PALEOECOLOGY OF LOWER SILURIAN BIOHERMS, MANITOULIN ISLAND, ONTARIO

Abstract

The largest and most extensive, known, Early Llandovery (Rhuddanian) biohermal complex is exposed within the Manitoulin Formation on Manitoulin Island. The complex comprises over three-hundred bioherms. Bioherms range in size from small mounds 4 m in diameter and less than 1 m in relief to large, elongate structures more than 1 km long and up to 11 m thick. They crop out in a narrow belt that trends northwest-southeast. The bioherms developed in generally clear, shallow, marine waters on the inner part of the northeast shelf of the Michigan Basin. Restricted, low-energy conditions predominated, infrequently interrupted by periods of increased energy, possibly storm-generated. Increased energy caused overturning of colonial corals, planation of the largest bioherms, and minor erosion of the sediments on the other mounds. Biohermal sediments primarily consisted of carbonate mud with admixtures of terrigenous clay. Much of the carbonate mud was produced in situ, although hydrodynamic deposition increased with the size of the bioherms. The fine-grained sediments were trapped and bound principally by benthic algae, with localized contributions by colonial corals, stromatoporoids, bryozoans, and crinozoans. Although the biohermal sediments generally were well aerated, reducing micro environments existed as a result of decomposition of organic matter. The reducing conditions, in conjunction with sulphur evolved by sulphate-reducing bacteria, caused replacement of many carbonate structures by pyrite. Bioherms became progressively larger and faunally more complex with increasing water-depth southeastward along the belt. Coincident with deepening water, energy increased somewhat and circulation became relatively more open. Salinity may have been high toward the northwest, descending to more normal levels as water-depth increased. The sedimentary surfaces of the bioherms were colonized mainly by bryozoans, crinozoans, and brachiopods. Lamellar favositids and stromatoporoids occurred in all mounds but were most abundant in the larger bioherms. Colonial rugose corals were late additions to the biohermal communityon some of the larger mounds. Gentle currents were directed from the southwest, possibly the result of flow out of the Michigan Basin toward the northeast Allegheny Basin. In response to these currents, suspension-feeding organisms preferentially inhabited southwest-facing slopes and most bioherms became elongated in a northeast-southwest direction. Several bioherms occupying a 'back-reef' position relative to the Manitowaning Bioherm, an elongate barrier, became irregular in shape because current directions were more variable in the lee of the barrier. Three generalized intergradational growth stages are recognized in the bioherms. In Stage 1, the bioherms became established as small, dense carpets of bryozoans, numerous brachiopods, and small numbers of lamellar favositids and stromatoporoids living on a substrate of soft carbonate mud. Most mounds began to developa lenticular shape, elongated northeast-southwest, soon after they became established. The distribution of organisms became asymmetrical on bioherms that attained relief of more than one meter, initiating Stage 2. At this stage, crinozoans became a common component in the biohermal community. Lamellar favositids and stromatoporoids increased in abundance and benthic algae probably became important in stabilization of the substrate. Stage 2 accounted for the largest proportion of the growth in most bioherms. Stage 3 was developed in bioherms with more than two meters of relief. Growth of the bioherms, and the distribution of organisms on them, remained asymmetrical. Nearly all epifaunal growth was concentrated on southwest slopes. Bryozoans, crinozoans, and brachiopods continued to dominate the community, but lamellar favositids and stromatoporoids became still more numerous than in preceding stages. Colonial rugosans occurred only on the larger bioherms toward the center of the belt, and were primarily confined to southwest slopes. Biohermal growthwas terminated by a phase of progradational sedimentation, which deposited terrigenous muds of the Cabot Head Formation over the shelf area.