The Shemshak Group (Lower–Middle Jurassic) of the Binalud Mountains, NE Iran: stratigraphy, depositional environments and geodynamic implications

Abstract

Abstract The Lower–lower Middle Jurassic non-marine sedimentary succession of the Binalud Mountains of NE Iran is correlated with the Jurassic part of the Shemshak Group of the Alborz Mountains and subdivided into three formations: the Arefi, the Bazehowz and the Aghounj formations. The succession rests, with angular unconformity, on a metamorphic basement deformed during the Late Triassic Eo-Cimmerian orogeny. The lowermost unit, the Arefi Formation, is subdivided into a lower Derekhtoot Member and an upper Kurtian Member. The Derekhtoot Member (up to 750 m thick) consists of very coarse-grained, chaotic boulder beds, breccias and conglomerates representing rock-fall deposits and proximal–middle alluvial fans, deposited along steep fault scarps. The succeeding Kurtian Member (>300 m) comprises finer-grained conglomerates with well-rounded clasts, reflecting deposition in a proximal braided river system. The overlying Bazehowz Formation is more than 1000 m thick and consists of vertically stacked, decametre-scale channel-fill cycles of the middle reaches of a braided fluvial system. The uppermost unit, the Aghounj Formation, consists of at least 400 m of granule- to pebble-size, thick-bedded and large-scale trough cross-bedded quartz conglomerates and sandy interbeds of a proximal braided fluvial system. The overall succession fines upwards due to erosion, down to metamorphic basement, of a high-relief source area in the NE, and rests on Cimmerian basement, suggesting that the strata are intramontane deposits of the Cimmerian mountain chain in NE Iran. This interpretation has important implications concerning the position of the NW–SE-trending Eo-Cimmerian suture in NE Iran, which should be placed further SW than formerly assumed.