Carbonate-Anhydrite Facies Relations in Otto Fiord Formation (Mississippian-Pennsylvanian), Canadian Arctic Archipelago

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The Otto Fiord Formation is a major evaporite deposit of Late Mississippian to Middle Pennsylvanian age occupying an axial position in the Sverdrup basin of the Canadian Arctic Archipelago. Where exposed in normal stratigraphic succession on northwestern Ellesmere Island, the Otto Fiord is composed of 350 to 600 m of rhythmically alternating limestone and anhydrite, with some interbedded sandstones near the top of the formation. The formation is underlain by continental to marginal-marine red beds, and is overlain by carbonate rocks and shales of deep-water aspect. Laterally, the Otto Fiord evaporites and limestones are coeval with marine-shelf carbonate rocks that flank the margins of the evaporite-carbonate basin. In the central and southern Sverdrup basin, a salt and anhydrite facies of the Otto Fiord is buried by thick Mesozoic rocks and is revealed at the surface only in large diapiric structures. At several localities on northwestern Ellesmere Island, limestone mounds are enclosed within the Otto Fiord Formation. At van Hauen Pass, mounds up to 30 m thick and 350 m long are built on erosional remnants of anhydrite. The mounds have a crinoid-rich marine limestone base, a main beresellid algal-buildup facies of hypersaline aspect, and several marine-limestone capping beds. Each of the marine-limestone phases has thin, marine-limestone equivalents in the intermound setting, each separated by a unit of anhydrite. Reconstructions of depositional events reveal that the mounds are the composite record of at least three transgressive-regressive cycles in which carbonate sediment was deposited during the marine phase and the algal facies and anhydrite units were formed under the hypersaline conditions imposed by evaporitic drawdown. At a facies transition from Otto Fiord evaporites to the basin-margin reef and shelf carbonate rocks exposed on northwestern Ellesmere Island, anhydrite units are interbedded with limestones at the foot of steeply dipping shelf-foreslope carbonate tongues. Depositional relief on these carbonate units increases upward through the evaporite succession from a few meters to more than 400 m. Anhydrite units interbedded with the limestones apparently formed subaqueously under conditions of depressed hypersaline sea level, probably shallower than the depths suggested by carbonate depositional relief, but far from a sabkhalike environment. The accumulated evidence supports earlier interpretations that the anhydrite evaporites of the Otto Fiord Formation were deposited in a hypersaline subaqueous environment under conditions of regional evaporative drawdown of sea level. The complex facies relations demonstrated by the Otto Fiord rocks may serve as guides for modeling evaporite-reef relations and major carbonate to evaporite-facies transitions in other basins and in subsurface studies related to hydrocarbon exploration.