Sequence stratigraphy and biostratigraphy of the prolific late Eocene, Oligocene and early Miocene carbonates from Zagros fold-thrust belt in Kurdistan region

Abstract

The Late Eocene-Oligocene-Early Miocene sequences of the Kurdistan foreland basin, considered as an important shallow, thick oil and gas reservoir in Kurdistan region/N-Iraq. Several diverse larger foraminifera (Alveolina, Orbitolites, Assilina, Archaias, Austrotrillina, Miogypsinoide, Nummulites, Operculina, Borelies, and Lepdicyclina) with important stratigraphic, paleo-ecological, and palaeo-biogeographical implications are described with respect to its position in the Neo-Tethys basin. The biozonation, using large benthonic foraminifera, leads to the reconstruction of the chronostratigraphic framework. For that purpose, more than 367 samples from six wells and twelve outcrops, from different tectonic domains, within Zagros fold-thrust belt were analyzed. Almost four sequence boundaries (SB) segmented the depositional sequences into three third-order sequences. The lower most sequence boundary was estimated to be of Bartonian age (37 Ma) that represents by more than 5-m conglomerates between the molasses facies of the Gercus and carbonates of the Pila Spi Formations. The second sequence boundary, known as Zagros Major Hiatus, was estimated to be of the age 34 Ma and was placed at the top of karstfied carbonates of the Pila Spi Formation (Pribonian), directly below the Fatha Formation (Burdigalian-Langhian) almost manifesting the boundary between Tectonomegasequence AP10 and AP11. The third one is almost between the Rupelian and the Chattian (28 Ma) within Kirkuk Group. The last recorded one of Aquitanian age (21 Ma) was placed between the Kirkuk Group and Jeribe or Euphrates Formations. The three third-order depositional sequences were recognized from deepening and shallowing trends in the depositional facies, stacking patterns, and sequence boundary features. The first third-order represents the Late Eocene sequence (Pila Spi-shelf facies latterly change to Avanah-Jaddala Formations-depocenter facies), the second one represents the Rupelian-Chattian-Early Aquitanian third order in which each of the lower cycle of the Kirkuk Group (Rupelian) and the upper sequence of the Kirkuk Group (Chattian-Early Aquitanian) represents a fourth-order sequence. The last third order manifests by the Late Aquitanian-Early Burdigalian sequence of the Euphrates shelf facies that is changed laterally to Serikagni depocenter facies and mostly change upwards to Jeribe Formation. The lateral and vertical facial changes point to the increasing of the sequence thicknesses from the high folded thrust zone towards the low folded thrust zone and across the transversal Khanqain fault. The carbonate depositional ramp mostly segmented into isolated highs (occasionally separated from the palaeo-shorelines), that are either due to the influence of the thrust propagation deformational front or reactivation of the deep cited faults.