NEW GEOLOGIC MAP OF THE SAGEHEN CREEK AND INDEPENDENCE LAKE HYDROLOGIC BASINS, NORTHEASTERN SIERRA NEVADA, CALIFORNIA

The UCSB 2007 summer field camp (13 students) mapped nearly 55 km2 in the northern Sierra Nevada where the only published comprehensive map was Waldemar Lindgren's 1895 USGS Truckee Folio. The UCSB map, compiled in ArcGeology format for ArcMap9, straddles the western boundary of the Tahoe-Truckee half-graben 10 km north of Truckee in the Independence Lake and Hobart Mills 7.5' quadrangles.

The rocks and alluvial deposits were subdivided into 30 map units. Cretaceous hornblende granodiorite (117 Ma; U-Pb) locally intruded Jurassic (?) hornfels. These bedrocks were uplifted, beveled, and cut by deep paleocanyons in late Cretaceous and Eocene time. Two rhyolitic ash flow tuffs of the Oligocene Valley Springs Formation were deposited in a paleocanyon that is the northern extension of one in Onion Valley 4 km west of Donner Pass. Erosional remnants of once extensive andesitic-dacitic lava flows and volcaniclastic deposits of the Mio-Pliocene Mehrten Formation lie upon the older rocks in upper Independence Lake basin. The lava flows include pyroxene andesite, hornblende andesite, and basaltic andesite, erupted from small plugs and domes. Pliocene or Pleistocene basalt flows overlie the andesitic flows in Sagehen Creek basin. Pleistocene drift from the Donner Lake, Tahoe, Tioga and Frog Lake glaciations, colluvium, and alluvium cover much of the bedrock in Sagehen Creek basin, but only in the lower half of Independence Lake basin.

The west parts of both basins are cut by pre- and post-Tahoe glaciation, northwest-striking normal faults, northeast side down with vertical separation of about 300 m. Faults were mapped on the basis of juxtaposition of dissimilar rocks, scarps, displaced streams, and alignment of springs and volcanic vents. The faults are part of the Medicine Lake-Tahoe deformation zone.

The presence of granitic enclaves and hornblende and plagioclase xenocrysts in andesitic lavas imply that 300 m-thick volcaniclastic deposits beneath Sagehen Creek basin are underlain by granodioritic bedrock.

Small lakes may have existed in the central reach of Sagehen Creek in Quaternary time, judging by a flight of four terraces along the creek, and by the presence of at least one meter of montmorillonitic clay beneath the fens. A pyroxene andesite flow crosses the three oldest terraces and suggests a Miocene age for the terraces.