Page 313 - Carbonate Facies in Geologic History

300 Shoaling upward Shelf Cycles and Shelf Dolomitization

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MONT"N"

Fig.X-15. Isopachous map in feet of Duperow cycles III a and IIIb showing gradual thicken-
ing from Montana shelf into axis of Williston basin in North Dakota and Saskatchewan.
Cross section of Fig.X-14 trends from southeastern Saskatchewan across Nesson anticline
(closed 70 foot contour north of Missouri River) and through western North Dakota into
southeastern Montana. After Wilson (1967b)

the peripheral shelves (Fig. X-16). The time of deposition of each cycle can be
estimated from 500000 to a million or so years assuming a constant rate of
sedimentation through the Late Devonian. Using the rate of rapid progradation
of very similar Holocene sediment along the Trucial coast (20 km in 5000 years)
and correcting for the thick Duperow cycles, it may be estimated that the 1000 km
width of the Williston basin might have been filled in by lateral progradation
during the half million to a million year span of a single cycle. That this repeatedly
and regularly happened in a vast non-orogenic area and in sediments lacking
evidence of erosion of previously deposited strata, suggests some climatic or
world-wide eustatic mechanism as a cause.

Middle Permian of West Texas Shelves

Very similar cycles are known in restricted marine shelf and platform strata of the
widespread Clear Fork and San Andres Formations of West Texas and New
Mexico. These beds represent one of the major marine transgressions through
southwestern North America, extending from central Texas far west to the Grand
Canyon (Kaibab Limestone) and south of central New Mexico into Chihuahua
(Concha Limestone). Extensive dolomitization has resulted in good porosity and
permeability in the subsurface and more than half the oil in this vast province has