STRATIGRAPHIC ANALYSIS OF EOCENE RIVER SYSTEMS: IMPLICATIONS FOR THE CENOZOIC UPLIFT HISTORY OF THE SIERRA NEVADA, CALIFORNIA

Stratigraphic analyses of paleochannel deposits located on the northwestern slopes of the Sierra Nevada suggest that early Cenozoic landsculpting processes varied markedly and that Eocene river systems were capable of moving large amounts of coarse material. Such analyses contradict the conventional idea that Sierra Nevadan landscape was characterized by low elevation, low relief features formed by broad, low-gradient river systems prior to Neogene uplift. In fact, the nature of Eocene fluvial deposits in the northern Sierra Nevada is consistent with a landscape of higher elevation and paleostream systems that varied climatically. Material from deepest thalwegs of the ancestral Yuba River is dominated by well-rounded, cobbled-sized clasts suggesting that Eocene river systems had gradients sufficient to move large clasts appreciable distances. In at least four locations, rounded boulders with b-axis diameters of 1-3 m were present in these thalweg deposits. These sediments characterize a non-steady state Paleogene Sierra Nevada of significant elevation and relief. Additionally, paleostream deposits of the ancestral Yuba River reveal multiple episodes of downcutting and backfilling. Such variation in sedimentological regime requires a variation in stream power, something which is generally caused by either surface tilting or a change in discharge. A documented shift in Cenozoic climate toward cooler, stormier conditions (Molnar and England, 1990; Wolfe, 1994) would have a great impact on discharge and, consequently, stream power. A changing climatic regime, therefore, is capable of not only explaining the episodic erosional and depositional features preserved in paleovalleys, but also of explaining changing river gradient, independent of tectonic uplift or tilting.