Palaeogeographical implications of an ecological paradox: cool-water carbonates in an Early Miocene mid-latitude warm realm (Qom Formation, Central Iran)

The shallow-marine Qom Formation outcropping in the mid-latitude Central Iran has sparked considerable interest among geologists owing to its hydrocarbon potential and palaeogeographical significance, being an important component of the relic Tethys seaway. The Ghalibaf succession located south-west of Semnan features a limestone-dominant lithology and archives a shallow-water tropical carbonate factory distinguished by prominent coral-dominated skeletal assemblages, also featuring variable incidences of coralline red algae and benthic foraminifera. The study section is dated Early Miocene (Late Aquitanian to Early Burdigalian) based on the characteristic index benthic foraminifera. Extensive photozoan coral framestones and boundstones with tropical coralline algae such as Sporolithon and Lithoporella, associated with common large symbiont-bearing benthic foraminifera, ooids and aggregates depict a typical shallow water tropical setting of the Neogene. However, distinct parts of this limestone sequence comprise cool-water carbonates represented by rhodalgal and bryorhodalgal skeletal associations that are rare to observe in the subtropical domain, except in highly eutrophic settings. The rhodalgal assemblage corresponds to an oligo-mesotrophic system dominated by coralline algae, while the bryorhodalgal association represents a highly productive system characterized by abundant bryozoans co-occurring with smaller benthic foraminifera, corallines and bivalves. High volumes of terrigenous influx besides rapid fluctuations in carbonate saturation levels and water temperature appear the most probable causes of this paradox. The palaeoceanographical changes linked to the Early Miocene glaciation-deglaciation cycles are also interpreted as having considerable influence in the process of cool-water carbonate genesis within an overall warm setting. The rapid community shifts pertinent to the Qom limestones indicate Central Iran as an epicentre of faunal migrations and turnovers in the Tethys seaway connecting western Tethys and the Indo-Pacific during the Early Miocene.