Matrix-Rich Sandstones in Base-of-Slope and Basin-Floor Strata of the Neoproterozoic Windermere Supergroup: Hydraulic Jump Deposits and the Initiation of Local Sedimentation

Terlaky, Viktor; Arnott, Bill

Recently matrix-rich sandstones, commonly termed hybrid event beds, have been increasingly recognized as a significant component of basin-floor stratigraphy in many deep-water systems. In deep-water slope and basin-floor deposits of the Neoproterozoic Windermere Supergroup (WSG) matrix-rich sandstones are similarly common and consist of structureless, coarse-tail graded, coarse to medium sandstone with 30-50% mud matrix. Bed contacts are sharp and planar, although rare several cm deep scours are observed locally. Mudclasts are several cm thick and up to several dm long, and locally are abundant. In some beds matrix content increases abruptly from 20-30% to up to 50% in the upper half of the bed, which then is commonly capped by a several cm-thick mud- and/or siltstone cap. Beds of matrix-rich sandstone are uncommonly observed isolated within the stratigraphy, but more commonly form laterally continuous (10s to 100s of meters wide) units that range up to several meters thick. In base-of-slope strata matrix-rich units are laterally adjacent to channels; in proximal basin-floor strata they are laterally adjacent to several-m deep scours, and typically underlie terminal splay and distributary channel fills; in more distal basin-floor strata they underlie terminal splays. Significantly, matrix-rich units are most abundant in proximal basin-floor strata where they occur laterally adjacent to coarse sandstone-filled scours. These sandstones are typically erosive based with anomalously large (up to several dm thick and several m long) tabular, internally stratified mudstone and sandstone clasts.

The matrix-rich, poorly-sorted nature of the beds and abundant mudstone clasts are most probably related to upflow erosion of a muddy substrate. Coarse-tail grading suggests very high rates of deposition from a turbulent flow. Similarly, adjacent deep scours and coarse sandstones with large tabular clasts indicate intense scouring and rapid deposition. Deposition of matrix-rich beds is most probably related to seafloor erosion within a hydraulic jump with subsequent rapid deposition from the quickly and now over-charged flow. Matrix-rich strata adjacent to channels in base-of-slope settings are interpreted to be crevasse splays. In basin-floor settings, on the other hand, matrix-rich units that underlie terminal splay and distributary channel elements are indicative of local channel avulsion.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013