--> Abstract: Hyperpycnal Versus Hypopycnal River Plumes and the Origin of Shelf Muds: Examples from the Cretaceous of the Western Interior Seaway, North America, by Janok P. Bhattacharya, James A. MacEachern, Boyan Vakarelov, Charles D. Howell, and Anne H. Covault; #90039 (2005)

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Hyperpycnal Versus Hypopycnal River Plumes and the Origin of Shelf Muds: Examples from the Cretaceous of the Western Interior Seaway, North America

Janok P. Bhattacharya1, James A. MacEachern2, Boyan Vakarelov1, Charles D. Howell1, and Anne H. Covault1
1 UT Dallas, Richardson, TX
2 Simon Fraser University, Burnaby, BC

Many sedimentological textbooks assume that most marine “shelf” mud is deposited by fallout from suspension under quiet water conditions. Work on modern active muddy shelves, such as Papua New Guinea and the Amazon Shelf, however, show that fine-grained sediment is predominantly supplied from river plumes that may be hyperpycnal or hypopycnal. These concepts have not been widely applied to the interpretation of ancient sedimentary systems, such as the North American Cretaceous Western Interior Seaway.

Comparison of the paleohydraulics of Cretaceous trunk river systems, such as those associated with the Ferron sandstone in Utah, or the Dunvegan Formation in Alberta, Canada, suggest that these systems frequently went hyperpycnal. Associated flood deposits show extremely high sedimentation rates, reaching 1 m per year. High sedimentation rates are indicated by abundant normally graded siltstone beds, aggradational current ripples, and soft-sediment deformation features. Associated environmental stresses also served to 1) inhibit pervasive infaunal burrowing, 2) favour deposit-feeding over suspension-feeding behaviours, and 3) instigate an abundance of fugichnia and equilibrichnia (re-adjustment) structures associated with permanent infaunal domiciles. Associated sandstones show well-developed Bouma sequences, suggesting hyperpycnal sandy river-flood events, or over-thickened HCS beds caused by storm-wave reworking of these flood deposits.

Mudstones associated with hypopycnal conditions show far higher abundances and diversities of ichnofauna. Where shelves experienced neither hypopycnal nor hyperpycnal processes, carbonate productivity tended to increase, producing carbonate-rich units like the White-Speckled shale in Alberta. In contrast, shale-dominated “shelf” mudstones, like the Shaftesbury shale in Alberta, display a strong prodeltaic overprint.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005