A novel approach for prediction of lithological heterogeneity in fluvial point-bar deposits from analysis of meander morphology and scroll-bar pattern

Meandering fluvial reaches exhibit a wide range of morphology, yet published interpretations of ancient meander-belt deposits do not reflect the stratigraphic complexity known to be associated with such variability. An improved understanding of processes that generate stratigraphic heterogeneity is important to improve predictions in sedimentary facies distributions in sub-surface settings. Quantification and classification of planform geomorphology of active fluvial point bars and their recently accreted deposits enables determination of spatio-temporal relationships between scroll-bar pattern and resultant meander shape. Scroll-bar deposits describe an overall pattern of lateral accretion that records how a meander has grown incrementally over time. Analysis of 260 active meander bends, from 13 different rivers, classified by a range of parameters including climatic regime, gradient and discharge, has been undertaken. Assessment of scroll-bar morphology and growth trajectory has been undertaken using remotely sensed imagery in Google Earth Pro. Twenty-two distinct styles of meander scroll-bar pattern are recognised within active meander bends. These are grouped into 8 types that reflect growth via combinations of expansion, extension, rotation, and translation. A novel technique for predicting the variable distribution of heterogeneity in fluvial point-bar elements integrates meander-shape and meander scroll-bar pattern. The basis for predicting relative lithological heterogeneity is the observation that deposited sediments fine downstream around a meander bend, and outwards as a barform grows and tightens due to bend expansion. Observations of these trends are seen in experimental models, modern fluvial systems, and in the ancient record at both outcrop and in the sub-surface. These trends permit planform geometries to be compared with distributions of bar-deposit lithology types. The method is applied to predict heterogeneity distribution in both sub-surface, and outcrop settings. Seismic-reflection data that image point-bar and related elements of the McMurray Formation (Cretaceous, Alberta, Canada) are used to test the predictive capability of the method by comparing predicted heterogeneity to trends known from analysis of gamma-ray data available from densely distributed well-log records. Outcrop data from a point-bar deposit in the Montanyana Group (Ypresian, southern central Pyrenees, Spain) are used to test the method by comparing heterogeneity predictions with observed lithologies seen in the outcrop. This novel method constrains heterogeneity predictions in fluvial point-bar deposits for which direct lithological observations are not possible or are limited. The method therefore provides the basis of a predictive tool for improving understanding of a fragmentary geological record, including prediction of lithological heterogeneity from outcrops of limited spatial extent, or from subsurface seismic datasets.