Depositional system and paleogeography of Miocene strata in the Northeast of Mahneshan (Zanjan)

In order to reconstruction of sedimentary conditions and paleogeography of Qom Formation, one stratigraphic outcrop in the Northeast of Mahneshan has been studied. In this study, microfacies analysis and microfossil investigations have been carried out on Forty-seven samples from one hundred sixty-two meters of sedimentary rock layers. Qom Formation in this section, is composed of carbonate, clastic and volcanoclastic layers that covers the andesitic strata of the Karaj Formation with a disconformity boundary and overlayed by clastic strata of the Upper Red Formation with gradual boundary. Microfacies analysis led to identification of one lithofacies (grain supported conglomerate) and three microfacies (1- Algal coral bioclast packstone, 2- Carbonate volcaniclastic and 3-Pelagic wack/packstone) that are related to the open carbonate shelf, submarine channels and submarine volcano in this section. Fossil investigations lead to identification if seventeen species of foraminifera fossils and two biostratigraphical zones. Based on foraminiferal index fossils (Borelis melo curdica, Meandropsina iranica, Praeorbulina glomerosa), Burdigalian-Langhian have been proposed as relative age for these strata. Facies change throw the stratigraphic column show deepening upward trend (during Langhian) in this study that can be correlated with paleogeographic events and relative sea level rise in Langhian age in this area.