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Scientific Investigations Map 3130

Geologic Map of the Fraser 7.5-minute Quadrangle, Grand County, Colorado

By Ralph R. Shroba, Bruce Bryant, Karl S. Kellogg, Paul K. Theobald, and Theodore R. Brandt

Thumbnail of and link to SIM 3130 Pamphlet (1.8 MB)

The geologic map of the Fraser quadrangle, Grand County, Colo., portrays the geology along the western boundary of the Front Range and the eastern part of the Fraser basin near the towns of Fraser and Winter Park. The oldest rocks in the quadrangle include gneiss, schist, and plutonic rocks of Paleoproterozoic age that are intruded by younger plutonic rocks of Mesoproterozoic age. These basement rocks are exposed along the southern, eastern, and northern margins of the quadrangle. Fluvial claystone, mudstone, and sandstone of the Upper Jurassic Morrison Formation, and fluvial sandstone and conglomeratic sandstone of the Lower Cretaceous Dakota Group, overlie Proterozoic rocks in a small area near the southwest corner of the quadrangle. Oligocene rhyolite tuff is preserved in deep paleovalleys cut into Proterozoic rocks near the southeast corner of the quadrangle. Generally, weakly consolidated siltstone and minor unconsolidated sediments of the upper Oligocene to upper Miocene Troublesome Formation are preserved in the post-Laramide Fraser basin. Massive bedding and abundant silt suggest that loess or loess-rich alluvium is a major component of the siltstone in the Troublesome Formation. A small unnamed fault about one kilometer northeast of the town of Winter Park has the youngest known displacement in the quadrangle, displacing beds of the Troublesome Formation.

Surficial deposits of Pleistocene and Holocene age are widespread in the Fraser quadrangle, particularly in major valleys and on slopes underlain by the Troublesome Formation. Deposits include glacial outwash and alluvium of non-glacial origin; mass-movement deposits transported by creep, debris flow, landsliding, and rockfall; pediment deposits; tills deposited during the Pinedale and Bull Lake glaciations; and sparse diamictons that may be pre-Bull Lake till or debris-flow deposits. Some of the oldest surficial deposits may be as old as Pliocene.

First posted October 5, 2010

For additional information contact:

USGS Geology and Environmental Change Science Center
Box 25046, Mail Stop 980
Denver, CO 80225

http://esp.cr.usgs.gov/

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Suggested citation:

Shroba, R.R., Bryant, Bruce, Kellogg, K.S., Theobald, P.K., and Brandt, T.R., 2010, Geologic map of the Fraser 7.5-minute quadrangle, Grand County, Colorado: U.S. Geological Survey Scientific Investigations Map 3130, 1 sheet, 26 p. pamphlet, scale 1:24,000.


Contents

Introduction

Description of Map Units

Surficial Deposits

Artificial Fill Deposits

Alluvial Deposits

Alluvial and Mass-Movement Deposits

Mass-Movement Deposits

Glacial Deposits

Bedrock Units

Post-Laramide Sedimentary and Volcanic Rocks

Pre-Laramide Sedimentary Rocks

Proterozoic Rocks

Eolian Silt Contributions to the Troublesome Formation in the Fraser Basin

Structural Setting of the Fraser 7.5-minute Quadrangle

Geologic History of the Fraser 7.5-minute Quadrangle

Proterozoic History

Paleozoic through Laramide History

Post-Laramide Cenozoic History

Acknowledgments

References Cited


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