Origin of the Paleoproterozoic Denault Dolomite, Labrador.
Zentmyer, Rebecca Anne
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The Denault Formation (2.1-1.9 Ga) outcrops in the Labrador Trough, northeastern Quebec and western Labrador. The Denault crops out around the town of Schefferville and at Marion Lake, fifty kilometers east-northeast of Schefferville. In June 2007, four stratigraphic sections were measured and samples were collected from eight locations. Occurrences of the Denault near the town of Schefferville contain textural characteristics consistent with deposition on the middle and outer portions of a storm-influenced shallow ramp. Mid-ramp facies consist of intraclastic grainstones with hummocky cross-stratification (HCS), swaley cross-stratification (SCS), current ripples, and graded event beds. Further outboard, grainstones grade into deeper water laminites that are composed of even, mm-scale couplets of flat-lying organic and micritic laminae. Scours within the laminites suggest occasional storm activity. Laminites gradually grade into outer ramp deep-water shales. Separated from these ramp facies by 50km (present day) is an isolated eastern stromatolitic accumulation. This outcrop can be interpreted as the remnant of a near-continuous margin or may simply represent an isolated accumulation that developed on a pre-existing topographic high. Microscopic (2-10µm) lathes of dolomite are interpreted to be pseudomorphs after gypsum. The presence of such pseudomorphs and solution-collapse breccias provide evidence for evaporative conditions during the deposition and early diagenesis of the Denault. Seepage-reflux of brines generated through evaporation during the deposition of the Denault and the overlying Fleming Formation are interpreted to have caused early, mimetic dolomitization in a shallow burial setting. Subsequent burial diagenesis resulted in recrystallization of some rock to fabric-destructive, anhedral dolomite.