82-4 Tsunami-lain Grainstone Beds in a Shallow, Tranquil, Muddy Epeiric Sea: The Selkirk Member of the Red River Formation (Upper Ordovician), Williston Basin of Southern Manitoba, Canada

Abstract:

Dolomitic limestones of the Selkirk Member of the Red River Formation (Katian, Upper Ordovician) of southern Manitoba were deposited in the shallow, equatorial, epeiric Williston Basin which was contiguous with a vast epicontinental sea that covered a large part of Laurentia. The Selkirk consists of thoroughly bioturbated, fossiliferous and bioclastic wackestone and packstone deposited within the photic zone. Northeast of Winnipeg, the member is 43 m thick and the lower part is quarried for Tyndall Stone because of its striking fabric (‘tapestry’) due to selective dolomitization related to Planolites and Palaeophycus burrows. Macrofossils include a diverse fauna of unusually large receptaculitids, stromatoporoids, gastropods, nautiloids, tabulate corals, solitary and colonial rugose corals, plus rare trilobites and brachiopods. Bioclasts include abundant, variably bioeroded crinoid ossicles, along with calcareous green algae and mollusc and coral fragments. The massive muddy limestone is punctuated by lenticular beds consisting of bioclastic and intraclastic grainstone to rudstone locally exhibiting variably distinct planar and unidirectional cross-lamination and crude normal grading. Dislodged shells and skeletons are commonly incorporated. These beds are ~0.5–5 m wide and up to 10 cm thick, have erosional bases, and are circular, elliptical, or lobate in plan view. Some beds appear concentrated along certain horizons whereas others are not. Paleocurrent direction cannot be determined precisely with the mostly two-dimensional quarry outcrop, but it seems to have been broadly towards the present-day north. Shallow subsurface cores indicate that grainstone beds are present up to about ~50 km to the southwest, west, and northwest, but are absent in the northern outcrops hundreds of kilometers away. They are also absent in cores in the deep subsurface of southern Saskatchewan. Like many comparable, high-energy beds punctuating low-energy facies, they have been reflexively assumed to be tempestites. However, evidence for storm wave-induced oscillatory currents is absent. Instead, the sedimentary attributes may be attributed plausibly to tsunamis triggered by movement on basement faults, those inferred in the central part of the basin as well as cryptic local structures. This interpretation suggests that other high-energy event beds in ostensibly tranquil epeiric sea deposits may not be due only to storms, thereby challenging interpretations of paleoclimate, bathymetry, and syndepositional tectonic activity.

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