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Paleohydrology and 3-D Facies Architecture of Ancient Point Bars Formed by a Low Sinuosity River, Ferron Sandstone, South-Central Utah: Implications for Evolution of Channel Bend

Abstract

Controls of fluvial process and facies architecture are thought to be very different for braided versus meandering rivers, but low sinuosity rivers may show characteristics that are transitional type between the two end members. Eight measured sections, cliff-photomosaics, and over 400 paleocurrents were taken from an ancient exhumed channel belts from the Cretaceous Ferron Notom Delta in south-central Utah. Unusually well preserved plan-view exposures, with local vertical cliff exposures, allowed documentation of 3D channel belt dimension, migration pattern, and facies architecture. Paleocurrents are highly consistent within individual architectural elements (such as a unit bar) and vary gradually at the scale of an individual channel belt. Paleocurrent variations were used to infer channel migration patterns and paleogeography. Average channel depth and channel width were determined to be 1.7 m-3.6 m and 23 m-89 m respectively. The discharge of the formative river is estimated to be 115 m/sec to 387 m/sec. Plan-view grain-size variation shows a distinct coarsening trend from inner bar to outer bar at both the scales of individual point bars and the meander loop complex. Bedding diagrams show distinct types of inclined strata including large scale foresets and unit bars, which are similar to the deposits of modern braided rivers. However the channel bend expanded laterally and then translated downstream with a continuous increase of sinuosity from 1.01 to 1.44, which is more typical of the migration pattern of meandering rivers. This migration pattern likely reflects the mud-dominated deposits underlying the channel belt. The channel abandonment process was directly associated with the formation of unit bars and mid-channel bars, which are a common cause of channel blocking in braided systems. The major controls on the formation of low sinuosity river are gradient and bank erosion, and the fluvial style is a bed-load to mixed-load, high net-to-gross river system with limited suspended content in the flow. Estimates of paleoslope are on the order of 0.02°-0.3° suggesting the Ferron was also a relatively steep gradient system, compared to much lower systems such as the Mississippi River.