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ABSTRACT: The Queenston Formation: Mixed Muddy-Carbonate Deposits of Semiarid Ordovician Shores in Ontario, Canada

P. Brogly, I. P. Martini

The well-known, but generally poorly understood, Queenston "delta" constitutes the westward expression of the last stages of the Taconic orogeny of eastern North America. In Ontario, the Queenston Formation is composed primarily of red mud, locally gypsiferous, alternating with storm layers of bioclastic carbonate and mixed quartzose-carbonate siltstones and sandstone. Few supratidal gypsiferous sequences are preserved: For the most parts, they and other sun-cracked muddy shoreline deposits have been reworked into a shallow shelf and interbedded with carbonate layers derived from reworking of offshore mounds. The ancient paleogeography is visualized as a flat land characterized by unvegetated coastal floodplains that trapped most of the red mud and were crossed by shallow muddy streams; these change basinward and laterally into vast muddy supratidal areas occasionally swept by storm surges, where thin layers of gypsum [and anhydrite(?)] and minor halite were formed in evaporating pans or within soil profiles near salty lagoons; the coast was further characterized by wide and flat muddy intertidal areas, perhaps locally displaying muddy wavy surfaces grading seaward into a clastic-carbonate mixed shelf, grading farther offshore into a shallow, more limpid sea where carbonate mounds flourished. Lateral migration of these settings and exceptional strong storms provided for reworking of most of the intertidal and subaerial mud-cracked sequences. These changing coastal situations were in part due to variation in mud supply from the Taconic orogen, and in part, at least in the middle and uppermost part of the sequence, to fluctuations in sea level owing to the contemporaneous Ordovician glaciations centered in what is now the Sahara desert.

AAPG Search and Discovery Article #90998 AAPG Eastern Section Meeting, London, Ontario, Canada, September 10-12, 1990