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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 923

Last Page: 923

Title: Oblique Ripples on Permian Point Bars in North-Central Texas: Helical Flow or Boundary Effect?: ABSTRACT

Author(s): K. A. Eriksson, Marc B. Edwards, R. S. Kier

Article Type: Meeting abstract


Fine-grained point-bar deposits are well-exposed both in cross section and in exhumed epsilon cross-beds. Above a basal lag of intrabasinal caliche and mudstone clasts, the epsilon cross-beds consist of cross-laminated very fine sand grading laterally and upward into mud. The epsilon cross-beds dip from 10 to 25°, are up to 2.5 m high and 15 m wide, and show a meander radius of curvature of 45 to 500 m, as measured on aerial photographs.

Sandstones in the epsilon sets are almost invariably structured into straight to slightly sinuous-crested ripples which are oriented obliquely, from 25 to 50°, to the main channel axis as inferred from the strike of the epsilon surface. The obliquity of these ripples greatly exceeds that of basal fluid flow related to a helical cell based on theory as well as observations of both experimental and natural stream conditions. An earlier study of the Muddy River, Wyoming, explained oblique bed forms by variation in boundary shear-stress.

The steep transverse slopes, probably resulting from the fine-grained sediment load of the Permian rivers, are associated with increasing downstream sediment transport and ripple migration rates at progressively lower levels on the point bar. Thus bed-form orientation in this situation may reflect boundary conditions rather than the local flow directions.

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