THE TALES OF LATE CRETACEOUS TO TERTIARY GRAVELS IN SOUTHWESTERN COLORADO

In southwestern Colorado fluvial-derived gravel deposits reveal a history of rising mountains, ancient rivers, and igneous events from the Late Cretaceous to Miocene. Formation of the ~70 Ma La Plata Mountains laccolithic dome was marked by deposition of mega breccias, debris flows and alluvial deposits on its southern flanks in streams and flows moving to the southeast. This event was likely triggered by oversteepening of the dome which led to flank collapse, gravity slides, and possible minor volcanic eruptions that contributed debris to the McDermott Formation.

Rise of the San Juan uplift led to erosive stripping of Proterozoic-cored blocks at about the same time as the La Plata Mountains were forming. Conglomeratic detritus created in this event was transported to the southwest and south onto the southern margins of this uplift from the Paleocene to the Eocene, producing the Animas Formation and San Jose Formation. These deposits contain clasts of felsic to intermediate volcanic rocks, indicating an early episode of volcanism in the area of the present-day San Juan volcanic field. Later reactivation of east-west basement blocks generated reverse-stratified debris flows and fluvial gravel deposits that were transported northeast on the northwestern edge of the San Juan uplift forming the Telluride Conglomerate. The presence of andesitic to rhyolitic volcanic clasts at the base of the Telluride Conglomerate is interpreted as evidence of contemporaneous erosion and early volcanism in the San Juan volcanic field. Ancient gravel deposits thus indicate that volcanic activity in the region happened from 60 Ma to 20 Ma as compressive strain fields varied from west-northwest to north-northeast.

The latest deposition of gravels identified in field studies on the northern edge of the San Juan basin formed by south and east flowing debris flows and stream deposits that generated a thick veneer on the bedrock in the area. Clasts in these deposits reveal varied source areas, including the San Juan volcanic field. These Miocene (?) to Pleistocene deposits reveal an older drainage system that was modified by later south-oriented glacial-carved drainage basins.