--> Chronostratigraphic Calibration of Mid-Cretaceous Depositional Cycles in the Western Interior by Graphic Correlation, by R. W. Scott, J. A. Bergen, M. J. Evetts, J. A. Stein, E. J. Kidson, and R. C. Franks; #90986 (1994).

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Abstract: Chronostratigraphic Calibration of Mid-Cretaceous Depositional Cycles in the Western Interior by Graphic Correlation

R. W. Scott, J. A. Bergen, M. J. Evetts, J. A. Stein, E. J. Kidson, R. C. Franks

A continuous core in western Kansas from the middle part of the Smoky Hill Member of the Niobrara Formation to the top of the Morrison Formation (Upper Jurassic) records several scales of depositional cycles in the Western Interior Seaway. Chronostratigraphy of this section has been calibrated by a multidisciplinary study of nannofossils, dinoflagellates, spore and pollen, foraminifers, and mollusks and has been graphed to a global composite standard consisting of key reference sections in Europe and North Africa. The first depositional cycle begin during the early part of the late Albian. The French Albian-Cenomanian boundary correlates approximately with a ravinement surface in the upper part of the Dakota Formation. The Cenomanian-Turonian boundary defined by nannofossils, dinoflag llates, foraminifers, inoceramids, and an anoxic carbon-13 event is within the basal part of the Bridge Creek Limestone Member of the Greenhorn Formation. The Turonian-Coniacian boundary is at the base of the Niobrara Formation at this site.

Four orders of depositional cycles arc recorded by lithologic changes and constrained by graphic correlation to the global composite standard. The

longest term depositional cycle/sequence is 2.0 to 3.4 Ma in duration: the Purgatoire Formation, the lower part of the Dakota Formation, the upper Dakota and Graneros formations, and the Greenhorn and Carlisle formations. The next order comprises transgressive-regressive subcycles in the Purgatoire that are about 1 Ma long. The third scale of cycles includes sandstone-clay cycles in the Dakota and limestone-marl cycles in the lower part of the Greenhorn that are about 100 Ka long. The shortest-term cycles are limestone-marl couplets in the upper Greenhorn that are about 41 Ka long. Cyclical strata in the Fort Hays Member of the Niobrara have approximately 113 Ka year periodicity. Graphic correlation is an independent method of measuring the durations of Milankovitch-scale sedimentary ycles. Graphic correlation is a useful technique for separating the 100 and 41 Ka short-term climatic cycles from long-term tectono-eustatic cycles. One wonders if the sandstone-clay cycles in the lower part of the Dakota may also reflect climatic control.

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