Facies-selective porosity in the Silurian (Wenlock) Lockport Group dolostone succession (Elora, Ontario): improving characterization of formation-level porosity

ABSTRACT This study improves porosity conceptualization in the Silurian (Wenlock) dolomitic Lockport Group using core-scale analysis of relationships between depositional facies and the distribution of macroporosity. This dolostone succession forms a crucial regional aquifer, and its shallowly buried to outcropping strata in the study region of Elora (Ontario, Canada) correlate with more deeply buried (> 400 m) petroliferous strata in the flanking Michigan and Appalachian basins. Periods of deposition recorded in the local Lockport Group lithofacies succession were influenced by eustasy and paleo-forebulge movement. Initial deposition established variably thick crinoid–tabulate coral shoals in rising accommodation space over a beveled paleosurface. Thickest shoals (∼ 30 m) are succeeded by tabulate coral mud mounds with stromatactis whereas thin shoals (∼ 15 m) are overlain by argillaceous cherty dolomudstone. Difference in water depth was eliminated through accumulation of subtidal dolomudstone between mounds while stromatoporoid-dominated buildups formed on mound tops. Continued shallowing resulted in regionally extensive fossiliferous peritidal facies. Inter-basin correlation suggests that shallowing in the Lockport succession was coeval with migration of mud mounds into the Michigan Basin and deepening in the Appalachian Basin. In the Lockport succession, about 40% of porosity is fabric-selective, represented by primary (fenestral, intraparticle) and early secondary (moldic) porosity. Fabric-selective porosity favors those facies dominated by tabulate coral, bivalves, and gastropods. An additional ∼ 10% of total porosity is non-fabric-selective (e.g., fractures, vugs, channels) yet exhibits a clear facies influence: solution-enhanced fractures bounding thin crinoid dolorudstone beds and channels developed along some facies boundaries. Remaining porosity is non-fabric-selective and exhibits a more subtle facies influence. This study demonstrates the importance of considering the influence of specific facies on porosity in formation-level hydrogeological analysis.