Biostratigraphy, microfacies and depositional environment of the Campanian-Lower Maastrichtian sequences in the Nasir Kandi area in southern part of the Moghan sedimentary basin, NW Iran

The Moghan area (N38º 30’ to 39º 42’ and E46º 39’ to 48º 10’), as a part of the Kura sedimentary-structural Basin, is located in the NW Iran. The geological and structural history of this area is related directly to its location in northern part of the Talysh-Lesser Caucasus folded and thrusted belt. It is positioned at the collisional zone between the Eurasia and Africa-Arabian continental plates. The convergence is active today, at an estimated rate of 20-30 mm per year.

The area is included in the world’s largest continental collision zone, the Alpine-Himalayan belt, and is marked by intense compression and faulting. The Moghan sedimentary Basin is part of the Para-Tethys Basin that formed in a back-arc and volcanic belt developed from southern France, in the Mediterranean to western China. Tectonic deformations during the Neogene favored the subdivision of Para-Tethys in three major subdomains namely the western, central and eastern Para-Tethys.

This study focuses on the Upper Cretaceous carbonate sequences of the Moghan area, considered as one of the most important intervals in view of their potential as reservoir rocks. Here, facies analysis and paleo-environmental reconstruction of these sequences are presented for the first time in eight surface sections across the study area. This study is based on the stratigraphic and petrographic analysis of Upper Cretaceous sedimentary sequences in Nasir Kandi surface section in the Moghan area, NW Iran. A total of 498 meters of sedimentary thickness were studied and logged. Macroscopic field-features descriptions combined with the results of microscopic studies have been used for facies analysis, paleoenvironmental reconstruction and biozonation. In total, 161 hand specimens were collected for subsequent studies. Sampling intervals were generally between 1 to 3 meters. Upper Cretaceous paleogeographic maps from the eastern part of the Para-Tethys Basin indicate that the Moghan area was located at 30-35º paleolatitude in northern hemisphere. During this time, sea-level was at one of its higher levels in geological history and carbonate platforms developed on continental margins all around the Tethyan realm (Miller et al., 2004). Shallow- to deep marine carbonates were deposited in such platforms in a tectonically active back-arc basin simultaneously to volcanic activity and siliciclastic influx. Based on it paleolatitude location, a temperate to cool subtropical paleoclimatic condition could be considered for this area. This interpretation is also supported by our observations, especially regarding the grain association (skeletal and non-skeletal) of the studied sequences.

Faunal and floral association of this formation (including rudists, other bivalves, echinoderms, benthic and planktonic foraminifera, red algae and bryozoan) represents a foraminiferal-mollusk (foramol) or bryozoan-mollusk (bryomol) association (Coffey and Read, 2007; Einsele, 2013). These communities live in temperate and cold waters and also at deeper settings in comparison to the tropical chlorozoan association (Lees, 1975; Flügel, 2013).

In Nasir Kandi section in westernmost part of the Moghan area, deep-marine pelagic facies are the dominant. Therefore, it seems that the shallow inner parts of the Upper Cretaceous platform were located in the central parts of Moghan area. This shallow platform turns into deep marine settings with a steep slope and without any remarkable marginal barrier, to the southeast and southwest.  Accordingly, biostratigraphic analysis of these facies, based on planktonic foraminifera resulted in recognition of four biozones: Globotruncana ventricosa Zone (middle to late Campanian), Globotruncanella havanensis Zone (late Campanian), Globotruncana aegyptiaca Zone (latest Campanian) and Gansserina gansseri Zone (latest Campanian to early Maastrichtian). We are grateful to the National Iranian Oil Company- Exploration Directorate for financial support and data preparation. The University of Isfahan and the Pars Petro Zagros (PPZ) Company are thanked for the provision of facilities for this research. Journal editor and anonymous reviewers are acknowledged for their kind helps.