THERE IS MORE TO CARBONATE ROCK POROSITY THAN COUNTING FOSSILS: INSIGHT FROM DEPOSITIONAL, DIAGENETIC, AND STRUCTURAL HISTORIES OF THE REGIONAL SILURIAN LOCKPORT DOLOSTONE AQUIFER IN ELORA, ONTARIO, CANADA

Conceptual site models that identify hydraulic, mechanical, and electrical properties of formation-scale lithostratigraphy are often used in hydrogeologic investigation. This approach is insufficient to resolve how these properties are controlled by higher-resolution sedimentological and diagenetic attributes. Analysis of facies and diagenesis is a common geologic characterization tool that is underutilized in informing important 3D characteristics.

Analysis of facies and diagenesis using core and thin sections was conducted at a site on the Algonquin Arch in Elora, Ontario, Canada. This work was motivated by a hydrogeologic investigation of the Silurian Lockport dolostone aquifer, a water source for ~1 million Ontarians, which is also present within the Michigan and Appalachian basins (USA). Facies associations record crinoid-coral shoals of variable thickness followed by stromatactis-bearing mud mounds on thick shoals. These strata are unconformably overlain by siliceous sponge-dominated lagoons over thin shoals and a stromatoporoid-dominated biostrome over thick shoals. Beveling by subtidal dolomudstone deposition equalized paleobathymetry, leading to peritidal cycles that cap the succession.

The succession records a complex diagenetic history related to regional structural evolution. Early paleoporosity developed through selective fossil dissolution, karstification, and burial gas expulsion. Invasion of the formation by saline brines dolomitized the succession and occluded paleoporosity with dolomite cementation and gypsum precipitation. Late Paleozoic orogenies and Mesozoic uplift reinvigorated porosity development through fracturing, creation of vugs, and renewed fossil dissolution. Quaternary glaciation likely solution-enhanced fractures.

Porosity patterns and types differ according to facies and diagenesis in each formation. For example: the Gasport Formation (crinoid shoals) is a widely recognized aquifer, but crinoid-rich facies are of low porosity when thickly bedded, yet fractured and high porosity when thinly bedded. Accounting for these controls could lead to ~20 % greater accuracy in local porosity prediction. Understanding geologic variability within formations is critical to the utility of geologic models in informing study of hydrogeologic and mechanical units and their properties.