LAKE STRATIGRAPHY AND CLIMATE: EOCENE PICEANCE CREEK BASIN, COLORADO, USA

Lake deposits, formed in the Green River Formation in the Piceance Creek Basin (PCB), indicate direct connection between lake stratigraphy and Eocene climate changes. PCB formed as a deep lake with deltas, shore sandstones, coquinas, and stromatolites on the lake margin, and fine-laminated, soft sediment disturbed, and brecciated kerogene-rich to kerogene-poor carbonates (oil shale) in the deeper parts of the lake. Six evolutionary lake stages and at least two levels of cyclicity occur in the PCB, and are bounded by sequence boundaries or correlative conformities. Lake stages and cyclicity are connected to variation in the runoff and vegetation due the climate changes, and variation between humid and arid conditions. Six lake stages are defined as: S1-Freshwater Lake, S2-High Siliciclastic Input, S3-Rapidly Fluctuating Lake, S4-Rising Lake, S5-High Lake, and S6-Closing Lake. Lake stages indicate an overall lake evolution and correlation with climate changes in the early to middle Eocene. This is illustrated by the Rapidly Fluctuating Lake Stage (S3), formed during the Eocene climate optimum, and composed of thin depositional units on the lake margin, and an abundance of oil shale breccias and evaporite beds in the deeper part of lake. The later stages, S4-Rising Lake, S5-High Lake, and S6-Closing Lake indicate cooler and more humid climate. Cyclicity is marked by oil shale richness and facies changes. Sequence boundaries (SB) separate, in many places, rich oil shale zones from lean oil shale zones, and are marked with channelized delta deposits or intraclastic carbonate beds in the lake margin and oil shale breccias, sand and evaporite beds in the deeper lake. SB’s are followed by leaner oil shale zones that suggest low vegetation and low runoff. Fine-laminated oil shale and evaporite deposits dominate in deeper lake areas. Rising lake level is marked by high occurrence of soft sediment deformation and oil shale breccias in the deeper lake suggesting higher energy during lake level rises. Shallow lake sandstones and stromatolites pass upwards to oil shale in lake margin areas. These rich oil shale zones deposited during high lake levels are interpreted to represent high runoff and high vegetation.