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Book Chapter

The Niobrara Transgressive Hemicyclothem in Central and Eastern Colorado: The Anatomy of a Multiple Disconformity

By
Cynthia G. Fisher
Cynthia G. Fisher
Department of Geological Sciences
University of Colorado
Boulder, CO
80309
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Erle G. Kauffman
Erle G. Kauffman
Department of Geological Sciences
University of Colorado
Boulder, CO
80309
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Laura Von Holdt Wilhelm
Laura Von Holdt Wilhelm
Micropaleontological Consultant
Houston, TX
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Published:
January 01, 1985

Abstract

The Niobrara Transgressive Hemicyclothem at Pueblo and to the south and west comprises four discrete lithostratigraphic members, separated by four and possibly five regional disconformities. The sequence disappears to the east and north as disconformities expand, anastomose, and become a single regional unconformity, bringing pelagic Fort Hays limestones, representing maximum transgression and eustatic rise, into erosional contact with maximum regressive shoreface sandstones (Codell) of the underlying Greenhorn Cyclothem. Eight molluscan biozones are defined within the complex Niobrara transgressive sequence in the Axial Basin. These biozones progressively disappear eastward within anastomosing disconformities. Near Pueblo only three molluscan biozones are recognized within the pre-Niobrara part of the cyclothem. A sequence of four foraminiferal biofacies are defined at Pueblo and along the Colorado Front Range which reflect rapid development of clear, warm, well circulated waters early in the transgressive history. These biofacies rise stratigraphically to the west, associated with eustatic rise in sealevel, and grade laterally along any time-line from diverse open water assemblages to environmentally stressed, less diverse, assemblages westward. This lateral gradation, and the rise of foraminiferal biofacies westward, is thought to reflect the westward migration of turbid watermasses associated with the active margin of the seaway during eustatic rise of the Niobrara Marine Cycle.

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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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