Surficial Geologic Map of the Upper Conejos River Drainage, Southeastern San Juan Mountains, Southern Colorado, USA

Abstract

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During the Last Glacial Maximum (LGM), the San Juan Mountains of southern Colorado, USA were covered by one of the largest ice caps in North America. The deposits formed subsequent to LGM retreat provide a record of the interaction between post-LGM climate change and late Quaternary landscape evolution. In order to determine the role of post-LGM climate change in alpine landscape modification, a high resolution surficial geologic map was produced for the four primary tributaries and the main stem of the upper Conejos River watershed in the southeastern San Juan Mountains.

Soil development, ¹⁴C ages, and stratigraphic relationships provide evidence for three distinct periods of hillslope, alluvial fan and/or stream terrace deposition (∼9.5–13 k.y.a., ∼1.2–2.1 ybp, and modern) in the upper Conejos River watershed. We interpret the first of these periods to be the result of paraglacial hillslope adjustment. The lack of subsequent early to mid-Holocene deposits suggests that an interval of relatively warm climate coincided with landscape stability. We attribute late-Holocene deposition to an interval of cooling that has been documented in nearby proxy records. Modern deposits in the field are limited both in extent and occurrence, and are interpreted to be the result of erosion localized outcrops of soft, volcaniclastic bedrock.

In addition to post-LGM depositional units, relatively flat erosional bedrock surfaces that sit 1–100 m above the modern stream, and that are capped with glacial till, are present in all four tributaries of the field area. These are interpreted as former glacial valley bottoms. The modern streams are incised into the floors of these hanging valleys. Headward erosion presumably initiated following deglaciation and is still active today. Modern channel long profiles differ between the four tributaries. We attribute the variability of incision into these hanging valleys to differences in basin size and bedrock erodibility, with more extensive incision occurring in basins characterized by more erodible rocks (poorly welded volcaniclastics) and larger basin size. Overall we see that the post-LGM landscape evolution in this alpine environment is the result of the complex interaction of Holocene climate change acting on basins of varying size and bedrock type.