ABSTRACT

Three genetic assemblages of rock types are distinguished in the Middle Ordovician to Middle Silurian Brodeur Group of northwestern Baffin Island, which represent a protected shelf and shore environment with low supply of clastic sediments, and a hot climate: 1) Richly to predominantly sparsely fossiliferous cryptocrystalline limestone. 2) Thinly interstratified microcrystalline limestone and dolomite. The dolomite formed by replacement at the sediment-sea water contact, and the limestone by recrystallization of probably inorganically precipitated calcium carbonate. Recrystallization is more advanced than in 1), probably because of a lower content of carbonaceous impurities. 3) Strata of assemblage 2) brecciated and deformed by early solution of thinly interstratified evaporites with associated stromatolitic beds. The three assemblages, listed in the order of increasingly evaporitic conditions of origin, are all partly replaced by dolomite which formed in the subsurface, and contain some clastic impurities.

The lower 460 feet of the composite reference section (upper Middle Ordovician) is composed mainly of 2); the overlying 1,140 feet [upper Middle or lower Upper Ordovician to lower (?) Niagaran] mainly of 1); and the upper 1,340 feet [Niagaran and (?) younger] of numerous successions of 1), 2), and 3) with the latter two comprising more than two thirds of the unit. This sequence represents a transgressive-regressive cycle with the deepest submergence mainly in the Late Ordovician and Early Silurian. The Late Ordovician submergence followed a weak uplift in the middle part of the Middle Ordovician, and corresponds to the widest known Palaeozoic advance of the Arctic seas. The cycle was probably terminated by positive movements which on adjacent Somerset Island culminated in the Early Devonian.