Carbonate-evaporite cycles in the silurian rocks of Somerset Island, Arctic Canada
Authors: Reinson G E 
Published: March 2016

The east coast of Somerset Island is characterized by spectacular cliffs, as much as 365 m high, consisting of Silurian strata which are nearly horizontal or dip westerly at a low angle. A cyclic sequence of carbonate-evaporite rocks forms the lower exposures,
but the upper two thirds of the cliff are composed primarily of carbonate rocks. The upper carbonate succession has been studied in detail and designated as the Leopold Formation. The lower cyclic carbonate-evaporite sequence is described in this paper. The carbonate- evaporite sequence is
considered to be Wenlockian in age, and correlative with the upper Cape Crauford Formation on northwestern Baffin Island and the lower Cape Storm Formation on Devon Island. It crops out only in the vertical cliffs of northeastern Somerset Island and, thus, cannot be mapped on a regional scale. This
cyclic carbonate- evaporite succession has a maximum exposure of 125 m in the coastal cliffs 18 km south of Elwyn Bay, and it is there that a detailed section was measured. The lower boundary of the ' sequence lies below sea level, and the upper boundary was placed at the top of the last continuous
evaporite bed, which marks the base of the Leopold Formation. Twenty-three carbonate- evaporite cycles were recognized in the measured section. Each cycle is considered to represent deposition during progradation of a lagoonal-sabkha shoreline. Three lithofacies make up the complete cycles;
subtidal, intertidal, and supratidal. Three rock types constitute the subtidal or lagoonal lithofacies; pelletoidal wackestone, finely laminated mudstone, and massive to laminated mudstone. Rocks interpreted to be algal stromatolites constitute the intertidal lithofacies. Two stromatolite forms are
recognized - digitate stromatolites and cryptalgal aminite. The supratidal lithofacies is characterized by birds-eye laminated mudstone, interlaminated gypsum and carbonate mudstone, and by bedded gypsum deposits. Not all the cycles contain all three lithofacies; in some, the subtidal lithofacies is
absent, and in others the supratidal lithofacies is absent. The middle of the measured section commonly contains subtidalintertidal cycles, whereas in the upper and lower parts, intertidal-supratidal cycles are prevalent . Where the lagoonal or subtidal facies of the cycles is absent, the carbonate
is dolomite but, where subtidal rocks are present in the cycl es , calcitic carbonate prevails. This dolomite distribution is consistent with that observed in the modern sabkha of the Trucial Coast, Persian Gulf, in that the dolomitization occurs as a diagenetic front which progresses seaward in the
prograding supratidal zone. The Silurian stratigraphic succession of northeastern Somerset Island, from the basal carbonate-evaporite sequence, through the Leopold Formation to the overlying Read Bay Formation, records a sequence of deposition in a shoreline- shelf environment that was subsiding
continually. During the evaporite depositional phase, conditions were restricted and hypersaline; these conditions changed gradually to a shallow, more open marine environment during deposition of the subtidal fossiliferous carbonates of the Read Bay Formation.