Studies of the upper Turonian to upper Coniacian or lower part of Niobrara Formation (in ascending order, the Fort Hays Limestone Member and the marlstone, shale, and limestone of the lower part of the Smoky Hill Shale Member) reveal a significant relationship between petroleum source rock potential and paleoclimate. Trends in bioturbation, ^dgrO¹⁸ (oxygen isotopic ratio), and C_org (organic carbon content) during lower Niobrara deposition suggest that paleoclimatic factors limited bioturbation of the sediment, favored high C_org contents, and resulted in excellent source rock potential in the shale unit of the lower Smoky Hill Member. From the Fort Hays through the overlying marlstone unit, ^dgrO¹⁸ in inoceramid shells shows a gr dual shift from nearly normal marine values (-2 to -4%) to lighter values (-4 to -6%); over the same interval, C_org changes from relatively low values (0.1-1.4%) to moderate values (0.5-2.5%). Bioturbation throughout these units is high. A marked shift toward heavier ^dgrO¹⁸ (-6.0 to -9.1%) occurs upward into the shale unit and coincides with increase in C_org (2.3-4.9%) and an absence of bioturbation. In the limestone unit, ^dgrO¹⁸ shifts to less negative values (-6.2 to -6.5%), C_org decreases (average 1.5%), and bioturbation returns. The trend toward more negative ^dgrO¹⁸ and higher C_org values in the shale unit is inferred to reflect a lowering of surface-water salinity in the Western Interior seaway due to cl matic warming and increased freshwater imput. Resultant salinity stratification of the water column apparently inhibited vertical mixing and oxgenation of the bottom waters, resulting in limited benthic activity and enhanced preservation of organic matter.

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