Paleoenvironmental and paleoecological reconstruction of the Chemahawin Member (Cedar Lake Formation; Silurian) at Lundar, Manitoba
Porter, D. Raegan
A mid-Early Silurian stromatoporoid-dominated biostrome within the upper, Chemahawin Member of the Cedar Lake Formation (Interlake Group; Silurian) is exposed within a quarry near Lundar, Manitoba. This highly dolomitized structure, previously classified as a bioherm, displays little evidence of topographic relief, but the density of skeletons within the unit, and the complex interactions among fauna, justify classification as a biostrome. Statistical tests were employed to assess the distributions of stromatoporoids and corals between localities in the quarry, and between stratigraphic intervals within localities, in order to recognize spatial and temporal trends within the biostrome. The faunal assemblage of the Chemahawin Member was analyzed in terms of abundance, species diversity and dominance, skeletal growth forms, and skeleton size. Within the Chemahawin biostrome, different microenvironments at various positions within the structure are interpreted from spatial variation in the distribution of fauna. Vertical trends in faunal distribution suggest short-term ecological succession as the biostrome transitioned from a pioneer community to a climax stage. In the southwest corner of the quarry, closely spaced facies represent a gradual progradation of the biostrome over previously unsuitable muddy substrates. It is interpreted from directional measurements of elongate fossils that a prevailing paleocurrent originated from the northeast, and the southwest corner likely represents a protected, back-reef setting. Examination of a subsurface drill core from Lundar Quarry North revealed facies arrangements similar to those within the quarry. A second core from nearby Mulvihill West Quarry revealed correlative facies, although the interval equivalent to the biostromal facies was not as well developed, indicating the limited lateral extent of the structure. The Chemahawin Member represents a transgressive-regressive cycle, with reefal development at the time of maximum transgression, corresponding to the interval of geological time when reefs were most widespread globally in the Silurian. Although large mid-Silurian reefs are common structures within other basins on the North American continent, reef development within the Williston Basin was limited and patchy, suggesting that conditions were less favourable for large-scale reef growth.