Mortaza Pirouz

Neogene sediments and modern depositional environments of the Zagros foreland basin system

A sedimentological investigation of the Neogene deposits of the Zagros foreland basin in SW Iran reveals a continuous and largely gradational passage from supratidal and sabkha sediments at the base (represented by the Gachsaran Formation) to carbonates and marine marls (Mishan Formation with basal Guri carbonate member) followed by coastal plain and meandering river deposits (Agha Jari Formation) and finally to braided river gravel sheets (Bakhtyari Formation). This vertical succession is interpreted to represent the southward migration of foreland basin depozones (from distal foredeep and foredeep to distal wedge-top and proximal wedge-top, respectively) as the Zagros fold–thrust belt migrated progressively southward towards the Arabian foreland. This vertical succession bears a striking similarity to modern depositional environments and sedimentary deposits observed in the Zagros region today, where one passes from mainly braided rivers in the Zagros Mountains to meandering rivers close to the coast, to shallow marine clastic sediments along the northern part of the Persian Gulf and finally to carbonate ramp and sabkha deposits along the southeastern coast of the Persian Gulf. This link between the Neogene succession and the modern-day depositional environments strongly suggests that the major Neogene formations of the Zagros foreland basin are strongly diachronous (as shown recently by others) and have active modern-day equivalents.

Post-collisional deposits in the Zagros foreland basin: Implications for diachronous underthrusting

Detailed sedimentology of the Neogene foreland basin deposits is investigated and classified into 11 lithofacies associations with respect to their paleo-sedimentary environments. The foreland deposits reveal a single coarsening-upward mega-sequence with continuous passage from back-bulge to forebulge, foredeep, and wedge-top sedimentary environments. The Gachsaran deposits form the base of the foreland strata and consist mainly of three different lithofacies associations including fluvial, marine, and sabkha deposits in the eastern Zagros in Fars, and are typically dominated with evaporites toward the west in the Dezful and Kirkuk embayments. The Mishan Formation has three different shallow-marine lithofacies associations in a vertical succession representing foredeep deposits in the eastern Zagros, which tapers toward the Dezful embayment and disappears in Iraq. The Agha Jari distal wedge-top deposits also contain three different lithofacies associations including delta deposits mostly in the Fars, tidal flat deposits in Dezful and Mesopotamia basin, and continental fluvial deposits across the entire Zagros. The uppermost synorogenic Bakhtiari Formation represents proximal wedge-top deposits and consists mainly of two main lithofacies associations including shallow marine and fluvial deposits, within which the fluvial succession is divided into three sub-lithofacies associations with respect to distance from the mountain front and hydraulic power of the river networks. Synthetizing sedimentary facies association with age constraints of the old foreland deposits near the Zagros suture in the High Zagros area suggests that a considerable part of the Arabian plate has been removed at the northern edge by underthrusting and erosion. Moreover, preservation of the young distal foreland deposits near the suture in the western Zagros implies that the magnitude and rate of removal of the proximal foreland deposits have been inconstant along-strike the belt and decreases toward the east.

Structural pattern for normal faulting of west central Iran

The studied area is situated in the North West Isfehan province in central Iran. The oldest outcropping rocks are upper Jurassic, the upper of which contacts are unconformable. Lower Cretaceous units are composed of conglomerate, sandstone and orbitolina limestone (Aptian-Abian). Upper Cretaceous units are composed of limestone and marl with interbeds of orbitolinaferous limestone. According to a study of ETM data, systematic conjugate mega-joints are generally observed in the whole area. These joints are also observed in Jurassic and Cretaceous units. Jurassic units are ductile and Cretaceous units are brittle. Transtensional faults developed in Pre-Jurassic units. Activation these faults formed new normal faults in Cretaceous units. Syntectonic Oligo-miocene limestone was deposited on the Cretaceous unit disconformably, so that the lower Cretaceous units show normal faults and Oligomiocene units show syncline structures. The fault system of the study area is a part of the Alpine orogeny. Based on the ETM data, a clockwise rotation could be observed in the whole area. Blocks between these mega-joints have experienced and elongation.