Sublacustrine-fan deposition in the Oligocene Creede Formation, Colorado, U.S.A.

Abstract

ABSTRACT The Oligocene Creede Formation represents mostly lacustrine deposits of an annular caldera lake, Lake Creede, within the Creede caldera in southwestern Colorado. The preserved record of Lake Creede reflects high sedimentation rates in a high-relief lake basin. The deep-water facies in the Creede Formation provide insight into sediment-gravity-flow and subaqueous-fan processes that are applicable to similar lacustrine and marine settings. The deep-water facies are dominated by sediment-gravity-flow and suspension-lamina deposits. Sediment gravity flows were deposited by four major processes: (1) lithic-rich debris flows generated by subaerial slope failure that deposited coarse-grained beds around the basin margin, (2) intraclast-rich debris flows generated by lacustrine-slope failure that deposited massive coarse-grained beds across the basin, (3) debris flows or high-density turbidity currents that deposited massive sandstone and conglomerate with fine-grained, turbidite caps in fan-delta, sublacustrine-fan, and profundal settings, and (4) low-density flows that deposited fine-grained tuffaceous turbidites. The compositional characteristics of the first two types of sediment-gravity-flow deposits suggest specific modes of initiation (e.g., subaqueous and subaerial slope failure); however, the latter two types were probably generated by a variety of mechanisms. The low-density flows were probably generated through flow transformations of the other flows. Exposures of fan-delta and sublacustrine-fan deposits are observed basinward of known or inferred paleovalleys. In the Antlers Park area, coarse-grained sediment-gravity-flow deposits are arranged into a hierarchy of lens-shaped units that are comparable to features observed in modern lacustrine and marine subaqueous fans along active tectonic margins. In the Wason Ranch area, coarse-grained deposits are arranged into broad lens-shaped units that are most consistent with intermediate to shallow-water deposition at the base of a fan delta, similar to the delta-fed ramp model of Heller and Dickinson (1985) or delta-fed apron of Galloway (1998). These exposures and others provide examples of the range of depositional environments in which coarse-grained deposition can occur in lacustrine basins, especially those influenced by high sedimentation rates. These exposures also provide insight into the sequence stratigraphic significance of sublacustrine-fan deposits in lacustrine versus continental-margin marine settings.