Articles
Publication Date: 2026
Journal of Asian Earth Sciences (18785786)297
The Upper Triassic–lower Middle Jurassic tectono-sedimentary megacycle in central Iran is represented by the Shemshak Group, deposited within the central part of the Cimmerian continent (the “Iranian Cimmerides”). It contains key information on the Mesozoic geodynamics of the Middle East during the collision of the Cimmerian terranes with Eurasia. However, detailed insights into the response of depositional systems to the complex contemporaneous tectonics are hampered by poor exposure, in particular of fine-grained strata. A new, 641-m-long core from the northern Tabas Block (east-central Iran) provides an unweathered and continuous subsurface record of the Shemshak Group. Detailed lithofacies and stratigraphic analyses refine previous lithostratigraphic and palaeoenvironmental interpretations, allowing to consider relative sea-level changes caused by Cimmerian tectonic events and/or global sea-level fluctuations. The thick siliciclastic succession of the Rhaetian Qadir Member of the Nayband Subgroup reflects rapid basin subsidence in advance of the Main Cimmerian Event (MCE). It comprises the dominant part of the Shemshak Group. In the aftermath of the MCE at the Triassic–Jurassic boundary, comparatively low subsidence rates and limited accommodation prevailed, resulting in low thicknesses of the Lower–lower Middle Jurassic mixed siliciclastic and carbonate Ab-e-Haji Subgroup. Process and stacking-pattern analyses of eleven clastic and eight carbonate lithofacies indicate a variety of continental and marine depositional environments, ranging from fluvial and deltaic to shallow-marine. Our findings significantly improve the knowledge of the tectono-stratigraphic setting of the Shemshak Group and contribute to a better understanding of the Cimmerian history of central Iran between the Late Triassic and the Jurassic. © 2025 Elsevier Ltd
Publication Date: 2024
Journal of Stratigraphy and Sedimentology Researches (20087888)40(2)pp. 1-26
The Middle Jurassic Hojedk Formation in the Parvadeh Coal Mine (90 km south of Tabas) has been studied. The study area is structurally located on the northern Tabas Block in the north of the Parvadeh–Nayband coal-bearing basin, east-central Iran. The Hojedk Formation has a thickness of 40 meters in the studied outcrop section. Five lithostratigraphic units have been identified, which mainly consist of sandstones, shales, conglomerates, siltstones and thin bioclastic sandy limestone. Based on field and petrographic studies, four groups of lithofacies including coarse- (Gcm and Gt), medium- (St, Sp, Sh and Sr) and fine-grained clastic (Fl and Fm) as well as carbonate facies (echinoid brachiopod grainstone-packstone) have been identified in the Hojedk Formation. The architectural elements of channels and point bars of a meandering tidal creek and coastal plain swamp have been identified in the Hojedk Formation and its palaeodepositional conditions can be attributed to the tidal flat of a marginal marine siliciclastic environment. To investigate the characteristics of the Hojedk Formation in the subsurface, the electrical facies were defined in an exploration borehole in the eastern part of Parvadeh Mine. In this borehole, the Hojedk Formation is 48 meters thick, and four electro/log facies have been identified by considering the gamma-ray motif and resistivity logs. These electro facies are compatible with the sub-environments of coastal sand flats, tidal channels and coastal plain swamps. © 2024 University of Isfahan.
