CAMBRO-ORDOVICIAN STROMATOLITE OCCURRENCES IN THE OTTAWA REGION

Early Ordovician (Ottawa Group, Pamelia Formation) stromatolites are exposed on the north shore of the Ottawa River in Gatineau, Quebec, Canada. This exposure has been known for >50 years, but only in the past few years has the full extent been revealed, due to low river levels. They form composite mounds up to 1.5 m in diameter, display upward branching and have a synoptic relief of less than 30 cm. Detailed mapping has revealed a distinct tendency for alignment of the stromatolite mounds, as well as both coalescence and a predominance of slight to distinct elongation of individual stromatolite heads in the same direction. Such features also are evident other reported localities in the U.S. and Canada. Clear evidence for shallow water to intertidal deposition (desiccation cracks, symmetrical ripple marks, erosional truncations) and evidence of hypersalinity (paucity of shelly fossils, in contrast to the underlying beds) is observed.

Characteristic of these stromatolites, and those exposed in the Hoyt Limestone of Saratoga Springs, are thin black siliceous ribs that mark successive stromatolitic growth laminae. Although most are decorated with convex-outward coliform ornamentation, outwardly angular deflections of laminae are sufficiently abundant locally to impart stellate outlines. Perhaps these v-shaped projections represent local outward growth of crystals of an evaporate mineral such as gypsum (similar to the outwardly projecting blades of gypsum sand roses)?

Most features described above bear a striking resemblance to matching features displayed by the classic stromatolites of Hamelin Pool, Shark Bay, Western Australia. Stromatolites and related biofilm structures are surprisingly abundant in Paleozoic strata of eastern North America, in view of the co-existence of a wide range of evolved grazing organisms, especially gastropods. This substantiates the proposition that hypersaline environments comparable to Shark Bay were a recurring theme during deposition of substantial accumulations of sedimentary rocks in eastern North America during the Cambrian and Ordovician Periods.