PRELIMINARY GEOLOGIC MAP OF THE GRASS LAKE AREA IN THE SOUTHERNMOST MOUNT TALLAC ROOF PENDANT; DESOLATION WILDERNESS AREA, ELDORADO COUNTY, CALIFORNIA

Preliminary geologic mapping of exposures of the Jurassic Tuttle Lake Formation in the vicinity of Grass Lake reveal complex crosscutting relationships within the southernmost Mount Tallac roof pendant. The Grass Lake study area is located approximately 8 km SW of Lake Tahoe. Here, the Tuttle Lake Fm. contains crossbedded strata of metamorphosed volcaniclastic conglomerate and breccia that dip gently to the southwest and are cut by at least five sets of dikes and the Keiths Dome pluton. The earliest four dike sets have intermediate compositions ranging from quartz andesite (D1), porphyritic amphibole dacite (D2), quartz diorite (D3), and porphyritic andesite (D4). The relative ages of these dike sets can be established with clear cross-cutting relationships. The Tuttle Lake Fm. and intermediate dikes are intruded by the Keiths Dome granodiorite pluton and a set of aplite dikes. Previous studies (Templeton and Hanson, 2003) suggest clasts in the Tuttle Lake Fm. were deformed during rapid deposition of unlithified sediments. However, the results of preliminary outcrop-scale strain analysis in the vicinity of Grass Lake suggest the local accumulation of considerable penetrative strains. Rocks in the vicinity of Grass Lake are also cut by arrays of 10 cm to 50 cm wide ductile shear zones, brittle-ductile faults, and brittle faults. These shear zones and faults strike NW-SE and dip steeply SW, sub-parallel to the contact with the Keiths Dome pluton, and appear to have a predominantly sinistral shear sense. Ductile shear zones, NW-SE striking joints, and clasts of the Tuttle Lake Fm. are affected by a Ca-Fe-rich alteration. Brittle faults typically follow pre-existing ductile shear zones, and are overprinted by a weak Fe-Mg-rich alteration (likely from secondary fluid enrichment of meteoric waters). Fe-Mg-rich alteration is especially well-developed along a 1 m to 2 m wide shear zone in the Tuttle Lake Fm. along the contact with the Keiths Dome pluton. Further detailed mapping is needed to resolve cross-cutting relationships among the intermediate dikes, the ductile shear zones and the Keiths Dome pluton. Similarly, additional study is needed to determine if the petrofabrics measured in the Tuttle Lake Fm. reflect tectonic strains or emplacement of the Keiths Dome pluton.