ABSTRACT

Partially dolomitized, fine-grained limestones make up most of the Middle Devonian (Eifelian) Elm Point Formation in the Paleozoic outcrop belt in southern Manitoba. Four lithofacies are recognized: 1) nodular limestone, composed of grey-brown, skeletal wackestone-packstone nodules in a tan-weathered matrix of skeletal wackestone-packstone; 2) mottled, skeletal packstone; 3) tabular stromatoporoid-coral boundstone, and 4) skeletal-intraclast rudstone-floatstone. These lithofacies lack evidence for subaerial exposure and contain abundant lime mud and a diverse fossil assemblage, suggesting that they were deposited in a low-energy, open subtidal setting which was subjected to intermittent storms.

The extent of dolomitization in the Elm Point platform was significantly controlled by sediment permeability and porosity. Burrowing, boring and microbial micritization enhanced permeability and porosity which, in turn, locally promoted later dolomitization. In contrast, early limestone lithification, evidenced by intraclasts and hardgrounds, effectively reduced permeability and porosity, inhibiting dolomitization.