Abstract

Two fluvial facies associations, braided and ephemeral fluvial, are recognized in strata of the Cambrian–Ordovician Potsdam Group in the Ottawa Embayment and Quebec Basin in northeastern North America. Braided fluvial sedimentation was dominated by the migration of coarse-grained sandy dunes and rare unit bars and local gravels that built up low-relief 0.4–5-m-thick downstream-accreting compound bars in very wide and shallow braided channels. Rare meter-scale confluence scours, incision, and filling of decimeter-thick bypass channels, and thin (≤ 7 cm) overbank deposition also occurred. Ephemeral fluvial sedimentation was dominated by the progradation of lobate terminal splays, most of which were deflated and/or reworked into thin (10–95 cm) sheets by eolian processes during the long recurrence intervals between floods. These strata were locally incised by rare, 0.3–1.9-m-thick scour-filling high-energy supercritical bedform strata and decimeter-thick bypass channels. Both braided and ephemeral fluvial deposits consist of meter-scale stacked, high-order stratal units separated by horizontal, laterally extensive surfaces recording the recurring buildup, avulsion, and finally abandonment of channel belts.

In braided fluvial strata of the Potsdam the dominance of thin (≤ 2 m) elongated compound bars with low-relief (≤ 5°) bar margins and internal accretion surfaces (≤ 15°) suggest that braided channels and in-channel compound bars expanded laterally with negligible increase in flow depth during periods of increased discharge in the absence of bank-stabilizing vegetation, similar to other examples of the pre-vegetated “sheet-braided” style. However, in areas where bedrock topography inhibited the lateral expansion of channels, thicker compound braid bars (1.2–3.5 m) with higher-relief accretion surfaces (up to 35°), like those observed in post-Silurian and modern systems, preferentially formed in relatively deep channels. Ephemeral fluvial deposits here are similar to ephemeral examples throughout geologic history in that they mostly comprise poorly confined sheet-like terminal splays that accumulated in topographic lows. However, compared to several post-Silurian examples, strata in the Potsdam exhibit considerably more eolian reworking of splays and preservation of supercritical-bedform strata, a manifestation of greater rates of surface-water infiltration due to a lack of sheltering by rooted plants and propensity for unconfined flooding. Finally, in the upper part of the Potsdam two regional ephemeral fluvial units are interstratified with two braided fluvial units, providing evidence for shifts in regional climate. The sharp, areally extensive surfaces that separate these fluvial systems are interpreted to approximate time lines, and therein provide a means for relative and absolute chronostratigraphic correlation. These changes are correlated with documented Late Cambrian to Earliest Ordovician global climate fluctuations, with semiarid conditions and related ephemeral fluvial systems corresponding to global cooling events at ca. 491 and 487 Ma.