Matrix-Rich Sandstones – an Essential Component of Base-of-Slope and Basin-Floor Avulsion Splays in the Neoproterozoic Windermere Supergroup

Terlaky, Viktor and Arnott, Bill
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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. Beds are typically flat based (non-erosive), and consist of structureless, coarse-tail graded, coarse to medium sandstone with 30-50% mud matrix. Mudclasts are locally abundant, several cm thick and up to several dm long. Beds are 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 overlain abruptly by thick, coarse-grained channel fills; in proximal basin-floor strata they are laterally adjacent to up 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, and transition laterally into thin-bedded, fine-grained Tcd turbidites. Near the scour margins matrix-rich beds are intercalated with coarse-grained sandstones that typically are 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. Structureless, coarse-tail graded sandstone 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 plane-wall jet and/or hydraulic jump with subsequent rapid deposition from the over-charged flow. Matrix-rich strata beneath channel fills in base-of-slope settings or terminal splay and distributary channel elements on the basin floor are a distinctive lithology harbinger of local channel avulsion.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013