Boletín de la Sociedad Geológica Mexicana

Volumen 67, núm. 1, 2015, p. 59-73

Foraminiferal paleoecology and paleoenvironmental reconstructions of the lower Miocene deposits of the Qom Formation in Northeastern Isfahan, Central Iran

 Maryam Nouradini1,*, Seyed Hamidreza Azami2, Mostafa Hamad3, Mehdi Yazdi4, Ali Reza Ashouri1


1 Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashad, POB. 9177948974, I.R. of Iran.
2 Department of Geodynamics and Sedimentology, Center for Earth Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria.
3 Department of Geology, Faculty of Science, Cairo University, Cairo University Road Oula, Giza, Egypt.
4 Department of Geology, Faculty of Science, University of Isfahan, Isfahan, POB. 81746- 73441, I.R. of Iran.

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The early Miocene Qom Formation is exposed in the Bagh section, northeast Isfahan, Central Iran, where it unconformably overlies Oligocene deposits and is unconformably overlain by the Upper Red Formation. The formation is mainly represented by carbonate deposits (marl, marly limestone facies) and subordinate siliciclastic facies. The planktonic foraminiferal biostratigraphical analysis led to recognition of two main planktonic foraminiferal zones: 1. the Globigerinoides primordius Zone in the lowermost part of the section indicating early Miocene (Aquitanian) and 2. the Globigerinoides trilobus Zone in the upper most part of the Qom Formation of Burdigalian age (early Miocene). Composition and abundance of benthic and planktonic foraminifers were examined for paleoenvironment reconstruction and paleoecology. The Q-mode cluster analysis performed on the benthic foraminifers led us subdivide the section into three distinct benthic foraminiferal clusters: I. the Cibicidoides ungerianus cluster in the lower part of succession, indicating predominantly inner neritic environments; II. the Lenticulina orbicularis cluster in the middle part of section, indicative of middle-outer neritic environments, and III. the Elphidium-Amphisteginacluster recorded in the upper part of the formation and characterizing a typical outer neritic environment. The planktonic foraminifers are interspersed in the upper part of the section with relatively low diversity, indicating open marine shelf segments, whereas the lower part of the section is characterized by an abundance of benthic foraminifers. As a point to consider for paleoecology and paleoenvironment, abundant although with low diversity benthic foraminifera with small tests, and calculated data based on these clusters are indicative of high nutrient and high oxygen environment with normal marine salinity during the early Miocene at the Bagh section.

Keywords: Aquitanian, Burdigalian, benthic foraminifera, paleoenvironmental condition.

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