AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Plan-View Paleochannel Reconstruction of Ancient Meanderbelts, Cretaceous Ferron Sandstone, Henry Mountains Region, Utah

Abstract

Few studies on reconstructing paleohydraulic parameters of ancient rivers obtained from outcrops have been examined by plan-view characteristics (meander amplitude and sinuosity). Plan view and cliff exposures of amalgamated ancient meander belts in the Cretaceous Ferron Sandstone member, Mancos Shale Formation, Utah, allow paleohydraulic reconstruction based on both vertical and plan-view outcrop data, evaluation of numerical models of facies variability in meander belts, as well as addressing the variability of paleocurrents and facies heterogeneity in complex fluvial analog reservoirs. This project integrates 3 measured sections from cliff facies, plan-view interpretive maps made from aerial photos, with 1259 areal grain-size measurements, 800 paleocurrents (mostly rib and furrows from dunes) and 75 strike and dip measurements on exposed bar accretion surfaces. Three amalgamated channel belts are marked by scour surfaces, an increase in grain size, and abrupt changes in paleocurrent orientations. The youngest is 2.0 m in depth, 90m in width, 435m in meander amplitude, and has a sinuosity of 2.9. The middle is 3.1m in depth, 1083m in meander amplitude, and has a sinuosity of 1.2. The oldest was insufficiently exposed to document its plan view style. Grain size showed systematic fining-upwards within each channel story, coarsening towards the bend apex along the bend axis, and fining downstream within some meander scrolls. Plan-form trends were less well developed, perhaps reflecting the fact that the exposures capture variable vertical position within the belts. Paleocurrents showed systematically varying trends within belts (from NE, to SW, to E), with abrupt changes between belts. Grain size and vertical facies associations vary as a function of the style of bar migration, as well as position within a bar (upstream vs. downstream). The outcrop showed the dominance of a meandering river style overall, according to lateral amalgamation of successive point bars within the belts. Compound braid bars, built by overlapping unit bars, constitute the youngest channel deposits, probably associated with channel abandonment. Independent measurements of meander wavelength based on plan view exposures match results from empirical equations. The Ferron rivers are small to medium in scale according to calculated paleohydraulic parameters (Qw = 135~225m3/sec).