GEOLOGIC CONSTRAINTS ON LANDSCAPE EVOLUTION OF THE SIERRA NEVADA, CALIFORNIA

The Sierra Nevada is the longest and tallest mountain chain in the coterminous U.S. It has also long been considered to be among the youngest, with uplift through Late Cenozoic tilting of a rigid block about faults along its eastern margin. More recent papers, however, have used a wide variety of laboratory techniques to propose a more complex uplift history for the Sierra Nevada, and some have argued for the antiquity of the range (Mesozoic). Two large NSF projects (SNEP and Sierra Drip CD Project) are currently applying mainly geophysical techniques to understanding the Sierra; yet some of the best constraints on our understanding of Sierran landscape evolution have come from field studies of dateable Cenozoic strata (e.g. Lindgren, 1911; Bateman and Wahrhaftig, 1966; Wakabayashi and Sawyer, 2001; Garside et al., 2005). We present new field and geochronologic data from Cenozoic strata in the central Sierra Nevada, largely preserved as volcanic-volcaniclastic fill of paleochannels, to provide constraints on Sierran landscape evolution.

Our detailed lithofacies mapping has revealed the presence of deep unconformities that record major paleochannel re-incision events: these can be closely bracketed in age by Ar/Ar dating of volcanic stratigraphy. We correlate these unconformities to infer that three episodes of uplift may have occurred in the central Sierra, in addition to Cretaceous uplift. The timing of these may correspond to: (1) onset of arc magmatism (≈15 Ma), (2) onset of Basin and Range faulting (≈10 Ma), and (3) arrival of the triple junction (≈ 6 Ma). We find evidence for greater local paleorelief and higher axial paleo-gradients than are inferred by previous workers. Furthermore, the paleocanyons in the central Sierra appear to be steep-walled. They are filled with braided stream deposits containing abundant large boulders, and lack meandering stream deposits, suggesting high axial gradients. To emphasize these differences, we refer to the central Sierran features we have studied as paleocanyons, rather than paleochannels.

The surface geology of the central Sierra does not appear to record root delamination, supporting a model for limited root removal (in the southern Sierra) rather than whole-Sierra removal.