DOLOMITES OF THE MIDDLE DEVONIAN ELM-POINT FORMATION, SOUTHERN MANITOBA - INTRINSIC CONTROLS ON EARLY DOLOMITIZATION

Partially dolomitized, nodular and mottled limestones comprise most of the Middle Devonian (Eifelian) Elm Point Formation in the Paleozoic o utcrop belt of southern Manitoba. Fabric-selective dolomite occurs in all of these platform limestones and consists of: (1) microdolomite in clusions in echinoderm grains; (2) microcrystalline dolomite in micrit ic components; and (3) fine- to medium-crystalline dolomite in micriti c components. The first two dolomite types are considered to be byprod ucts of the stabilization of high Mg calcite precursors. The third dol omite type is interpreted to be of early diagenetic origin and formed from Middle Devonian seawater. The findings also tentatively suggest t hat there is no genetic linkage between dolomites of the Elm Point and overlying Winnipegosis Formations, implying that the Elm Point was no t a conduit for dolomitizing fluids flowing upward into Winnipegosis c arbonate buildups. The fabric-selective nature of Elm Point dolomites suggests that the nature of the host rock, i.e., intrinsic factors, pl ayed a major role in determining the extent of dolomitization. Formati on of dolomite Type 3 occurred only in permeable and porous sediment w hich locally had been modified by burrowing, boring and microbial micr itization. Early lithification of lime mud, indicated by intraclasts a nd hardgrounds, locally reduced permeability and porosity, inhibiting dolomitization.