Sulphates of Submarine Origin in Pennsylvanian Otto Fiord Formation of Canadian Arctic

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ABSTRACT The Pennsylvanian Otto Fiord Formation in the Hare Fiord area, Ellesmere Island, is a succession of interbedded limestone, anhydrite and minor clastics of over 600 m thickness. Ancient anhydrite deposits have been widely assigned to supratidal or sabkha environments largely by analogy with the well-known Trucial coast sabkhas. Further, it is commonly held that in basinal environments precipitated sulphate will not accumulate as extensive deposits because of sulphate reduction in the presence of organic matter. The Otto Fiord Formation provides an example of a thick calcium sulphate deposit which apparently accumulated in a submarine rather than a supratidal environment. Evidence for a submarine origin of calcium sulphate comes first from interbedded carbonates which maintain a marine fauna to the transitional contacts with overlying anhydrite. At these contacts, limestone and anhydrite are regularly interlaminated on a mm scale without disruptive or displacive nodular or enterolithic structures, which are typical of sabkhas. Intertidal sediments normally underlie the displacively grown sulphates of sabkhas, yet algal stromatolites or mats, flat pebble conglomerates or such desiccation structures as mudcracks are lacking at the dozens of limestone-anhydrite contacts examined. Extensive dolomitization is a normal consquence of lateral brine movements in sabkhas, but dolomite is a minor constituent (less than 10 per cent) in Otto Fiord limestones. The deficiency of dolomite can be explained by lack of lateral or downward movements of dolomitizing solutions after burial of carbonate by sulphate in a submarine environment. Sabkha cycles are the product of successive progradations on almost flat surfaces, and the carbonate-sulphate couplets are typically of the order of 3 or 4 m. Nowhere in the Otto Fiord are carbonate-anhydrite couplets repeated on this scale. These couplets are typically 15 to 30 m or more thick and anhydrite units of up to 60 m thick are common. Although it is possible for anhydrite units thicker than a few metres to be generated in a supratidal environment, they should be the exception rather than the rule. The primary precipitate in the submarine environment probably was gypsum, and evidence of large, elongate gypsum crystals pseudomorphed by anhydrite occur in very regularly bedded sequences within the Otto Fiord. The most common anhydrite structure, however, is nodular mosaic or chickenwire. Finally, the sulphate deposits, on a regional scale, are axially rather than marginally situated in the basin and therefore were not laid down on basinmargin shelves. A basin axis position itself, however, does not preclude the possibility of formation in a supratidal environment. End_Page 149------------------------