The Otto Fiord Formation is a major evaporite deposit of Late Mississippian to Middle Pennsylvanian age that occupies an axial position in the Sverdrup basin of the Canadian Arctic Archipelago. Where it is exposed in normal stratigraphic succession on northwestern Ellesmere Island, the formation is composed of 350 to 600 m of rhythmically-alternating limestone and anhydrite, with some sandstones interbedded near the top. The formation is underlain by continental to marginal-marine red beds, and overlain by carbonates and shales of deep-water aspect. Laterally, the Otto Fiord evaporites and limestones are coeval with marine shelf carbonates that flank margins of the evaporite-carbonate "basin." In the central and southern Sverdrup basin, a facies of salt and anhydrite is b ried by thick Mesozoic rocks and 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 as thick as 30 m and as long as 350 m 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. The marine limestone phases of the mounds have thin, marine limestone equivalents in the intermound setting, separated from each other by units of anhydrite.

Reconstruction of depositional events reveals that the mounds are the composite record of at least three transgressive-regressive cycles, in which limestones were deposited during the marine phase and the algal facies and anhydrite units were formed under hypersaline conditions imposed by evaporitic drawdown.

In the Otto Fiord Formation of northwestern Ellesmere Island, in a facies change from evaporites to basin-margin reef and shelf carbonates, anhydrite units are interbedded with limestones at the feet of steeply-dipping tongues of shelf-foreslope carbonate rock. Depositional relief on these carbonate units increases upward through the evaporite section from a few meters to more than 400 m. Anhydrite interbedded with the limestones apparently formed subaqueously under conditions of depressed hypersaline sea level, in water probably shallower than depths suggested by depositional relief on the carbonates, but far from a sabkhalike environment.

The complex facies within the Otto Fiord Formation may serve as guides elsewhere in modeling evaporite-reef relations and major carbonate-evaporite facies transitions, and in studies of the subsurface related to exploration for hydrocarbons.