The position of the Oligocene/Miocene boundary in Iraq has been debated for more than 50 years. This stratigraphic note reviews various proposed positions and presents paleontological evidence for its suggested position.
Van Bellen et al. (1959, Figure 1) interpreted the Tertiary succession of northern Iraq in terms of several transgressive-regressive sequences. They positioned the Oligocene/Miocene boundary between the Ibrahim-Azkand-Anah sequence (Ibrahim Sequence) and the Serakangi-Euphrates-Dhiban sequence (Euphrates Sequence). These authors interpreted the ages of the Ibrahim as probably “upper” Oligocene, and the Euphrates Sequence as probably “lower” Miocene, but stated that this dating does not imply strict correlation with the European Oligocene and Miocene stages. Their age attributions were highlighted with quotation marks and were based on stratigraphic position rather than on paleontological evidence.
Indeed, in an earlier 1953 unpublished report (IPC Ltd, Table 1 by van Bellen, 1953) the Azkand and Anah formations were interpreted as Early Miocene, whereas the Bajawan and Baba formations (Tarjil sequence) were interpreted as Late Oligocene, thus placing the Oligocene/Miocene boundary between the Tarjil and Ibrahim sequences. This note summarizes the results of further studies that support this lower position for the boundary. The implication is that in several regional stratigraphic frameworks, which adopted the higher position (e.g. Buday, 1980; Goff et al., 1995; Sharland et al., 2004; Jassim and Goff, 2006), the boundary is mispositioned, and its bounding sequences are incorrectly dated and regionally miscorrelated.
Youhanna (1983) was among the first biostratigraphers to study the Ibrahim Formation in its type locality (Ibrahim-1). He concluded that its faunal assemblages mainly consist of planktonic foraminifera genus Globigerinodes, considered to be diagnostic for the Miocene. In the Kirkuk area, Al-Eisa (1992) also interpreted the age of the Ibrahim Formation as Early Miocene. These studies revised the previously assigned “upper” Oligocene age suggested by van Bellen et al. (1959).
Al-Banna (1997) and Al-Banna et al. (2002) studied the paleontology of the Oligocene-Miocene interval between the Tarjil and Ibrahim formations from three core holes that were drilled in the Sheikh Ibrahim and Sasan areas in western Mosul, near to the Ibrahim-1 well (Figure 2). The cores represent a complete succession, 150-180 m thick, which includes the Tarjil, Ibrahim, Hamrin (new formation described by Al-Banna and Amin, 2000; and first formally named by Ahmad and Al-Eisa, 2003), Serikagni, Euphrates and Dhiban formations. They identified 24 planktic foraminiferal species and subspecies, and recognized four biostratigraphic zones (Table 2). On the basis of lithololgical and paleontological evidence they positioned the Oligocene/Miocene boundary between the Tarjil and Ibrahim formations.
In the outcrop type section of the Serikagni Formation, van Bellen et al. (1959) considered the formation as “lower” Miocene, and placed it unconformably above the Eocene Jaddala Formation. They considered the Oligocene to be absent. A paleontological and sedimentological re-examination of a succession near to the type section (Al-Banna, 2004; Ismail, 2006) provided a complete statigraphic succession in which the Ibrahim, Tarjil and Palani formations were recognized (Table 3). Again the Oligocene/Miocene boundary was found between the Tarjil and Ibrahim formations.
ABOUT THE AUTHOR
Nabil Y. Al-Banna obtained a BSc in Geology from Mosul University, Iraq, in 1978 and received an MSc in Sedimentology (Clastic rocks) and a PhD in Sedimentology (Carbonate rocks) from Mosul University in 1983 and 1997, respectively. Between 1983 and 1998 Nabil worked as a Geologist in the Underground Storage Team of the Iraq Oil Company. In 1999 he moved to Mosul University as a Lecturer at the Dams and Water Resource Research Center. Since 2000 Nabil’s main research interests are in sedimentology and sequence stratigraphy of the Mesozoic and Cenozoic.