Influence of channelized-flow density structure on the stratal architecture of deep-marine levee deposits

Abstract

ABSTRACT Deep-water channel and levee deposits are common depositional elements on modern and ancient continental slopes. Unlike their channel counterparts, the spatial and temporal evolution of levee stratigraphy is much less well understood, in part because of the typically more recessive nature of levee deposits in the ancient sedimentary record, and sparse, widely spaced core control or seismic images of insufficient resolution in the modern. Moreover, it is generally inferred that levee development, at least in part, precedes the main phase of channel filling, the reasons for which remain largely unknown. In the Isaac Formation of the Windermere Supergroup (Neoproterozoic) of east-central British Columbia, Canada, well-exposed levee deposits are divided vertically into packages, each consisting of a sand-rich lower part overlain sharply by a mud-rich upper part. The lower part (3–10 m thick) consists mostly of medium- to thick-bedded, upper medium- to coarse-grained, lower-division turbidites intercalated with thin-bedded, fine-grained, upper-division turbidites. Along depositional strike away from channel-fill margins, the thickness of lower-division turbidites exhibit a distinctive thickening and then thinning over a few hundreds of meters that results in a similar thickening and thinning of the entire lower part of a package. The upper part (3–16 m thick) consists mostly of thin-bedded, fine-grained, upper-division turbidites intercalated with uncommon medium- to thick-bedded, medium-grained, lower-division turbidites. Significantly, the thickness of very thin- and thin-bedded turbidites in the upper part generally decreases stratigraphically upward whereas the thickness of intercalated medium- and thick-bedded turbidites changes little. The lateral and vertical changes in these deposits suggest that channelized flows were initially coarse grained and moderately well-sorted, causing them to exhibit negligible density stratification, and therefore high flow efficiency. We interpret that the velocity maximum occurred above the height of the incipient channel margins, thereby allowing the lower, coarse-grained, dense part of flows to easily overspill and deposit thick-bedded, coarse-grained turbidites in the lower part of each package. The sharp contact with the upper part of each package marks the point when relief from channel floor to levee crest exceeded the height of the velocity maximum in average throughgoing turbidity currents. Above this height, density of the flow decreased abruptly and consisted of significantly finer-grained sediment that overspilled to form the upper, finer-grained part of each package. Later the makeup of the sediment supply changed to a more polydispersed grain-size distribution, which caused the throughgoing currents to be more density stratified. This enhanced near-bed stratification and concentration effects, which in addition to intense interfacial mixing, resulted in rapid kinetic energy loss, and promoted deposition in the channel.