AN ASSESSMENT OF THE DEPOSITIONAL HISTORY OF THE TELLURIDE CONGLOMERATE AND IMPLICATIONS FOR MID-TERTIARY TECTONIC-VOLCANIC EVENTS IN THE WESTERN SAN JUAN MOUNTAINS, COLORADO

Our detailed analysis of selected stratigraphic sections in the Telluride Conglomerate supports the hypothesis that this unit originated as debris flows transported from a highlands adjacent to the Needle Mountains (southeast to northwest) at the onset of Tertiary volcanism. No evidence was found to support the idea of Cross and Purington (1899) that the distal part of this system was dominated by lacustrine deposits.

The Telluride Conglomerate shows a distinct lateral change in grain size, sedimentary structure, and style of deposition from proximal to distal deposits. Proximal deposits are dominated by clast-supported boulder and cobble conglomerate with subordinate amounts of finer-grained material present. Distal deposits have approximately equal proportions of fine-grained and coarse-grained facies, and sections examined contain three distinct fining-upward sequences ranging from 20 to 50 meters in thickness. Individual distal sections of these fining-upward sequences display abundant scoured channels, planar and trough cross-beds, and dewatering structures in poorly sorted sandstones.

Vertical sections in the Telluride Conglomerate display a younger to older sequence of clast types (from bottom to top) reflecting regional unroofing of the uplifted Needle Mountains. Erosion was synchronous with high-energy fluvial events, with the trends and transitions of deposits in the Telluride Conglomerate indicating that eroded sediments were transported to the northwest. The occurrence of intermediate to felsic volcanic clasts in this unit suggest that eruptions had occurred before or during fluvial deposition. This suggests that either the volcanic record in the San Juan Mountains is older than previously reported, or that the Telluride Conglomerate is younger than currently documented.