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Depositional Continuum of Sand to Mud-Rich Basin-Floor Sandstones Associated with a Submerged Hydraulic Jump Downflow of an Avulsion Node, Upper Kaza Group, Neoproterozoic Windermere Supergroup, B.C. Canada

Abstract

Matrix-rich sandstones (30 up to >50% mud/silt matrix) have been increasingly recognized in ancient deep-marine basin-floor strata. Beds are generally thought to be deposited from mud-rich transitional flows but mechanistic details are still poorly understood, partly because details of any lateral lithological changes are poorly known. Similar strata are common in proximal basin-floor strata (Upper Kaza) of the Windermere Supergroup (British Columbia, Canada). Here, vertically-dipping, periglacial exposures provide an excellent opportunity to illustrate the lithological make-up and lateral evolution of these strata in a 40m thick unit over a distance of ∼800m. Three major facies are recognized: matrix-rich and matrix poor sandstones, and thin-bedded, mostly upper division turbidites. Matrix-rich beds are structureless, massive or coarse-tail graded, with grain size ranging from lower medium to lower coarse sand. In thin section the matrix, which represents up to as much as half the bed volume, consists of chlorite, muscovite and silt. Typically, beds are 15-40cm thick, flat based and stack to form units up to a few meters thick. Matrix-poor beds consist of amalgamated, dm-thick, coarse grained Ta sandstone. The outcrop is divided into 3 parts (S1, S2, S3). S1 (∼400m wide) comprises amalgamated, structureless, matrix-poor sandstone. In S2 (∼280m wide), however, bedding is discernible and meter thick, matrix-poor Ta beds overlain sharply by one to more, up to 70cm thick, sandy (medium sand) mudstones are common. The transition from S1 to S2 occurs over several Dm and is marked by the abrupt occurrence of abundant, large (>2m x 1m), irregularly shaped clasts composed of thinly-bedded turbidites or matrix-rich sandstones. The transition from S2 to S3, on other hand, is marked by a rapid (over a few Dm) and dramatic decrease in the thickness and abundance of matrix-poor beds but an increase in matrix-rich sandstones interstratified with thin-bedded turbidites, which stack to form units up to ∼1m thick. The lateral change from matrix-poor to matrix-rich sandstones is interpreted to represent a depositional continuum outboard of a submerged hydraulic jump that formed immediately downflow of an avulsion node. Fine sediment eroded in the jump abruptly changes flow rheology. In addition, the flow becomes highly stratified, with coarse sand remaining in the axis, but medium and finer sand and the abundant mud being partitioned towards its margins.