Palaeoenvironment, palaeogeography, and palaeoclimatology of the Mississippian carbonate ramp in the Alborz tectonostratigraphic zone: Implications for deciphering the controlling factors on ramp evolution

Abstract

The Mississippian successions of the Alborz tectonostratigraphic zone have been studied in several sections (Dozdehban, Naserabad, Shahmirzad, and Mighan) extending from north to southeast. The sedimentological data indicate three main facies associations (FA): (FA1) a basin to distal outer ramp of low‐energy, low‐oxygen environment with low bioturbation and less developed in situ fossils; (FA2) a middle ramp facies accompanied by storm‐induced sediments (tempestites), echinoderm‐dominated mud mounds, and high fragmentation of skeletal components; and (FA3) an inner ramp facies, represented by sand shoal, and lagoonal/back‐barrier, mainly characterized by the dominance of oolitic, bioclast, and intraclast grainstones. The sedimentary features imply the formation of the succession on a broad flat‐topped, high‐to‐moderate‐energy shelfal carbonate ramp which had been open to the N/NE, the downslope direction, as revealed by local palaeoenvironmental reconstructions. Analysis of several proxies indicates a subtropical palaeoclimate evidenced by (a) widespread tempestite facies and occurrence of storm‐depositional processes, (b) diverse and abundant pellet‐forming thalassinoideans and burrowing echinoids trace fossil assemblages, (c) geochemical proxy indicating a palaeotemperature of 23°–25°, and (d) a rich subtropical heterozoan fossil association. The data provided here imply the palaeolatitudinal position of the Alborz Mississippian ramp is 30–40°S along the southern margin of the Palaeotethys.