Deposits of high-density turbidity currents on fan-delta slopes: an example from the upper Visean Szczawno formation, Intrasudetic Basin, Poland

Abstract

The sedimentary succession of the Szczawno Formation (upper Visean, Intrasudetic Basin) comprises several lithofacies which reflect different transport/deposition mechanisms. Six facies associations interpreted as deposits of (1) alluvial-delta plain, (2) fan-delta front, (3) prodelta, (4) subaqueous fan, (5) basin plain and slope, and (6) shallow-marine shelf are distinguished. Coarse-grained, fan-delta-front deposits form large-scale (several tens of metres thick and 2–3 km in cross section) lensoidal bodies enveloped by fines of prodeltaic to basin plain settings. They represent channel-and-lobe complexes developed on frontal faces of fan deltas which entered the submerged, intramontane basin from the north. Fan deltas evolved from alluvial fans built along tectonically active, steep northern basin slope. Sedimentation on the delta fronts was dominated by gravity flows. Most sediments deposited from gravelly and/or sandy, high-density turbidity currents. The sediments display a diverse suite of facies related to variable flow parameters (sediment concentration, grain size, velocity, slope gradient, etc.). Sedimentary units commonly amalgamate, which is explained in terms of surging nature of flows. The growth of the fan deltas ceased due to rejuvenation/uplift of the southern edge of the basin, which caused a northeastward progradation of a large fluvial/deltaic system and, consequently, a marine regression.