Investigation on Bioerosion in Miocene Coral-rich sequences in Western Makran, Bashagard area

Due to difficult access to Makran Basin, a few geological investigations have been performed so far. The successions of Band-e-Chaker Unit in Tejek, Kermestann, and Irer sections were measured and studied. Bioerosion is frequently seen in the corals of the studied sections. Through this paper, we investigate on bioerosion and its related factors in carbonate coral-bearing parts of Band-e Chaker Unit in studied sections around Band-e Chaker Syncline with Burdigalian age. Tejek section is composed of pure limestone and Kermestan and Irer sections are mostly composed of marly limestone and clastic.

Based on the presence of corals, foraminifera, and previous studies (McCall et al., 1994), the studied sections are attributed to Burdigalian. According to diverse coral morphotypes, these sections indicate shallowing upward trends. In the studied sections, the effect of bivalves and polychaetes (serpula worms) in bioerosion is more obvious. Serpula worms are often the first organisms that colonize the coral surfaces (Hutchings & Peyrot-Clausade, 2002). Trypanites sp. and honeycomb borings are the most observed serpula traces on corals. In lower parts of sections that are concordant with deeper parts, serpula worm borings are thicker in diameter in comparison with upper and shallower parts. The other trace fossil that could be seen in the sections is Gastrochaenolites sp. which is created by bivalves; these are less frequent than serpula worms and mostly are present in shallower parts of the sections. Beside nutrient level, different environmental factors such as oxygen and salinity can affect in bioerosion process. Clastic and siliciclastic inputs decrease water transparency and increase nutrient level; increasing nutrient level is concordant with decreasing large benthic foraminifera and enhancement of filter-feeders and encrusters and finally bioerosion (Morsilli et al., 2012). Bioerosion, encrustation, and abundance of bryozoa are the result of an increase of nutrient availability. Based on the mentioned notes, a nutrient condition in the Tejek section is oligotrophic to slightly mesotrophic, and in Kermestan and Irer sections mesotrophic nutrient condition is identified. Size measurements of bioeroders show that increasing depth reduces the diameter and abundance of them. In this case, it seems that the dissolved oxygen in the water can play an important role in bioeroders’ diameter size.

We would like to thank Dr. A. Bahrami, Dr. E. Mohammadi, and Dr. C. Chaix who helped us in this investigation.