Stratal Architecture and Morphodynamics of Downstream-Migrating Fluvial Point Bars (Jurassic Scalby Formation, U.K.)

Abstract

Abstract Assessments of depositional architecture and paleodrainage in planform are increasingly recognized as critical for the reconstruction of ancient fluvial morphodynamics; however, planview exposures of ancient, undeformed meandering-fluvial deposits are scattered, and exceptional occurrences such as the exhumed meander plain of the Scalby Formation (Yorkshire, UK) deserve particular attention. Here we specifically focus on the planview style and morphodynamics of downstream-migrating point bars, providing parallels with modern meandering rivers sharing scale and processes. The Middle Jurassic Scalby Formation lies in the Cleveland Basin of Yorkshire (UK), and accumulated in incised valleys eroded onto shallow-marine strata. Five well preserved downstream-migrating point bars are exposed along a modern tidal platform. Through 1:2000-scale mapping, point bars are analyzed for sedimentology, stratal architecture, vectors of paleoflow and bar migration, and nature of adjacent and underlying deposits. Results indicate that downstream migration was facilitated by—but not strictly related to—the occurrence of resistant outer banks. Meander-bend migration promoted cannibalization of upstream-bar portions, subordinate preservation of central-bar portions, and widespread development of downstream-bar scrolls. The most reliable indicators of regional transport are upstream-bar paleoflow indicators and the dip direction of downstream-bar inclined beds. The generated sectional architecture overlaps in part with that of other fluvial depositional models, such as those for braid bars, indicating that lack of planform exposures may introduce bias when reconstructing ancient fluvial morphodynamics.