Abstract The Southern Rocky Mountain volcanic field contains widespread andesite and dacitic lavas erupted from central volcanoes; associated with these are ~26 regional ignimbrites (each 150–5000 km 3 ) emplaced from 37 to 23 Ma, source calderas as much as 75 km across, and subvolcanic plutons. Exposed plutons vary in composition and size from small roof-zone exposures of porphyritic andesite and dacite to batholith-scale granitoids. Calderas and plutons are enclosed by one of the largest-amplitude gravity lows in North America. The gravity low, interpreted as defining the extent of a largely concealed low-density silicic batholith complex, encloses the overall area of ignimbrite calderas, most of which lack individual geophysical expression. Initial ignimbrite eruptions from calderas aligned along the Sawatch Range at 37–34 Ma progressed southwestward, culminating in peak eruptions in the San Juan Mountains at 30–27 Ma. This field guide focuses on diverse features of previously little-studied ignimbrites and caldera sources in the northeastern San Juan region, which record critical temporal and compositional transitions in this distinctive eastern Cordilleran example of Andean-type continental-margin volcanism.

Abstract 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 (CaCO 3 ·6H 2 O) in the lake sediments and tufa, and the paleolimnology of ancient Lake Creede.