A Cenomanian-Turonian drowning unconformity on the eastern part of Kopet-Dagh basin, NE Iran
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ORIGINAL PAPER
A Cenomanian-Turonian drowning unconformity on the eastern part of Kopet-Dagh basin, NE Iran Behnaz Kalanat & Mohammad Vahidinia & Hossein Vaziri-Moghaddam & Mohamad Hossein Mahmudy-Gharaie
Received: 29 September 2014 / Accepted: 6 January 2015 # Saudi Society for Geosciences 2015
Abstract A Cenomanian-Turonian interval in the Mozduran pass section, eastern part of Kopet-Dagh basin (NE of Iran), was measured and studied in order to interpret the biostratigraphy and paleoecology (based on distribution of planktonic foraminifera). This section consists of 52.5-m light and gray marl and coincides with the Aitamir and Abderaz formation boundary. Based on study of 47 samples, 11 planktonic foraminifera genera and 27 species were encountered, and the following zones have been defined: Rotalipora cushmani Total Range Zone and three zones of Helvetoglobotruncana helvetica, Dicarinella primitiva-Marginotruncana sigali, and Dicarinella concavata as condenced zones. This study shows that the sediment record is incomplete in the section. This unconformity is just after a drowning succession, places in the Late Cenomanian-Early Tur onian W hi te in e l la archaeocretacea Zone, and interpreted as a drowning unconformity. A paleoenvironmental model has been formulated to explain the successive paleocommunity changes during this drowning of the platform. We attribute the drowning of some parts of the platform to the occurrence of the Cenomanian-Turonian oceanic anoxic event. The impingement of anoxic waters over the platform could produce the drastic reduction of the carbonate producing observed in the stratigraphic section and therefore a reduction in carbonate accumulation rates. Subsidence and the Late Cenomanian-Early Turonian sea level rise were then able to drown the platform. B. Kalanat : M. Vahidinia (*) : M. H. Mahmudy-Gharaie Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran e-mail: [email protected] H. Vaziri-Moghaddam Department of Geology, Faculty of Science, Isfahan University, Isfahan, Iran
Keywords C-T boundary . Drowning unconformity . Planktonic foraminifera . Kopet-Dagh basin
Introduction The middle to late Cretaceous is characterized by very warm climate and high sea levels, which is attributed commonly to high concentrations of greenhouse gases related to increased tectonic activity (Larson 1991; Larson and Erba 1999). The Cretaceous also includes one of the most conspicuous and well-documented mass extinction in Earth’s history identified at the Cenomanian-Turonian boundary. The mechanisms put forward to explain this extinction are diverse and include the following: oceanic anoxia (Schlanger et al. 1987; Arthur et al. 1985), rise in sea level (Jarvis et al. 1988), salinity stratification (Brass et al. 1982), and an increase in productivity (Jarvis et al. 1988; Vogt 1989; Hilbretch et al. 1992). Global sea level rise, compounded with forcing factors associated with the oceanic anoxic events, induces the first step of drowning events. Close re
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