Sedimentological Study of an Early Pleistocene Upland Lake Core, Unaweep Canyon, Colorado

Abstract

The recent recovery of a core in Unaweep Canyon, Colorado, penetrated ~140 m of lacustrine sediment of what is here termed paleo-Lake Unaweep. The lake is inferred to have formed as a result of mass wasting that blocked the ancestral Gunnison River during the early Pleistocene, causing partial filling of Unaweep Canyon before the ancestral Gunnison River abandoned the canyon. This core has been correlated to previous core that also penetrated paleo-Lake Unaweep, and captures a sediment record that dates from ~1.4-1.3 Ma, enabling a glimpse of the Early Pleistocene before the mid-Pleistocene transition— a time interval rarely captured in an upland (or any) setting of the greater Rocky Mountains. The lacustrine section begins atop an interval of ~21 m of inferred ancestral Gunnison River gravels, and comprises a series of mass flows that, overall, decrease in thickness (2 to 60 cm) from the bottom to the top of the core. The entire lacustrine section exhibits an alternation of two intervals: one exhibiting an olive-gray color and containing siderite, and intervals exhibiting red and ochre colors, possibly recording climatic variations. The basal ~20 m consists of reddish-brown, graded (granules to fine sand) mass flows. Above this is a ~15 m interval of thinner (2-10 cm) and finer-grained mass flows exhibiting ochre colors with pink/white clay caps. Next is an olive-gray interval (~48 m) with mass flows exhibiting basal loading, convolute bedding, sand injections, and mud clasts with thin (<1 cm) clay caps. Siderite layers also occur locally in the olive gray interval. Above this, there is ~18.5 m of the same ochre-colored interval. The upper interval (~38.5 m) is also olive-gray and comprises 2-5 cm beds of upwardly fining, sandy clay. Macroscopic charcoal occurs commonly at the bases of event beds and in transitional units of the olive-gray interval but is absent from the intervals that exhibit an oxidized color. Many of the mass flows exhibit normal grading capped by silt, interpreted as partial turbidite sequences. Preliminary palynological results exhibit changes in pollen and spore assemblages that track the large-scale color variation observed, supporting the inference of a climate driver for these alternations. Analyses are ongoing to determine whether mass flows reflect autogenic (e.g., deltaic failure) events, or allogenic (e.g., flooding) events. The primary goal of this study is to utilize this recently recovered UDR1 core to understand the drainage history and the origin and evolution of this paleo-lake. These Quaternary paleo-lake sediments provide a continuous record of sedimentological processes within this unique upland lake that dates from before the Mid Pleistocene Transition. This study also provides new data points to help reconstruct a possible basement profile that mandates a remarkable shallowing of the gradient of the ancestral Gunnison River in westernmost Unaweep Canyon.