Stratigraphy and paleogeographic significance of the Pennsylvanian-Permian Bird Spring Formation in the Ship Mountains, southeastern California

A thick sequence of limestone, dolomite, and minor sandstone assigned to the Pennsylvanian and lower Permian Bird Spring Formation is exposed in the Ship Mountains about 85 kilometers (km) southwest of Needles, California, in the eastern Mojave Desert. These strata provide a valuable reference section of the Bird Spring Formation in a region where rocks of this age are not extensively exposed. This section, which is about 900 meters (m) thick, is divided into five informal members.

Strata of the Bird Spring Formation in the Ship Mountains originated as shallow-water marine deposits on the broad, southwest-trending continental shelf of western North America. Perpendicular to the shelf, the paleogeographic position of the Ship Mountains section is intermediate between those of the thicker, less terrigenous, more seaward section of the Bird Spring Formation in the Providence Mountains, 55 km to the northwest, and the thinner, more terrigenous, more landward sections of the Supai Group near Blythe, 100 km to the southeast. Parallel to the shelf, the Ship Mountains section is comparable in lithofacies and inferred paleogeographic position to sections assigned to the Callville Limestone and overlying Pakoon Limestone in northwestern Arizona and southeastern Nevada, 250 km to the northeast.

Deposition of the Bird Spring Formation followed a major rise in eustatic sea level at about the Mississippian- Pennsylvanian boundary. The subsequent depositional history was controlled by episodic changes in eustatic sea level, shelf subsidence rates, and sediment supply. Subsidence rates could have been influenced by coeval continental-margin tectonism to the northwest.