SagehenAbstract

Cretaceous hornblende-biotite granodiorite (U-Pb age 117 ma), Miocene and Pliocene andesitic lava flows and volcaniclastic deposits, and Pleistocene basalt are the principal rock types in the study area. Pleistocene glacial drift, colluvium, and alluvium cover much of the bedrock in the south half of Sagehen Creek basin, whereas only the lower half of Independence Lake basin is so covered. At least five northwest-striking normal faults cut, or are inferred to cut, the west halves of each basin.
A small outcrop of pre-batholithic biotite hornfels, which is exposed extensively west and southwest of the study area, lies adjacent to granodiorite about 1 km southwest of Independence Lake. Both rock units are overlain there by sparse outcrops of Valley Springs rhyolite ignimbrite of Oligocene age. The ignimbrite erupted from calderas in central Nevada, then coursed westward down, and deposited in, Late Cretaceous and Eocene paleocanyons. Erosional remnants of once extensive andesitic lava flows and volcaniclastic breccia and tuff of the Mehrten Formation of Miocene and Pliocene age lie upon the Oligocene and older rocks in upper Independence Lake basin. Only the lava flows are exposed in Sagehen Creek basin; therefore, we infer that volcaniclastic rocks underlie most of the basin at no great depth. That granodiorite also underlies the basin is inferred from the presence of granitic enclaves and hornblende and plagioclase xenocrysts in some of the andesitic rocks in the Sagehen Hills. Basalt of presumed Pleistocene age underlies much of the north part of Sagehen Creek basin. Isolated outcrops of basaltic andesite in and north of the Sagehen Hills suggest extensive flows of that rock unit are present in the basin but are now obscured by colluvium and heavy vegetation.
All these rock units were eroded by at least three stages of Pleistocene temperate valley glaciers that deposited drift and erratics over much of the study area. Independence Lake experienced severe glacial erosion in Tahoe-time, judging from the extensive moraine and drift at the north end of the lake. Much smaller glaciers deposited moraines in Tahoe, Tioga, and Frog Lake times at the head of Sagehen Creek. Pre-Tahoe and Tahoe glaciers spilled into Sagehen Creek basin from Independence Lake and Carpenter Valley, respectively, and deposited granodiorite erratics onto the northwest and south parts of Sagehen Creek basin.
A system of northwest-striking normal faults cut the west parts of the two basins before and after glaciation. They were mapped on the basis of juxtaposition of dissimilar rocks, scarps, displaced streams, and alignment of springs and volcanic vents. Vertical separation across the system is inferred to be no more than 500 m, northeast side down. The faults are part of the Medicine Lake-Tahoe deformation zone and represent the westward incursion of Basin and Range faulting into this part of the northeastern Sierra Nevada.
Small lakes may have existed along 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. The dam for the lakes was probably a now breached paleo-landslide near the present intersection of the creek and Highway 89.
Several springs lie along the base of the Tahoe moraine that separates Sagehen Creek basin from Independence Lake basin. Each is at or below the elevation 6944' (2116 m) of Independence Lake. Their distribution relative to the geology and location of the lake suggest that groundwater from the lake seeps southeastward through or beneath the moraine to feed some of the springs in Sagehen Creek basin.