Biostratigraphy of Blancan and Irvingtonian mammals in the Fish Creek-Vallecito section, southern California, and a review of the Blancan–Irvingtonian boundary

Abstract

ABSTRACT Rocks of the Palm Spring and uppermost Imperial formations in the Fish Creek-Vallecito Creek area of southern California preserve diverse Blancan and Irvingtonian land mammal faunas in stratigraphie superposition. The 4,300 m section of predominantly clastic rocks spans about 3.6 Ma, preserves a continuous paleomagnetostratigraphic record, and documents the local stratigraphic ranges of terrestrial mammals during the Blancan and Irvingtonian. Improved stratigraphic and paleontologic data suggest that the distinctions among the Layer Cake, Arroyo Seco, and Vallectio Creek local faunas no longer exist and that these terms be abandoned. The Fish Creek-Vallecito Creek section has the potential to serve as the Irvingtonian boundary stratotype. The Irvingtonian possesses characterization and identification, but not definition because no genus or species has been proposed to define the Irvingtonian boundary, a situation that must be rectified by field data. The present state of knowledge indicates that none of the possible boundary-defining taxa, Mammuthus, Smilodon, Euceratherium, and Lepus, are appropriate to define the boundary. Current data suggest that the Irvingtonian boundary lies between 1.95 and 2.15 Ma. Only the Fish Creek-Vallecito Creek area and the San Pedro Valley in Arizona provide long, continuous stratigraphic sections that span the Blancan-Irvingtonian boundary. Definition of the Irvingtonian and documentation of the biochron by a chronostratigraphic unit can be accomplished in these two sections.