ABSTRACT: Models to Interpret Bed-Form Geometries from Cross-Bed Data

Stefan M. Luthi, Jayanth R. Banavar, Ulf Bayer

To improve the understanding of the relation of cross-bed azimuth distributions to bed-forms, geometric models were developed for migrating bed forms using a minimum number of parameters. Semi-elliptical and sinusoidal bed-form crestlines were modeled with curvature and sinuosity as parameters. Both bedform crestlines are propagated at various angles of migration over a finite area of deposition. Two computational approaches are used, a statistical random sampling (Monte Carlo) technique over the area of the deposit, and an analytical method based on topology and differential geometry. The resulting foreset

azimuth distributions provide a catalog for a variety of situations. The resulting thickness distributions have a simple shape and can be combined with the azimuth distributions to further constrain the cross-strata geometry. Paleocurrent directions obtained by these models can differ substantially from other methods, especially for obliquely migrating low-curvature bed forms. Interpretation of foreset azimuth data from outcrops and wells can be done either by visual comparison with the catalogd distributions, or by iterative computational fits. Studied examples include eolian cross-strata from the Permian Rotliegendes in the North Sea, fluvial dunes from the Devonian in the Catskills (New York state), the Triassic Schilfsandstein (Federal Republic of Germany), and the Paleozoic-Juras ic of the Western Desert (Egypt), as well as recent tidal dunes from the German coast of the North Sea and tidal cross-strata from the Devonian Koblentzquartzit (Federal Republic of Germany). In all cases the semi-elliptical bed-form model gave a good fit to the data, suggesting that it may be applicable over a wide range of bed forms. The data from the Western Desert could be explained only by data scatter due to channel sinuosity combined with the scatter attributed to the ellipticity of the bed-form crestlines. These models, therefore, may also allow simulations of some hierarchically structured bed forms.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990