NEW CONSTRAINTS ON THE LATE CENOZOIC FILL HISTORY OF UNAWEEP CANYON, CO

Unaweep Canyon represents a geomorphic oddity incised into the modern Uncompahgre Plateau. Canyon formation has been attributed to Cenozoic fluvial incision and/or Cenozoic (Pleistocene) glaciation and more recently to Paleozoic glaciation and Cenozoic exhumation. Work on cores collected from within the inner Precambrian gorge indicates a thick sediment fill (329 m) comprising three units: a lower Paleozoic-aged diamictite, unconformably overlain by a middle (150+) lacustrine interval, and an upper (160+) conglomerate unit. The base of the lacustrine section is dominated by organic-rich sediments, ranging from 1 to 7% TOC, and implies, along with an absence of fauna, anoxic conditions. Grain size studies of the middle lacustrine unit reveal an overall coarsening upward trend, consistent with the infilling of a lake, and the provenance indicates an ancestral Gunnison River source for the sediment. The lacustrine interval grades upward into an approximately 8 meter series of paleosols that alternate between argillisols and well developed calcisols, indicating multiple stabilization surfaces on the order of 10,000+ years. Paleosols are intercalated and overlain by stream deposits not likely derived from the Gunnison River. The stream deposits are overlain by conglomerates formed by mass wasting of canyon walls that may also reflect a change in climate. These data are consistent with the hypothesis that Unaweep Canyon was carved glacially in the Paleozoic and exhumed by the Gunnison River during the Cenozoic. The present work suggests that after almost complete exhumation, the Gunnison River became blocked, leading to aggradation of a lake system and ultimate abandonment of the canyon during the Plio-Pleistocene, as inferred from regional incision rates. Ongoing paleomagnetic analyses of the middle lacustrine unit and tephrochronology on two inferred ash deposits within the upper conglomerate will help constrain the age of the lacustrine fill and timing of Gunnison River abandonment.