--> Prediction of Fluvial Point-Bar Form, Internal Architecture and Heterogeneity From Outcrop and Morphometric Analysis of Meander Bends

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Prediction of Fluvial Point-Bar Form, Internal Architecture and Heterogeneity From Outcrop and Morphometric Analysis of Meander Bends

Abstract

Meandering fluvial reaches are particularly dynamic and diverse, yet published interpretations of ancient meander belts do not reflect the stratigraphic complexity known from modern settings. Quantification and classification of plan-form geomorphological details, integrated with internal architecture and heterogeneity of exhumed systems, permits development of predictive tools to improve interpretation of ancient systems. Active fluvial systems studied from different physiographic regions help to quantify spatio-temporal relationships between scroll-bar behavior and meander shape. Quantitative comparison of meanders with markedly differing morphologies was allowed by development of a novel method. Measurements of 35 morphometric parameters of 520 active meander bends and 130 abandoned loops from 13 different rivers were acquired using Google Earth Pro. Twenty-one scroll-bar geometries were assessed according to a scheme independent of meander shape. The rivers studied were classified by a range of parameters (e.g., climatic regime, gradient, discharge). Ancient point-bar successions (Jurassic, England; Lower Cretaceous, Isle of White) served as a test data set for accurate reconstruction of meander morphology. From the Pennsylvanian in Wales, lithofacies distribution and paleocurrent readings highlight subtle yet predictable variations in ripple, dune and bar growth histories. Overall dimensions were used to infer barform aspect ratio and possible shapes. Changes in lithological heterogeneity, and flow and migration directions were used to determine scroll-bar styles. The following findings arise: (i) the most likely scroll-bar configuration relates to expansion and rotation, which accounts for 18% of meanders observed; (ii) meander forms can be classified into four groups, and 25 specific shapes; (iii) fluvial systems with different gradients, sediment caliber, channel sizes, accumulation rates and climate regime exhibit different yet predictable characteristics in meander and scroll-bar development; (iv) the most likely paths and transitions in a bend becoming mature and cutting off are quantifiable, thereby improving understanding of a fragmented geologic record. This method can be applied to seismic time slices to predict up- and down-stream meander shapes, and to predict lithological heterogeneity beyond seismic resolution. It also provides constraints to reservoir models, and can be used predict stratigraphic heterogeneity prior to drilling.