Page 355 - Carbonate Facies in Geologic History

342 The Rise of Rudists; Middle Cretaceous Facies in Mexico and the Middle East

Toward the beginning of the Cretaceous, evaporite seas withdrew from the Ara-
bian shield into the basin. Not much of the Early and Middle Cretaceous terrig-
enous clastic influx reached it and the section deposited during this time, is
mostly limestone. In places farthest removed from the source of clastics in north-
western Arabia, shoal carbonates of Cretaceous age tended to develop on the
eastern extremity of the Rub al Khali basin forming grainstone and rudist reser-
voirs for oil along the Trucial Coast and in the Persian Gulf itself. Some of the
most detailed analyses of Middle East Cretaceous facies derive from studies here.

Facies Patterns

Mosul Block: The Aptian and Albian facies surrounding the Mosul Block have
been described and mapped regionally by Dunnington (1958). Previously, exten-
sive thin-section studies, which laid the groundwork for biostratigraphy and envi-
ronmental analysis, were carried out by F.R.S. Henson of Iraq Petroleum Com-
pany. Detailed information on the Mesozoic geology of Iraq rests almost entirely
on the published work of these two men, which also reflects a large amount of
man-years by a talented staff of British and Swiss geologists.
The area over the subsurface Mosul block consists of about 200 m of partIy
eroded Lower and Middle Cretaceous, but almost 1000 m is present eastward
along the edge of the shoal limestone facies. Still farther east the beds thin into the
Zagros geosyncline passing into a dark argillaceous limestone facies. Along the
Iranian-Iraqi border in the Pir-i-Mugrun ridge, Middle Cretaceous limestones are
seen grading to globigerinid marls in a southerly direction (Henson, 1950, Fig. 14).
The strike of the oil-producing structures of northern Iraq cuts directly across the
belts of thick carbonate facies. Figure XI-12 is a greatly exaggerated cross section
representing 700 m oflimestone from the Mosul block southeast down the axis to
Kirkuk anticline to the outcrops in the frontal Zagros Mountains of Kurdistan, a
distance of 200 km. This section includes Albian, Aptian, and Barremian strata
whose age was determined from extensive micropaleontology by the Iraq Petro-
leum Company and whose carbonate petrography was determined by the author
from the same thin sections.
The Albian western facies (Jawan Formation) is essentially shelf evaporite.
This gives way eastward to a dolomitized carbonate mudbank which farther
northeast of Kirkuk grades to Orbitolina peloidal wackestone and in the Kurdi-
stan outcrops is represented by thin ammonite-bearing argillaceous limestone.
The Albian carbonate bank is 300 m thick and the outcropping dark limestone
measures 80 m.
Aptian-Barremian strata east of Mosul show more complex facies. The most
shelf ward strata are not preserved, having been eroded in both pre-Albian and
pre-Senonian time. These western beds are foraminifera-rich shoal mudstones
and wackestones with an algal flora of Lithocodium (codiacean) and Diplopora
(dasycladacean). The foraminiferal genus Orbitolina is also common along with
other large forms such as ChofJatella and Pseudocyclammina. Under the eastern
end of the Kirkuk anticline lies the edge of the Aptian-Barremian bank whose full,
uneroded thickness is 450 m. The shelf margin consists of grainstones and some