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High Resolution Stratigraphy and Detositional History of the Greenhorn Regressive Hemicyclothem, Rock Canyon Anticline, Pueblo, Colorado

By
Linda M. Glenister
Linda M. Glenister
Department of Geological Sciences
University of Colorado CB250
Boulder, Colorado
, 80309
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Erle G. Kauffman
Erle G. Kauffman
Department of Geological Sciences
University of Colorado CB-250
Boulder, Colorado
, 80309
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Published:
January 01, 1985

Abstract

The Greenhorn Cyclothem has become the model for eustatically-generated, third-order, cyclic sedimentation in Cretaceous epicontinental settings. The Rock Canyon Anticline sequence near Pueblo, Colorado, is the standard reference section for this cyclothem. The Greenhorn transgressive hemicyclothem (including the Muddy Sandstone, Mowry Shale, Graneros Shale, and the Greenhorn Formation through the middle Bridge Creek Limestone Member) has been the subject of detailed stratigraphic and paleontologic study for many years. In contrast, the Greenhorn regressive hemicyclothem (including the upper Bridge Creek Limestone Member, Greenhorn Formation, and the Fairport Shale, Blue Hill Shale, and Codell Sandstone Members of the Carlile shale) has received only general stratigraphic study. In this paper, high-resolution lithostratigraphic, geochemical, event-stratigraphic, and paleobiologic data for the Greenhorn regressive hemicyclothem are presented and analyzed in terms of facies and depositional history. The regressive sequence at Pueblo is nearly complete, and in general displays a graded, upward-coarsening suite of facies cut by rare disconformities. Cyclically bedded pelagic carbonates (Bridge Creek Limestone Member) grade upward through chalky and calcareous shales (Fairport Member) to dark non-calcareous shales becoming more silty and sandy upward (Blue Hill Shale Member), and the sequence terminates in lower, middle, and upper shoreface sands of the Codell Sandstone Member. The top of the cyclothem is truncated on middle-upper shoreface sandstones of the Codell by a regional transgressive unconformity at the base of the Niobrara Cyclothem. A second regional disconfor-mity may occur at the Fairport - Blue Hill member contact. Although the Greenhorn regressive facies sequence generally mirrors that of the transgression (the symmetrical cyclothem model), significant differences were recorded in this study which suggest that the Western Interior Basin was tectonically passive during eustatic fall and regional regression; subsidence did not outstrip or equal sedimentation rates and the basin filled; shallow water environments predominated. Thus, the regressive sequence of the Greenhorn Cyclothem is coarser-grained, more bioturbated, less laminated, and contains lower percentages of organic carbon than equivalent transgressive facies. Greater current circulation and higher levels of benthic oxygenation are implied during regression. Volcanic ash (bentonite) beds are extremely common in transgressive facies, and comparatively rare in regressive facies; this suggests a major slowdown in tectonism and associated volcanism during eustatic fall and regional regression. The number and extent of disconformities cutting the regressive sequence is far less than during either the Greenhorn or the Niobrara transgression; eustatic fall and regression were more continuous than the highly punctuated transgressive events.

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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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