1.08 Ga diabase sills in the Pahrump Group, California: Implications for development of the Cordilleran miogeocline

Abstract

U-Pb baddeleyite ages of 1087 ±3 and 1069 ±3 Ma for twodiabase sills that intrude the Crystal Spring Formation of the Pahrump Group, southeastern California, provide a minimum estimate for the depositional age of the Crystal Spring Formation. The new ages high-light the potential range in deposition ages for the Pahrump Group, from ≥1087 Ma to possibly as young as 600 Ma for Kingston Peak glaciogenic sediments. Consequently, there probably are significant disconformities within the Pahrump Group that separate unrelated, tectonically distinct episodes of basin subsidence. The Crystal Spring and Beck Spring Formations are considered to be platform-cover rocks preserved as a result of block faulting during deposition of the upper Kingston Peak Formation. The age of the Pahrump sills is in excellent agreement with the age of sills that intrude the Apache Group (central Arizona) and the Unkar Group (Grand Canyon) and supports earlier correlations of those units.