Sedimentology and Depositional Context Structural Characteristics of a Shallow Homoclinal Ramp During the Middle Ordovician Period in the Lake Simcoe Region, Ontario

الملخص

During the Middle Ordovician, the southern Ontario region was primarily defined by an extensive shallow-water carbonate platform, where a carbonate ramp system developed in the Lake Simcoe vicinity. Sixteen outcrops and quarry sections of the Middle Ordovician Black River-Trenton Group were analyzed to clarify the origins of meter-scale cycles and depositional sequences, thereby improving the comprehension of platform history. The Middle Ordovician marine transgression features a clear stratigraphic sequence that progresses from supratidal and tidal-flat carbonates to lagoonal and shoal carbonates, ultimately leading to offshore and deep-shelf carbonates. Fifteen microfacies have been identified and categorized into six depositional facies (associations): (1) supratidal regolith and supratidal pond, (2) shallow intertidal and supratidal sabkah, (3) lower supratidal to upper intertidal of low to moderate energy, (4) shallow subtidal low energy and shallow intertidal environment, (5) shallow lagoonal to lower intertidal flats, and (6) high energy marine environment in the photic zone. The lithofacies are arranged in vertically stacked, meter-scale cycles that transition from subtidal to shallowing-upward peritidal habitats. Ramp stratigraphy, facies correlations, and bounding surfaces indicate the presence of four third-order depositional sequences within the Black River-Trenton Group. Furthermore, it can be classified into five fourth-order cycles. Their representation illustrates the shift from a clastic-dominated ramp during the Chazyan-Blackriverian to a carbonate-dominated ramp in the Blackriverian-Trentonian period. The nearest contemporary counterparts of these Ordovician habitats are found on the Arabian shelf of the Persian Gulf for the Black River Group and the Recent West Florida Shelf for the Trenton.