Abstract

In this investigation, field observations and geomorphic reconstructions of stratigraphic-time surfaces test ideas about post-Laramide landscape evolution in the western San Juan Mountains. Our findings offer further insight into the nature and relations of processes that crafted geologic events over the past ∼35 million years.

Laramide uplift and magmatism (75–60 Ma) was followed by a protracted period of fluvial incision prior to ∼35 Ma. The landscape was mostly sculpted into undulating terrain with less than 200 feet (∼60 meters) of relief, but rugged canyons up to 1,500 feet (460 meters) deep were carved near Ouray. This was followed by deposition of the 34 to 32 Ma Telluride Conglomerate in westward-flowing rivers that formed in response to rapid tectonic uplift and erosional unroofing of highlands to the east and southeast. Eruptions from stratovolcanoes between 32 to 30 Ma deposited thick successions of the San Juan Formation conformably on the Telluride Conglomerate and infilled pre-Telluride canyons. The highlands that emerged prior to deposition of the Telluride Conglomerate remained elevated in the Oligocene as revealed by thinning of the San Juan Formation along their flanks.

The paleogeographic record after 30 Ma in the western San Juan Mountains chronicles episodic tectonic uplift coupled with magmatic inflation that warped and tilted the pre-Telluride erosional surface and overlying San Juan Formation eastward. Emplacement of 27 to 4 Ma plutons contributed to regional doming and uplift especially in the San Miguel and Rico Mountains where magmatic inflation was ∼3,000 feet (∼915 meters). Recurrent tectonic uplift after 35 Ma was focused in the Needle Mountains. The driving forces for this tectonism are not well constrained but were possibly related to upwelling of low-velocity asthenosphere into the crust from latest Paleogene to Neogene, particularly in the past 10 to 6 million years.