139 GeoArabia Special Publication 4, 2010, v. 1, p. 139-197 Gulf PetroLink, Bahrain Carbon-isotope stratigraphy, biostratigraphy and organic matter distribution in the Aptian – Lower Albian successions of southwest Iran (Dariyan and Kazhdumi formations) Benoit Vincent, Frans S.P. van Buchem, Luc G. Bulot, Adrian Immenhauser, Michele Caron, Darioush Baghbani and Alain Y. Huc ABSTRACT The lateral transition from carbonate platforms to intra-shelf basin in Aptian – Early Albian times is documented along a regional transect in the excellent exposures of the Zagros Mountains. An integrated dataset, including carbonate carbon-isotope curves, and ammonite and planktonic foraminifera biostratigraphy constrains the regional correlations, and forms the basis for an age revision of the Kazhdumi and Dariyan formations. Of particular importance in this study is the Kuh-e-Bangestan section, wherein a continuous succession of Aptian – Albian intra-shelf basinal deposits was used to erect a detailed ammonite and planktonic foraminifera biostratigraphic zonation scheme, in combination with a carbon-isotope curve and organic-matter measurements. Key observations are: (1) the oldest Kazhdumi intra-shelf deposits are of Early Aptian age (D. deshayesi Zone), indicating a time- equivalent initiation of the Kazhdumi and neighbouring intra-shelf Bab Basin. (2) The presence of well-developed organic matter-rich sediments in the N. nolani and H. jacobi zones is interpreted as equivalent to part of the OAE1b set. (3) The presence of an exceptionally well-developed Upper Aptian – Lower Albian succession (80 m thick) shows a clear positive carbon-isotope excursion and a faunal crisis, with the turn-over of the planktonic foraminifera assemblage (only small and low diversity Globigerinelloides and Hedbergellids) and the total absence of ammonites. A composite carbon-isotope curve is proposed based on sections measured in both the platform and basin settings. This curve deserves attention because it has an expanded Upper Aptian - Lower Albian section, which is well dated (ammonites, planktonic foraminifera and orbitolinids). INTRODUCTION Whereas recently much progress has been made in the understanding of the mid-Cretaceous stratigraphy of the Arabian Plate in the United Arab Emirates (UAE) and Oman (e.g. Harris and Frost, 1984; Hughes Clarke, 1988; Hughes, 1997; Murris, 1980; Vahrenkamp, 1996; Sharland et al., 2001; Davies et al., 2002; van Buchem et al., 2002; Hillgärtner et al., 2003; Immenhauser et al., 2004; Yose et al., 2006), no detailed stratigraphic and paleontological work has been published of the time-equivalent successions in southwest Iran since the standard work by James and Wynd (1965). As documented from previous work in Oman and the UAE, the Aptian sedimentary system is particularly complex. Any detailed study aiming at a larger scale regional and over-regional correlation of field and subsurface data must hence aim at integrating biostratigraphic, chemostratigraphic and sequence-stratigraphic data in a multidisciplinary manner. The focus of this study is on the Upper Barremian to Albian deposits of the eastern part of the Arabian Plate exposed in world-class exposures along the Zagros Mountain chain in southwest Iran. This paper presents – for the first time – a detailed Aptian carbon-isotope record from southwest Iran combined with new biostratigraphic data based on ammonite and planktonic foraminifera stratigraphy. In addition, basinal deposits have been analysed for carbonate and TOC (Total Organic Carbon) content. In two related papers, a revision of the orbitolinids in southwest Iran is presented by Schroeder et al. (2010), providing excellent age control for the shallow-water platform successions of the Dariyan Formation, and a sequence-stratigraphic model is proposed for the Barremian – Aptian interval by van Buchem et al. (2010a). The results of these twin studies are applied – where appropriate – in this paper, but the reader should refer to the original manuscripts for more detailed information.