Matrix‐rich and associated matrix‐poor sandstones: Avulsion splays in slope and basin‐floor strata

A common facies observed in deep‐water slope and especially basin‐floor rocks of the Neoproterozoic Windermere Supergroup (British Columbia, Canada) is structureless, coarse‐tail graded, medium‐grained to coarse‐grained sandstone with from 30% to >50% mud matrix content (i.e. matrix‐rich). Bed contacts are commonly sharp, flat and loaded. Matrix‐rich sandstone beds typically form laterally continuous units that are up to several metres thick and several tens to hundreds of metres wide, and commonly adjacent to units of comparatively matrix‐poor, scour‐based sandstone beds with large tabular mudstone and sandstone clasts. Matrix‐rich units are common in proximal basin‐floor (Upper Kaza Group) deposits, but occur also in more distal basin‐floor (Middle Kaza Group) and slope (Isaac Formation) deposits. Regardless of stratigraphic setting, matrix‐rich units typically are directly and abruptly overlain by architectural elements comprising matrix‐poor coarse sandstone (i.e. channels and splays). Despite a number of similarities with previously described matrix‐rich beds in the literature, for example slurry beds, linked debrites and co‐genetic turbidites, a number of important differences exist, including the stratal make‐up of individual beds (for example, the lack of a clean sandstone turbidite base) and their stratigraphic occurrence (present throughout base of slope and basin‐floor strata, but most common in proximal lobe deposits) and accordingly suggest a different mode of emplacement. The matrix‐rich, poorly sorted nature of the beds and the abundance and size of tabular clasts in laterally equivalent sandstones imply intense upstream scouring, most probably related to significant erosion by an energetic plane‐wall jet or within a submerged hydraulic jump. Rapid energy loss coupled with rapid charging of the flow with fine‐grained sediment probably changed the rheology of the flow and promoted deposition along the margins of the jet. Moreover, these distinctive matrix‐rich strata are interpreted to represent the energetic initiation of the local sedimentary system, most probably caused by a local upflow avulsion.