pedregosa Basin. With sparse control at present, it is difficult to show whether or not the Pedregosa Basin, which should be between these localities, actually existed this early in Pennsylvanian time.