EARLY ANTICLINES OF THE ZAGROS FOLD BELT, SOUTH WEST IRAN

The classic anticlines of the Zagros Fold Belt are a result of the Miocene collision of Arabia with Eurasia. However, strata revealed in the exposed cores of anticlines suggest that some of the anticlines started to grow as early as Late Cretaceous and Eocene. Stratigraphic variation and facies changes in the Lurestan province of the Zagros sedimentary basin shows that upper and post Cretaceous Formations are affected by both regional uplift in the northeast portion of the basin, and growing anticlines, near the basin center.

The impressive Sefid and Khoshab anticlines near the cities of Khorram-abad and Pol-e-dokhtar display the best evidence for early anticline growth. The Amiran and Kashkan Formations are flysches and conglomerates of late Cretaceous and Eocene age. These formations surround the high relief, early and middle Cretaceous limestones in core of the Sefid Anticline. Stratigraphic studies of the Amiran and Kashkan Formations in this location indicate that:

-The Amiran and Kashkan Formations thin from the core of the northern syncline southward toward the flank of the Sefid anticline.

-Some of the underlying Cretaceous beds and older clastic layers of Amiran Formation are onlapped by the younger conglomerate beds of the same Formation. Also, the Cretaceous and Eocene beds are truncated and overlapped by Oligo-Miocene Asmari Formation along portions of the north limb of the anticline.

-There is more than 800 meters of massive bedded coarse conglomerate within the Amiran Formation along the north limb of the anticline, while only fine sandy marine shale and flysch are present at the same horizon on the southern limb.

-In the Khoshab Anticline at the southwestern edge of Mountain Front, the thickness of Amiran and Kashkan Formations changes from 1000 meters in the northern limb of the anticline to 100 meters in the southern limb.

-Lower beds of Kashkan are truncated and onlapped by younger beds.

These data, present in the Sefid and Khoshab anticlines, and lack of similar data around the other anticlines of the region suggest that growth of the Sefid and Khoshab anticlines started in the Cretaceous and Eocene. The anticlines acted as a sedimentary barrier and prevented the regular distribution of the clastics in the basin. Their early uplift may have affected the migration and accumulation of early cooked hydrocarbons.