REVISED AGE OF THE PROXIMAL BAKHTIYARI FORMATION AND IMPLICATIONS FOR THE EVOLUTION OF THE HIGH ZAGROS

The syntectonic Bakhtiyari Formation is the youngest clastic sedimentary unit of the Zagros foreland basin in southwestern Iran. It consists of conglomerate with subordinate sandstone and marl, and was deposited in proximal depocenters flanked by older uplifted formations of the Zagros fold-thrust belt. The basal strata of the Bakhtiyari sit either parallel to or in erosional angular unconformity over tilted older formations of Paleozoic to Oligocene age.

According to many publications (including the Stratigraphic Lexicon of Iran), the Bakhtiyari Formation is considered a Pliocene-Pleistocene conglomerate developed after Miocene-Pliocene shortening and associated deposition of eroded material from uplifted mountains. It is further believed that rapid growth of the Zagros folds and deposition of the Bakhtiyari Formation started during Pliocene time in the northeastern part of the basin and progressed systematically to the southwest during Pleistocene to Recent time. Thus, the Bakhtiyari Formation is generally interpreted as an unmistakable time indicator for the Zagros orogeny, mountain uplift, and advance of deformation from northeast to southwest.

However, the present study shows that in the High Zagros south of Shahr-e-kord the Bakhtiyari conglomerate was deposited during the early Miocene or earlier. In this locality, more than 50 meters of gray marl, thin limestone, and sandstone with marine pelecypod, gastropod, coral, and coralline algae fossils are intercalated with heterogeneous Bakhtiyari conglomerate beds. Palynology and foraminiferal paleontology of these strata demonstrate deposition during early Miocene time. Therefore, this succession of Bakhtiyari conglomerate and interbedded marl must belong to the early Miocene. In addition, these lower Miocene Bakhtiyari beds unconformably overlie an older, thick, heterogeneous Bakhtiyari conglomerate which in turn caps an Oligocene marine sequence. These erosionally truncated conglomerates of the older Bakhtiyari Formation could have an age even older than early Miocene, possibly late Oligocene.

These findings reveal that the structural evolution of the High Zagros, the hinterland portion of the Zagros Mountains, was underway by early Miocene time, as recorded by initial foreland basin development and deposition of the proximal Bakhtiyari Formation.