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Volcano Hazards Program

##### U.S. Geological SurveyGeologic Investigations Series I-2799Version 1.0

# Geologic Map of the Central San Juan Caldera Cluster,  Southwestern Colorado

### *By* Peter W. Lipman

### *Including the* Database for the Geologic map of the Central San Juan Caldera Cluster, Southwestern Colorado by Joel E. Robinson, Dillon R. Dutton, David W. Ramsey, Peter W. Lipman, and Tracey J. Felger

#### 2006

*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.

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### Files for Users

#### PDFs(Geologic map of the Central San Juan Caldera Cluster, Southwestern Colorado by Peter W. Lipman)

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)

#### Database(Database for the Geologic map of the Central San Juan Caldera Cluster, Southwestern Colorado by Joel E. Robinson, Dillon R. Dutton, David W. Ramsey, Peter W. Lipman, and Tracey J. Felger)

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: