Abstract: Example of Carbonate Banks in Lower Proterozoic Belcher Group, Hudson Bay, Canada

Brian D. Ricketts

Detailed mapping of sedimentary facies in the Belcher Islands is possible where, in addition to excellent exposure and preservation, individual rock units display remarkable continuity (traceable up to 50 km along strike).

An example of one such unit is the wedge-shaped carbonate bank deposits making up the Rowatt Formation (390 m thick) and forming part of an offlap sequence within the middle part of the Belcher Group (> 1,800 m.y. old). Two main subfacies can be recognized: (1) an intertidal subfacies on the seaward margin of the bank comprising both terrigenous and carbonate sediments which display abundant primary sedimentary structures indicative of shallow-water deposition (herringbone cross-beds, ripples including late-stage runoff forms, reactivation surfaces, lenticular and flaser bedding, channel deposits, and mud cracks); (2) a carbonate breccia subfacies consisting of crudely bedded dolarenites and dolorudites, and thin well-bedded dololutites that in places are highly contorted. Primary avities in the lutites show up to five stages of cavity-cement lining. Structures in the coarse clastic carbonate rocks include parallel laminations, avalanche-type planar cross-beds, lutite rip-ups, and sparse graded beds. Lenses of cross-bedded dolarenites occupy channels up to 200 m wide. Fracturing of these carbonate rocks was part of a continuous sequence of events involving numerous cycles of deposition, erosion, soft-sediment deformation, lithification, and postlithification brecciation. Algae played an important part in binding the carbonate deposits.

An orthoquartzite facies (Mukpollo Formation) conformably overlies Rowatt strata and contains intertidal and possibly subaerial deposits. The Mukpollo is overlain by banded ironstones (Kipalu Formation) that reflect low-energy depositional conditions. Directly below the Rowatt are red argillites (Laddie Formation) representing deposition below wave base. Using Walther's law of facies correlations, a depositional model has been erected in which carbonate and terrigenous clastic materials have been deposited on a broad, shallow-marine platform as a system of banks and offshore barrier bars that protected a lagoonal basin.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma