Mud‐rich low‐density turbidites in structurally‐controlled intraslope mini‐basin: The influence of flow containment on depositional processes and sedimentation patterns (Szczawa, Oligocene, Polish Outer Carpathians)

Abstract

ABSTRACTThis study demonstrates the effectiveness of sedimentological–statistical multivariate analysis of one‐dimensional‐section in the unravelling depositional processes and sedimentation patterns recorded in a contained and partially ponded succession. Turbidite deposition of a 100 m thick mud‐rich Oligocene‐age sequence at Szczawa, the Polish Outer Carpathians, was primarily controlled by topographic confinement and magnitude of incoming flows. Only the largest turbidite flows were subjected to the true flow ponding, while in smaller volume flows the silty/sandy part behaved as unconfined flow and only muddy suspension cloud developed a ponded character. True ponding of low‐density turbidity currents is interpreted for sandstones that show sedimentary structures attesting to flow reflections and combined‐flow processes, and are associated with abnormally thick co‐genetic mudstones, which in turn results in low sandstone‐to‐mudstone ratio, irrespective of turbidite bed thickness. These features testify to pronounced interaction of flows with confining topography. Tractional structures associated with a considerable proportion of fines, namely banded structure and heterolithic bedding, are interpreted as produced during transitional (turbulent to laminar) flow phase in the condition of mud‐rich turbidite flow confinement. These structures are also considered here as indicators of proximity to source area and location on the slope. The co‐occurrence of banded sandstones, hybrid event megabeds and an olistostrome resulted from flows‐trapping, confined basin geometry, which precluded downslope propagation and further transformation of large‐volume flows. Therefore, a structurally‐controlled contained intraslope mini‐basin is proposed here as the most likely depositional setting. This work provides robust field and statistical evidence for ponding processes of low‐density turbidity currents in a structurally‐controlled mini‐basin. The results of this study are consistent with experimental data and recent field studies on ponded turbidites. Therefore, the Szczawa succession may serve as a new reference example of containment processes of low‐density turbidity currents and represents a valuable depositional model for intraslope turbidite succession.