Investigating the characteristics of “Calcite eyes” in large benthic foraminifera of the Aptian-Albian carbonate deposits, North and Southwest Neo-Tethys Ocean

The Zagros structural zone has undergone part of its evolution and sedimentation trend during the Cretaceous on the northwestern margin of a vast carbonate platform called the Arabian Plate (Sharland et al. 2001; Ziegler, 2001). Darian Formation Lower Cretaceous in age (Aptian-Albain) (Schroeder et al. 2010) is the last formation of the Khami group named by James & Wynd (1965) and is important in terms of hydrocarbon reservoir potential (Habibi et al., 1994; Rahmani et al., 2010). An essential feature of the Darian Formation is the abundance of different species of orbitolina, so that at the beginning of studies, this formation is named "Orbitolina limestone" (James & Wynd, 1965). The study of benthic foraminifers in carbonate sediments of the Dariyan Formation revealed round to oval subjects in Orbitolina tests. Since these subjects have been field by sparry calcite cement, known as “calcite eyes” (Galloway 1933; Schroeder & Cherchi 1979), various theories have been proposed about the mechanism and cause of the formation of "calcite eyes" (Colom, 1955; Mohammed, 1996). The present study tries to provide a report on the characteristics of calcite springs and the process of their changes in terms of frequency, size, and dispersion in Darian Formation sediments. In the following, the hypotheses about the formation mechanism of these structures are examined.

Banesh and Sangsiah stratigraphic sections were selected based on geological maps. Total 351 samples were collected and named according to Dunham (1962) classification for carbonate rocks. Different species of “calcite eyes” were identified based on Haslett (1992) and Schlagintwait (2007). To investigate the relationship between calcite eyes and adaptive reactions of Orbitolina to the environmental condition such as low oxygen states, the abundance trend of calcite eyes was correlated with early Aptian oceanic anoxic event (OAE1a) (Moosavizadeh et al. 2014) interval.

Because there are still uncertainties about the mechanism for the formation of these structures, the present study tries to compare the morphology, frequency, and distribution of "calcite eyes" in Orbitolina, the sedimentation, and paleoecological conditions during the formation of sediments of the Daryian Formation with the processes discussed concerning the formation of "calcite eyes" and the best mechanism to specify. The structural features of "calcite springs" include morphology, their position on the orbitolina tests, and their stratigraphic distribution. These structures are seen in two main forms, spherical and oval shapes. However, boring in these two forms has caused irregular shapes in these structures. In terms of stratigraphy, these structures are abundant in the Upper Aptian sediments and are simultaneous with highstand facies of the seawater. On the other hand, in terms of abundant facies and magnificence, "calcite springs" have been observed in elongated orbitolina related to orbitolina rudestone of open marine setting.

Investigation in orbitolina of the carbonate sediments of the Dariyan formation revealed the existence of round to oval subjects called "calcite eyes". Examining the structural properties and based on the evidence available, it was found that these objects cannot be created as a result of an adaptive response to environmental changes, and these are, in fact, sponge spicules that form as foreign objects in the tests of benthic foraminifers in the Dariyan formation.