Geologic map of the Cochetopa Park and North Pass Calderas, northeastern San Juan Mountains, Colorado

Lipman, Peter W.

Abstract:

The San Juan Mountains in southwestern Colorado have long been known as a site of exceptionally voluminous mid-Tertiary volcanism, including at least 22 major ignimbrite sheets (each 150-5,000 km³) and associated caldera structures active at 33-23 Ma. Recent volcanologic and petrologic studies in the San Juan region have focused mainly on several ignimbrite-caldera systems: the southeastern area (Platoro complex), western calderas (Uncompahgre-Silverton-Lake City), and the central cluster (La Garita-Creede calderas). Far less studied has been the northeastern San Juan region, which occupies a transition between earlier volcanism in central Colorado and large-volume younger ignimbrite-caldera foci farther south and west. The present map is based on new field coverage of volcanic rocks in seventeen 7.5‘ quadrangles in northeastern parts of the volcanic field, high-resolution age determinations for 120 new sites, and petrologic studies involving several hundred new chemical analyses. This mapping and the accompanying lab results (1) document volcanic evolution of the previously unrecognized North Pass caldera and the morphologically beautifully preserved but enigmatic Cochetopa basin, including unique features not previously described from ignimbrite calderas elsewhere; (2) provide evidence for a more rapid recurrence of large ignimbrite eruptions than previously known elsewhere; (3) quantify the regional time-space-volume progression from the earlier Sawatch magmatic trend southward into the San Juan region; and (4) permit more rigorous comparison between the broad mid-Tertiary magmatic belt in the western U.S. Cordillera and the type continental-margin arc volcanism in the central Andes.

Genre:

USGS Numbered Series

ProdID:

70040032

Citation Author:

Lipman, Peter W.

Citation Contributing Office:

Volcano Science Center

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Citation Language:

English

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Citation LatN:

0383000

Citation LatS:

0380000

Citation LonE:

-1062230

Citation LonW:

-1070000

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Citation Phsyical Description:

Pamphlet: ii, 48 p.; 2 Sheets: 46.4 x 47.2 inches and 38.2 x 47.3 inches; GIS Database

Citation Projection:

Lambert Conformal Conic projection

Citation Public Comments:

Data to accompany the map, by Joel E. Robinson and Peter W. Lipman. This GIS database contains all the geologic information used to publish the map, and additional photographs. Please see the Database link above for more information.