Late Oligocene-Early Miocene Basin-Center Evaporites in Relationships to Multi-stage Tectonic Inversions in the Sinjar Trough, NW Iraq and NE Syria

Abstract

Abstract The Sinjar Trough is a major east-west trending extensional feature in Northwest Iraq and Northeast Syria. It began to develop in the Late Cretaceous (the Maastrichtian) due to transtensional tectonics and was inverted during the late Pliocene-Pleistocene as a result of the Zagros Orogeny. Through biostratigraphic, Sr-isotope age dating, petrographic, and sequence-stratigraphic studies of two late Oligocene-earliest Miocene basin-center evaporite intervals in Northwest Iraq and adjacent Northeast Syria, we recognized several minor episodes of inversion in the Sinjar Trough during the Paleogene. The Basal Serikagni Anhydrite (BSA) is a thin basinal anhydrite unit imbedded between the middle and late Chattian deep-marine carbonate sequences. The BSA extends into Northeast Syria but is missing in several adjacent wells within the Sinjar Trough. The Dhiban Formation is a thicker late Aquitanian-early Burdigalian evaporite-dominated interval mixed with carbonates. It overlies the Serikagni Formation and onlaps onto the carbonate ramp margins of the Euphrates Formation, which prograded towards basin center from the northeast and southwest. In Northeast Syria, the same basin-center evaporite is called the Dibbane Formation and shows local thickening and thinning. The overlying Jeribe Formation, however, has a uniform thickness across Iraq and Northeast Syria. The areal distribution, facies, and stratal geometry of these basin-center evaporite-bearing intervals reflect the antecedent topography during their deposition. Minor inversions within the Sinjar Trough before or during the late Oligocene caused non-deposition or erosion of the BSA in Northeast Syria. Another episode of inversion before the early Miocene created low-relief highs and differential accommodation within the Sinjar Trough. The Dhiban/Dibbane Formation simply filled the remnant basin and was able to cover the highs during the lowstand stage, resulting in local variations of the basin-center evaporite accumulation. This study may shed some light on the timing of early trap formation within the Sinjar Trough. Tectonic Setting, Tectonic History, and Hydrocarbon System of the Sinjar Trough The Sinjar Trough is an east-west trending Late Paleozoic-Mesozoic rift basin (~250 km long, 50 km wide) that is located in Northwest Iraq and Northeast Syria (Best et al., 1993; Brew et al, 1999; Figure 1). It is a possible eastern extension of the Palmyride rift system but has less intense Paleozoic history of extension, thermal subsidence, and sediment accumulation (Best et al., 1993). The major extension took place during Late Campanian-Maastrichtian time and the east-west striking faults formed the Abd el Aziz and Sinjar grabens, in which thick Maastrichtian Shiranish Formation (up to 5000 ft) was deposited (Brew et al., 1999, 2001). Normal faulting ceased abruptly by the end of the Cretaceous, and the Paleogene was mostly a time of tectonic quiescence (Brew et al., 1999). Full-scale inversion by reactivation of east-west striking normal faults formed fault propagation folds during late Plio-Pleistocene time and inverted the grabens as surface anticlines (Kent and Hickman, 1997; Brew et al., 1999, 2001). Deformation increases to the east in the Iraqi Sinjar region (Brew et al, 2001).