Sequence Stratigraphy, Climate, and Organic-Richness: Green River Formation, Piceance Creek Basin
The Green River Formation in the Piceance Creek basin is comprised of kerogen-rich and kerogen-poor mudstones i.e. oil shales, siliciclastics, and carbonates, formed in a deep (10’s of meters), stratified lake environment. Small-scale (meters thick) and large-scale (10’s meters thick) depositional cycles are composed of deepening-upward depositional sequences that mark abrupt changes in lithofacies and oil shale richness. Cyclicity is controlled by variations in runoff and vegetation that influence the inflow of siliciclastics and nutrients, and therefore also the distribution of facies and formation of organic-rich deposits. Cycles are bounded by sequence boundaries and correlative conformities, and are divided into units that represent low, rising, and high lake levels. During times of low runoff, lake level was low, vegetation decreased, fewer nutrients were brought into the lake, and lean oil shales formed. Thin marginal deposits formed during low lake level, and at times, evaporite deposition occurred in the deeper part of the basin. During the change to a wetter period, runoff increased and nutrient input increased. The rising lake level is, in many places, marked by delta front sandstones on the lake margin. Microbial and shoaling grainstones occur above and adjacent to the sand input areas. In the profundal area, rich oil shale breccias, as gravity flow deposits, formed. Subsequent wet periods increased vegetation and runoff, bringing high lake levels, and increased nutrients, resulting in rich oil shales. Profundal units are composed of gravity flow and laminated oil shale deposits. Correlating cycles to published age dates, the large-scale cycles are interpreted to represent 400ky eccentricity cycles. Published early Eocene hypothermals correlate with five of these sequence boundaries. The small-scale cycles may be a combination of the 100ky eccentricity and 25ky precession cycles.
Depositional cycles are grouped into lake evolution stages that reflect longer-term changes in the basin, controlled by both climate and tectonics. Stage 1, Fresh Lake, was deposited during decreasing tectonic activity and increasing climate control. Climate dominated Lake Stages 2 and 3, Transitional and Rapidly Fluctuating Lakes. These stages are characterized by high frequency cyclicity. Stages 4, 5, and 6, Rising, High, and Closing Lakes record the change to a wet climate and tectonic activity, resulting in increasing siliciclastic input and closing of the lake.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California