An upper Carboniferous shelf-to-basin transect: Nansen and Hare Fiord Formations, NW Ellesmere Island.

Abstract

An ancient transition from shallow marine platform to deep basin is preserved in the Upper Carboniferous strata of the Sverdrup Basin on northwestern Ellesmere Island. The shelf break is delineated by an approximately 200 km long discontinuous reef tract extending from Audhild Bay to the head of Hare Fiord within the Nansen Formation. The reef tract is marked by a series of massive, coalesced shelf edge buildups which developed during a major second order regression within the Sverdrup Basin. The shelf edge buildups are largely composed of phylloid alga-rich boundstones with abundant early marine synsedimentary cement. Fenestellid bryozoans, however, dominate buildups that grew in deeper slope settings. Shoreward of the reef tract lie cyclic, shallowing-upward carbonate shelf deposits, also of the Nansen Formation. The outer shelf deposits include bedded oolitic and foraminiferal grainstones representing a shallow water sand apron located immediately behind the reef tract. Basinward of the shelf edge reef tract lie correlative slope-to-basin rhythmic mixed deposits of the Hare Fiord Formation. The upper slope deposits are dominated by fine grained calcisiltites while the lower slope/basin facies comprise calcisiltites, siltstones and shales interbedded with abundant turbidities and debrites. The massive prograding shelf edge buildups reach stratigraphic thicknesses up to 1 km. The Nansen reef tract appears to be a reef complex in which the reefal rim remained relatively deeply submerged, but still within the photic zone, through much of its growth. The carbonate buildups on the shelf, shelf edge and slope all represent potential petroleum-bearing stratigraphic traps within the Sverdrup Basin. If these occur in the subsurface they would be likely exploration targets but unfortunately within the study area little porosity remains. Where the buildups are extensively dolomitized, higher porosity and permeability create excellent potential hydrocarbon reservoirs. (Abstract shortened by UMI.)