Page 303 - Carbonate Facies in Geologic History

290 Shoaling upward Shelf Cycles and Shelf Dolomitization

fossils (brachiopods, mollusks, echinoderms, foraminifers), a middle grainstone
member of clean, washed calcarenite composed of rounded skeletal particles, in a
few beds oolitic, and a thin, tidal flat, dolomitic mudstone grading within a few
feet to a thick anhydrite bed. Figure X-9 is a lithologic cross-section of Arab D,
the lowest and thickest of the cycles possessing a persistent and widespread
anhydrite above it. Capping anhydrites ofthe several older cycles beneath Arab D
are confined to the more basinal areas and cannot be recognized in the J ubaila
Formation of the Saudi Arabia and Qatar shelf areas. A very detailed petro-
graphic description of the Arab D cycle was published by Powers (1962) whose
regional study shows it to consist in places of two cycles. The upper one consists
basically of a lower, well-developed grainstone unit grading up to a tidal flat
dolomite and thence to a thick anhydrite which forms the essential seal of the
reservoir. The lower Arab D subcycle is more normal marine; its upper member is
composed of dense dolomitic lime mudstone, no evaporite being present. This
dense lime mudstone was used by Powers (1962) as a key marker. It is stratigraph-
ically equivalent to upper Jubaila beds on outcrops. Below this dense micrite cap
is a grainstone facies best developed along the northern Arabian Hasa Coast. The
lowest beds of the lower subcycle are lime wackestones which merge impercepti-
bly into the open marine, dark gray, slightly bioclastic Jubaila or Darb Forma-
tion. Thus no clearly defined base exists for the Arab D cycle and it cannot be
accurately isopached. It is generally less than 100 m thick.
Arab C cycle contains rock types similar to those of Arab D. It is also a double
cycle south of the Qatar-Surmeh high and its lower half bears a thin capping
anhydrite like that of its upper subcycle. Of particular interest in the cycle is the
basal grainstone which is thin but persistent. The maximum development of this
rock type in both subcycles coincides with the Hasa Coast, shifted slightly east-
ward from that of Arab D, and indicates a regression away from the Arabian
shield out into the Rub al Kali basin and up the flanks of the Qatar-Surmeh high.
The cycle is generally indicative of shallower, more restricted seas than is Arab D.
More oolite is present. Little or no normal marine wackestone occurs. The trend
toward more restricted and evaporitic conditions is continued in the overlying
cycles B and A.
Geographic variations of the cycles: ARAMCO work (Steinecke et aI., 1958;
Powers, 1962) indicated that lime mudstone, dolomite, and anhydrite dominated
Arab zone outcrops in the Arabian interior and graded downdip to lime sand in
the subsurface along the Hasa Coast. The cross-section (Fig. X-9) trends down the
coast from the Basrah basin of the northern Persian Gulf, crosses the major
grainstone area of Saudi Arabia, and passes toward a tidal flat and dolomite facies
underlying the Persian Gulf eastward from Qatar. Figure X-l0 shows that the
major grainstone development in the Arab D and C cycles occupies a passageway
between two basins north and south and two positive areas east and west. It was a
wide threshold area in which strong currents must have passed between the two
basins and which lay between the shallower tidal flats on the positive elements.
The more clearly defined shelf margin at the northern end of the grainstone area
was apparently the site of the most active tidal exchange because only in wells
located along this margin does any true oolite appear. The northern facies change
from basin to oolitic shelf must occur within 35 km (Fig. X-9). Most of the shelf