Sublacustrine-fan deposition in the Oligocene Creede Formation, Colorado, USA

The Oligocene Creede Formation represents mostly lacustrine deposits of an annular caldera lake, Lake Creede, within the Creede caldera in southwester n 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 l acustrine and marine settings, The deep-water facies are dominated by sedim ent-gravity-flow and suspension-lamina deposits. Sediment gravity Bows were deposited by four major processes: (1) lithic-rich debris Bows generated b y subaerial slope failure that deposited coarse-grained beds around the bas in margin, (2) intraclast-rich debris Bows generated by lacustrine-slope fa ilure that deposited massive coarse-grained beds across the basin, (3) debr is Bows or high-density turbidity currents that deposited massive sandstone and conglomerate with fine-grained, turbidite caps in fan-delta, sublacust rine-fan, and profundal settings, and (4) low-density Bows that deposited f ine-grained tuffaceous turbidites, The compositional characteristics of the first two types of sediment-gravity-flow deposits suggest specific modes o f initiation (e.g., subaqueous and subaerial slope failure); however, the l atter two types were probably generated by a variety of mechanisms, The low -density Bows were probably generated through flow transformations of the o ther flaws. Exposures of fan-delta and sublacustrine-fan deposits are observed basinwar d of known or inferred paleovalleys. In the Antlers Park area, coarse-grain ed sediment-gravity flow deposits are arranged into a hierarchy of lens-sha ped units that are comparable to features observed in modern lacustrine and marine subaqueous fans along active tectonic margins, In the Wason Ranch a rea, coarse-grained deposits are ar ranged into broad lens-shaped units tha t are most consistent with intermediate to shallow-water deposition at the base of a fan delta, sim ilar to the delta fed ramp model of Heller and Dic kinson (1985) or delta-fed apron of Galloway (1998), These exposures and ot hers provide examples of the range of depositional environments in which co arse-grained deposition can occur in lacustrine basins, especially those in fluenced 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.