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Abstract

The Lincoln Limestone Member of the Greenhorn Limestone is characterized by calcareous shale and calcarenite beds deposited during the transgressive phase of the Greenhorn Cyclothem. At Rock Canyon anticline the Lincoln Member is conformably underlain by the Graneros Shale and overlain by the Hartland Shale Member of the Greenhorn Limestone. A detailed study of the Lincoln divides the member into a lower and an upper interval, based on lithostratigraphic, geochemical and paleobiologic criteria. The lower unit is characterized by laminated, weakly calcareous shales, relatively high organic carbon content (Corg ∼ 2.5%), and a macrofaunal assemblage transitional between the Graneros Shale and overlying Greenhorn deposits. The upper Lincoln interval is characterized by frequent alternation of laminated shale and ripple-bedded calcarenite beds with associated high Corg values in the shale beds (>2.0%) and lower Corg values in the calcarenite beds (<1.0%). Faunal composition through this upper interval records development of the first typical Greenhorn biota in the calcarenite beds. In the upper interval, the interbedding of contrasting lithofacies and biofacies suggests a dynamic paleooceanographic system poised between a relatively quiet water, oxygen-deficient environment during shale deposition, and well-circulated, well-oxygenated benthic conditions during calcarenite deposition. The Lincoln Limestone Member thus represents one complete 4th order marine cycle within the Greenhorn Cyclothem beginning with lithofacies and biofacies characteristic of a stillstand event in the lower Lincoln interval, progressing to deposits characteristic of an overall transgressive pulse in the upper Lincoln interval.

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