Evidence for basin restriction during syn-collisional basin formation in the Silurian Arisaig Group, Nova Scotia

摘要

The Arisaig Basin in Nova Scotia, Canada provides a regional sedimentary record of an Early Silurian basin subsidence on the microcontinent of Avalon. The lower members of the Arisaig Group (Beechill Cove and Lower Ross Brook Formations) are likely to be deposited in a basin formed during syn- to post-collisional rifting/subsidence driven by the collision of Avalon and Baltica. The evolution of this constricted basin can be tracked by examining the geochemistry of the sedimentary rocks deposited within the basin. Here we report on the chemostratigraphy of the Arisaig Group, a series of siliciclastics, with a focus on understanding the redox chemistry during sedimentation. The trace and major-element geochemistry of this group provides insight into the oceanographic and geological processes that were influencing its depositional environment. In particular, concentrations of U, V, Mo, Ni, Cd, and Pb indicate regional shifts in ocean redox state, which serve as a proxy for changes in ocean circulation and regional tectonic activity. We find evidence for anoxic conditions near the base of Ross Brook Formation, and attribute this anomaly in redox status to a significant change in physical mixing in the region. Trace element concentrations indicate that this anoxic event was associated with poor circulation rather than elevated productivity and organic matter burial. Based on these data, we confirm that these sediments are deposited in a tectonically formed basin during a period of limited ocean circulation.