Palynology of the Upper Triassic strata from Tabas, east-central Iran: Biostratigraphic, paleoenvironmental, and paleobiogeographic inferences

Abstract

Subsurface data from Well no. 954, Tabas, east-central Iran, constitute the foundation for an integrated palynological, paleoenvironmental, and paleobiogeographical analysis of the Upper Triassic (Rhaetian) Qadir Member of the Nayband Formation. The assemblage consists of diverse, moderately preserved spores and pollen. Of the latter, such taxa as Falcisporites nuthallensis, Lunatisporites rhaeticus, Ovalipollis ovalis, Ricciisporites tuberculatus, and Araucariacites australis dominate the assemblages. Representatives of such trilete spores as Dictyophyllidites mortonii, Gleicheiniidites senonicus Kyrtomisporis laevigatus, Limbosporites spp., and Lophotriletes bauhiniae, are essentially abundant in the palynofloras examined. Vertical distribution of miospores allows recognition of Dictyophyllidites-Striatella seebergensis-Ricciisporites tuberculatus assemblage zone (Rhaetian). The inferred natural relationships of the sporae dispersae indicate a parental flora derived from, in descending order: Pterophyta, Cycadophyta, Pteridospermophyta, Coniferophyta, Lycopodophyta, Ginkgophyta, and Bryophyta. Fern abundance suggests a moist, warm climate. Sporomorph EcoGroup (SEG) analysis shows dominant lowland/plain communities over upland and coastal/tidal ones, indicating deposition during relatively low sea level in shallow marine conditions. Palynological analysis identifies three palynofacies following Tyson's methodology. Palynofacies I represents very proximal, shallow shelf environments during sea-level fall and regression. Palynofacies II suggests dysoxic to anoxic, restricted shallow-water settings. Palynofacies VI indicates conditions from low-oxygen coastal to anoxic shelf environments. Key proxies—the transparent/opaque AOM ratio, equidimensional/blade-shaped opaque phytoclasts (P1/P2) ratio, and terrestrial/marine (T/M) ratio—support this interpretation. The palynofloras show strong biogeographic affinities with both northern Gondwana and southern Laurasia, highlighting Iran's position as a biogeographic crossroads on the southern margin of Eurasia during the Late Triassic.