Abstract Abstract 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 buried 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.

A thick succession of upper Paleozoic carbonate rocks and minor chert crops out north of the head of Otto Fiord (northwest [NW] Ellesmere Island, Nunavut) in the Canadian Arctic Archipelago. These rocks accumulated in a tectonic subbasin—the Otto Fiord Depression (OFD)—of the Sverdrup Basin that likely originated through rifting during late Early Carboniferous (Serpukhovian). Following a long interval of passive subsidence that allowed a thick succession of Moscovian–Kasimovian carbonate rocks to fill the OFD, tectonic activity resumed during the Gzhelian (Late Pennsylvanian). This resulted in rapid collapse of the depression along its axis and simultaneous uplifts of its margins, a style of tectonism in accord with the inferred basin-wide shift to a transpressional–transtensional stress regime at that time. Late Pennsylvanian–Early Permian sedimentation in the OFD led to the development of four long-term (second-order) transgressive–regressive sequences of early Gzhelian–middle Asselian (<1200 m), late Asselian–late Sakmarian (<380 m), latest Sakmarian–late Artinskian (<160 m) and latest Artinskian–late Kungurian (<60 m) age. These ages are supported by integration of biostratigraphic data from conodonts, fusulinaceans, and small foraminifers. The development of each sequence-bounding unconformity was associated with renewed tectonism in the OFD. Each sequence recorded the development of a depositional system characterized by high energy peripheral shoreface grainstones passing basinward across a gently dipping ramp into deep-water basinal calcareous and siliceous mudstone. The ramp portion of the early Gzhelian–middle Asselian system comprises both cool-heterozoan to warmphotozoan carbonates (Nansen Formation) suggesting a relatively shallow thermocline at that time. These rocks are arranged in a series of high-order cyclothems of glacio-eustatic origin. Cyclothemic sedimentation ended at the Asselian–Sakmarian boundary, simultaneous to a major depositional system shift to cool-water heterozoan sedimentation (Raanes Formation), a change presumably brought on by the closure of the Uralian seaway linking NW Pangea with the Tethyan Ocean. This event led to the destruction of the permanent thermocline, and disappearance of photozoan carbonates by the early Sakmarian despite rising temperatures globally. Cool-water heterozoan sedimentation, associated with relatively shallow outer-ramp to midramp spiculitic chert resumed in the Artinskian and then again in the Kungurian (Great Bear Cape Formation) when the OFD was filled up. The depression ceased to exist as a separate tectonic/subsidence entity with the widespread sub-Middle Permian unconformity, above which sediments were deposited during a passive subsidence regime across most of the Sverdrup Basin. The Pennsylvanian–Lower Permian succession that accumulated in the OFD along the clastic-free northern margin of the Sverdrup Basin is essentially identical, both in terms of tectonic evolution and stratigraphic development, with the coeval succession of Raanes Peninsula, southwest (SW) Ellesmere Island, the type area of the Raanes, Trappers Cove, and Great Bear Cape formations along the clastic-influenced southern margin.