Geol. Mag. 148 (5–6), 2011, pp. 925–939. c Cambridge University Press 2011. The online version of this article is published within an Open Access environment subject to the conditions of the Creative Commons Attribution-NonCommercial-ShareAlike licence <http://creativecommons.org/ licenses/by-nc-sa/2.5/>. The written permission of Cambridge University Press must be obtained for commercial re-use. 925 doi:10.1017/S0016756811000471 Significance of angular unconformities between Cretaceous and Tertiary strata in the northwestern segment of the Zagros fold–thrust belt, Kurdistan Region, NE Iraq ∗ ∗ KAMAL H. KARIM ,HEMINKOYI‡†, MUSHIR M. BAZIANY & KHALED HESSAMI§ ∗ Department of Geology, College of Science, University of Sulaimani, Sulaimani, Kurdistan Region, Iraq ‡Hans Ramberg Tectonic Laboratory, Department of Earth Sciences, Uppsala University, Uppsala, Sweden §International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran (Received 10 November 2010; accepted 3 May 2011; first published online 8 July 2011) Abstract – In this study, two angular unconformities are found and analysed for the first time in the Mesozoic–Cenozoic succession in the northwestern segment of the Zagros fold–thrust belt (ZFTB) in the Kurdistan Region. The first unconformity exists between Lower Cretaceous and Paleocene– Eocene rocks and the second between the Campanian Shiranish Formation and the Maastrichtian Tanjero Formation. Each of these unconformities is found in two different localities in the Zagros Imbricate Zone (i.e. the highly deformed zone immediately SW of the Zagros Suture) of the ZFTB of the Kurdistan Region near the border with Iran. The study uses recent geological mapping, structural and stratigraphic analyses in addition to using previous biozonation of the stratigraphic units that bound the two unconformities. The first unconformity was initiated with obduction of the ophiolite and Lower Cretaceous radiolarite onto the passive margin of the Arabian plate. This unconformity formed during an early phase of the Zagros orogeny, which is associated with the developing of a foreland basin, and resulted in the folding of the radiolarites and their uplift to form high-relief land. The erosion of this high-relief land resulted in the formation of the Paleocene–Eocene Red Bed Series and their deposition on the folded radiolarite. The timing of the deformation that caused this unconformity is hard to determine; however, its stratigraphic position may suggest that it possibly is related to post-Cenomanian movements. The second unconformity is between the tilted Campanian Shiranish Formation (hemipelagite) and Tanjero Formation (500 m of conglomerate in the more proximal area). These unconformities indicate that deformation and uplift of the sedimentary units was variable during ophiolite obduction in this part of the ZFTB. We argue that deformation, ophiolite obduction and collision are likely to have varied in space and time along the c. 2000 km long ZFTB. Keywords: angular unconformity, Zagros fold–thrust belt, Kurdistan Region, obduction, Tanjero Formation, Red Bed Series, Qulqula Radiolarian Formation. 1. Introduction and Central Iran did not occur simultaneously; they suggest that obduction, as one of the mechanisms for The Zagros fold–thrust belt (ZFTB; Koyi, 1988) emplacement of ophiolites, did occur sometime in Late formed owing to convergence and collision between Cretaceous time (Berberian & King, 1981; Ziegler, the northeastern part of the Arabian plate and Central 2001) before the final closure of the Tethys Ocean Iran/Eurasia, with associated SW-directed obduction of that occurred in Oligocene–Miocene time (Koop & ophiolitic material derived from Neo-Tethyan oceanic Stoneley, 1982). crust during Late Cretaceous time (Berberian & King, In general, the sedimentary record shows that the 1981). North-northeastward movement of the Arabian Late Cretaceous represents a period of major change plate relative to Central Iran during Late Cretaceous in the area of the present-day Zagros foreland basin and later times resulted in the formation of the fold– (Loutfi, Baslaib & Abu Hamd, 1987; Burchette, 1993; thrust belt, which has since migrated southwestwards Droste & Steenwinkel, 2004; Farzadi, 2005, 2006a). (Hessami et al. 2001). Different estimates for the onset Rapid thickness variations and facies changes in Albian of deformation in the ZFTB have been proposed (e.g. and Cenomanian sedimentary rocks, which have been Falcon, 1967; Stöcklin, 1968; Wells, 1969; Ricou, documented both from seismic data and in the field, 1970; Nowroozi, 1972; Haynes & McQuillan, 1974; have been related to the reactivation of deep-seated Alavi, 1980, 2004; Alavi & Mahdavi, 1994; Berberian pre-existing N–S-trending normal and strike-slip faults & King, 1981; Ni & Barazangi, 1986). (Koop & Stoneley, 1982; Sepehr & Cosgrove, 2004; Geological evidence has led early workers to suggest Sherkati & Letouzey, 2004). that emplacement of ophiolites and collision of Arabia This paper documents the presence of two angular unconformities within the Zagros Imbricate Zone (i.e. †Author for correspondence: [email protected] the highly deformed zone immediately SW of the http://journals.cambridge.org Downloaded: 11 Aug 2015 IP address: 130.238.171.136 926 K. H. KARIM AND OTHERS Figure 1. (a) Simplified geological map of the Qandil Mountain area in the Kurdistan Region showing the Lower Cretaceous–Paleocene angular unconformity which is located between outcrops of the Qulqula Radiolarian Formation and Red Bed Series (modified from Sissakian, 1997). Cross-section A–A is shown in Figure 8. (b) Names of areas and towns that are mentioned in the text, X1 – SE of Mawat town, X2 – south of Chwarta town. (c) Simple tectonic map of the studied area showing the tectonic zone of the Kurdistan Region in NE Iraq (Jassim & Goff, 2006). Zagros Suture) (Figs 1, 2) of the northwestern part However, some variations should perhaps be expected, of the ZFTB in the Kurdistan Region. The aim of the given the lengthy evolution of the orogen and its present study is to indicate the temporal and spatial complex tectonic history. Glennie (2000) proposed that beginning of the deformations that are associated with the Neo-Tethys began to close with Early Cretaceous obduction of ophiolites upon the northern margin of (Aptian?) subduction of oceanic crust beneath Central Arabia. Iran. A major relative sea-level fall, attributed to a compressional event in Late Aptian time, is recognized as an unconformity throughout the Middle East (Harris 1.a. Timing of deformation et al. 1984; Sharland et al. 2001). Considerable A range of different timings has been suggested crustal-scale subsidence occurred on the NE margin for both the obduction and collision phases of the of the Arabian plate between Turonian and Maastrich- Zagros orogeny. Some of these differences may arise tian times (Sherkati & Letouzey, 2004). Radio- from imprecise data and/or erroneous interpretation. metric dating (Haynes & Reynolds, 1980) confirms a http://journals.cambridge.org Downloaded: 11 Aug 2015 IP address: 130.238.171.136 Unconformities in NW Zagros, Kurdistan Region 927 Figure 2. Geological map of the Chwarta area showing the location of the Lower Cretaceous–Paleocene angular unconformity (black stars) (Modified from Sissakian, 2000). Cross-section B–B is shown in Figure 10. X1 and X2 are locations of outcrops of red beds. syn-post-Cenomanian emplacement age. Berberian & at 63 Ma (Sharland et al. 2001) marks the cessation King (1981) suggested a Santonian–Maastrichtian of ophiolite obduction along the eastern margin of age for the emplacement of ophiolites, radiolarites the Arabian plate. Beydoun (1999) proposed that and turbidites. Koop & Stoneley (1982) indicated obduction in the form of an ophiolite belt took place as truncation and onlap on a pre-Late Cretaceous un- a result of a collision between the leading edge of the conformity (Middle Turonian, dated at 92 Ma). The NE Arabian passive margin and an oceanic ridge (or Late Cretaceous has also been proposed as the time possibly an island arc system) during Campanian time of obduction of the oceanic crust onto the Arabian (c.75Ma). plate by Alavi (1994), Haynes & McQuillan (1974) and Different timings, on the other hand, have been Berberian & King (1981). Sediments were deposited proposed for the continental collision between Ara- within a compressional foreland basin setting following bia and Central Iran/Eurasia. Takin (1972) sugges- onset of Mid-Turonian ophiolite obduction along the ted that the earliest compressional movements of eastern margin of the plate (Ziegler, 2001). Ziegler Zagros-related deformation occurred in Turonian to (2001) suggested that the pre-Cenozoic unconformity Campanian times. Collision has been suggested to http://journals.cambridge.org Downloaded: 11 Aug 2015 IP address: 130.238.171.136 928 K. H. KARIM AND OTHERS have occurred during Eocene (Numan, 1997, 2001; these conglomerates (in time and according to lateral Numan & Al-Azzawi, 2002; Al-Qayim, 2000); facies changes) consist of sandstone and calcareous Eocene–Oligocene (Hempton, 1987; Beydoun, Hughes shales. These conglomerates (as the main materials of Clarke & Stoneley, 1992; Hooper et al. 1994); this study) and their equivalents are mapped in the field Oligocene–Miocene (Koop & Stoneley, 1982); Late and in thin-sections. Miocene (McQuarrie et al. 2003; Stoneley, 1981); and Early Pliocene (Falcon, 1969) times. More recently, 3. The nature of the unconformities on the basis of magnetostratigraphic