Page 306 - Carbonate Facies in Geologic History

The Smackover Formation 293

grainstone consists oflightly cemented rounded and micritized skeletal fragments,
many only identifiable now as peloids. Some of the sediment is very coarse. The
biota is restricted to dasycladacean algae, rare foraminifers, algal onkoids, gastro-
pods, and rare, worn fragments of normal marine organisms, such as brachiopods
and crinoids. By analogy with Recent carbonate sands, such a grainstone sheet
might be composed of a series of individual tidal bars but well spacing is not close
enough to discern this.
Percentage lines on the facies map (Fig. X-lO) indicate relative thickness of the
capping anhydrite of Arab D Cycle. The evaporite cap is thinnest over the eastern
edge of the lime sand buildup but may constitute about half the cycle westward
over the Arabian shield. Eastward it thickens across the Qatar-Surmeh high and
then thins, probably because of later dissolution. The thickening of the anhydrite
toward the positive areas on each side of the grainstone sheet, and its develop-
ment there in association with laminated tidal flat dolomite, affords evidence that
it was developed as a sabkha deposit on the surrounding highs. In addition, the
areas of thickest anhydrite along with interbedded salt (more than 75% of the
total cycle) lie within the two basins. These beds also contain thin dark shale and
carbonates. They change shelfward to lime grainstone through an intermediate
facies of dark brown, more or less homogeneous lime mudstone. These strata
have even planar laminations and contain scattered lenses of fine peloidal grain-
stone. Wells penetrate this facies in the Basrah basin. A few scattered wells in the
Rub al Kali basin show dark lime mudstone of the Dijab-Darb formations,
mainly along the Trucial Coast. Both east and west from the lime sand sheet the
cycle changes to restricted marine carbonate facies: light-colored, dolomitic and
chalky lime mudstone with sedimentary structures characteristic of tidal flats is
found in wells in Qatar and the eastern Persian Gulf (Wood and Wolfe, 1969). As
shorelines on old positive elements are approached, this section becomes thor-
oughly and finely dolomitized.

The Smackover Formation

The Arab zone facies have much in common with those of the Jurassic Smackover
Formation, a major sedimentary cycle of Oxfordian age which surrounds the Gulf
of Mexico and lies in the deep subsurface from Mississippi to Tamaulipas, Mex-
ico. It crops out only in northern and east-central Mexico,whereit is termed the
Zuloaga Formation. It forms a 100 m thick belt of shelf oolite and grainstone
about 30-80 km wide with a narrow slope facies of peloidal bioclastic wackestone.
This belt grades gulf ward into much thicker basinal facies (300-600 m). The distri-
bution of facies and thickness indicates that an underlying tectonic hinge line,
probably the edge of the eroded roots of the Ouachita Mountain chain, controlled
this narrow grainstone strip .

... Fig.X-lO. Facies map of ArabD cycle indicating evaporite and dark shaly limestone in
northern Basrah basin and southern Rub al Kali basin. Oolite and grainstone buildup is in a
shelf area between these basins and between the Arabic shield on west and Qatar-Surmeh
high on east