Biogenic structures in test of large benthic foraminifers (Orbitolina): Example from Aptian-Albian sediments of SW Neo-Tethys Ocean, Zagros, SW Iran

Study of carbonate sediments of the Dariyan Formation (Aptian-Albian) in northwest of Shiraz releases different types of biogenic structures in forms of encrustation and drilling in hard substrates. Generally biogenic structures are referred to as changes in soft or hard sediments, which are created by certain organisms and through biological processes (Neumann, 1966). If drilling is performed on soft substrates, it is called Burrow, and if it is created in hard substrates, it is called Bore (Golubic et al., 1975). The organisms can cause macroscale destruction and cavitation on hard surfaces, known as macroboring, which including as certain bivalves, gastropods, echinoids, sponges, polychaetes, arthropods, bryozoans or benthonic foraminiferans (Bromley, 2004). Certain organisms, like certain groups of cyanobacteria, green and red algae, as well as fungi, create microscale digestion, which, due to their small size, have been called microboring (Bromley, 2004). From paleoecology perspective, studying the hard substrates can help determine the spatial distribution and how to orientate the organisms during their lifetime. For example, an accurate investigation of encrustation or drilling on brachiopod shells can provide information on the orientation of the bedding, the pattern of feeding, coexistence, as well as the sequences of occupations by the diggers, their priority for specific mechanisms in the host shells, and that how these organisms interacted throughout life (Taylor and Wilson, 2003). In order to study the biological structures, 267 thin sections were studied the facies were named by Dunham (1962) and Embry and Klovan (1973) methods. For distinguished calcite and dolomite, thin sections were stained with Alizarin Red-S and Dickson (1966). In order to identify the types of existing biotechnology, their factors and classification of these effects have been used from references such as Bromley (1970), Taylor and Wilson (2003), Schlagintweit and Bover-Arnal (2013) and Schlagintweit et al (2015). Finally, the conditions of formation and characteristics of these biological structures are discussed. One of the most selected substrates by borer organisms is orbitolina test, as an index large benthic foraminifer in this episode. Both conical and discoidal orbitolina have been subjected to these type of biogenic structure. The most likely options for producer organisms of these bioerosive structures are “colinid sponges” and “calcimicrobes” and much of them can be attributed to Entobia Ichnogenus (mostly produced by sponges). In the studied samples, Entobia has been created in parallel with the outer surface of the orbitolina wall, which is also vertically viewed in the longitudinal section and horizontally in the transverse section while others have no orientation. In these cases they also extend from the wall to the central part. Some of these types of drills are a single central cavity with radial channels leaving it. Other identified biogenic structures include tubes of serpulid worms, bacinellid encrustation, thaumatoporellaceans encrustation and channel shape borings by unknown producers. Tubes of serpulid worms are seen both in the form attached to shells and colonies within the limed mud matrix. Bacinellid encrustation has created a dark wall around the cellar and in some rare cases, the two adjacent orbitolinids form a tangential state and connect both of them, so that the encrustation extended to the limestone between two orbitolins. Thaumatoporellaceans encrustations were observed on the tests of the orbitalins (upper and lower levels) or even attached to the shells of brachiopods. The size of the chamber wall occupied by this algae is about 8 microns and in appearance is similar to Entobia. Borings by unknown producers is characterized by a straight-to-slightly irradiated channel pattern, which shows continuous increase in the diameter of the channel from the beginning to the end of the path. Often, these drills are single channels, but in some cases they are seen as branching channels. In morphological aspects, the most boring structure is parallel tunnels to external side of orbitolina test and in terms of facies distribution, they mostly found in the foraminiferal wackestone and orbitolinid floatstone facies contain orbitolina and green algae. This dispersal pattern accompanied by Thalassinoides ichnogenus somehow reflects low sedimentation rate and providing the opportunity for organism’s activities.