New Mexico Geological Society Annual Spring Meeting — Abstracts

Tuffaceous and alluvial sedimentary rocks, pyroclastic rocks, and a variety of intermediate to felsic lavas are commonly observed to fill moat depressions between the topographic margin and resurgent dome of modern and ancient resurgent calderas. The sedimentary fill of the Creede caldera in the central San Juan Mountains in southwest Colorado has been studied in detail in outcrop and scientific drill core. Similar sequences have been examined in other calderas in the san Juan Mountains, and in the Bursum and Emory cauldrons in New Mexico.

The oligocene Creede Formation is the sedimentary fill of the Creede caldera in the Tertiary San Juan volcanic field in southern Colorado. Following emplacement of ash-flows and ash-fallout from the caldera-forming eruption, up to 121 meters of coarse grained debris-flow strata and rockfall debris were deposited along with interstratified basinward ephemeral lake deposits. Subsequently, up to 13 meters of lacustrine margin strata were deposited, followed by up to 550 meters of turbiditic and laminated lacustrine strata with numerous 1 to 15 meter thick ash-fallout tuffs. The fallout tuffs are of broadly similar compositions but tend to progress from dacitic to rhyolitic compositions through time. The tuffs are thought to have been derived from dacitic to rhyolitic ring-fraction lava domes that interfinger with sedimentary rocks, and ash-flow and ash-fallout tuffs in the southern part of the caldera. Back-filled alluvial valleys in the walls of the caldera suggest that the caldera filled with approximately 1025 meters of sedimentary and volcanic strata. Intracaldera fill of the Platoro and Uncompahgre calderas also contain thick deep water lacustrine strata but are dominated by intermediate to felsic lavas and outflow ash-flow tuffs from later calderas.

Intracaldera fill of the Bursum and Emory (south) cauldrons is dominated by ring-fracture lava domes and ash-flow with intercalated alluvial
sedimentary rocks draped against the topography of the domes. Lacustrine strata are of minor significance and represent only shallow water facies. Similar intracaldera fill sequences are described for the Muir, Geronimo Trail, and Turkey Creek (AZ) cauldrons.

Broad similarities exist between all intracaldera sedimentary fill sequences examined, however the thick lacustrine sequences that are prominent in some San Juan Mountain calderas are absent in those of southwestern New Mexico. This discrepancy may be partially explained by climatic differences but may also be due to differing physiographic expression of calderas in the two volcanic fields.