NEW GEOLOGIC MAP OF THE INDEPENDENCE LAKE AND HOBART MILLS 7.5’ QUADRANGLES, TRUCKEE AREA, NORTHERN SIERRA NEVADA, CALIFORNIA

Geologic mapping by UCSB undergraduate students reveals Cretaceous hornblende-biotite granodiorite (U-Pb age 117 ma), Miocene and Pliocene andesitic lava flows and volcaniclastic deposits, and Pleistocene basalt are the main rock types in the study area that are overlain locally by Pleistocene glacial drift and colluvium.

An Eocene paleocanyon in granodiorite contains sparse outcrops of Valley Springs rhyolite ignimbrite (early Miocene) erupted from calderas in central Nevada. Miocene pyroxene- and hornblende andesite crop out extensively over much of the map. Pyroxene andesite (⁴⁰Ar/³⁹Ar age 5.5 ma) overlies two fluvial terraces in Sagehen basin that are overlain by basaltic andesite (⁴⁰Ar/³⁹Ar age 4.5 ma), indicating that Sagehen has been a basin since Miocene time. The basaltic andesite is overlain, in turn, by a pair of Recent and modern fluvial terraces.

At least three stages of Pleistocene temperate valley glaciers deposited drift and erratics over much of the study area during Donner Lake, Tahoe and Tioga times. Independence Lake glaciers deposited moraines and outwash in the Little Truckee River drainage in the north part of the map area. Carpenter Valley glaciers deposited extensive moraines and outwash over the southeast part of the map. Donner Lake and Tahoe glaciers also deposited automobile-size granodiorite erratics onto the northwest and south parts of Sagehen basin.

Several northwest-striking normal faults cut the west part of the area before and after glaciation. They were mapped by juxtaposition of dissimilar rocks, subtle scarps, offset moraines, alignment of springs and volcanic vents, and LiDAR imagery. Inferred cumulative vertical separation is less 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 the northeastern Sierra Nevada. The active Polaris fault strikes NNW through the east half of the map, approximately coincident with Highway 89, with about 2,300 m of cumulative right slip.

Many springs lie along the base of the Tahoe moraine separating Sagehen basin from Independence Lake basin. Each spring is at or below the 2116 m elevation of the lake. Their distribution suggests that lake water seeps southeastward through or beneath the moraine to feed some of the Sagehen springs.