LATE ORDOVICIAN STROMATOLITES IN THE TACONIC FORELAND BASIN (OTTAWA EMBAYMENT): FABRIC, DEPOSITIONAL ENVIRONMENT, AND GEOLOGIC SIGNIFICANCE

The Middle to early Late Ordovician marks the early phase of Taconic foreland basin development within the Ottawa Embayment, a narrow closed seaway that extended inland from the open marine Laurentian shelf. A prominent ~1.2-m stromatolitic interval near the base of the Pamelia Formation (lowermost Ottawa Group) stands out as an anomaly relative to the preceding and following foreland basin succession. The stromatolitic interval can be traced over at least 80 km along the axis of the embayment, and denotes a relatively unique set of environmental controls triggering microbial activity. Mounds consist of stratiform, domal, nodular and laterally linked hemispheroids form beds. Their internal structure consists of poorly laminated clotted and filamentous fabric of varying densities. These probably represent microbial communities that induced calcium carbonate precipitation, thereby forming stromatolitic fabric. Variations in both details of internal fabric and external form are interpreted to define variations in depositional environment, but overall remaining part of a muddy peritidal setting. The boundaries of each of the formations (Carillon, Rockcliffe, Hog’s Back) within the foreland succession that precede the Pamelia Formation, and the base of the Pamelia itself, are marked by evidence for earthquake or local structural disturbance suggesting that abrupt changes in lithology, as now preserved by formations, was influenced by an evolving regional structural framework. Stromatolites form two prominent stratigraphic intervals within the foreland basin succession in the Ottawa Embayment: one occurs within the Middle Ordovician Carillon Formation, the base of the succession, marking initial regional foreland flooding; the Pamelia stromatolites form the second interval, and appear to coincide with another stage of regional flooding that backstepped the embayment to the northwest, allowing for initiation carbonate deposition that would predominate this part of the foreland basin through to the end of the Mohawkian.