Palaeobiogeography, palaeoecology, and sequence stratigraphy of the Upper Jurassic carbonate succession of the Lar Formation, central Alborz Zone, Iran

Abstract

Foraminifera, ammonites, and calcareous dinoflagellates were used for stratigraphy and, together with microfacies, for the assessment of the palaeoenvironmental conditions of the Upper Jurassic deposits in the central Alborz Zone of northern Iran. The Lar Formation (Lar Fm.) in the Polur section is of latest Oxfordian to early Kimmeridgian age. The ammonite Subnebrodites planula and the calcareous dinoflagellate Colomisphaera nagyi have been introduced as new biomarkers of the lower Kimmeridgian in the central Neo-Tethys. The distribution of calcareous dinoflagellates reflects possible dispersal routes along a narrow seaway between the western Neo-Tethys and the Alborz Zone in the central Neo-Tethys. The Terebella-Crescentiella associations of the Lar Fm. represent a low-energy setting under dysoxic conditions in the Central Neo-Tethys Ocean. The benthic foraminiferal assemblages in this formation show a high dominance of infaunal taxa and r-selected strategists. This assemblage is reminiscent of eutrophic conditions and low oxygen levels in the lower part of the Lar Fm. Good preservation of the hexactinellid sponges in the upper part of the Lar Fm. also indicates an oxygen-minimum zone. Three third-order depositional sequences can be distinguished in the study area based on six microfacies. Depositional sequence 1 (DS1) is composed mainly of argillaceous limestone and medium- to thick-bedded limestone, corresponding to an outer ramp-to-middle ramp environment. Depositional sequence 2 (DS2) comprises breccia limestone and thick-bedded limestone facies in its lower part and thin-bedded limestone to massive limestone in its upper part. The breccia limestone facies may be associated with subaerial exposure and reworking of previously deposited sediment during a relative sea level fall. The thin-bedded limestone to massive limestone of DS2 consists mainly of bioclastic mudstone to wackestone (outer ramp). These represent an deep-water outer homoclinal ramp facies. Depositional sequence 3 (DS3) consists mainly of massive limestone to thick-bedded limestone with a bioclastic peloidal microbial Crescentiella packstone (middle ramp). The relative stratigraphic positions of DSs1–3 and sequence boundaries in the uppermost Oxfordian to lower Kimmeridgian of the Polur area show a fair match to the upper Oxfordian to lower Kimmeridgian sequences (JOx7, JOx8, JKi1 and JKi2) on the global sea level curve.