LANDSCAPE EVOLUTION OF THE NORTHERN SIERRA NEVADA (U.S.A.): INSIGHTS FROM THE AMERICAN RIVER DRAINAGE

Oligocene and Miocene volcanic flows blanketed most of the central and northern Sierra Nevada leaving only few basement highs exposed. These deposits capped channel networks and topographic lows, and now form summits and tables on interfluves. Several Pliocene volcanic deposits west of the Sierra Nevada crest have been mapped and age-dated in the context of regional studies of volcanism and petrology. Field relations, chronology, and correlation of these Pliocene volcanic deposits help constrain models of the landscape evolution of the northern Sierra Nevada. Located west of the Sierran crest and the glacial extent, a 3.8 Ma basalt is inset into andesitic deposits of the Mehrten Formation, and the base of this basalt is more than 800 meters above the present-day river. On and to the west of the Sierran crest, Pliocene andesitic flow deposits compose several summit highs atop other late Cenozoic volcanic flows. Our preliminary analysis correlates a deposit located on the crest with a 3.3 Ma deposit to the west across a valley incised about 580 m below the base of these deposits. The disposition of these Pliocene volcanic flow deposits suggests: (1) approximately 800 meters of bedrock river incision occurred after 3.8 Ma, (2) incision into basement upstream did not begin until after the deposition of a 3.3 Ma andesitic flow, and (3) by 3.3 Ma, range-front faulting had not developed a topographic barrier that restricted the deposition of volcanic flows west of the crest from eastern sources.