FLUVIAL RESPONSE TO A COMPRESSIONAL GROWTH CYCLE: AN EXAMPLE FROM THE LATE NEOPROTEROZOIC FLOODPLAINS OF AVALONIA, EASTERN NEWFOUNDLAND

Clastic growth sequences are common elements of tectonically active basins, and because growth strata occur near active faults at the edge of sediment hinterlands, alluvial deposits dominate them. The architecture of alluvial growth sequences (thickness, dip variations, and bounding unconformities) and their relationship to compressional deformation is well understood. However, details of how rivers adapt to cycles of uplift and aggradation that characterize growth sequences are not. Presented here is a detailed three-dimensional reconstruction of alluvial growth sequences of the Ediacaran Flatrock Cove Formation, Avalon Zone, Newfoundland. Here, areal imagery and sedimentary logging reveal details of six unconformity-bound growth sequences, with three basal braided fluvial sequences overlain by three upper alluvial fan sequences. The basal three growth sequences onlap a higher order erosional unconformity overlying relatively inclined, slightly older strata, and each growth sequence thickens away from folded pre-growth strata, consistent with sedimentation accompanied by limb rotation. Facies and architectural analysis of the most well exposed growth sequence reveals an upward decrease in the average grain size accompanied by changes in fluvial morphodynamics. Specifically, channel belts in the lower part of the growth sequence consist mainly of low-angle, upstream-accreting gravel elements filling relatively narrow channels, suggesting rapid avulsion and abandonment of low-sinuosity anabranches and deposition of sheet-like upstream-accreting bars. Conversely, upper channel belts of consists mainly of sandy, high-angle downstream- and laterally-accreting elements filling broad and internally complex channel successions, reflecting greater channel sinuosity and lateral migration of channels. Assuming consistent discharge throughout deposition, the upward change records a drop in river competence and an increase in channel stabilization coinciding with a decrease in stream power and floodplain slope. This record of fluvial morphodynamic change is consistent with the cycle of peripheral uplift and aggradation evident from the stratal architecture of the growth sequence, and provides a template for the evolution of modern alluvium in tectonically active basins.