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GEOREF RECORD

Upper Cretaceous shelf sandstones, northwestern Colorado; migrating sand-ridge model

J. Michael Boyles and Alan J. Scott
Upper Cretaceous shelf sandstones, northwestern Colorado; migrating sand-ridge model (in Association round table; 1981 AAPG annual convention with divisions; SEPM/EMD/DPA; technical program summaries and abstracts, Anonymous)
AAPG Bulletin (May 1981) 65 (5): 905

Abstract

An N45E shoreline trend and five major shelf facies were interpreted from surface and subsurface data in the upper Mancos (Campanian) shelf sands in northwestern Colorado. The central bar, up to 20 m thick, coarsens upward from fine to medium-grained sandstone containing large-scale landward-dipping accretion surfaces. Smaller cross-beds have reactivation surfaces and indicate two transport directions (west and southwest). Hummocky surfaces are also present. The ramp unconformably overlies the seaward edge of the central bar. Large-scale seaward-dipping ramp units have cross-strata indicating only westward transport. Up to 15 m of bioturbated back bar occurs landward and below the central bar. Overall, the back bar coarsens upward but contains (1) fining-upward sequences(< 2 m thick); (2) coarsening-upward sequences (< 2 m thick); and (3) bioturbated isolated sand waves. Paleocurrents were to the west. Shelf bioturbated clayey siltstones gradationally underlie the back bar and overlie the reworked bar top. These facies represent large-scale migrating sand ridges with axes parallel to the shoreline. The dominant processes were: (1) major storms that eroded the seaward edge and crest of the sand ridges and transported sediment landward (westward paleocurrents). Back-bar deposits include storm washovers and graded beds. Channels, hummocky cross-strata, and hummocky erosion surfaces were formed on the bar crest. (2) Between storms, contour currents (southwesterly) transported sand onto and along the bar crest. The ramp facies represents sand waves climbing onto the eroded seaward face of the bar. The sand-ridge migrated over the back-bar deposits causing that facies to thicken landward.


ISSN: 0149-1423
EISSN: 1558-9153
Coden: AABUD2
Serial Title: AAPG Bulletin
Serial Volume: 65
Serial Issue: 5
Title: Upper Cretaceous shelf sandstones, northwestern Colorado; migrating sand-ridge model
Title: Association round table; 1981 AAPG annual convention with divisions; SEPM/EMD/DPA; technical program summaries and abstracts
Author(s): Boyles, J. MichaelScott, Alan J.
Author(s): Anonymous
Affiliation: Univ. Tex. at Austin, Austin, TX, United States
Pages: 905
Published: 198105
Text Language: English
Publisher: American Association of Petroleum Geologists, Tulsa, OK, United States
Meeting name: 1981 AAPG annual convention with divisions; SEPM/EMD/DPA
Meeting location: San Francisco, CA, USA, United States
Meeting date: 19810531May 31-June 3, 1981
Summary: Y
Accession Number: 1983-028921
Categories: Sedimentary petrology
Document Type: Serial Conference document
Bibliographic Level: Analytic
N37°00'00" - N41°00'00", W109°00'00" - W102°00'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute.
Update Code: 1983
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