Sedimentology and stratigraphy of the Lower Cretaceous Isachsen Formation: Ellef Ringnes Island, Sverdrup Basin, Canadian Arctic Archipelago

摘要

The Lower Cretaceous Isachsen Formation is a widespread, siliciclastic succession up to 1.4 km thick that records the northwestward progradation of fluvio-deltaic environments into the Sverdrup Basin in the Canadian Arctic. Although the Isachsen Formation is a prominent unit with proven oil and gas reserves, detailed outcrop descriptions of its sedimentology and stratigraphy, and correlation of these features to subsurface data are rare. We examined outcrop exposures of the Isachsen Formation on Ellef Ringnes Island in the province of Nunavut in the Canadian Arctic in order to document and interpret the sedimentology and stratigraphy of this unit in the central portion of the Sverdrup Basin. Interpretations are based on observations from over 5000 m of measured stratigraphic sections, local mapping of fades contacts, palynology, and sandstone petrography. Deposits of the Isachsen Formation on Ellef Ringnes Island can be divided into six broadly-defined facies associations that record deposition in braided-fluvial, meandering-fluvial, floodplain, tidally-influenced fluvial, shoreface, and offshore environments. Sandstones within the Isachsen Formation are typically quartzose, commonly with compositions of >90% well-rounded, monocrystalline quartz. Although the sedimentary details between outcrop locations vary, a generalized stratigraphic trend is present within the Isachsen Formation. Where exposed, the base of the unit consists of marginal marine sandstones abruptly overlain by braided stream deposits, which grade into mudstone-rich, meandering-fluvial and floodplain deposits. These nonmarine deposits are overlain by a thin marine mudstone (Rondon Member), which is in turn overlain by marginal marine sandstone and fluvial deposits. The uppermost portion of the Isachsen Formation records a transgression and is overlain by marine mudstones of the Christopher Formation. The stratigraphic succession of the Isachsen Formation in outcrops correlates to data from nearby hydrocarbon exploration wells. Strati-graphically, the Isachsen Formation on Ellef Ringnes Island represents a regressive-transgressive sequence, but contains within it two higher-order regressive-transgressive sequences, which is consistent with observations from elsewhere in the Sverdrup Basin. The detailed descriptions of facies within the Isachsen Formation on Ellef Ringnes Island and their correlation to subsurface data can be extrapolated to other parts of the Sverdrup Basin where data are lacking.