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

Foraminifera, ammonites, and cal car e ous dinoflagellates were used for stra tig ra phy and, to gether with microfacies, for the assessment of the palaeoenvironmental con di tions of the Up per Ju ras sic de pos its in the cen tral Alborz Zone of north ern Iran. The Lar For ma tion (Lar Fm.) in the Polur sec tion is of lat est Oxfordian to early Kimmeridgian age. The ammonite Subnebrodites planula and the cal car e ous dinoflagellate Colomisphaera nagyi have been in tro duced as new biomarkers of the lower Kimmeridgian in the cen tral Neo-Tethys. The dis tri bu tion of cal car e ous dinoflagellates re flects pos si ble dis persal routes along a nar row sea way be tween the west ern Neo-Tethys and the Alborz Zone in the cen tral Neo-Tethys. The Terebella-Crescentiella as so ci a tions of the Lar Fm. rep re sent a low-en ergy set ting un der dysoxic con di tions in the Cen tral Neo-Tethys Ocean. The ben thic foraminiferal as sem blages in this for ma tion show a high dom i nance of infaunal taxa and r-selected strategists. This assemblage is reminiscent of eutrophic con di tions and low ox y gen lev els in the lower part of the Lar Fm. Good pres er va tion of the hexactinellid sponges in the up per part of the Lar Fm. also in di cates an ox y gen-min i mum zone. Three third-or der depositional se quences can be dis tin guished in the study area based on six microfacies. Depositional se quence 1 (DS1) is com posed mainly of ar gil la ceous lime stone and me dium-to thick-bed ded lime stone, corre spond ing to an outer ramp-to-mid dle ramp en vi ron ment. Depositional se quence 2 (DS2) com prises brec cia lime stone and thick-bed ded lime stone fa cies in its lower part and thin-bed ded lime stone to mas sive lime stone in its up per part. The brec cia limestone facies may be associated with subaerial exposure and reworking of previously deposited sediment during a relative sea level fall. The thin-bed ded lime stone to mas sive lime stone of DS2 con sists mainly of bioclastic mudstone to wackestone (outer ramp). These rep re sent an deep-wa ter outer homoclinal ramp fa cies. Depositional se quence 3 (DS3) con sists mainly of mas sive lime stone to thick-bed ded lime stone with a bioclastic peloidal microbial Crescentiella packstone (mid dle ramp). The rel a tive strati graphic po si tions of DSs1-3 and se quence bound aries in the up per most Oxfordian to lower Kimmeridgian of the Polur area show a fair match to the up per Oxfordian to lower Kimmeridgian se quences (JOx7, JOx8, JKi1 and JKi2) on the global sea level curve.