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 Special thanks are due Jim Points, general r.1anager, and Charles Ponchak, chief geologist, for allowling use of maps and records of past and present development and production at Camp Bird mine. Ihformation for this paper has been taken freely from several sources including the author's own work at Camp Bird as geologist. Sources used are Langston, D.J., 1978 and Paul, A.H.; 1974, Colorado School of Mines M.S. theses (unpublished), and Burbank, w.s., 1941, Colorado Sci. Soc. Proc., vol. 14, no. 5, p. 141–261.