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The Importance of Grain Size and Grain Size Distribution on Deep-Marine Channel Evolution

Abstract

In the fluvial sedimentary record sinuous channels and their associated fills are typically manifest as laterally-accreting point bar deposits. In the deep-marine, observations in modern systems and high-resolution seismic time slices indicate that channels there too are commonly sinuous. However, much less frequently reported from the modern or ancient sedimentary records are the associated meter- to several meter-scale laterally-spaced features that typify fluvial lateral accretion surfaces. In the Neoproterozoic Windermere Supergroup (WSG) exceptional exposure and vertically-dipping strata allow the easy recognition of surfaces resembling lateral accretion surfaces in a number of channel fills. These deposits tend to form at the top of much larger, aggradationally-filled (sinuous) channels, or as isolated clusters. Channel fills are 10–15 m thick and consist of amalgamated beds of decimeter-thick, very coarse sandstone/granule conglomerate. These strata, then, are overlain abruptly vertically and obliquely-upward by mudstone interbedded with thin-bedded turbidites. These finer, thinner strata are interpreted to be the inner-bend levee deposits onto which the channel-filling, thicker-bedded, coarser grained strata onlap. Moreover, the successive several-meter-scale lateral-offset stacking of these strata is interpreted to be caused by the continuous lateral migration of a single channel. This, then, begs the question as to why this style of channel filling is common here, but uncommon in much of the deep-marine sedimentary record. Laterally-accreting channels in the WSG are typically filled with very coarse sandstone, granule and even rare pebble conglomerate. These strata are notably coarser than those that fill the many other WSG channels that lack lateral accretion. The coarseness, but also the bimodal grain size distribution of the sediment supply, is interpreted to have had two interrelated consequences: channelized flows were highly density stratified, and accordingly most of the flow's momentum resided in the basalmost part of the flow. This, then, enhanced erosion along the outer bend, which in turn promoted active lateral migration of the channel. The general lack of well-developed lateral accretion in the deep-marine sedimentary record, therefore, may simply be the general lack of a sufficiently coarse and bimodally distributed sediment supply to alter the structure of the channelized flows and encourage lateral channel migration.