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U.S. Geological Survey Scientific Investigations Map 3123



This digital publication contains all the geologic map information used to publish U.S. Geological Survey Scientific Investigations Map 3123 and additional photographs.

The San Juan Mountains in southwestern Colorado have long been known as a site of exceptionally voluminous middle 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, western calderas, and the central cluster. This study focuses on 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. The new mapping and 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; and (3) quantify the regional time-space-volume progression from the earlier Sawatch magmatic trend southward into the San Juan region. SIM 3123.







U.S. Department of the Interior
KEN SALAZAR, Secretary

U.S. Geological Survey
Marcia K. McNutt, Director