RESTRICTED PALEOPROTEROZIC BASIN: A CANDIDATE ENVIRONMENT FOR SYNSEDIMENTARY DOLOMITE PRECIPITATION?

The Denault Formation (2.1-1.9 Ga) outcrops in the Labrador Trough, northeastern Quebec and western Labrador. As presently understood, open ocean was located to the east. An elevated high formed the shelf margin, isolating a basin whose inboard western margin had a distally-steepened ramp morphology. The elevated margin is composed of stromatolitic dolomite, with disorganized domal, cuspate, digitate, and columnar stromatolites. This obstruction may have contributed to restricted conditions in the basin to the west. Interpreted as basinal, the deepest water facies consist of 2-40 cm thick beds of thinly-laminated dolostone intercalated with black shale. These grade into distal ramp facies composed of remarkably even, mm-scale couplets of flat-lying microbial and micritic laminae that drape preexisting topography. Scours within the microbial units suggest occasional storm activity. Mid-ramp facies consist of oolitic and peloidal grainstones with HCS, SCS, current ripples, and occasional intraformational conglomerates. These facies are punctuated by dolostone boulder conglomerates with a mudstone-to-grainstone matrix and collapse breccias up to 4 m thick. These conglomerates, as well as disturbed bedding and synsedimentary deformation point to active tectonism during carbonate deposition. Lack of stromatolites along the ramp suggests an environmental stress, potentially elevated salinity. The Denault Formation is entirely composed of fine-grained dolomite with excellent preservation of detail, and its depositional setting is consistent with the conditions necessary for the direct precipitation of dolomite from hypersaline seawater: elevated salinity, high organic content, and low sulfate concentrations. The early Paleoproterozic was a time of profound oceanographic change and carbonates from this period provide a window into ocean chemistry and depositional processes under non-actualistic chemical conditions.