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Cyclic Steps and Hydraulic-Jump Unit Bar Sedimentation in Deep-Marine Sandstones of the Neoproterozoic Windermere Supergroup, Cariboo Mountains, British Columbia, Canada


Although ubiquitous in fluvial strata, dune cross-stratified sandstone is comparatively rare in the deep-marine sedimentary record. In basin floor deposits of the Neoproterozoic Windermere turbidite system, graded, but generally poorly sorted, structureless coarse-grained sandstones (Bouma Ta division) are commonly, but locally, overlain by well-sorted, high-angle cross-stratified, coarse-grained sandstone that resembles dune cross-stratification. Moreover, these sandstones commonly have high intragranular porosity and a distinctive red color related to a pervasive ferroan calcite cement with depleted (i.e. bacterially mediated) C13 values, both suggestive of early (near seabed) cementation. Cross-stratification occurs as a single set that generally is of the order of a few up to several decimeters thick -- multiple sets occur but are less common. Set bases show negligible angle of climb, but more commonly angle downward into the underlying Ta part of the bed. In contrast, the top of the set remains more or less horizontal. Of particular note also is that high-angle cross-stratification commonly transitions upflow into planar-laminated sandstone. Here, individual planar lamina pass abruptly downflow into a single cross-lamina, which with aggradation of the upflow planar laminated unit forms a downward thickening (wedge-shaped) cross-stratified set. Further downflow, the cross-stratified unit rapidly thins and becomes replaced laterally by planar laminated sandstone, which in many cases terminates abruptly in an overturned fold ('swirl’ structure) or a single ripple cross-stratified set. Both terminate rapidly downflow. Planar to high-angle cross-stratified sandstones form localized units separated by several Dm along the top of discrete Ta beds. As noted earlier localized development is not caused by post-depositional erosion. Alternatively it could be related to limited available sediment, however these structures overlie a sand-rich, areally extensive Ta bed substrate. Planar leading downflow to high-angle cross-stratified sandstone is interpreted to be antidune unit bars formed in a submerged hydraulic jump associated with cyclic steps. More specifically, the planar laminated unit forms the upflow part of a hydraulic-jump unit bar that on its downflow side transitions sharply into a prograding depositional wedge forming well-sorted, “dune-like” cross-stratification.