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Variations in Braided Fluvial Styles Related to Topography and Climate in the Cambrian-Ordovician Potsdam Group, Ottawa Embayment and Quebec Basin


Siliciclastic braided fluvial deposits are common in the lower part of the Potsdam Group in the Ottawa Embayment and Quebec Basin. Four end-member types of braided fluvial deposits are recognized in the Potsdam, including: A: poorly channelized perennial, B: well channelized perennial, C: poorly channelized ephemeral and D: well channelized ephemeral. Type A consists of 30-90cm thick sheet-like packages of coarse cross-stratified sandstone forming low-angle downstream and lateral accretion elements bounded by sharp sub-horizontal bounding surfaces. It forms 30-100 m thick units that can be correlated over 10's of km. Type B consists of 2–4m packages of coarse sandstone and conglomerate that form steep downstream- and lateral accretion elements, well-defined channel margins and scour hollows. Type C comprises medium to coarse sandstone dominated by upper stage planar lamination, antidune stratification and hydraulic jump scour fills forming sheet-like packages bounded by sub-horizontal bounding surfaces and interbedded with 5-30cm thick eolian sand flat deposits. Type D is characterized by 20-100 wide and 1.5-3.5m deep channel scours filled with 10-30cm thick waterlain dune and ripple deposits at the base overlain by medium to coarse grained eolian wind ripple stratification onlapping the channel margins. Deposition of each end-member type is controlled primarily by two important variables: climate and topography. Types A and B are interpreted to represent steady, quasi-stable fluvial systems fed by perennial runoff in a humid climatic setting; types C and D, on the other hand, record brief, episodic high-energy flows during periods of aridity. Nevertheless, types A and C exhibit a similar sheet-like geometry, most likely reflecting deposition on broad open channels on extensive braidplains. Types B and D, however, suggest flow confinement, most probably related to local topography, including fault scarps. This is supported by stratigraphic data that show a consistent intercalation of Type A and C deposits and Type B deposits D deposits, suggesting that topographical effects (or lack thereof) persisted irrespective of changes in climate, and accordingly fluvial conditions. Moreover, surfaces that separate strata deposited during humid versus arid can be traced regionally suggesting that they can form reliable markers that can aid in correlating coeval braided fluvial deposits separated by paleotopography.