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

• Autorzy: Taheri J. , Fursich F. , Wilmsen M.
• Konferencja: International Congress on the Jurassic System (7 ; 06-18.09.2006 ; Kraków, Poland)
• Języki publikacji: EN

Abstrakty

EN

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.

Słowa kluczowe

EN

Iran; Bajocian-Bathonian; Kashafrud Formation; biostratigraphy; lithostratigraphy; facies analysis

• Wydawca: Państwowy Instytut Geologiczny - Państwowy Instytut Badawczy
• Czasopismo: Volumina Jurassica
• Rocznik: 2006
• Tom: Vol. 4, no. 1
• Strony: 105--106

Twórcy

autor

Taheri J.

autor

Fursich F.

autor

Wilmsen M.

• Institut fur Palaontologie der Universitat Wurzburg, Pleicherwall 1, D-97070 Würzburg, Germany,