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 Recent debris flow studies in Colorado indicate that the state is most susceptible to debris flows that initiate from surface-water runoff that erodes and entrains hillslope and channel sediment. These runoff-initiated debris flows grow in size by entraining sediment along travel paths, thereby increasing their destructive potential. Yet, the mechanics of initiation, erosion, and entrainment processes for runoff-initiated debris flows are poorly understood. The steep, bedrock-dominated flanks of the formerly glaciated Chalk Creek Valley near Nathrop, Colorado, generate an average of two runoff-initiated debris flows per year, making the valley an ideal natural laboratory for debris-flow research. This two-day field trip to the Chalk Creek Valley will examine debris-flow initiation areas, transport zones, deposits, and the impact of large pulses of debris-flow sediment on the morphology of Chalk Creek. On the first day, participants will hike into a particularly active basin at Chalk Cliffs where debris flows are being monitored by the U.S. Geological Survey, the University of Colorado, and East Carolina University. The second day will focus on debris-flow deposits in Chalk Creek and on recent debris flows in and near the community of Alpine in the central part of the valley.

Early Proterozoic supracrustal rocks near Gunnison and Salida, Colorado, include sequences of tholeiitic metabasalt, metarhyolite to metadacite, and interbedded volcaniclastic turbidite. These rocks were intruded by synchronous gabbroic sheets, complexly folded and metamorphosed in upper greenschist to upper amphibolite facies, and intruded by plutons ranging from quartz diorite to granite. U-Pb ages of zircons show that an early period of volcanism in the Gunnison area occurred from 1,770 to 1,760 Ma and was followed by emplacement of plutons from 1,755 to 1,750 Ma. A younger sequence of volcanic rocks was formed in both the Gunnison and Salida areas between 1,740 and 1,730 Ma. In the Gunnison area these rocks were intruded by major plutons from 1,725 to 1,714 Ma. Near Howard, Colorado, southeast of Salida, metarhyolite yielding ages of 1,713 and 1,668 Ma is believed to be part of the younger sequence. Late, post-tectonic granite plutons were emplaced in both areas from 1,700 to 1,670 Ma. The age data, petrography, and geochemistry of these rocks indicates that they are part of a broad belt of juvenile, arc-related terranes, exposed from southern California across Arizona, New Mexico, Colorado, and known in the subsurface as far east as western Missouri, that was accreted to the southern edge of the continent during the Early Proterozoic.

The Wet Mountains–southern Front Range region is underlain by high-grade granitic gneiss, amphibolite, and schist of Early Proterozoic age. These rocks were intruded by granitic to granodioritic plutons during four episodes: one in the Early Proterozoic (1,660 to 1,700 Ma) and three in the Middle Proterozoic (1,485 to 1,440 Ma, 1,370 to 1,360 Ma, and about 1,060 Ma). We also report here a zircon age determination (536 ± 4 Ma) for syenite of the Cambrian McClure Mountain alkaline-mafic complex. The granitic gneiss was clearly formed before 1,700 Ma. Its protolith was probably pelitic to psammitic sedimentary rocks, in contrast to the volcanogenic rocks of this age farther west in the Gunnison and Salida areas of Colorado. The early Proterozoic plutons emplaced within the granitic gneiss are mostly somewhat younger than those emplaced within volcanogenic rocks to the west, although some are coeval. The middle Proterozoic rocks are representatives of the widespread “anorogenic granite-rhyolite suite” which is known in the St. Francois Mountains of Missouri and the subsurface of the midcontinent.

The Wall Mountain Tuff was deposited about 36 m.y. ago in paleovalleys extending from west of Salida to beyond Castle Rock, Colorado, a distance of at least 140 km. The Gribbles Run paleovalley, 16 km northeast of Salida, is exceptionally well exposed by modern dissection and reveals a complex interplay of primary and secondary structures formed during deposition of the Wall Mountain Tuff. Deposition occurred in laminar boundary layers between the bottom and sides of the channel and ash flows passing above. The overlapping sequence of events during deposition was (1) agglutination and incipient collapse of glassy particles, (2) laminar shearing of the compacting and welding mass to form a primary foliation analogous to flow banding in lavas, (3) expulsion of gases from the collapsing spongy mass and concentration of these gases along shear planes, (4) formation of gas pockets in places where the volume of gases expelled exceeded that which could be accommodated on shear planes, (5) elongation of gas pockets and pumice to form a primary lineation in the plane of the foliation, (6) statistical alignment of the long axes of solid particles parallel to the direction of flow and imbrication of the long axes so that they dip sourceward relative to the foliation planes, (7) development of primary flow folds with axes perpendicular to the lineation, and (8) end of forward motion. By the time forward motion ceased within a given layer, the tuff had the rheological properties of a rhyolite lava. The high viscosity of the welded tuff preserved open cavities and prevented differential compaction over lithic fragments or primary folds. More rapid deposition along the sides of the paleovalley than along its axis caused inward accretion of welded tuff with steep primary flow foliation to form a U-shaped cross-channel profile. Secondary folds, whose axes parallel the lineation, and concurrent growth faults formed locally by creep toward the valley axis. Spectacular internal unconformities developed where undeformed tuff was deposited over primary or secondary folds, yet all the tuff welded together to form a remarkably uniform simple cooling unit. Episodic downstream movements of a few metres, as the Wall Mountain Tuff adjusted to its bed and to its rising center of gravity, opened swarms of tension cracks along certain horizons. These tension cracks dip steeply in the downstream direction and provide a useful indicator of flow direction. Other structures useful in determining flow direction are imbricated crystals, streamlined ridges and grooves in lineated gas cavities, and upstream dip of axial planes on asymmetric primary folds. Emplacement temperature is the dominant factor in determining whether a tuff undergoes primary or secondary welding. If the temperature is well above the softening point, the glassy particles will agglutinate and collapse during deposition in the laminar boundary layer, and the tuff will show megascopic laminar flow structures (primary welding). If the temperature is below the softening point, deposition occurs as loose ash, and welding is a postemplacement process (secondary welding). Most tuffs undergo secondary welding and only the preferred orientation of solid particles and the grain-size distribution remain as evidence of laminar flow during deposition. The type of welding may vary both laterally and vertically within some ash-flow sheets and impose a significant facies variation.