COMPOSITION AND ARCHITECTURE OF BRAIDED AND SHEETFLOOD-DOMINATED EPHEMERAL FLUVIAL STRATA IN THE CAMBRIAN-ORDOVICIAN POTSDAM GROUP: A CASE EXAMPLE OF THE MORPHODYNAMICS OF EARLY PHANEROZOIC FLUVIAL SYSTEMS AND CLIMATE CHANGE

摘要

Two fluvial facies associations, braided and ephemeral fluvial, are recognized in strata of the Cambrian-Ordovician Potsdam Group in the Ottawa Embayment and Quebec Basin in northeastern North America. Braided fluvial sedimentation was dominated by the migration of coarse-grained sandy dunes and rare unit bars and local gravels that built up low-relief 0.4-5-m-thick downstream-accreting compound bars in very wide and shallow braided channels. Rare meter-scale confluence scours, incision, and filling of decimeter-thick bypass channels, and thin (<= 7 cm) overbank deposition also occurred. Ephemeral fluvial sedimentation was dominated by the progradation of lobate terminal splays, most of which were deflated and/or reworked into thin (10-95 cm) sheets by eolian processes during the long recurrence intervals between floods. These strata were locally incised by rare, 0.3-1.9-m-thick scour-filling high-energy supercritical bedform strata and decimeter-thick bypass channels. Both braided and ephemeral fluvial deposits consist of meter-scale stacked, high-order stratal units separated by horizontal, laterally extensive surfaces recording the recurring buildup, avulsion, and finally abandonment of channel belts.