CRATONIC ARCH ACTIVITY AND BASIN DISSECTION IN EARLY PALEOZOIC SOUTHEAST LAURENTIA RECORDED BY PRE- AND SYN-TRANSGRESSIVE STRATA OF THE POTSDAM GROUP

In the Ottawa Embayment (OE) and Quebec Basin (QB) the Potsdam Group records Cambrian-Ordovician siliciclastic deposition preceding and in the early stages of marine transgression. The OE and QB are separated by a structural arch called the Oka-Beauharnois Arch (OkBA). Because faults along the OkBA are post-Ordovician it is unclear whether it was present during Potsdam sedimentation; however, the Potsdam isopach and make-up of its constituent lithofacies change dramatically in the vicinity of the OkBA, forming unique eastern and western successions (ES and WS) and implying arch involvement.

The ES is 100-700 m thick and comprises three conformably stacked units: E1, marine mudstone, arkose and dolostone; E2, arkose of braid delta origin; and E3, eolian, marginal and shallow marine quartz arenite. Detrital zircon ages from E1 and E2 form single peaks at ~1.16 Ga consistent with the timing AMCG plutonism and the Shawinigan Orogeny in the region. In contrast, ages from E3 indicate regional sourcing including diverse Grenville sources and minor input from the Superior Craton.

The WS is 10-50 m thick and consists of three unconformity bounded units of quartz arenite and conglomerate. W1 comprises eolian and fluvial deposits; W2, mainly fluvial; and W3, marginal and shallow marine. Detrital zircon ages from all three units are similar to those from E3, recording regional sourcing from the Grenville and Superior provinces.

Dramatic differences in thicknesses and the nature of the internal bounding surfaces between the WS and ES reflect profound differences in accommodation space, likely due to major differences in rates of subsidence across the ancient OkBA. Moreover, although three units are identified in both the ES and WS, it is unlikely that the lower two units of each correlate due to differences in facies, composition and provenance. We suggest that through much of the early Potsdam deposition the ancient OkBA acted as a barrier that separated continental conditions and regionally sourced sediments in the west from marine conditions and locally sourced sediments in a rapidly subsiding sub-basin east of the OkBA. Later, the OkBA was breached and sediments from regional sources moved eastward. Finally, the arch was transgressed, and siliciclastic replaced by carbonate sedimentation across the entire basin.