BETWEEN TWO BASINS: FACIES ANALYSIS OF ONTARIO SILURIAN CARBONATE ROCKS NEAR A BURIED BEDROCK VALLEY

As key factors in determining porosity, analysis of bedrock facies and diagenesis provides essential context for a hydrogeologic investigation into the Elora buried bedrock valley. This study site provides an interesting opportunity for sedimentological study through its position on a structural arch bounding the Michigan and Appalachian Basins, a combination of fabric destructive and retentive dolomitization of the carbonate, and a history of nomenclature confusion that has made correlation with regional stratigraphy difficult. Four bedrock boreholes were cored along two transects orthogonal to the valley (NE and SW; 3.5 km apart) using a PQ3 triple tube core barrel from the Guelph dolostone interface aquifer to the depth of the regional Cabot Head shale aquitard (~150 mbgs). Detailed sedimentological logs were used to construct a local facies model and to study cyclicity within cored Formations. Petrographic study of recovered core samples supplements this analysis and enables a reconstruction of major diagenetic events in the system’s history.

Data reveals a laterally heterogeneous net-shallowing succession that records a maturing carbonate accumulation adjacent to thick inter-reef deposits. At the NE transect, building on top of the shale datum are multiple meter-scale crinoid accumulations that diversify upwards to include robust tabulate coral and later stromatoporoids while crinoids gradationally disappear. At the SW transect, basal crinoid accumulations are truncated early by chert deposits that grade upwards from bedded to nodular and are embedded in a dolomudstone matrix that becomes laminated upwards. Chert-bearing beds are overlain by ~60 m of dolomudstone with rare tabulate coral and stromatoporoid dolofloatstones. Both transects are capped by mollusc-dominated skeletal deposits and coral-stromatoporoid accumulations. Integrating the facies model with ongoing petrographic analysis enables a tie between facies associations and porosity-enhancing or destroying diagenetic events as well as providing insight that helps distinguish between competing paleogeographic reconstructions. Future integration with hydraulic datasets (head profiles, etc.) will determine the importance of facies and diagenesis in altering flow pathways of the local hydrogeologic system.