Characterization of an enigmatic sedimentary dike in southern Ontario, Canada

The injection age and emplacement mechanism of sedimentary dikes can be enigmatic. Understanding these ages and mechanisms can help us to better understand the surface processes (e.g., erosion, deposition, soil formation, glaciation, etc.) and broader geologic setting during dike formation. In this study, we investigated a sedimentary dike cross-cutting Mesoproterozoic basement from southern Ontario, Canada. It is hypothesized that the top of the dike lies along or near the Precambrian-Paleozoic unconformity (Di Prisco and Springer, 1991); therefore, the results of this research may have implications for better understanding this unconformity and more broadly, the Great Unconformity. We analyzed this dike for its mineral composition and internal structure with the goal of better constraining its age and origin. To do this, a thin section was made and visually characterized using a petrographic microscope. It was also viewed using cathodoluminescence (CL) to better identify internal structures that are not visible with standard optical microscopy. With this analysis, we identified that this rock is a carbonate sedimentary rock. This carbonate includes unusual accessory minerals such as biotite and various oxides alongside large carbonate clasts in a fine-grained matrix. Internal variation and flow pattern is visible in the hand sample and thin section. These observations suggest that this dike is a remobilized carbonate sediment that was injected, most likely from above, into the felsic igneous Mesoproterozoic basement. Because this area was once covered in late Cambrian and Ordovician carbonate sediments (Lavois, 2012; Paton and Brett, 2019), it is possible that these cover rocks are the source of the remobilized sediment seen here, which would provide a Cambrian maximum date of emplacement. To our knowledge, this is the first carbonate sedimentary injectite identified and studied in the region. Further research is needed to better understand the surface conditions that generated this dike, but a carbonate injectite in this region of southern Ontario implies an injection event and surface conditions that have not been previously identified.