PRELIMINARY RESULTS FROM A SEDIMENTOLOGICAL STUDY OF AN EARLY PLEISTOCENE UPLAND LAKE (UNAWEEP CANYON, CO)

Recent recovery of a 161 m core in Unaweep Canyon, Colorado revealed lacustrine sedimentation of a former “Lake Unaweep,” potentially formed as a result of mass wasting that blocked the ancestral Gunnison River. Using previous work, this core is hypothesized to capture a record that dates from ~1.4-1.3 Ma, enabling a glimpse of the early Pleistocene— a time interval rarely captured in an upland setting of the greater Rocky Mountains. The lacustrine section begins atop inferred ancestral Gunnison River gravels, and -overall- comprises a series of mass flows that decrease in thickness (2 to 60 cm) upward. The basal ~19 m consist of oxidized (red), graded (granules to fine sand) mass flows yielding upward to a sub-equal proportion of silt/clay. Above this is a ~16 m interval of thinner (2-10 cm) and finer mass flows exhibiting ochre (less oxidized) colors with pink/white clay caps. Next is a reduced interval (~48.5 m) with mass flows exhibiting basal loading, convolute bedding, sand injections, and mud clasts with thin (<1 cm) clay caps. Siderite layers also occur here. Above this the ochre colored interval returns for ~18 m. Finally, the reduced, olive-grey color returns, and the upper interval (~38.5 m) comprises 2-5 cm beds of upwardly fining, sandy clay. Charcoal occurs at bases of beds and in transitional units of the olive-grey intervals. Many of the mass flows exhibit normal grading capped by silt, consistent with turbidites. But the convolute bedding, flame structures, and locally chaotic nature of beds in other sections of the core are interpreted as hybrid event beds (HEBs), in which both cohesive and non-cohesive flow predominated. Generally speaking there appears to be an alternation of (two) intervals exhibiting an olive-grey color and siderite (anoxic conditions) with (two) intervals exhibiting red and ochre colors suggestive of oxidized conditions, possibly recording climatic alternations. Palynological results also suggest alternations potentially attributable to climatic cycles. Analyses are ongoing to determine whether mass flows reflect autogenic (e.g. deltaic failure) events, or allogenic (e.g. storm) events.