FAUNAL RESPONSE TO EMPLACEMENT OF THE EARLY PERMIAN LAST CHANCE ALLOCHTHON, EAST-CENTRAL CALIFORNIA

From Pennsylvanian to early Sakmarian (Early Permian) time the Bird Spring Shelf on the continental margin of east-central California was fringed on the west by the Keeler turbidite basin, part of an unstable borderland of basins and uplifts. In middle Sakmarian time, deep-water deposits of the Keeler Basin were thrust onto the western margin of the Bird Spring Shelf as part of the Last Chance allochthon. Emplacement of the allochthon from west to east resulted in a complex antiformal uplift, a new foreland turbidite basin (Darwin Basin), and retreat of the Bird Spring Shelf margin about 25 km east of its former position. By late Sakmarian time, turbidites more than 1.5 km thick built up in the foreland basin against the antiformal uplift. The lack of major conglomerates in these turbidites suggests that the uplift was mostly submerged, but reworked Keeler Basin fossils suggest the presence of islands.

By Artinskian time, the Bird Spring Shelf margin had receded farther eastward, and an isolated carbonate platform developed on the antiformal uplift. In late Artinskian(?) time an extremely rich, almost totally endemic fusulinid fauna developed on this platform, which is well exposed at Conglomerate Mesa in the southeastern Inyo Mountains. This fauna includes three new genera, two of which are completely endemic with 12 new species, two unusual unassigned forms, and two other new species assigned to previously named genera. This fauna bears almost no resemblance to those of similar age elsewhere on cratonal North America to the east and south, and it bears very limited similarity to the faunas of the Eastern Klamath Terrane to the west.

The great diversity of species on the carbonate platform at Conglomerate Mesa probably was due to a large number of environmental niches in and around this unusual tropical island. The high degree of endemism in the late Artinskian(?) is difficult to explain as the Bird Spring Shelf margin probably was less than 60 km to the east. It may, however, have been caused by restriction of carbonate-platform environments on the Bird Spring Shelf to the east, limiting habitats appropriate for fusulinids, the large distance from the Eastern Klamath Terrane to the west, and/or the direction of the prevailing currents.