--> ABSTRACT: Architectural Studies of Jurassic-Cretaceous Fluvial Units, Colorado Plateau, by Andrew D. Miall, M. H. Bromley, E. J. Cowan, and C. E. Turner-Peterson; #91022 (1989)

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Architectural Studies of Jurassic-Cretaceous Fluvial Units, Colorado Plateau

Andrew D. Miall, M. H. Bromley, E. J. Cowan, C. E. Turner-Peterson

A sixfold hierarchy of architectural elements and bounding surfaces evolved from outcrop studies of three fluvial units: Westwater Canyon member (WCM), Morrison Formation, Upper Jurassic; Torrivio sandstone member (TSM), Gallup Sandstone, Upper Cretaceous, northwestern New Mexico; and Kayenta Formation (KF), Lower Jurassic, southwestern Colorado.

Three submembers in the WCM are separated by sixth-order surfaces. They are up to 30 m thick, extend for more than 200 km across the San Juan basin, and display distinct paleocurrent trends and pebble suites. They probably formed in response to subtle regional tectonism and are crudely chronostratigraphic. The bounding surfaces represent widespread events of nondeposition, pedimentation, or slight erosion. A sixth-order surface in some outcrops of the TSM separates a braided fluvial submember below from a tidally influenced unit above and is interpreted as a sequence boundary separating regressive and transgressive phases of fluvial aggradation.

Fifth-order surfaces define a variety of channel-fill geometries. In the WCM, ribbon sandstones 125-250 m wide and 10-20 m thick represent rapidly cut narrow channels filled by alternate bars. Tabular sheet sandstones hundreds of meters wide represent broader, braided channels. Sheetlike channels dominate in the KF.

Lateral- and downstream-accreting macroform units bounded at the top by convex-up fourth-order surfaces commonly occur in the same outcrop in all three units. Internal third-order surfaces separate growth increments and reveal pauses in accretion accompanied by slight erosion. Macroforms in the WCM and KF show little evidence of upward fining but are commonly draped by thin mud units and probably formed by rapid flash-flood deposition in arid environments.

First and second-order surfaces bound individual and multistory lithofacies units, respectively.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.