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SEDIMENTOLOGY, TAPHONOMY, AND PALEOECOLOGY OF METER-SCALE CYCLES FROM THE UPPER ORDOVICIAN OF ONTARIO

¹ Dept. of Geology, University of Cincinnati, Cincinnati, Ohio 45221-0013, USA
² Dept. of Earth Science & Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
³ Dept. of Earth Sciences, University of Western Ontario, London, Ontario, N6A 5B7, Canada
⁴ Dept. of Education, Raymond M. Alf Museum of Paleontology, Claremont, California 91711, USA carlton.brett{at}uc.edu

The Upper Ordovician (Caradoc) Collingwood Member of the Lindsay Formation, southern Ontario, Canada, is a strikingly cyclic package of clastics and carbonates. Cycles are 50–150 cm thick and comprise four major components: (1) dark gray to black, organic-rich, laminated shales that grade upsection into (2) dark to light gray calcareous shales or mudstones, (3) lenticular to tabular concretionary argillaceous limestones, and typically (4) light gray calcareous, fossiliferous mudstones or shales and marls. Black shale units have a characteristically sharp basal contact and overlie a condensed shelly pavement. Fossil in shales are preserved as pavements or stringers of trilobite, ostracode, and brachiopod debris, with strong taphonomic bias as a result of prolonged exposure at the sediment-water interface. Gray mudstones and marls are bioturbated and contain numerous low-diversity orthid brachiopod pavements. Persistent tabular concretionary limestone bands were formed by early diagenetic cementation. These concretionary units include shelly beds that alternate with less fossiliferous calcareous mudstones containing noncompressed, spar-filled burrows and articulated, sometimes in situ fossils. Variation in fossil abundance is the result of cyclic variation in sedimentation, ranging from periods of condensation to rapid burial.

Collingwood cycles involve upsection changes including: (1) benthic oxygenation from lower dysoxic to fully oxic biofacies, (2) increased frequency and episodicity of sedimentation, (3) higher net sedimentation rate within gray mudstone to carbonate intervals, (4) increased environmental energy level, and (5) diagenetic cementation of muds a few centimeters below cycle tops. Consistency of these variations suggests an allocyclic mechanism for the Collingwood cycles related to short-term fluctuations in eustatic sea level or climate.