Fluvial Architecture and Paleohydraulic Analysis of Ancient River Deposits in Plan-view and 2-D Vertical Outcrops of the Turonian Ferron Notom Delta, Utah: From Allogenic Valleys to Autogenic Bars
Ullah, Mohammad Shahid 1981-
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This study documents the facies architecture of large valley-scale, intermediate bar-scale, and small bed-scale fluvial elements in a compound incised-valley fill in the Cretaceous Ferron Notom Delta in southeastern Utah, U.S.A. The results are based on aerial LiDAR surveys, 24 measured sections, photomosaics, and 1800 paleocurrent measurements based on both plan-view and cliff exposures of exhumed meander belts within a compound valley-system. Cross-sectional observations from outcrops and comparisons with modern rivers reveal that autogenic confluence scours have diagnostic fill facies composed of single set of large steep foresets, and produces a single-story body in which a fifth-order scour is filled with unit-bar foresets, which in turn are overlain by a fourth-order surface capped by compound-bar deposits. Although the downstream persistence of fluvial sandy facies along the entire stretch of the incised valley suggests a greater upstream control, it could not entirely erase the imprint of downstream sea-level rise as indicated by the “backwater effect”. The backwater transition is observed in fluvial deposits at the most downstream location and is marked by thickening of channel belt deposits tied to channel deepening towards the river mouth, reduction in average channel belt width related to a reduced rate of lateral migration, increased proportion of finer grains in channel belt deposits tied to reduced bed material flux, and a vertical translation from fluvial to tidal facies in the valley deposits. Findings from this study support the idea that the backwater zone is a fundamental transition in coastal rivers, across which preserved sedimentary bodies display predictable geometric changes that can be observed in ancient fluvial systems. Paleogeographic reconstructions of the meander plain from plan-view data in the upper part of the valley fill show the evolution of three channel belts formed by successive phases of dominantly lateral expansion. Three types of architectural facies or macroforms were identified in the point bars in the channel belts. They are classified as – Type A or small-scale macroforms, the amalgamation of which produce large-scale compound bars; Type B macroforms, which are interpreted as unit bars; and Type C macroforms, which consists of large, steeply dipping upper flow regime (UFR) planar beds. Plan-view bedding architectural analysis, therefore, can be a useful method to identify various fluvial macroforms.