Upper cretaceous stratigraphy of Yukon coastal plain and northwestern Mackenzie delta
Authors: Young F G 
Published: March 2017

Upper Cretaceous and older Mesozoic sedimentary rocks were studied for three firld sensors by the writer on Yukon Coastal Plain and regions to the south. These studies, combined with those made in the office and laboratory, including the close examinations of the
I.O.Z. Ellice 0-14 and I.O.Z. Blow River YT E-47 boreholes form the basis for this report. Upper Cretaceous strata comprise the youngest rocks of Yukon Coastal Plain and consist predominantly of shale, sandstone and conglomerate. Comparatively rare are coal, carbonate rocks and pyroclastic
volcanics. These rocks are economically important for their potential reserves of coal and petroleum. The Upper Cretaceous sequence of formations consists of the Boundary Creek Formation at the base, followed upwards by the Fish River Group, which includes the Tent Island and Moose Channel
formations, and the Reindeer Formation. The Boundary Creek Formation is mainly bituminous; marine shale which was apparently deposited during a considerable portion of middle Late Cretaceous time. It is preserved only east of Blow River where it unconformably overlies Lower Cretaceous strata, and is
commonly 500 to 1000 feet thick. This unit is characterized by its brightly coloured sulphate encrustations, bentonite beds and hematite bands, and its tendency to deform into complex, disharmonic folds. The Fish River Group is a molassoid terrigenous clastic wedge, approximately 6000 feet thick,
consisting of grey mudstone, siltstone, quartz-chert-lithic sandstone, conglomerate and rare coal seams. It is extensively preserved on the coastal plain, in the subsurface of lower Mackenzie Delta, and probably beneath the continental shelf under Mackenzie Bay. The Tent Island Formation consists
mainly of soft mudstone approximately 3000 feet think immediately west of Mackenzie Delta, and possibly twice as thick farther west in the area of Deep Creek. A basal sandstone and conglomerate unit, here named the Cuesta Creek Member, occurs sporadically at the base of the formation. This coarse
clastic unit is a potential reservoir for hydrocarbons because of its lenticular nature and position between two thick marine shale formations. Although macrofossils are extremely rare, microfossils are relatively common in the Tent Island, and include foraminifers, dinoflagellates, pollen and
spores. These assemblages indicate the formation is Campanian-Meastrichtian in age. The Moose Channel Formation is subdivided into two members which include the basal sandstone member, and Ministicoog Member. The basal sandstone member consists of sandstone, conglomerate, sand mudstone, interpreted
to represent alluvial, deltaic and littoral depositions cited. The Ministicoog is dominated by mudstone, but contains sandstone beds as well, and is believed to represent tidal-flat to shallow marine environments. The Reindeer Formation (Aklak Member) is exposed in a few small outcrops west of
Mackenzie Delta where it comformably overlies the Moose Cahnnel Formation. A delta-plain ? prevails in these exposures as well as in the I,C,E, Ellice C-14 borehole, and consists of sandstone, conglomerate, mudstone and coal. Fossils in the Moose Channel and Reindeer Formations include wood
fragments, lead and fruit impressions, pollen and spores, and minor foraminifers. They indicate a latest Cretaceous (Maratrichtim) age, axtending into earliest Tertiary in the ? sections of the Reindeer Formation. Sandstones of the Fish River Group and Reindeer Formation are mainly fracture, chert
litiarenites. The basis in this study was placed on attempting to definestratigraphic and ? trends in sandstone composistions in order to exsist in the ?. The greatest variations occur on the proportions of chert, plagioclase and volcanic road fragments, the latter two being roughly ?. The Cruesta
Creek Member sandstones are relatively high in chert content (average 34) compared with stratigraphically higher sandstones (basal Moose Channel 21; Aklak 24). Plagioclase and volcanic fragments are particularly enriched in the basal sandstone member of the Moose Channel. Paleocurrents, dispersal
trends and petrography of clastic particles indicate that in general the source of sediment and heads of drainage systems lay west and south of the present coastal plain, and that the Moose Channel Formation was not deposited by a proto-Mackenzie River. Deltaic sedimentation and this location of
depocantres appear to be concentrated in the lower coastal plain and subsurface of the present Mackenzie Delta. Evidence from polymorphic discoloration, coal reflectance, and mineral stabilites indicate that these areas are highly favourable sites for petroleum generation and
entrapment.