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

Geologic Map of the Breckenridge Quadrangle, Summit and Park Counties, Colorado. The map shows distribution of various rock types and their configuration and provides information about potential resources and hazards. Includes cross section, map unit correlation, shaded relief map with geology overlay, booklet of extended descriptions of map units, economic geology and selected references. 44 pages. 1 color plate (1:24,000). Digital ZIP download. OF-02-07D

From the authors notes:
The Breckenridge quadrangle is located in southcentral Summit County and northwestern Park
County, Colorado, mostly west of the Continental Divide. The quadrangle includes numerous peaks that have elevations of 13,000 to more than 14,000 ft. The Breckenridge ski area is in the northern part of the intensely glaciated Tenmile
Range. Quandary Peak, at 14,265 ft above sea level, in the southwestern quadrant of the map area, is a popular peak for hikers and climbers. The town of Breckenridge is located in the northern part of the quadrangle. The Blue River flows
north through the middle of the map area in a glacial valley. The Breckenridge area has a long history of mining and mineral processing; lode mines produced gold, silver, zinc, lead, and copper, and placer mines produced large amounts
of gold. Nearly $300 million worth of metal were produced from the Breckenridge mining district at approximate current metal prices.

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.

The Breckenridge area has a well-documented history of placer and lode mining. Placer mines were active for one hundred years, from 1859 to 1959, with the greatest production occurring during the period 1906 to 1924. Lode
mining commenced in 1869 and eventually 25 to 30 mines produced ore of at least 1000 tons. The Wellington Mine, in French Gulch, was the most prolific producer, and gold, silver, lead and zinc were the principal metals extracted. None of the
metal mines in the quadrangle are currently active. Analysis of Quaternary deposits has identified four categories of geologic hazards in the Breckenridge quadrangle: (1) landslides and rock slides; (2) floods and debris flows; (3) abandoned mines and placer deposits; and (4) seismicity. Landslides are widespread in the map area, most commonly in till
and in glaciated cirques. Rock falls and rock slides are common in rugged valleys of the Tenmile Range. Areas of incipient slope failure are identified by “sackung” structures, which are deep seated tension gashes in bedrock caused by gravitational spreading on high ridges and mountains (Varnes and others, 1989). Floods are local hazards of known extent in the map area, but debris-flow hazards can occur in ephemeral streams or on Holocene alluvial fans. Abandoned mines pose a hazard from collapse, and areas that were dredged can form cavities at the surface. Mine water also poses a hazard because this water is usually acidic and can carry toxic elements. If it percolates into soil it may be corrosive to metal and concrete in foundations.

Author: Wallace, C.A., John W. Keller, James P. McCalpin, Paul J. Bartos, E. E. Route, Natalie N. Jones, Francisco Gutierrez, Cindy L. Williams, and Matthew L. Morgan. “OF-02-07 Geologic Map of the Breckenridge Quadrangle, Summit and Park Counties, Colorado.” Geologic. Open File Reports. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 2003.
Status: Digital
Year: 2005