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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 989

Last Page: 990

Title: Trace Fossil Assemblages as Indicators of Shelf-Sandstone Facies, Upper Cretaceous, Northwestern Colorado: ABSTRACT

Author(s): Alan J. Scott, J. Michael Boyles

Article Type: Meeting abstract


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Five facies of upper Mancos shelf-sandstones have been identified using sedimentary structures and subsurface data. Microfossil and subsurface studies suggest that all five facies were mid-shelf deposits, yet each of the facies has a distinctive trace fossil assemblage. The Low Energy Shelf Facies is intensely bioturbated by horizontal indistinct forms with some Helminthoida, "armored" burrows (Diopatra-like), Terebellina (plural curving tubes), and "donut" burrows. The intensely bioturbated lower Back Bar Facies contains abundant Helminthoida with common donut and some armored burrows, and Trichichnus. This grades vertically into the more diverse assemblage of abundant Teichichnus, Terebellina, "nest structures," and common Thalassinoides with some Ophiomorpha, armored burrows, and H lminthoida near the highly bioturbated top. The overall bioturbation drops dramatically in the Central Bar Facies in which Teichichnus and nest structures are the most common. Ophiomorpha and Thalassinoides are rare. The upper part of the Central Bar contains only rare Ophiomorpha and nest structures. The Ramp Facies, seaward of the Central Bar, contains only rare Thlassinoides and nest structures. The High Energy Shelf Facies is more diverse than the Low Energy Shelf Facies and is moderately bioturbated with abundant Ophiomorpha, Thalassinoides, Teichichnus, nest structures, Terebellina, and armored burrows.

The distinctive trace fossil assemblages found in the five facies that were deposited on the mid-shelf suggest that physical energy and substrata characteristics control trace fossil distribution.

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Copyright 1997 American Association of Petroleum Geologists