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Directional Scours on a Transgressive Surface: Examples from the Silurian Whirlpool Sandstone of Southern Ontario, Canada
Richard J. Cheel (1), Gerard V. Middleton (2)
The Whirlpool Sandstone (basal Silurian of southern Ontario and New York) consists of two units: a lower fluvial unit, deposited by braided rivers on a paleoslope inclined to the NW, and an upper marine unit, consisting of interbedded sandstone and shale deposited offshore during a marine transgression. The transgressive surface that separates these two units has been mapped across the whole outcrop belt; at outcrop scale it is characterized by scours, several meters across and a few decimeters deep, cutting into the fluvial sandstone below. The scoured transgressive surface locally displays a thin, discontinuous shale-clast lag, marine burrows extending down into the fluvial sandstone, and extensive fields of symmetrical wave ripples. Wave ripples on this surface and higher within th marine unit show a regionally consistent NNW-SSE crest alignment, indicating the trend of the shoreline during transgression.
Exceptional exposures of the transgressive surface, produced during quarrying operations near Georgetown, show that the transgressive scours include parabolic, linear, and irregular forms. Parabolic scours are several meters long and have a SW-pointing apex with arms opening towards the NE. Linear scours are cigar-shaped to channel-shaped and trend NE-SW: one channel-like scour deepens towards the NE. The scours clearly were formed by marine erosion (probably by storm-induced, offshore-directed currents) during transgression at a time when the fluvial sandstone was somewhat compacted but not cemented. They indicate flow offshore towards the NE, suggesting that the transgression proceeded from NE to SW. The change from a regional paleoslope to the NW, during deposition of the fluvial l wer Whirlpool, to a transgression moving westwards across a shore trending NNW-SSE implies tectonic tilting of the basin towards the east, probably as a result of thrust emplacement at the continental margin.
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