Climate Control on Sequence Stratigraphy in Organic-Rich Lake Basin: Green River Formation, Lake Uinta, Colorado and Utah
Kati Tanavsuu-Milkeviciene, J. Frederick Sarg, and Yuval Bartov
The Green River Fm. in Lake Uinta of Colorado and eastern Utah is comprised of oil shales (kerogen-rich and kerogen-poor mudstones), siliciclastic and carbonate mudstones, sandstones, and microbial and non-microbial carbonates. Large-scale (10's metres) 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 associations 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. Organic richness varies vertically and laterally within the depositional cycles. The richest deposits occur in the middle and upper portion of the depositional cycles, and laterally, in the deeper parts of the basin. During times of low runoff, lake level was low, vegetation decreased, and fewer nutrients were brought into the lake. Deposition is characterized by thin marginal deposits, and in the deeper part of the lake with lean oil shales or at times with evaporites. During the change to a wetter period, runoff and nutrient input increased. Rising lake level is marked by marginal delta sandstones, microbialites, and shoaling oolitic grainstones. In the profundal area, rich oil shales formed. Subsequent wet periods increased vegetation, runoff, and nutrients, bringing high lake levels, resulting in mudstones or lean oil shales along the basin margin and rich oil shales in the deeper part of the basin. Correlating cycles to published age dates, we interpret the large-scale cycles to represent 400ky eccentricity cycles. Published early Eocene hypothermals correlate with five of these sequence boundaries. Depositional cycles are grouped into lake 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. Lake Stages 2 and 3, Transitional and Highly Fluctuating Lake, are interpreted to be dominated by an increasingly warm and arid climate. Stages 4, 5, and 6, Rising, High, and Closing Lake periods record the change to a wetter climate and increasing tectonic activity, resulting in a wide-spread deep lake and increased siliciclastic input that extended across Lake Uinta.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012