Abstract: Cyclicity and Paleoenvironments of the Niobrara Formation in the Western Interior Seaway

T. White, M. Arthur, W. Dean, T. Bralower, C. Savrda, M. Leckie

The Fort Hays Limestone (Ls.) and lower Smoky Hill Chalk members of the Niobrara Formation (Fm) are characterized by distinctive cyclic beds of bioturbated, organic-carbon-poor chalk or limestone alternating with beds of laminated, organic-carbon-rich (up to 8% TOC) marlstone. The carbonate-rich beds were probably deposited in well-oxygenated waters with considerable bottom-water circulation, whereas carbonate-poor beds were deposited in a salinity-stratified seaway with dysoxic to anoxic bottom-water and little circulation.

The Fort Hays Limestone and lower Smoky Hill Chalk formed on the falling limb of the Niobrara sea-level rise; progradational, shoreline parasequence sets provided a ready supply of detritus to the seaway. Dilution led to decreasing %CaCO₃ through the units, while dysoxic to anoxic conditions, and higher sedimentation rates may account for a general increasing trend in %TOC from the Fort Hays Limestone through the lower Smoky Hill Chalk. Interpreted oxygenation profiles show a general trend from well-oxygenated to poorly oxygenated conditions.

In 1895, G.K. Gilbert suggested that distinctive cyclic bedding in the Niobrara Formation is related to orbital forcing of global climate change. The newly revised Late Cretaceous time scale of Obradovich (1991 and this volume) shows that the Niobrara Formation in eastern Colorado spans about 5 m.y., with about 0.5 m.y. assigned to the Fort Hays and about 4.5 m.y. assigned to the Smoky Hill. Using this time scale, decimeter-scale bedding cycles in the Niobrara have an average periodicity of about 19 ka per cycle, similar to precessional periodicity of 21 ka as pointed out by Fischer et al. (1985). These appear to be bundled into 100- and 400-ky eccentricity cycles.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994