OF-02-07 Geologic Map of the Breckenridge Quadrangle, Summit and Park Counties, Colorado

Access to the quadrangle from the north is via I-70 and State Highway 9 south through Frisco to Breckenridge. From the south, U.S. Highway 285 provides access to State Highway 9 to the north via Alma and Hoosier Pass.

The oldest rocks are exposed in the western part of the quadrangle where Early and Middle Proterozoic metamorphic and igneous rocks are widespread. Proterozoic rocks are overlain unconformably by Paleozoic clastic and carbonate sedimentary rocks, which are exposed mainly in the eastern part of the map area. Mesozoic rocks occur mainly in the northeastern part of the map area and are composed of shale and subordinate limestone and sandstone. Intermediate and siliceous Tertiary sills and dikes intrude Paleozoic and Mesozoic sedimentary rocks, and lode and disseminated mineral deposits are commonly associated with these intrusive bodies. In the western part of the map area, Proterozoic rocks are intruded by a Tertiary stock and numerous Tertiary dikes. Quaternary deposits are common in valleys and are dominated by till, outwash, and lacustrine deposits. The topography of the Tenmile Range was sculpted by at least two alpine glacial events. Northwest- and north-trending faults and shear zones that cut Proterozoic through Tertiary rocks, and in some cases Quaternary deposits, are the dominant structural feature of the Breckenridge quadrangle. At least two of these northwest- trending faults, in the western and northern parts of the map area, have large vertical separation. Many other faults in Phanerozoic rocks form a mosaic of intersecting faults of small separation. A less obvious but important east-northeast-trending zone of faults, dikes, and fracturing in the vicinity of the Humbug stock in the Tenmile Range likely represents a structural expression of the Colorado Mineral Belt and may have been active periodically since Proterozoic time. This zone appears to be continuous with several faults of similar trend in the Illinois Gulch area in the Breckenridge mining district east of the Blue River. Most faults east of the Blue River trend northeast, with a subordinate number of faults that trend northwest. Lower Paleozoic rocks rest unconformably over the Proterozoic basement and are exposed on east-facing dipslopes on the east side of the Tenmile Range only in the southern half of the quadrangle. In the eastern part of the map area the Upper Paleozoic and Mesozoic sequences are generally homoclinal, dipping eastward, except where small fault blocks have locally rotated beds to other orientations. Prominent folds of regional scale occur in Paleozoic rocks in the Blue River Valley; these folds are covered by Quaternary deposits. Metamorphic rocks in the Tenmile Range exhibit strong foliation that has a well-defined and consistent north-northeast orientation.