Geologic map of the Frisco quadrangle, Summit County, Colorado

New 1:24,000-scale geologic mapping along the Interstate-70 urban corridor in western Colorado, in support of the USGS Central Region State/USGS Cooperative Geologic Mapping Project, is contributing to a more complete understanding of the stratigraphy, structure, tectonic evolution, and hazard potential of this rapidly developing region. The 1:24,000-scale Frisco quadrangle is near the headwaters of the Blue River and straddles features of the Blue River graben (Kellogg, K.S., 1999, Neogene basins of the northern Rio Grande rift?partitioning and asymmetry inherited from Laramide and older uplifts: Tectonophysics, v. 305, p. 141-152.), part of the northernmost reaches of the Rio Grande rift, a major late Oligocene to recent zone of extension that extends from Colorado to Mexico. The Williams Range thrust fault, the western structural margin of the Colorado Front Range, cuts the northeastern corner of the quadrangle.

The oldest rocks in the quadrangle underlie the Tenmile Range and include biotite-sillimanite schist and gneiss, amphibolite, and migmatite that are intruded by granite inferred to be part of the 1,667-1,750 Ma Routt Plutonic Suite (Tweto, Ogden, 1987, Rock units of the Precambrian- basement in Colorado: U.S. Geological Survey Professional Paper 1321-A, 54 p.). The oldest sedimentary unit is the Pennsylvanian Maroon Formation, a sequence of red sandstone, conglomerate, and interbedded shale. The thickest sequence of sedimentary rocks is Cretaceous in age and includes at least 500 m of the Upper Cretaceous Pierre Shale. The sedimentary rocks are intruded by sills and dikes of dacite porphyry sills of Swan Mountain, dated at 44 Ma (Marvin, R.F., Mehnert, H.H., Naeser, C.W., and Zartman, R.E., 1989, U.S. Geological Survey radiometric ages, compilation ?C??Part five?Colorado, Montana, Utah, and Wyoming: Isochron/West, no. 53, p. 14-19. Simmons, E.C., and Hedge, C.E., 1978, Minor-element and Sr-isotope geochemistry of Tertiary stocks, Colorado mineral belt: Contributions to Mineralogy and Petrology, v. 67, p. 379-396.). Surficial deposits include (1) an old, deeply dissected landslide deposit, possibly as old as Tertiary, on the south flank of Tenderfoot Mountain, (2) deeply weathered, very coarse gravel deposits, mostly along Gold Run and underlying Mesa Cortina; the gravels are gold bearing and were mined by hydraulic methods in the 1800's, (3) glacial deposits of both Bull Lake (middle Pleistocene) and Pinedale (late Pleistocene) that were derived from large valley glaciers that flowed down Tenmile and North Tenmile Creeks; the town of Frisco is underlain mostly by Pinedale-age glacial outwash, (4) recent landslide deposits, including one large (about 1 square kilometer) area just downslope from Lilly Pad Lake, west of I-70, and (5)extensive colluvial and alluvial deposits.

The latest seismic events appear to be middle Pliestocene in age and are associated with small scarps that cut Bull Lake till but do not cut Pinedale till.