Page 296 - Carbonate Facies in Geologic History

Oolite Cycles on the Mississippian Shelf around the Williston Basin 283

climates and with considerable relief and good underground drainage form special
areas for development of this type of cycle. Both marine and meteoric vadose-
phreatic alteration is possible.
Table X-I outlines the idealized sequence of upward shoaling environments
displayed by the above cycles. Few individual cycles follow this pattern com-
pletely but the schema is useful in predicting sequence once careful petrographic
description has been made.

Oolite-Grainstone Cycles

Shoaling cycles with rounded, worn, coated, bioclastic grainstones and some well-
formed oolite are especially common in Jurassic and Early Carboniferous strata
of the northern hemisphere. Grainstone members of many other cycles lack well-
developed oolite and have only coated particles, particularly in Devonian and
Cretaceous strata.

Oolite Cycles on the Mississippian Shelf around the Williston Basin

Description of the cycles: The Williston basin and Montana area contains a 400-
700 m thick Mississippian sequence consisting of the Lodgepole, Mission Can-
yon, and Charles formations of the Madison Group. These units together repre-
sent a major upward shoaling sequence in which the sea spread out of the shallow
Williston basin over the Central Montana high and at first deposited dark argilla-
ceous limestone alternating in several cyclic sequences with oolite. Above strata
which indicate extensive inundation, the cycles become highly oolitic with a light-
colored, peloidal unit at the top. In later Mississippian (Charles) time, the cyclic
pattern continued to form as the sea retreated gradually from the shelves, confin-
ing its deposits to the center of the Williston basin. During this period, the upper
part of each cycle became more evaporitic and finally salt was deposited in the
basin center.
The Lodgepole-Mission Canyon cycles are best developed along the margins
of the very large, shallow, marginal cratonic Williston basin in North Dakota,
Saskatchewan, and its adjoining shelves on the west. A prominent east-west ele-
ment on the Montana shelf west of the basin (Central Montana high) today
coincides geographically with major Rocky Mountain uplifts exposing the shelf
equivalents of the Williston basin strata. Very detailed stratigraphic work was
done in these outcrops and in wells throughout the Williston basin in the two
decades from 1950-1970 because of successful petroleum exploration. Both the
regional stratigraphic framework and sedimentological details are well under-
stood. The first good analysis of the facies pattern was by Edie (1958) and the
most recent work is by Smith (1972).
More than 15 cycles can be traced in the Madison Group along the part of the
Montana shelf exposed in the Big Snowy and the Little Belt Mountains uplifts of