Cretaceous–Paleogene transition interval in the north flank of the Alborz Mountains (N Iran); Planktic foraminiferal biostratigraphy and stable isotopes evidence

Highlights

• The studied interval has been deposited across the K–Pg interval of the Alborz Basin.

• A short-term gap across the K/Pg boundary is marked by lack of P.hant and P0 biozones.

• The base of Danian is accompanied by a gentle Ir anomaly and δ¹³C negative shift.

• The Paleocene is initiated by planktic foraminiferal extinction and G. cretacea bloom.

Abstract

A Cretaceous/Paleogene (K/Pg) boundary interval in the north flank of Alborz Mountains (Aboksar section), northern Iran, has been studied to investigate the biological and geochemical variations of this event in the eastern Tethys. Abathomphalus mayaroensis at the late Maastrichtian, Pα (Parvularugoglobigerina eugubina), P1a (Parasubbotina pseudobulloides) and P1b (Subbotina triloculinoides) at the early Danian are the recognized planktic foraminiferal biozones and subzones in the studied area. The Plummerita hantkeninoides subzone at the uppermost Maastrichtian and P0 Biozone at the base of Danian were not defined in the studied section. This proposes a short-term hiatus and sedimentary discontinuity at the base of Paleocene and probably uppermost Maastrichtian, which most likely caused by winnowing of sediments after a cooling phase and development of bottom current in the Albors Basin. The K–Pg transition in the Aboksar section is characterized by a bloom in the Guembelitria cretacea, evolution of first Danian species (Pv. eugubina, Pv. longiapertura), δ¹³C negative shift (about 0.5‰), and a gentle Ir anomaly (0.1 ppb). Planktic foraminifera were diverse and abundant during the late Maastrichtian of studied section in a warm (relatively lower δ¹⁸O contents) and well stratified environment. Bloom of the opportunist species, G. cretacea, and decrease of planktic foraminiferal abundance and diversity at the base of Paleocene interval propose an abnormal stressful environment in this time probably after the asteroid impact and its following temperature decline. The appearance of more complete species with more specialized shells and larger sizes, such as Globoconusa daubjergensis, Parasubbotina pseudobulloides, Eoglobigerina eobulloides and Subbotina triloculinoides in the P1a and P1b subzones and return of carbon isotope diagram into the pre-boundary values testify an environmental recovery and optimization of ecosystem conditions.

Introduction

The Cretaceous/Paleogene boundary (K/Pg) is testified as one of the five large mass extinctions in the Earth's history (David et al., 1982). Many theories have been proposed to explain the causes of extinction at this boundary (e.g., Alvarez et al., 1980; Schulte et al., 2010; Hull et al., 2020). The volcanic eruption and asteroid impact are among the most famous theories. For the first time, Alvarez et al. (1980) proposed the extraterrestrial impact hypothesis based on the iridium anomalies across the K/Pg boundary of two sections in the Italy and Denmark. Another theory for the K/Pg boundary extinction is related to the Deccan eruption, which is one of the largest volcanic activities in the Earth's history (Chenet et al., 2007). There is therefore no surprise that the Deccan volcanism is nominated as the potential cause for the K/Pg event even before the impact theory was proposed (McLean, 1978).

Evidence of the late Maastrichtian warming has been proven by oxygen isotope studies that would be directly related to the Deccan volcanism in western India (Li and Keller, 1998; Chenet et al., 2007). Some scientists suggest that despite the widespread lava outflow, this eruption could not cause severe climate changes in the earth system and a mass extinction at this time (e.g., Hull et al., 2020). The Deccan volcanism might have contributed to the shape of the Cenozoic species and communities after extinction (Hull et al., 2020). It has been estimated that about 40% of genera (Sepkoski, 1996) and 70% of species went extinct at the K/Pg boundary, which ranked the third among the Phanerozoic mass extinctions (Stanley, 2016).

Despite the importance of K/Pg boundary in the world, not very comprehensive studies have been conducted in Iran (Asgharian Rostami et al., 2012, 2013a, 2013b, 2018, 2020; Beiranvand and Ghasemi-Nejad, 2013; Beiranvand et al., 2014; Zaghbib-Turki and Beiranvand, 2017). The active tectonics and dense vegetation limited geological investigations on the K/Pg boundary in the Alborz sedimentary Basin. In this study, we aim to consider biostratigraphy, planktic foraminiferal turnovers, carbon and oxygen stable isotopes, and iridium anomaly of a K/Pg boundary sequence in the Alborz Basin (Fig. 1) to better understand the sedimentary conditions at the east of the Tethyan realm in this time.