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

Abstract:

Like continental environments, sinuous channels are common geomorphic features on deep-marine slopes. However unlike their fluvial counterparts well developed lateral accretion surfaces related to episodes of lateral channel migration are comparatively rare. Instead most deep-marine channels fill aggradationally. This, then, begs the question as to the nature and origin of the seemingly uncommon sedimentological conditions that result in laterally accreting deep-marine channels.

In the Neoproterozoic Windermere Supergroup (WSG) channels filled with well developed lateral accretion surfaces are well exposed and occur 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, in turn, 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. Notably also these strata are generally coarser than those that fill the many other WSG channels that lack lateral accretion. The coarseness, but more importantly the bimodal grain size distribution of the sediment supply, is interpreted to have had caused the channelized flows to be highly density stratified, and for density to be equally distributed throughout the lower part of the flow. Together these conditions caused the momentum and related fluid circulation patterns in the lower part of the flow to resemble those observed in rivers, and hence sediment transport patterns to be meandering-river-like with deposition along the inner bend and erosion along the outer bend.