TB or not TB: banding in turbidite sandstones

Abstract

Recognition and interpretation of sedimentary structures is fundamental to understanding sedimentary processes. Banded sandstones are an enigmatic sedimentary facies comprising alternating mud-rich (as matrix and/or mud clasts) and cleaner sand layers. The juxtaposition of hydrodynamically different grain sizes contradicts established models of cleaner-sand bedform development. Here, outcrop, subsurface core, and petrographic data from three deep-water systems, with well-constrained paleogeographic contexts, are used to describe the range of sedimentary textures, bedform morphologies, and facies associations, and to quantify the mud content of banding. Banding can occur in any part of a bed (base, middle, or top), but it typically overlies a structureless basal sandstone or mud-clast conglomerate lag, and is overlain by clean parallel-laminated sandstone and/or ripple cross-lamination. Banding morphology ranges from sub-parallel to bedforms that comprise low-angle laminae with discontinuous lenses of mudstone, or asymmetric bedforms comprising steeply dipping foresets that transition downstream into low-amplitude bedwaves, or steeply dipping ripple-like bedforms with heterolithic foresets. This style of banding is interpreted as a range of bedforms that form progressively in the upper-stage plane-bed flow regime via tractional reworking beneath mud-laden transitional plug flows. The balance of cohesive and turbulent forces, and the rate of flow deceleration (aggradation rate), govern the style of deposit. Banded sandstones and linked debrites are rarely found juxtaposed together in the same bed because they are distributed preferentially in proximal and distal settings, respectively. Understanding the origins of banding in turbidite sandstones, the conditions under which it forms, and its distribution across deep-water systems and relationship to linked debrites, is important for it to be used effectively as a tool to interpret the geological record.

Sole marks reveal deep-marine depositional process and environment: Implications for flow transformation and hybrid-event-bed models

“…Thus, the H1 divisions may represent not only high-density turbidity currents, but also low-density turbidity currents in the presence of plane-parallel lamination, and transitional flows in the presence of grain-size banding, large ripples, and low-amplitude bed waves (Lowe and Guy 2000;Baas et al 2011Baas et al , 2016aStevenson et al 2020). The missing H4 divisions are inferred to indicate a stable, stratified debris flow without significant upper-boundary mixing with ambient water (cf.…”

“…Bedload sediment travels slower than suspended sediment, with the velocity of clasts decreasing as a function of increasing grain diameter (e.g., Bridge and Dominic 1984). Therefore, the mud clasts move backwards relative to the head, with the smallest mud clasts (sub-mm to mm in diameter; Stevenson et al 2020) moving fastest, presumably via saltation, whilst the larger clasts undergo segregation as a function of size, as well as angularity, during bedload transport (Fig. 20).…”

“…This gradual aggradation matches the aforementioned sustained nature of the SGFs, inferred from the presence of migrating dunes in the submarine-channel fill. Where present, the H2 division represents bedform development under slow to moderately decelerating flows in the lower or upper transitional plug-flow regimes (Baas et al 2011(Baas et al , 2016aStevenson et al 2020). Such banded layers frequently incorporate large numbers of small mud clasts, representing the fastest moving part of the bedload component closest to the head of the flow.…”

Substrate Entrainment, Depositional Relief, and Sediment Capture: Impact of a Submarine Landslide on Flow Process and Sediment Supply

“…The location of these facies in the westernmost sector, and the palaeoflow measurements, suggests that the western sector was relatively proximal. Banded sandstones represent the deposits of mud-rich transitional flows formed by tractional reworking (Stevenson et al, 2020). The bipartite beds with a basal structureless to planar laminated sandstone division and linked mudstone clast-rich upper division are interpreted as hybrid event beds (HEBs), formed from transitional flows deposited under high-deceleration rates (Haughton et al, 2009;Hodgson, 2009;Kane & Pontén, 2012) in more distal environments than the banded sandstones (Stevenson et al, 2020).…”

“…Banded sandstones represent the deposits of mud-rich transitional flows formed by tractional reworking (Stevenson et al, 2020). The bipartite beds with a basal structureless to planar laminated sandstone division and linked mudstone clast-rich upper division are interpreted as hybrid event beds (HEBs), formed from transitional flows deposited under high-deceleration rates (Haughton et al, 2009;Hodgson, 2009;Kane & Pontén, 2012) in more distal environments than the banded sandstones (Stevenson et al, 2020). The gradual and diffuse boundary between the basal turbidite and the upper debrite suggest vertical segregation of particles within the cohesive flow (Kane et al, 2017).…”