Bedforms and associated facies of an Early Permian grounding-line fan: Exploring flow regime transitions of glacigenic density flows during the Late Paleozoic Ice Age

Abstract

Grounding-line fans and their associated supercritical-flow bedforms are still poorly recognized in the upper Paleozoic glacial record. In order to fill this gap and to explore the products of the transition between upper and lower flow regimes, we examined a gravel-rich succession composed of conglomerates, sandstones, and diamictites deposited during the Late Paleozoic Ice Age (LPIA) in southeastern Paraná Basin, Brazil. To investigate this succession, we described the main bedforms and associated facies in laterally persistent outcrops and measured several vertical logs that were subsequently merged into a composite log. From base to top, the succession is composed of a muddy glacitectonized substrate, and a subglacial to ice-marginal facies association including a basal, boulder-rich, subglacial lag of local extent, and submarine deposits belonging to a grounding-line fan. In the grounding-line fan association, we identified bedforms related to concentrated flows that represent the complete spectrum of flow regime transition from supercritical to subcritical in a waning wall jet model. The proximal zone was dominated by highly turbulent supercritical flows, resulting in highly scoured conglomerates and deposition of unstable and stable antidunes. A transitional zone under transcritical flow conditions resulted in deposits represented by humpback dunes. In the distal zone, the flow behaved as subcritical, resulting in the deposition of dunes and climbing ripples. Debris flows were also active. These cohesive flows underwent dilution by water entrainment and erosion of the head, leading to the generation of turbulent portions in the head of the debris flows. This bipartite behavior of the flows resulted in thin sandstones capping diamictites, which compose the fabric of the glacigenic debrites observed in the study area. The described facies and bedforms point to the existence of a grounded ice margin at the final stages of the LPIA in the SE portion of the Paraná Basin.