Blackriveran (lower Mohawkian, Upper Ordovician) lithostratigraphy, rhythmicity, and paleogeography: Ottawa Embayment, eastern Ontario, Canada

The Ottawa Embayment contains erosional remnants of a shallow-water carbona te platform (St. Lawrence Platform) of Late Ordovician (Caradocian) age. St ratigraphy of three Blackriveran formations - in ascending order, Pamelia, Lowville, and Chaumont - documents regional changes in continuity and types of depositional facies. The Pamelia Formation contains two members, each c ontaining basin-wide basal siliciclastic units overlain by interbedded lime stone, dolostone, and shale. An alternate division recognizes six shallowin g-upward units; regionally extensive, intertidal to supratidal, dolomitic s andstone and (or) sandy dolostone define their tops. Sedimentary structures and isotope (C, O) geochemistry support a syngenetic origin of this strati graphic (bedded) dolomite. The Lowville Formation contains two facies assoc iations: subtidal to lower intertidal bioclastic and oolitic packstone - gr ainstone followed by lagoonal to intertidal mudflat facies. Lateral facies continuity is reduced compared to the Pamelia Formation. The Chaumont Forma tion contains thick beds of burrowed, bioclastic, peloidal mudstone to pack stone, and minor shale. No rhythmic pattern is recognized in these subtidal facies. Upsection decrease of rhythmic sedimentation, with a decrease in l ateral facies continuity of the studied strata reflects a progressive incre ase in net accommodation space related to Taconic transgression. Higher ord er rhythmicity of dolostone and sandstones of the Pamelia Formation can be traced into adjacent regions (New York and Kingston, Ont.). Dolomitic units may identify basin-wide chronostratigraphic markers, potentially useful fo r future sequence analysis. Regional correlation reveals a good oceanograph ic linkage between the Ottawa Embayment and the Appalachian Basin during Pa melia time and a restricted access across a paleohigh in the Montreal regio n. By the time of Lowville deposition, Taconic transgression had breached t his restriction.