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1 2006

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The Kashafrud Basin (NE Iran): a key for understanding the Jurassic geodynamic evolution of the Iran Plate

Wilmsen M., Fürsich F., Taheri J., Brunet M.-F., Seyed-Emami K., Majidifard M.

Volumina Jurassica

2006

Vol. 4, no. 1

69-69

The boundary between the Koppeh Dagh and the Binalud Mountains in northeastern Iran corresponds to the suture of the Palaeotethys, an ocean which, in the area of present-day Iran, had been completely subducted below the Turan Plate as part of Eurasia in the north towards the end of the Triassic (Early Cimmerian orogeny). At this boundary between the Turan Plate and the Iran Plate, the latter a part of the so-called Cimmerian Microcontinent Collage, a strongly subsiding, NW-SE-trending basin formed during the Late Bajocian-Bathonian, which became infilled with a thick (>2000 m) pile of fluvial to deep-marine siliciclastic sediments, combined in the so-called Kashafrud Formation. This Kashafrud Basin is a key for understanding the geodynamic history of the Iran Plate during the Middle-Late Jurassic. The Kashafrud Formation overlies, often with angular unconformity and a thick basal conglomerate, Triassic and older rocks. In the area of the southwestern basin margin (Binalud Mountains), coarse-grained fluvial sediments grade into marine sediments (fan deltas, deltas, storm-influenced shelf). Short transport distances and steep relief are indicated by high compositional and textural immaturity. In the northeastern part of the outcrop belt, towards the Koppeh Dagh Mountains, the Kashafrud Formation is marine throughout and rapidly grades into deep-marine, dark shales with turbidite intercalations, indicating a slope to basin plain environment. By the Early Callovian, siliciclastic sedimentation was gradually replaced by carbonates and the Kashfrud Basin was infilled with carbonate platform and slope sediments of the Chaman Bid and Mozduran formations (Callovian - Upper Jurassic). Estimates of subsidence rates indicate very high values of 700 m/my and more during the Late Bajocian-Bathonian, indicative of young continental rift zones, and the Kashafrud Basin is thus interpreted as a rift basin. Integrated facies and stratigraphic analyses indicate that the bulk of the sediments entered the basin from the SW, derived from erosion of the uplifted rift shoulders in the Binalud Mountains. Deeper marine, basinal areas extended to the NE, probably far below the Cretaceous cover of the Koppeh Dagh. A coeval subsidence pulse of similar magnitude, related to the Mid-Cimmerian tectonic movements, also occurred in northern Iran (deep marine marls of the Dalichai Formation in the Alborz Mountains). From a geodynamic viewpoint, the Kashafrud Basin is the southeastern extension of the rapidly subsiding South Caspian Basin (SCB) which, in northern Iran, started to develop already in the Toarcian-Aalenian. In the Bajocian-Bathonian, this basin was enlarged towards the E-SE (opening of the Kashafrud Basin), leading to a renewed separation of the Iran and Turan plates after the Early Cimmerian collision. The reactivation of a former ocean suture for the development of a strongly subsiding basin is rather exceptional (the SCB possibly also reached the spreading stage) and the reasons for its opening are still poorly understood.