Volcano Hazards Program
Wheeler Geologic Monument (Half Moon Pass quadrangle) provides exceptional exposures of three outflow tuff sheets erupted from the San Luis caldera complex. (Photograph by P.W. Lipman)
The San Juan Mountains are the largest erosional remnant of a composite volcanic field that covered much of the southern Rocky Mountains in middle Tertiary time. The San Juan field consists mainly of intermediate-composition lavas and breccias, erupted about 35-30 Ma from scattered central volcanoes (Conejos Formation) and overlain by voluminous ash-flow sheets erupted from caldera sources. In the central San Juan Mountains, eruption of at least 8,800 km3 of dacitic-rhyolitic magma as nine major ash flow sheets (individually 150-5,000 km3) was accompanied by recurrent caldera subsidence between 28.3 Ma and about 26.5 Ma. Voluminous andesitic-dacitic lavas and breccias erupted from central volcanoes prior to the ash-flow eruptions, and similar lava eruptions continued within and adjacent to the calderas during the period of more silicic explosive volcanism. Exposed calderas vary in size from 10 to 75 km in maximum dimension; the largest calderas are associated with the most voluminous eruptions.
Download the 37-page pamphlet for this map (i2799pamphlet.pdf; 1.6 MB)
Download Sheet 1 (north) as a ~47" x 33" PDF file (i2799_sheet1.pdf; 19 MB)
Download Sheet 2 (middle) as a 48" x 32" PDF file (i2799_sheet2.pdf; 12.7 MB)
Download Sheet 3 (south) as a 48" x 42" PDF file (i2799_sheet3.pdf; 36.3 MB)
Download Sheet 4 (map units) as a 42" x 40" PDF file (i2799_sheet4.pdf; 36.3 MB)
This section contains all the geologic map information used to publish U.S. Geological Survey Geologic Investigations Map Series I-2799 and additional photographs. The main component of this digital release is a geologic map database prepared in ArcInfo GIS.
This digital database was compiled with ArcInfo version 7.2.2 and consists of files containing geologic map and topographic base data in ArcInfo coverage format. ArcInfo-compatible GIS software is therefore required to use the files of this database. The database consists of parts of 29 24,000 scale USGS topographic quadrangles. For this data relase, the individual quadrangles were merged to correspond to the printed map sheets and stored as WinZip files. Each zip file contains the metadata and the following layers: quadrangle boundaries, geology, struture, alteration, and caldera boundaries. The scale of the source map limits the spatial resolution (scale) of the database to 1:24,000. The topographic bases are raster files in ArcInfo Grid format. Each base is a 1000 dpi scan of the 1:50,000 county bases for Colorado.
Available ArcInfo coverages and grids are described below:
|Map Sheet||Quadrangles||WinZip File|
|Sheet 1 (north)||Baldy Cinco||sheet 1.zip||Metadata||Cannibal Plateau||Elk Park||Half Moon Pass||Mesa Mountain||Mineral Mountain||San Luis Peak||Saguache Park||Slumgulion Pass||Stewart Peak|
|Sheet 2 (middle)||Bristol Head||sheet 2.zip||Metadata||Creede||Hermit Lakes||Lake Humphreys||Little Squaw Creek||Pool Table Mountain||Spar City||South Fork West||Wagon Wheel Gap||Workman Creek|
|Sheet 3 (south)||Beaver Creek Reservoir||sheet 3.zip||Metadata||Cimarrona Peak||Mount Hope||Pagosa Peak||Palomino Mountain||Saddle Mountain||South River Peak||Wolf Creek Pass|
View the photograph collection (22 MB total).
Below are some links to on-line abstracts relating to the San Juan caldera cluster.
Chemical Analyses Of Tertiary Volcanic Rocks, Central San Juan Caldera Complex, Southwestern Colorado - USGS Open-File Report 2004-1194
Petrologic evolution of the San Juan volcanic field, southwestern Colorado: Pb and Sr isotope evidence - Geological Society of America Bulletin: Vol. 89, No. 1, pp. 59-82, 1978.
Recurrent eruption and subsidence at the Platoro caldera complex, southeastern San Juan volcanic field, Colorado: New tales from old tuffs - Geological Society of America Bulletin: Vol. 108, No. 8, pp. 1039-1055, 1996.
Comagmatic granophyric granite in the Fish Canyon Tuff, Colorado: Implications for magma-chamber processes during a large ash-flow eruption - Geology: Vol. 25, No. 10, pp. 915-918, 1997.
The Fish Canyon Magma Body, San Juan Volcanic Field, Colorado: Rejuvenation and Eruption of an Upper-Crustal Batholith - Journal of Petrology, Vol. 43, No. 8, pp. 1469-1503, 2002.
Rejuvenation of the Fish Canyon magma body: A window into the evolution of large-volume silicic magma systems - Geology: Vol. 31, No. 9, pp. 789-792, 2003.
Contemporaneous Trachyandesitic and Calc-alkaline Volcanism of the Huerto Andesite, San Juan Volcanic Field, Colorado, USA - Journal of Petrology, Vol. 46, No. 5, pp. 859-891, 2005
For questions about the content of this map, contact Peter Lipman. For questions about the database, contact Joel Robinson.
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