Page 44 - Carbonate Facies in Geologic History

Carbonates Developed Adjacent to Platforms in Areas of Moderate Subsidence 31

DOMINANT FACIES
Ed 10 '" .. ~ ~D
BaSInal lim. 8,ociO$l1C Ool,t. Dolomlt. TIdal Anhydrlle
mudstone wacke stan. F at lsi Dolom't.

Fig. 11-8. Generalized Early Mississippian facies of Williston basin and Montana shelf. The
map is of a single interval (Nesson zone) about 30 m thick in a carbonate sequence of about
500 m (Mission Canyon facies of Madison group). Such intervals are subdivided on the basis
of electrical log characteristics and cyclic lithofacies. The dominant facies patterns are indi-
cated where the rock type represents more than 50% of the interval. Note the evaporite and
dolomite across the central Montana high, the tidal flat-lagoonal limestone and dolomite
south of the positive area, and the irregular "basinal" lime mudstone north, with scattered
large banks of bioclastic wackestone and oolite sands. This cyclic progression of facies was
built northward off the low Wyoming shelf into the gently subsiding Williston basin. The
facies belts are wide and somewhat irregular

America, a great shoal water and intertidal wedge of carbonates bordering the Shield and
extending into miogeosynclines east, west, and south of it (Fig. 11-7). The Cretaceous border-
ing the Gulf of Mexico and the Mesozoic section east and south of the Arabian shield also
typify this deposit. In the latter case, the present Persian Gulf lies in the shallow foredeep
formed between the Arabian Shield and the rising Zagros chain.
b) Narrow fringes or halos at cratonic edges: When tectonic activity is strong and rapid,
basin subsidence occurs at borders of old orogenic ridges or cratonic plates and a relatively
narrow, but thick, fringe of carbonate facies forms. Such patterns are common around tec-
tonic lands within geosynclines or on steep borders of some platforms. If the slope is suffi-
ciently steep, no barrier and no back reef lagoon develops. An example of the latter would be
the halo of high energy Smackover (Late Jurassic) sediments crowded against the roots of the
Ouachita orogenic belt and the northern Gulf of Mexico (Bishop, 1968, Fig. X-12).

3. Carbonates Developed Adjacent to Platforms
in Areas of Moderate Subsidence

These are built out mainly within shallow intracratonic basins. They are similar in construc-
tion to large carbonate ramps and platforms but the strata are thinner and all facies belts may
be tens ofkm wide; the gentle depositional slopes into the basin center may be on the order of
only 20-30 cm per km. Somewhat irregular facies patterns occur along platform margins and