Middle Jurassic palaeoenvironment and palaeobiogeography of the Tabas Block, Central Iran: palynological and palaeobotanical investigations

Abstract

We performed a palynological study on Jurassic successions of the Hojedk Formation, Central Iran. The sampled material was recovered from well no. 233, South Kuchak-Ali area belonging to the Tabas Block. The lithology comprises 475.80 m of green–grey shales and sandstones. Palynological assemblages were recovered including spores, pollen grains, dinoflagellate cysts, woody debris and amorphous organic matter (AOM). Furthermore, 22 taxa of plant macrofossils were identified. We analysed 48 samples which resulted in identification of 13 spore species (belonging to 11 genera) and three pollen species (from three genera) with fair preservation quality. Trilete spores dominate while pollen grains only represent 11.5% of the total particulate organic matter. Representatives of the fern spore Klukisporites are particularly abundant and the K. variegatus acme zone, which is of biostratigraphical importance, is identified in the host strata. The abundance of K. variegatus and K. scaberis suggests a Middle Jurassic (Bajocian–Bathonian?) age for the host strata. This age designation corroborates previous stratigraphic interpretations, mainly based on plant macrofossils for this formation. The palynomorphs indicate deposition in an oxygenated environment close to the shoreline in a deltaic system. The presence of subordinate marine palynomorphs (proximate dinoflagellate cysts) is probably indicative of intermittent sea level transgression. The affinity of the palynological assemblages from the Hojedk Formation indicates a diverse parental flora of, in descending quantitative order, ferns, bryophytes and gymnosperms, the latter mainly represented by conifers and ginkgophytes. Comparisons with modern plant ecology indicate accumulation of palynomorphs under a moist warm climate during the Middle Jurassic (Bajocian–Bathonian?) time. The comparison of parental plants with previous palaeofloristic studies indicates that the Tabas Block was located in the Mid-Asian part of the Indo-European floristic province of the Northern Hemisphere.