LARSEN, DANIEL¹ and GARY A. SMITH^2
1Univ. of Memphis, Dept. of Geological Sciences, Memphis, TN
²Univ. of New Mexico, Dept. of Earth and Planetary Sciences, Albuquerque, NM

Abstract: Depositional Geometry of Sublacustrine-Fan Deposits In an Ancient Caldera Lake: Origin and Comparison to Other Subaqueous-Fan Deposits

Sublacustrine-fan deposits are present in caldera-lake deposits of the Oligocene Creede Formation, San Juan Mountains, Colorado. Three types of facies stacking patterns are recognized: (1) At Antlers Park, laterally and vertically stacked depositional lobes were emplaced near the base of the slope and were fed by individual slope channels; no evidence for an extensive subaerial fan delta is observed. (2) At Wason Ranch laterally stacked depositional lobes were emplaced in a ramp setting and fed by one or more channels from an upslope fan delta. (3) Sublacustrine-fan lobes were also emplaced basinward of the Bachelor paleovalley; limited exposures suggest sublacustrine-fan deposition alternated with fan-delta deposition at the mouth of the paleovalley depending on lake level.

Modern lacustrine and marine analogs to the Creede sublacustrinefan deposits are present in deep, rift or alpine, glacial lakes, fjords, and steep, active-tectonic continental margins. Few similar ancient lacustrine examples have been described, although sublacustrine-fan deposits of the Miocene Ridge basin, CA; Carboniferous Lake Narvaez, Argentina; and Cretaceous Uhangri Formation, Korea, show comparable facies.

The geometry and organization of the fan deposits was influenced more by stream-sediment discharge, lake-level, and geomorphic evolution of the basin, than by tectonic processes. All of these factors need to be considered when evaluating deepwater reservoir potential in irregularly-shaped, high-relief lacustrine basins.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas