Scaling Relationships of Rib-and-Furrow Structures in Modern and Ancient Dunes and Bars in Rivers

Abstract

A common problem in the interpretation of fluvial paleoenvironments is the estimation of river scales and properties using stratigraphic features. A scaling relationship between either the wavelength or sinuosity of duneforms in plan view and features of the bars or rivers in which they appear is hypothesized to aid in the interpretation of ancient fluvial systems using rib-and-furrow measurements. Modern, high-resolution remote sensing satellite data with sub-meter scale resolution via GoogleEarth is of sufficient quality to allow individual dunes and larger bars to be resolved. The cross-flow dune-crest width and wavelengths (assumed to be equivalent to rib and furrow width) of several bars were measured for the 400 m wide single-thread Sewa River of Sierra Leone and the 4 km wide braided Orinoco River in Venezuela, as part of a previous study. We hypothesize that rib and furrow widths should broadly scale to river paleohydraulics. This initial study showed dune widths of 20 - 40 m, indicating large dunes, with little measurable difference between the two rivers. This suggest that dunes scale to individual threads rather than the entire river. Unit bars were about 200 m wide in both rivers, but these amalgamated in the Orinoco to form km-wide compound bars, absent in the Sewa. We also measured the scales of dunes and bars in ten other rivers in North America. These rivers have been chosen due to the availability of hydrographic data. Dune-crest sinuosity, cross-flow wavelength, and down-flow spacing were analysed using linear regression in order to investigate the scaling relationships between dune widths, bar wavelength, channel width, and various other hydrologic or topographic variables. These scaling relationships were also examined using over 1000 measurements of rib and furrow width taken from the top of plan-view exposed dunes and bars in fluvial and mouth bar deposits of the Cretaceous Ferron Sandstone in southern Utah, for which we have also conducted paleohydraulic evaluations. Dune rib widths in the Ferron are on the order of 1 m, estimate channel widths are about 80 m, nearly an order of magnitude smaller than the Sewa and Orinoco, and unit bar widths are on the order of 10 m. It appears that the larger rivers are composed of relatively uniform scale dunes and bars that amalgamate to form compound architectural elements as rivers become wider, and braided. The smaller rivers appear to contain smaller dunes and unit bars.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015