Salehi, M.A.,
Wilmsen, M.,
Yahya sheibani, V.,
Zamanian, E.,
Keyvanpoor, K. Publication Date: 2024
Marine and Petroleum Geology (02648172)167
The Upper Triassic to lower Middle Jurassic Shemshak Group of the Central-East-Iranian-Microcontinent is important for understanding Mesozoic geodynamics of the Middle East during and in the aftermath of the collision of the Cimmerian microplates with Eurasia, resulting in the massive burial of organic matter and the formation of significant coal reserves. The present study thus aims to elucidate the stratigraphic patterns, lithofacies inventory, depositional environments and geodynamic significance of the Shemshak Group in the Halvan area of the Kalmard sub-block of the northern Tabas Block where the succession is most completely developed. Lithostratigraphic studies through detailed logging resulted in a differentiation of the siliciclastic Upper Triassic Nayband and Lower Jurassic Ab-e-Haji formations, bounded at their bases by the Eo- and Main Cimmerian unconformities, respectively, and the mixed carbonate-siliciclastic Badamu Formation (upper Lower to lower Middle Jurassic). The lithofacies analysis and stacking patterns of siliciclastic strata indicate deposition in a variety of environments ranging from (proximal to distal) coastal and delta plains to delta front and prodelta settings for the Nayband and Ab-e-Haji formations. Detailed litho- and microfacies studies of the Badamu Formation revealed inner to middle parts of a mixed carbonate-siliciclastic ramp system. Careful petrographic studies indicate that sandstones of the succession are generally lithic-rich (ranging from phyllarenite to sedarenite) and, up-section, grade into quartz-rich sublitharenite, providing important information for sedimentary provenance analysis. The sandstones rich in metamorphic- and sedimentary lithic fragments indicate a proximal source on the western Yazd Block as well as rocks exposed on the Kalmad sub-block itself. The integrated studies also led to the recognition of the Eo- and Main Cimmerian orogenic phases, characterized by rapid uplift and/or subsidence phases. Late Triassic and Early Jurassic deposition took place in a back-arc extensional basin characterized by normal faulting. During the Toarcian–Aalenian, the reduced input of clastic material due to source-area denudation and/or a major transgression resulted in the development of the mixed carbonate-siliciclastic system of the lower Badamu Formation in the western part of the northern Tabas Block. This ramp system developed into an attached, ocean-facing carbonate platform with a prograding margin of ooid shoals. © 2024 Elsevier Ltd
Studies of relict Upper Cretaceous obducted ophiolites provide information on the evolution of the Neotethys suture belts in central Iran. Studies of the sediment derived from their erosion will complement this information by addressing questions related to sandstone provenance, ophiolite emplacement history, and, perhaps most importantly, the mode and timing of their erosion. The ca. 3200 m thick Middle Eocene to lowermost Oligocene fine- to medium-grained siliciclastic beds of the Akhoreh Formation are superbly exposed along the western border of the Central-East Iranian Microcontinent. They cover the northeastern flank of the Cretaceous Nain ophiolite mélange massif and lie adjacent to the Paleogene Urmieh–Dokhtar Magmatic Arc. The common occurrence of chrome (Cr)-spinel in the sandstones indicates the presence of mafic to ultramafic rocks in the source region. However, the timing, extent and nature of contributions of the potential source rocks to the sedimentary succession are not well understood. We conducted a petrographic and chemical compositional study of Cr-spinel in samples of Akhoreh Formation sandstones to outline the type of ocean lithospheric source. Our results document variable Cr# (0.09 to 0.79) and Mg# (0.21 to 0.88) while Fe3+ and TiO2 (average 0.12 wt%) are generally low. The wide range of Cr# and Mg# indicates provenance from a variety of (ultra-)mafic rock types, including supra-subduction peridotites and partially volcanic rocks of an arc-forearc setting. Cr-spinels are chemically similar to those of the lherzolites, harzburgites and dunites of the Nain ophiolite. While 70 % of the Cr-spinel have Cr# > 0.6, suggesting a harzburgitic peridotite provenance, the minimum Cr# < 0.4, observed in the samples from the base of the succession, were likely sourced from lherzolite. The chemical composition of detrital clinopyroxene grains shows high SiO2 (52.76 wt%), CaO (20.15 wt%), and MgO (16.36 wt%), while FeO has moderate (6.03 wt%), Al2O3 has low (2.23 wt%), and TiO2 has very low (0.25 wt%) concentrations (commonly augite and diopside), indicative of crystallization from subalkaline magma in an arc setting. The provenance data trace the erosion history of the obducted ophiolite and indicate that the Nain supra-subduction ophiolite supplied detritus to the Akhoreh Basin northwards from the Middle Eocene to the earliest Oligocene. © 2023 Elsevier GmbH
