Banding in Turbidite Sandstones 1 2

9 Recognition and interpretation of sedimentary structures is fundamental to understanding 10 sedimentary processes. Banded sandstones are an enigmatic sedimentary facies comprising 11 alternating mud-rich (as matrix and/or mud clasts) and cleaner sand layers. The juxtaposition of 12 hydrodynamically different grain sizes contradicts established models of cleaner sand bedform 13 development. Here, outcrop, subsurface core, and petrographic data from three deep-water 14 systems, with well-constrained paleogeographic contexts, are used to describe the range of 15 sedimentary textures, bedform morphologies and facies associations, and to quantify the mud 16 content of banding. Banding can occur in any part of a bed (base, middle or top), but typically 17 overlies a structureless basal sandstone or mud clast conglomerate lag, and is overlain by clean 18 parallel-laminated sandstone and/or ripple cross-lamination. Banding morphology ranges from sub19 parallel to bedforms that comprise low-angle laminae with discontinuous lenses of mudstone, or 20 asymmetric bedforms comprising steeply dipping foresets that transition downstream into low21 amplitude bedwaves, or steeply dipping ripple-like bedforms with heterolithic foresets. This style of 22 banding is interpreted as a range of bedforms that form progressively within the upper stage plane 23 bed flow regime via tractional reworking beneath mud-laden transitional plug flows. The balance of 24 cohesive and turbulent forces, and the rate of flow deceleration (aggradation rate), govern the style 25 of deposit. Banded sandstones and linked-debrites are rarely found juxtaposed together in the same 26 bed as they are distributed preferentially in proximal and distal settings, respectively. Understanding 27 the origins of banding in turbidite sandstones, the conditions under which it forms, and its 28 distribution across deep-water systems and relationship to linked-debrites, is important for it to be 29 used effectively as a tool to interpret the geological record. 30

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