STRATIGRAPHIC ARCHITECTURE OF THE LOCKPORT GROUP IN ONTARIO AND MICHIGAN - A NEW INTERPRETATION OF EARLY SILURIAN 'BASIN GEOMETRIES' 'GUELPH PINNACLE REEFS'

摘要

The Lockport Group, as recognized in southern Ontario, is a succession of open to restricted marine carbonates of Wenlock age, comprising in ascending order, the: Gasport, Goat Island, Eramosa, and Guelph formations. Despite having been studied for more than 150 years, efforts toward improving the understanding of the relative ages and facies composition of stratal units of the Lockport Group have proven challenging to stratigraphers / sedimentologists, oil/gas explorationists, and biostratigraphers. A few of the major factors that have hindered progress in this regard, include: 1) the paucity of good outcrops featuring the entire Lockport succession; 2) correlation difficulties related to lateral changes of similar lithologies of stacked dolostones due to short-lived tectophases and associated sea-level fluctuations; 3) regional variability in ephemeral forebulge (Arch) migrations; 4) diagenetic alteration and post-depositional karst processes; 5) nomenclatural confusion between subsurface and surface stratigraphic studies; and 5) processing of dolostones for conodont biostratigraphic studies. This paper forms part of a regional bedrock aquifer mapping program by the Ontario Geological Survey to better understand the stratigraphic controls on bedrock groundwater flow systems in Early Silurian carbonates across southern Ontario. It also incorporates data from a recently completed MSc thesis by Brintnell (2012) on the Guelph Formation - providing detailed descriptions of type sections and clarification of the stratigraphic and regional architectural elements of pinnacle reefs or elevated dolostone banks in the subsurface of Michigan Basin. Data from more than 100 rock cores drilled since 2004 and hundreds of deep bedrock water well and oil/gas well records, including selected classic cores from Michigan, have been incorporated to provide the first comprehensive regional stratigraphic perspective of the Guelph Formation and its relationships to underlying Lockport Group strata. The Guelph Formation possesses distinctive facies that collectively contribute to a predictable architectural theme regionally. It can be divided into two Members: 1) basal Wellington Member characterized by skeletal rich and reef mound-bearing carbonates in eastern Michigan Basin (Ontario), and more restricted marine character to non-existent in Michigan; and 2) upper Hanlon Member mat records numerous shallowing-upward cycles that reflect increasingly more restricted marine conditions temporally and laterally from east to west. This does not make sense if the inferred ramp geometries for the Wenlock-age carbonates are supposed to deepen into the inferred centre of the Michigan Basin bulls-eye! New regional drilling reveals that the Guelph Formation is thickest in the eastern outcrop belt of the Niagara Escarpment region of Ontario and becomes more restricted marine and paleo-karsted to non-existent in central Michigan Basin. Eight major facies have been delineated in the Guelph Formation. The most open-marine Guelph facies with minimal subaerial exposure occur in the eastern portion of the Michigan Basin and the Appalachian Sub-Basin. Alternatively, the most restricted marine Guelph strata, with multiple subaerial exposure episodes (paleokarst horizons) that extend downward into the underlying Goat Island Formation, occur in the central portion of the Michigan Basin. Much of what has been called Guelph Formation in the central and northwestern parts of the Michigan Basin actually are stacked carbonates of older Lockport Group formations. These findings lead us to reinterpret the depositional history of the Lockport Group succession in the following scenario: deposition of Eramosa and Guelph carbonates were deposited on an easterly dipping carbonate ramp during part of the Wenlock. The non-reefal, upper Member of the Guelph Formation was subjected to regional erosion/karstification in late and post-Guelph time that is reflected in extensive brecciation of the overlying basal Salina Group strata (A0, A1 and A2 facies). The proximal juxtaposition of younger Salina Group strata enveloping Lockport Group strata (Guelph through Gasport facies) is due to the regional development of Lockport Group karst towers. As such, the paleotopographic low of the upper Lockport Group in central Michigan is an erosional depression and not a deeper water depositional basin feature with seaway outlets and mythical rimmed pinnacle reefs!