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Book Chapter

Comparative Geochemical and Mineralogical Studies of Two Cyclic Transgressive Pelagic Limestone Units, Cretaceous Western Interior Basin, U.S.

By
Michael A. Arthur
Michael A. Arthur
Graduate Schoool of Oceanography
University of Rhode Island
Narragansett, Rhode Island
02882
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Walter E. Dean
Walter E. Dean
U.S. Geological Survey
Branch of Oil and Gas Resources
Denver, Colorado
80225
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Richard M. Pollastro
Richard M. Pollastro
U.S. Geological Survey
Branch of Oil and Gas Resources
Denver, Colorado
80225
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George E. Claypool
George E. Claypool
U.S. Geological Survey
Branch of Oil and Gas Resources
Denver, Colorado
80225
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Peter A. Scholle
Peter A. Scholle
Gulf Research and Development Company
Houston, Texas
, 77236
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Published:
January 01, 1985

Abstract

Pelagic limestone units were deposited in the North American Western Interior seaway during two major Cretaceous transgressive episodes. The Bridge Creek Limestone Member of the Greenhorn Formation, deposited during the Late Cenomanian-Early Turonian transgression, and the Smoky Hill Member of the Niobrara Formation, deposited during the overall Early Coniacian-Early Campanian transgression, are both enriched in organic-carbon and exhibit smallscale carbonate cycles representing periodicities in the range 20 to 40 ky. The distinct periodicity and overall unusual depositional milieu of both units are reflected in their sedimentary structures, mineralogy, and geochemistry.

The Bridge Creek Limestone at Pueblo, Colorado, averages 78% CaCO3 and 1.75% organic carbon with ranges of 42-96% and 0.06-6.97%, respectively, across small-scale cycles. High concentrations of Al, Fe, Mg, K, Ti, Na, Cr, Ni, V; higher Sr/Ca and lower Si/Al ratios; and lighter δ180 in CaCO3 in dark-colored clay-rich beds all suggest periodic influx of terrestrial clay minerals during times of peak fresh water runoff from uplifted highlands to the west. Higher Sr/Ca ratios in marlstone beds than in limestone beds suggest that the marlstone beds have undergone less diagenetic removal of Sr. Higher concentrations of organic carbon, hydrogen, and sulfur, and preservation of some lamination in the clay-rich beds also suggest that the times of enhanced runoff may have induced stable salinity stratification in the water column, which led to gradual depletion of dissolved oxygen in the bottom waters and enhanced preservation of organic carbon in sediments. The geochemistry also suggests that a significant change in sedimentation occurred at the Cenomanian-Turonian boundary.

The geochemical characteristics of the Niobrara Formation near Fort collins, Colorado, are very similar to those of the Bridge Creek Limestone at Pueblo, suggesting similar depositional conditions and source of clastic materials. However, the small scale cycles are present but more subdued in the Niobrara Formation than in the Bridge Creek Limestone, and the Niobrara Formation in the Fort Collins area has not been as altered by diagenesis.

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Contents

SEPM Field Trip Guide

Fine-Grained Deposits and Biofacies of the Cretaceous Western Interior Seaway: Evidence of Cyclic Sedimentary Processes

Lisa M. Pratt
Lisa M. Pratt
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Erle G. Kauffman
Erle G. Kauffman
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Frederick B. Zelt
Frederick B. Zelt
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SEPM Society for Sedimentary Geology
Volume
4
ISBN electronic:
9781565762794
Publication date:
January 01, 1985

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