Geologic mapping combined with the tephrochronology of spatially isolated sedimentary sections along the western Black Mountains piedmont adjacent the Death Valley fault zone (DVFZ) improves the late Cenozoic stratigraphy from relative age to correlated age. Pliocene tephra layers identified in Funeral Formation conglomerates at Artist Drive and Copper Canyon include a “Nomlaki-like” tephra bed (ca. 3.4 Ma), the tuffs of Mesquite Spring (3.1–3.3 Ma), and a tuff of the lower Glass Mountain family (1.86–1.92 Ma). We informally name the early(?) to middle Pleistocene Mormon Point formation1, which contains tephra layers correlated with the upper Glass Mountain/Bishop family of tephra layers (0.76–1.2 Ma), the Lava Creek B ash bed (ca. 0.66 Ma), and the Dibekulewe ash bed (ca. 0.51 Ma). Identification of these tephra layers indicates that the maximum age of the overlying and inset lacustrine gravel and alluvial fan deposits is 0.51 Ma.

The correlated age stratigraphy indicates that the dextral-oblique DVFZ has stepped basinward at Mormon Point and Copper Canyon since the late Pliocene. In contrast, during that same time the DVFZ at Artist Drive has not stepped basinward, but developed into a graben. The age of faulting on the low-angle (~19°–40°) Mormon Point turtleback fault is bracketed between 0.76 and 0.18 Ma, and the overlying Mormon Point formation shows no evidence of tilting, indicating slip on the turtleback fault was at a low-angle. Early Quaternary slip on the low-angle turtleback fault conflicts with the present versions of the pure shear, rolling-hinge, and detachment/rift models for Death Valley extension. Early Quaternary slip is most compatible with turtleback faults as folded or warped detachment fault. We propose that the warping is thermally driven and related to the Black Mountains igneous complex.