The Triassic Evolution of the Aghdarband Basin (Koppeh Dag, NE Iran)

Balini, Marco ¹; Nicora, Alda ¹; Zanchi, Andrea ²; Ghassemi, Mohammad R.³; Bahrammanesh, Maryamnaz ³; Hosseiniyoon, Maryam ⁴; Norouzi, Mehdi ³; Soleimani, Sara ³
(1)Dipartimento di Scienze della Terra, Università degli Studi di Milano, Milano, Italy. (2) Dipartimento di Scienze Geologiche e Geotecnologie, Università degli Studi di Milano-Bicocca, Milano, Italy. (3) Geological Survey of Iran, Theran, Iran, Islamic Republic of. (4) Geological Survey of Iran, Mashhad, Iran, Islamic Republic of.

The Aghdarband Basin, consisting of a strongly deformed arc-related Triassic marine succession, is a key-area for the study of the Cimmerian events, and has been investigated in detail in the framework of a project sponsored by the DARIUS Programme. The evolution of this basin was previously reconstructed only from scattered investigations carried out in the 1970s and 1990s.

The basin developed during the Triassic in a highly mobile tectonic context suggested by abrupt facies variations and local unconformities. In addition, syn-sedimentary tectonic activity is documented by the occurrence of carbonate olistholiths in the deepest parts of the basin.

The marine succession, spanning from Olenekian to lowermost Carnian, shows at the base continental conglomerates and sandstones, as well as basaltic lava flows, of Early Triassic age. They are followed by the shallow water Sefid Kuh Lmst., Olenekian in age, in which an intraformational unconformity has been now identified.

The carbonate ramp of the Sefid Kuh Lmst. shows complex stratigraphic relationships with the pelagic Nazarkardeh Fm. that are described in detail for the first time. In the northern part of the basin the shallow water carbonate sedimentation persists until the Early Anisian and the carbonate ramp interfingers with the cherty limestones, volcaniclastic sandstones and tuffs of the Nazarkardeh Fm. In the middle and southern part of the basin the drowning of the Sefid Kuh Lmst. is late Olenekian and the unit is unconformably overlain by the Nazarkardeh Fm.

In the Late Anisian the succession is uplifted, tilted and deeply eroded due to a tectonic event that is documented in the different part of the basin. A complex paleomorphology is sealed by the basal conglomerates and volcaniclatic sandstones of the Sina Fm. A Late Ladinian drowning is recorded at the base of the Faqir Marl Bed, which marks the beginning of a shale dominated sedimentation of the upper part of the Sina Fm. This unit is in turn unconformably covered by the coal bearing shales of the Miankhui Fm., characterized by an up to 10m thick basal coal layer. This coal bed is Norian-Rhaetian in age, supported by plant megafossils, and marks the eo-Cimmerian unconformity, i.e. the end of marine sedimentation and of volcanic-arc activity in the Aghdarband area.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.