Page 359 - Carbonate Facies in Geologic History

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

MUUAN MU ..... N MUU ... N
so. au HASA NO. IU H ... SA II.I
__ ---------------- 30k .. ----------------------+ 30k .. NE
N

I.

'ocl".lIa bound .'on.
o

Fig.XI-14. South to north cross section of uppermost Shuaiba beds along Bu Hasa dome to
Bab dome south of Murban on Trucial Coast of Persian Gulf. Vertical exaggeration X 200.
For legend see Fig. III-I. For location see Fig.XI-13. After Harris et al. (1968)

Bacinella. This was followed by a deposit of chalky lime mudstone. Next
a mat of Lithocodium developed across the whole area; at the northern end
of Bu Hasa this grew rapidly enough to form a boundstone reef. This early reef
stage shed some debris off to the deeper area to the north. This debris graded
through Orbitolina wackestone to basinal lime mudstone. The Lithocodium reef
was followed by a rudist reef, consisting of monopleurids and caprin ids ? in a
chalky micrite with much fracturing. Much rudist forereef debris accumulated on
the open sea side. As reef growth stopped, presumably upon growth into wave
base or to sea level, a detrital shoal facies developed which capped the forereef
slope. This consists of bioclastic and lithoclastic grains. The southern end of Bu
Hasa dome contains a facies interpreted as open backreef. It began with a lower
Orbitolina wackestone and developed a miliolid wackestone at the top, probably
indicating more restricted circulation. The facies described in these Aptian beds
are almost identical to those of Mexico. The environmental significance of the
double reef and why the higher rudist buildup replaces the Lithocodium reef is not
clear.
A distinct facies sequence is also discernible in Cenomanian strata in both the
Rub al Khali basin and in the northern Persian Gulf. These are associated with
carbonate buildups also forming large banks such as are found in the Bangestan
limestone of the Zagros in Iran and within major sedimentary cycles of the same
age in Iraq. The typical Cenomanian bank facies consist of patches of whole
rudists in a lime-mud matrix, or rudist-algal packstones (peri-reef deposits) or
miliolid mudstones-wackestones of ponds or tidal flats. More open channels and
lagoons on the banks are marked by wackestones with miliolids and the abun-