1: Exploring the ancient volcanic and lacustrine environments of the Oligocene Creede caldera and environs, San Juan Mountains, Colorado
Daniel Larsen, Peter Lipman, 2016. "Exploring the ancient volcanic and lacustrine environments of the Oligocene Creede caldera and environs, San Juan Mountains, Colorado", Unfolding the Geology of the West, Stephen M. Keller, Matthew L. Morgan
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The 26.9-Ma Creede caldera is a large resurgent ignimbrite-subsidence structure that hosts world-class epithermal Ag-Pb-Zn deposits set within the San Juan locus of the Southern Rocky Mountain volcanic field (SRMVF). This field guide reviews the understanding of the SRMVF and central San Juan magmatic locus and then provides a summary of the Oligocene Creede Formation, the caldera moat fill, and ancient Lake Creede, which occupied the caldera for ~100 k.y. after the caldera-forming eruption. The summary of the volcanic setting and Creede Formation provides a framework for discussing the hydrothermal system that deposited Ag-Pb-Zn ores in the Creede district ~1.8 m.y. after the caldera formed. This guide also highlights controversial aspects of the Lake Creede environment, such as the duration of the intracaldera sedimentation, proposed pseudomorphs after ikaite (CaCO3·6H2O) in the lake sediments and tufa, and the paleolimnology of ancient Lake Creede.
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Prepared in conjunction with the 2016 GSA Annual Meeting in Denver, Colorado, this volume contains sixteen guides to field trips in this rich geologic region. The four “Great Surveys” of the late 1800s ventured west to explore and document the region’s unknown natural resources and collect valuable geologic information. Many of the field guides in this volume, aptly titled Unfolding the Geology of the West, will cover the same hallowed ground as the early geologic expeditions. Organized into four sections, this volume spans some of the major subdisciplines of geology: (1) stratigraphy, sedimentology, and paleontology; (2) structure and metamorphism; (3) Quaternary landscape evolution; and (4) engineering and environmental geology.