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

IMAP 2799

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Abstract

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.

Geospatial Extents

Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Geologic Map of the Central San Juan Caldera Cluster, Southwestern Colorado
Series title:
IMAP
Series number:
2799
Edition:
Version 1.0
Year Published:
2006
Language:
ENGLISH
Contributing office(s):
Western Volcano Hazards Program
Description:
37 p.; 4 oversized sheets (3 map sheets); metadata files; photo collection
Scale:
24000
Additional Online Files(Y/N):
Y