Abstract

The Kugmallit Formation was deposited in the Beaufort-Mackenzie Basin by prograding deltas during the Oligocene. One of several thick successions of deltaic rocks in the clastic fill of the basin, the Kugmallit has large reserves of oil and gas, hosted principally in delta-front facies.

The Kugmallit delta front is present seaward of the fringes of the modern Mackenzie Delta and extends offshore in a broad zone some 50 km wide. Within this zone, the sand-dominated Kugmallit Formation is bounded, above and below, by shales of the Mackenzie Bay Formation and the Richards Formation, respectively. Widespread shale tongues within the Kugmallit permit a breakdown into three informal units: A, B and C. The Kugmallit Formation — a lithostratigraphic unit — corresponds closely with the seismically-defined Kugmallit Sequence within the same boundaries and limits. In certain areas, the Kugmallit Sequence is internally subdivided by widely traceable seismic markers. These define “subsequences”, a term chosen to imply similarity, in both morphology and genesis, with the Sequence, albeit on a smaller, more local scale. Well control and data establish, in part, the lithological character of these subsequences and their equivalence with the A, B and C units.

We distinguish proximal and distal delta front facies on the basis of log character and lithology. The former facies is characterized by blocky aggradational sandstones in an abbreviated coarsening-upward section; the latter by thin bedded sandstone and siltstone in a much thicker coarsening-upward section. The move from proximal to distal facies correlates with the appearance of foreset events visible on seismic sections.

This paper discusses the origin of Kugmallit delta-front depositional facies and suggests, as a preferred interpretation, that the subsequence boundaries primarily reflect eustatic change, rather than delta lobe switching or tectonic movement of basin or hinterland.