USGS-EDMAP STUDY OF TERTIARY PALEOGEOGRAPHY AND PALEOTECTONIC EVOLUTION OF THE SIERRA NEVADA IN THE CARSON PASS-KIRKWOOD AREA

Tertiary volcanic and sedimentary rocks along California Route 88 from the Sierra Nevada crest at Carson Pass, west through the Kirkwood Valley ski area to Silver Lake occupy a paleocanyon cut into Mesozoic granitic rocks. As the canyon filled, it was reincised seven times, producing seven unconformity-bounded sequences . The lowest five sequences are dominantly braided stream and minor debris flow deposits with silicic to intermediate-composition volcanic clasts. These clasts and interstratified ignimbrites were derived largely from volcanic centers to the east in Nevada, prior to initiation of Sierra Nevada range-front faulting. The upper two sequences are dominated by andesitic volcanic debris flow deposits, interstratified with andesitic to dacitic lava domes, flow breccias, block-and-ash-flow tuffs and peperite intrusions, as well as basalt lava flows. The upper two sequences thus record the westward sweep of the arc from Nevada into the present-day Sierra crest, and development of high-relief centers that probably blocked volcanic and sedimentary supply from the east. We map no significant faults in the region. On north side of Carson Pass, a N-S trending paleocanyon wall cut into metamorphic basement and filled by sequence 2 was mistaken for an active fault by previous workers. Ar/Ar dating in progress on volcanic rocks of sequences 2, 4, 6 and 7 will constrain the timing of the seven canyon reincision events. Some of the unconformities (3 and 7) cut through all older strata into granitic basement, suggesting regional tectonic controls on their formation.