Page 357 - Carbonate Facies in Geologic History

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

Id .1 Shargi PERSIAN GUlf

AREA

TRUCIAl STATES

7
lOOkm

Fig.XI-13. Suggested Lower Cretaceous facies of the Upper Thamama Limestone of the
Trucial Coast, Arabia. Shuaiba is uppermost unit of Thamama (see Fig.XI-14). The shoal
grainstone area includes the whole Upper Thamama. It is not clear to what extent these shoal
areas represent regional trends or to what extent they merely represent local shoal facies
induced by structural growth over individual domes

reefoid strata with Lithocodium and rudists interstratified with shaly tongues of
Orbitolina limestone and marl. Off the bank the facies changes rapidly to pelagic
argillaceous limestone and in the outcrop to the east the equivalent is radiolarite
and ammonite limestone totalling only 200 m. The facies pattern is typically
regressive off the Mosul block, the Albian change of facies into thinner, true
basinal pelagic strata occurring farther east than that of Aptian-Barremian beds.
Rudists appear to be present but not extremely common in either Albian or
Aptian strata as interpreted from the well data, but subsurface studies may not
give a correct impression. Henson observes major buildups and scattered patches
of rudists forming what he termed "shoal banks or reefs." He interpreted the facies
transition so remarkably displayed in the Pir-i-Mugrun, not far from the line of
section on Fig. XI-12, as consisting of massive, much recrystallized rudist reef
which grades through microdetrital forereef limestone with bioclastic rudist de-
bris and Orbitolina to globigerinid lime mudstones.
Upper Thamama shoal area in the northern Rub al Khali basin : Over the central
part of the Persian Gulf and into the basin, the upper or Aptian part of the
Thamama, the Shuaiba Limestone, is separated from the main body of the Tha-
mama (Karaib) by a widespread shale unit (Hawar). The Shuaiba was formed
during an extensive inundation of the northern Arabian shield. During this event,
extensive shoal deposits formed to the southeast in the Rub al Khali basin
(Fig. XI-ll). A series of thick grainstone-rudist buildups probably exists along this