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Ammonites from the Oxfordian-Kimmeridgian boundary and the Lower–Upper Kimmeridgian of Kachchh, western India

Pandey D., Alberti M., Fürsich F., Głowniak E., Olóriz F.

Volumina Jurassica

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2013

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Vol. 11, no. 1

97–-146

EN

Several new specimens of ammonites from the Oxfordian and Kimmeridgian of Kachchh, western India, are described and illustrated. The Oxfordian ammonites ?Subdiscosphinctes Malinowska, Perisphinctes Waagen, Dichotomoceras Buckman, and ?Larcheria Tintant, all from Bharodia in the Wagad Uplift, enable tentative biochronostratigraphic correlations with the uppermost Middle Oxfordian up to the lower Upper Oxfordian of the unified Submediterranean zonation, whereas the Kimmeridgian ammonites Streblites Hyatt, “Orthosphinctes” Schindewolf, Torquatisphinctes Spath, Pachysphinctes Dietrich, Katroliceras Spath, Aspidoceras Zittel, and Schaireria Checa reconfirm a stratigraphic gap embracing incompletely known parts of the uppermost Oxfordian and the Lower Kimmeridgian as known from the ammonite records of the Kachchh Mainland of the Kachchh Basin.

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The Upper Triassic - Middle Jurassic Shemshak Group of Iran: foreland basin or rift basin?

Fürsich F., Wilmsen M., Seyed-Emami K., Majidifard M.

Volumina Jurassica

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2006

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Vol. 4, no. 1

41-41

EN

The Shemshak Group is a predominantly siliciclastic rock unit occurring over much of the Iran Plate. The group is sandwiched between two unconformities: the lower one resulted from the late Middle Triassic Early Cimmerian Orogeny, which documents the collision of the Iran Plate with Eurasia, the upper one results from the still poorly understood Bajocian Mid-Cimmerian tectonic movements. At the base, the group overlies, with distinct stratigraphic gap documented by karst features and locally extensive bauxite deposits, the platform carbonates of the Shotori and Elikah formations. At the top, the group is overlain by the marls of the Dalichai Formation, part of a widespread carbonate system existing for most of the late Middle-Late Jurassic and locally extending into the Early Cretaceous. The Shemshak Group reaches enormous thicknesses (up to 4000 m) and consists largely of fluvial sediments with common intercalations of coal beds. At the base and at the top, the group is represented by marine sediments which include reef and lagoonal carbonates, deposits of the storm-influenced shelf, deltas, and basinal mudstones. Another characteristic feature are volcanic and volcanoclastic rocks, which are commonly intercalated at or near the base of the group. Conventionally, the Shemshak Formation has been interpreted as the fill of a foreland basin. However, detailed sedimentological and stratigraphic studies over much of the outcrop belt of the group show that such a simple interpretation can no longer be upheld. Instead, the Shemshak Group is thought to reflect the following major tectono-sedimentary settings: during the Late Ladinian - Carnian initial collision of the Iran Plate with Eurasia in northeastern Iran (Koppeh Dagh-Binalud area), the Triassic carbonate platforms of the Iran Plate suffered warping and extensive lateritic weathering. In the Alborz area, a remnant basin of the Palaeotethys persisted, in which basinal sediments were deposited. During the Norian-Raethian, distension of the Iran Plate in connection with the onset of subduction of the Neotethys led to formation of rapidly subsiding marine grabens which were filled with sediments of neighbouring horsts (Nayband Formation). In the Alborz area, the remnant basin of the Palaeotethys became gradually infilled with partly shallow marine and partly fluvial sediments. The major collision between the Iran Plate and Eurasia took place around the Triassic/Jurassic boundary. Concomitant with the rising Cimmerian Mountains, a foreland basin developed, which received a thick conglomeratic fill in the north which southwards graded into a fluvial system with widespread coal swamps. During the Toarcian-Aalenian, this foreland basin changed into a rift basin, which is documented by transgression and a rapid change to basinal deposits, indicative of high rates of subsidence. By the end of the Early Jurassic, the Cimmerian Mountains were more or less eroded. The following Mid-Cimmerian tectonic movements are seen as recording strong distensional tectonics in connection with the opening of the South Caspian Basin in the north.

3

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

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2006

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Vol. 4, no. 1

69-69

EN

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.