Unidirectional and combined transitional flow bedforms: Controls on process and distribution in submarine slope settings

Abstract

Mixed grain-size bedforms comprise alternating sand-rich and poorly sorted mud-rich laminae and bands. These bedforms have been identified in distal submarine settings formed underneath unidirectional flows. This study documents mixed grain-size bedforms in a proximal submarine slope setting formed beneath both unidirectional and combined flows. Core and outcrop data with well-constrained palaeogeographical context are used to describe two types of mixed grain-size bedform. Type A bedforms comprise mud-rich current ripples and low-amplitude bed-waves with alternating concave and planar sandstone–mudstone foresets that pass into mud-rich troughs, and aggradational sinusoidal laminasets. Type B bedforms consist of sandstone–mudstone laminasets that comprise rounded, biconvex ripples with sigmoidal-shaped foresets and swale and hummock-like laminasets and banded sets. These bedforms occur in channel-margin, internal-levée and external-levée, intraslope and disconnected lobe environments, and represent 27 to 63% by stratigraphic thickness of the studied successions. They are interpreted as deposits of clay-rich transitional flows, whose depositional style is governed by the balance of cohesive and turbulent forces, and the rate of flow deceleration. Type B bedforms are further interpreted as combined transitional flow deposits, resulting from flow deflection and ponding processes by seabed topography. Upward and lateral transitions between different bedforms create distinct bedform sequences, demonstrating progressive spatio-temporal transformations in flow properties and their topographic interactions. By using a well-constrained palaeogeographical setting, mixed grain-size bedforms are shown to be situated close to sites of erosion into muddy substrates, abrupt losses in confinement, and/or changes in slope gradient. These bedforms demonstrate that flow transformation and transitional flow behaviour are not restricted to distal submarine settings. Furthermore, mixed grain-size bedforms are not a diagnostic criterion for bottom currents, because such flows cannot account for the high mud content in laminasets, or the interlamination of sand and mud.