A GEOMORPHIC STUDY OF THE FLUVIAL-INCISION HISTORY OF THE SAN MIGUEL RIVER FROM TELLURIDE TO PLACERVILLE, COLORADO

Following eruption of the Specie Mesa basalt at 605 ka there was a period when the dominant fluvial systems in the western San Juan Mountains flowed west off of the flanks of the western San Juan Mountains. In the middle to upper Pleistocene, incision of the San Miguel and Dolores Rivers coincided with a major shift in the transport directions of the drainage systems in the area.

Although previous studies provided general descriptions of fluvial terraces on the San Miguel, no comprehensive studies of terrace gravels were attempted. In this investigation, the sequence of terraces on the San Miguel River between Telluride and Placerville were mapped, and the deposits were studied to determine clast populations, provenance, and fluvial mechanisms. Three stages of river incision and terrace development were identified from distinct sets of strath terraces. Deposits that define the terraces sourced Paleozoic to Cenozoic sedimentary and igneous rocks exposed near Telluride, with an overall increase in contributions of fragments from post-70 Ma plutons in the lower terraces. This implies that there was a progressive entrenching of the headwaters into shallow plutons as the San Miguel River evolved.

The San Miguel River beheaded the post-605 ka west-flowing streams on the west flanks of the San Juan Mountains. Factors that may have led to the shift in drainage-transport directions is uncertain, but likely involved glacial-interglacial events or changes in base level of the regional Colorado River system. It is reasonable that over steepening by the “Cerro glaciation” of the lowlands on the flanks of the western San Juan Mountains gradually led to an early San Miguel pathway which became the dominant stream system after a major warming period. The timing of fluvioglacial events in the area are not well constrained, but incision of the San Miguel River likely happened before deposition of outwash terraces near Ridgway at ~80 ka. Field data, combined with pending ⁴⁰Ar/³⁹Ar age constraints on sanidine from terraces in Disappointment Valley, may provide insight into the timing of stream capture and an average rate of incision for this segment of the San Miguel River.