Geology, Belcher Islands, Nunavut

Basement rocks are not exposed on the Belcher Islands, but might be relatively close to the surface below Tukarak Island and east of Laddie Island. The initial miogeoclinal stage of the Belcher Group is represented by the 1200 m thick Kasegalik Formation composed mostly of evaporitic and stromatolitic dolostones deposited on a transgressive-carbonate shelf or platform that sloped west to southwest. Depositional environments range upward from supratidal and evaporitic at the bottom to shallow subtidal near the top. Sedimentation was halted by abrupt extrusion of the Eskimo contaminated, tholeiitic, continental basalts, possibly at 1960 ma. They thicken north-northeast to 950 m at Eskimo Harbour and extrusion ended almost as abruptly as it began. They are coeval and likely correlative with the Persillon and Nastapoka mainland tholeiitic basalts and the Qingaaluk Formation in the Richmond Gulf Graben, based mainly on chemistry and paleopole positions. Sedimentation during the second miogeoclinal stage followed the volcanism, but was carried out in a more unstable environment than the first miogeoclinal stage. The first six formations of the second stage (Fairweather - Laddie, about 1730 m) were deposited in cyclic fashion on a west- to southwest-dipping transgressive carbonate shelf or platform. Strata include clastic and stromatolitic dolostones and minor limestones, mudstones to quartzarenites and local channel conglomerates. Depositional environments range from supratidal to subtidal, slope and basin. Fantastic stromatolite buildups, large biohermal domes, thin-, rhythmically- interlayered dolostone and limestone and soft-sediment deformation in slope deposits are present. Deposition of the top three formations (Rowatt-Kipalu, about 560 m) of the second miogeoclinal stage changed to progradation in a restricted basin. Sedimentation ranged from a tidal flat carbonate buildup to deposition of the Kipalu iron-formation in slightly deeper water in a restricted environment. The second miogeoclinal stage was terminated as abruptly as the first stage by the extrusion of the Flaherty slightly contaminated contintental, tholeiitic basalts. These flows are part of a volcanic arc that extends from west of the Sutton Inlier east and north to the Cape Smith Belt and beyond. Thicknesses range from 200 m on eastern Tukarak Island to at least 1650 m in the western Belchers. The flows were accompanied by intrusion of the Haig diabase-gabbro sills and a few dykes in all of the underlying formations and are not known to be present in the uppermost part of the Flaherty Formation. Precise, identical baddeleyite ages of 1870 Ma for two Haig sills, chemical data and paleopole positions indicate the sills are the same age as and are probably all related to the Flaherty volcanism. These data help to correlate these flows and sills and the underlying Flaherty Iron-Formation and Mukpollo quartzarenite with similar strata present in the Sutton Inlier, along the offshore islands of the Nastapoka Arc and on the Sleeper Islands. Accumulation of the volcanic pile to the west led to depression of the land to the east, changed the paleoslope from southwesterly to easterly and helped to transform the miogeoclinal stage into a foredeep of a foreland basin system. When volcanism ceased, the foredeep continued to be depressed and the thick turbidite greywacke flysch of the Omarolluk Formation (about 3300 m) was deposited in a submarine fan. As the basin began to fill, deep water deposits graded upward into shallow-water crossbedded arkoses of the Lower Loaf Member (about 500 m), that underlie most of the Bakers Dozen and King George islands. They are interpreted to be fluvial and shallow marine distal molasse strata and are overlain on a few Bakers Dozen Islands by about 215 m of mostly red and pink crossbedded arkoses interpreted to be terrestrial molasse. Deposition of molasse has been taken to indicate overfilling of the basin and these strata may be close to the original top of the Belcher Group. Most of the region in the southeastern Hudson Bay embayment that is under water is interpreted to be underlain by little-deformed Upper Omarolluk and Lower Loaf members. Field, topographic, and geophysical data indicate that relatively intense folding dies out abruptly east of a line extending south from the north end of the Nastapoka Arc along the Marcopeet and Sleeper islands. Continuing southerly, the line becomes arcuate and convex to the east as it passes southeast by the North Belcher Islands.to the north end of Tukarak Island whereit turns south and finally south-southwest toward the northeast corner of James Bay. The relatively intense deformation west of this arcuate belt in the southern part of the southeast Hudson Bay embayment is interpreted to have resulted from forces acting in a southeast direction. The thrusting resulting from two continental masses colliding has been interpreted to have terminated just west of the Belchers, except for movement along a sole fault or decollement that may have continued to east of the "eastern islands". This movement may have been facilitated by occurring along a relatively low topographic belt that may mark the extension westward of the Richmond Gulf Graben (Aulacogen) to the Belcher Islands. Most of the region in the southeastern Hudson Bay embayment that is under water and east of the deformed belt is interpreted to be underlain by little-deformed Upper Omarolluk and Lower Loaf members. This is substantiated by the little-deformed nature of the islands examined in a large part of this region. The features noted above are probably considerably younger than the Nastapoka Arc and suggest that the intense deformation to the west of the line noted above is younger than the Nastapoka Arc and had little to do with the latter's formation. The Flaherty Iron-Formation has received considerable attention in the past, but nothing of serious possible interest has yet been found. Sizeable areas underlain by the iron-formation are drift covered, although indications of leaching and enrichment in outcrop areas are meagre. The drift is probably not thick and trying modern geophysical methods and drilling a few shallow holes might be worthwhile. Some drilling of copper showings was not encouraging, although hand-size specimens (10-15 cm across), looked interesting. Some traces of anthraxolite occur at a few localities, but have not been reported by others. However, they are probably not indicative of the presence of a subsurface hydrocarbon deposit.