Stratigraphy, depositional environments and provenance of the Shemshak Group (Upper Triassic–lower Middle Jurassic) on the northern Tabas Block; evidence for the emergent Cimmerian ridge in Central Iran

Abstract

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.