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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.

Stratigraphy and depositional environments of the Upper Bajocian-Bathonian Kashafrud Formation (NE Iran)

Taheri J., Fursich F., Wilmsen M.

Volumina Jurassica

2006

Vol. 4, no. 1

105-106

The Upper Bajocian-Bathonian Kashafrud Formation is a thick (>2 km), siliciclastic sedimentary unit, distributed in a NW-SE-trending, 200 km long and 80 km wide outcrop belt in the area between the Koppeh Dagh and the Binalud Mountains (NE Iran). It was deposited in a strongly subsiding rift basin developing between the Iran Plate and Eurasia (Turan Plate) in response to Middle Jurassic crustal extension following the Mid-Cimmerian tectonic movements. The Kashafrud Basin is a southeastern prolongation of the South Caspian Basin, opening along the ocean suture which has been formed by the Late Triassic closure of the Palaeotethys (so-called Early Cimmerian orogeny). The stratigraphy and depositional environments of the Kashafrud Formation are the scope of this integrated analysis. Ten sections of the Kashafrud Formation were logged in detail and sampled for lithology, macro- and trace fossils, and facies analysis. At several localities, ammonites occur near the base of the formation, in every case providing a Late Bajocian age. Within the middle and upper parts of the Kashafrud Formation, ammonites are rare. However, in its uppermost part and at the base of the overlying unit (marls of the Chaman Bid Formation), ammonites of the genus Macrocephalites indicate an Early Callovian age. Thus, the Kashafrud Formation can be mainly assigned to the Upper Bajocian-Bathonian. Commonly, the Kashafrud Formation rests with angular unconformity on rocks affected by the Early Cimmerian orogeny. Based on thicknesses and principal facies development, two NW-SE-striking (basin-axis-parallel) zones can be differentiated, i.e., a more proximal one close to the Binalud Mountains in the SW and a distal one towards the Koppeh Dagh in the NE. Basal conglomerates tend to be thickest and coarsest in the proximal zone, at the southwestern basin margin. Rapid lateral thickness changes indicate a fault-controlled deposition (proximal alluvial fans or rockfalls). Up-section, the conglomerates are replaced by highly immature arcosic sandstones and pebbly sandstones of a short-headed braided river system grading into marine fan delta deposits. Rapid deposition and subsidence are indicated by the lack of significant maturation of the sediments even in settings above the fair-weather wave base. In some sections, basal conglomerates are thin and fluvial sediments missing, and the Kashafrud Formation directly starts with coarse-grained marine fan delta deposits. In all cases, a distinct fining-upward indicates a significant deepening trend. In the distal zone, basal conglomerates are thin and commonly marine as indicated by marine fossils. They are vertically replaced by basin plain (dark shales with rare ammonites) and intercalated submarine fan deposits (including coarse upper fan feeder channels, mid-fan leveed channel and inter-channel deposits, and well bedded shale-sandstone outer fan intercalations). Bioturbation is common in turbidite sequences and a deeper marine environment is indicated by rare graphoglyptids of the Nereites ichnofacies. Also in the distal part, a general fining- and thinning-upward trend indicates a deepening. All the observations can be integrated into a rift basin model for the Kashafrud Formation. The basin axis trend was roughly NW-SE and the stratigraphic and sedimentologic data indicate a main sediment input from the southwestern basin margin (Binalud). The northeastern basin margin is inferred below the Koppeh Dagh.