Cimmerian tectonostratigraphic events in east-central Iran revealed by a long drill core of the Shemshak Group (Upper Triassic–Middle Jurassic)

Abstract

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