The Evaporites of the Ordovician Baumann Fiord Formation, Ellesmere Island, Arctic Canada

The Lower Ordovician Baumann Fiord Formation is a basin-margin evaporite sequence that forms part of the Franklinian Miogeosyncline-Arctic Platform sedimentary wedge. In the study-area on central Ellesmere Island, formation thickness ranges from about 200 m along the edge of the Precambrian craton to about 475 m at the basinward margin of the depositional wedge. The outcrop belt is about 110 km wide, extending more than 320 km along strike. The fundamental genetic unit is the carbonate-anhydrite cycle. A typical cycle is 3.5 m thick and consists of a basal lagoonal lime mudstone facies, succeeded by an intertidal stromatolitic facies, in turn overlain by a supratidal anhydrite facies. Each of these regressive sequences is interpreted as a discrete sabkha cycle, analogous in all essential regards to the Recent Trucial Coast sabkha cycle. Some sections of the B4umann Fiord Formation on Ellesmere Island contain up to 120 superposed sabkha cycles. Rhythmic successions of cycles were compounded during periods of steady and constant subsidence. Departures from conventional rhythmic cyclicity in any one vertical section may be explained in terms of temporal fluctuation in the rates of subsidence. Lateral changes in the character or thickness of individual cycles are explained as a function of spatial fluctuation in rates of subsidence. A regional stratigraphic synthesis, based on correlations between eight measured sections, reveals that, given appropriate subsidence differentials, the facies pattern in the formation as a whole can be explained in terms of sabkha model genesis. Most of the original nodular anhydrite of the Baumann Fiord Formation underwent early-stage compactional flow to produce layered and laminar anhydrite. Deep burial, accompanied by pervasive recrystallization, further modified the textural character of some of the anhydrite, notably that of the seaward part of the sedimentary wedge. The Ellesmerian Orogeny (Middle Devonian to mid-Pennsylvanian) produced folding and, where deformation was intense, it caused remobilization and flow of the anhydrite. Subsequent thrust-faulting (Cretaceous-Tertiary Eurekan Orogeny) produced shear zones at various levels within the formation. During exhumation, the basal part of the formation locally underwent hydraulic fracturing. Water from the underlying strata moved up into the evaporites and hydrated the anhydrite to form secondary gypsum. The excess calcium sulphate liberated during this volume-for-volume replacement process precipitated in the fracture system, forming satin-spar veins. The latest stages of exhumation of the formation permafrost regime of Pleistocence and Holocene times. water locally promotes gypsification of the outermost a rule, it is unaltered anhydrite that outcrops. have taken place during the Hydration by present-day meteoric veneer of the exposures but, as a rule, it is unaltered anhydrite that outcrops.