Cenozoic stratigraphy of the Mackenzie delta, Northwest Territories

Abstract
Over 10 km of Cenozoic sediments accumulated beneath the modern Mackenzie Delta within the Richards Island Basin, which is flanked to the southeast by the northern Interior Platform and to the southwest by the Northern Cordillera. Northward, this basin forms part of the continental terrace wedge of the Beaufort Sea. Cenozoic deltaic sediments of the Richards Island Basin are divisible into two main fades: muddominant and sand-dominant, corresponding generally to prodeltaic and delta front to delta plain depositional environments respectively. These two fades, along with major unconformities, form the basis for defining formations within this sequence. In ascending order, the Cenozoic formations (new units in heavy print) are: Reindeer Formation, including the Aklak and sandstone-mudstone members (the latter being the major Cenozoic hydrocarbon reservoir), Richards Formation, Kugmallit Formation including the lvak and Arnak members, Mackenzie Bay Formation, Beaufort Formation, and Nuktak Formation. The Reindeer Formation, formed by northward progradation of a delta plain to delta front fades complex during mid-Paleocene to Early Eocene times, consists of sandstone, silty mudstone and minor amounts of conglomerate and coal. Its major source area was the western tectonic highland, but the uplifted Eskimo Lakes fault zone to the southeast also contributed elastic sediments. Deltaic depocentres at this time developed in the Richards Island and northern Tuktoyaktuk Peninsula areas. The Reindeer Formation is rich in temperate terrestrial palynomorphs and yields sparse, brackish water foraminifers of the Saccammina-Trochammina spp. assemblage. The Reindeer is conformably overlain by the Richards Formation, a northward thickening prodeltaic fades consisting of mudstone and shale deposited after subsidence and transgression in Early to Middle Eocene time. The formation is marked by the Haplophragmoides spp. agglutinated foraminiferal assemblage and, in the lower part, by a rich zone of dinoflagellates dominated by Wetzeliella spp. The upper part of the Richards Formation probably represents the northeastward-migrating toe of a major upper Paleogene deltaic wedge (Kugmallit Formation), which thickens markedly north and northeastward. The Kugmallit Formation consists of sand, mudstone, gravel, and lignite. It is conformable (north) to probably disconformable (south) over the Richards Formation. A rich terrestrial palynoflora, dated latest Eocene to Oligocene, occurs in the Kugmallit. Following deposition of the Kugmallit deltaic wedge, possibly in Middle to Late Oligocene time, the immediate area of the Mackenzie Delta was uplifted; a major unconformity now separates Neogene strata from Paleogene. To the north and northwest, however, deposition seems to have been continuous and the Neogene-Paleogene contact is in a conformable marine mudstone sequence (Mackenzie Bay Formation). A basin in the Richards Island area still existed with the advent of early Neogene deposition. Gravelly fluvial and fan-delta sediments of the Beaufort Formation are intertongued with, and replaced seaward in the basin by, the marine muds of the Mackenzie Bay Formation, which conformably overlies the Kugmallit. The Mackenzie Bay Formation carries cool-temperate to boreal terrestrial palynomorphs and the neritic Cibicides spp. foraminiferal assemblage, which is rich in calcareous benthonic species. During the Pliocene, the sea may once again have retreated from the Mackenzie Delta area, with subsequent scouring of the upper surface of the Beaufort-Mackenzie Bay sequence causing a widespread unconformity. The Pliocene-Pleistocene Nuktak Formation records the last offlapping deltaic sequence, prior to final marine transgression over the area, and Late Pleistocene complexities caused by advancing and retreating continental ice sheets. The Nuktak consists of nonmarine (lower, gravel member) and marine (upper, mud member) fades and contains the cool water, inner shelf, Elphidium spp. foraminiferal assemblage, and a variety of in situ fossils, as well as conspicuous reworked foraminifers and palynomorphs.

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