Abstract: Tertiary Deltaic Cycles on Unstable Craton Margin, Banks Island, Arctic Canada

A. D. Miall

Mesozoic and Tertiary sediments in Banks Island were deposited in a series of small pericratonic basins that were active during sedimentation, as shown by facies trends and paleocurrent data.

Upper Maestrichtian to Eocene rocks comprise a mainly nonmarine-deltaic sequence as much as 1,200 m thick. Single-dependency Markov-chain analysis of surface sections indicates that the following vertical succession is of strong statistical significance: prodelta shale; delta-front interbedded shale, silt, and very fine sand; distributary-mouth bar or beach sand; soil; lignite. This repeat unit ranges from 0.2 to 35.9 m thick, with an average thickness of 7.4 m. The cyclicity changes slightly with time, transitions into soil and lignite becoming statistically more significant in the upper part of the succession.

Gamma-ray logs of four subsurface sections through the same rocks show repeated, serrated-funnel-shaped curves indicating the presence of upward-coarsening cycles with a similar thickness distribution to those at the surface. The similarity is confirmed by Fourier analysis of the logs, which reveals a relatively prominent harmonic in the amplitude spectra, with a wavelength ranging from 5.8 to 6.8 m. Variations in cycle thickness appear to depend on position within the sedimentary basins, thicker cycles being nearer the basin centers.

The origin of the cycles probably is related mainly to autocyclic mechanisms including distributary-mouth progradation and avulsion; cycle thickness also depended on rates of basin subsidence.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA