Prediction of Internal Architecture and Heterogeneity Within Fluvial Point Bars Through Analysis of Modern and Ancient Examples

Abstract

Point-bar elements that represent preserved remnants of meandering fluvial channel systems are widely recognized in the rock record. Although the fundamental mechanisms responsible for determining the facies architecture of these bodies have long been known, the specific processes that give rise to the wide variability in internal facies compositions and arrangements remain poorly constrained. This study documents and accounts for the facies architectures of a range of ancient outcropping point-bar successions via comparisons to a GIS-based analysis of the morphology of modern point bars. Results from four ancient case studies are presented here: a Bashkerian delta-plain succession, Pembrokeshire, UK, and 3 Campanian lower alluvial-plain successions from the Mesa Verde Group, Utah. Each accumulated under different inferred accommodation regimes and in positions that vary in position along the fluvial profile. Fifty studied point-bar elements are each 3 to 12 m thick and internally comprise of sand-prone strata with subordinate proportions of gravel, silt and clay. Many sandstone packages are delineated by muddy and organic drapes on surfaces inclined up to 12° in the direction of point bar propagation. Carbonaceous mud drapes vary in composition, thickness, spacing, lateral and downslope continuity as a function of both position within individual point-bar elements and position along the alluvial profile. For individual point-bar elements, the abundance, thickness and lateral continuity of mud drapes is greater on the downstream side of the apex of reconstructed meander bends, especially in high-accommodation systems. Regionally, the incidence of mud-draped surfaces is greater in the lower parts of alluvial plains. Forty meanders have been assessed from each of 20 modern fluvial systems: 800 point-bar shapes have been recorded to devise a novel classification scheme. Identified end-member types form the basis for a predictive model of controls on point-bar heterogeneity. The integration of outcrop characterization and GIS study of variability of point-bar morphology in modern river systems has enabled development of a classification framework that can be used as a tool for the prediction of point-bar architecture as a function of multiple autogenic and allogenic controls (e.g. climatic setting, subsidence rate, scale of system, gradient, sediment load type, degree of tidal influence).

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