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Origin of the Phacoid Structures within the Chia Gara Formation, Kurdistan Region, Iraq


Sirwan H. Ahmed1, Ibrahim M. J. Mohialdeen1, Hemin F. Muhammed1

1Department of geology, College of Science, University of Sulaimani, Sulaimani, Kurdistan Region- Iraq


Original: 3 November 2019       Revised: 4 January 2019      Accepted: 30 January 2020         Published online: 20 June 2020  

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Special structural shapes exhibit occurrence in the sequence of Chia Gara Formation in the northwestern part of the Zagros belt in Kurdistan region-Iraq. These structures name as Phacoids that have spherical to semi-spherical shape. Constructing concentric zonal features in the carbonate/shale of Chia Gara Formation. These structures are characterized by the presence of ball-like bodies within the host rocks. The diameter of phacoids ranged from 20cm to about 120cm. They are harder than the host rock. Seven outcrop sections, of Chia Gara Formation are reviewed and two of them studied in detail to determining the origin and process of their formation. The phacoids can be conceder as sedimentary key marker to recognize the Chia Gara Formation in the outcrops. Petrographically, the phacoids are mostly organic matter–rich bioclastic lime wackestone. The bitumens are present in sub-parallel lines in a calcareous groundmass. However, the host rock is generally composed of calcareous shale rich in organic matter but barren of fossils. The carbonate phacoids were forming in a competent/incompetent sequence. The mechanism of creating these carbonate phacoids structures are related to the compression stress and diagenesis processes on the carbonate/shale successions. Some phacoids directly formed from the carbonate strata after separated and rolled due to continue compression shear stress that in the final stage form the carbonate phacoid with the aids of diagenesis.

Key Words: Chia Gara Formation, Phacoid structures, Jurassic,  Kurdistan,


[1] Ahmed S. H., "Tectonostratigraphic evolution of the northeastern Arabian Plate in Kurdistan since the Jurassic", Unpublished PhD Thesis, L’UNIVERSITE PIERRE ET MARIE CURIE Spécialité Géosciences et Ressources Naturelles. France-Paris. (2013).

[2] Ahmad S. H., Barrier É., Muller C., "Basin evolution model during Cretaceous in the northeastern of Arabian Plate in Kurdistan region". Arab J Geosci. Vol. 9, pp. 645, doi:10,1007/s12517-016-2677-2. (2016).

[3] Al-Beyati, F.M. "Organic Geochemical and Environmental Evaluation study of Chia Gara Formation from selected boreholes in middle Iraq", Ph D thesis, University of Baghdad, Unpublished, 172 p. (1998).

[4] Al-Ameri T.K. and Zumberge J. "Middle and Upper Jurassic Hydrocarbon potential of the Zagros Fold Belt, North Iraq". Journal of Marine and Petroleum Geology, Vol. 36, pp.13-34. (2012).

[5] Aqrawi A.A.M., Goff J.C., Horbury A.D., and Sadooni F.N. "The petroleum geology of Iraq". Scientific press Ltd., Oxford, 424 p. (2010).

[6] Barrier E. and Vrielynck B., "Paleotectonic Maps of the Middle East, Middle East Basins Evolution Programme", CGMW, Atlas, Maps, p. 1-14. (2008).

[7] Buday, T. "The Regional Geology of Iraq", Vol 1: Stratigraphy and Paleogeography. Publications of Geological Survey of Iraq, Baghdad, 445 p. (1980).

[8] Bellen, R.C. van, Dunnington, H.V., Wetzel, R. and Morton, D.M., "Luxique Stratigraphique International", Asia, Fascicule10a, Iraq, Paris, 333 p. (1959).

[9] Edilbi, A.N., "Stratigraphy and Petroleum Potential of Chia Gara Formation (Tithonian- Berriasian) in Selected Sections in North Iraq", MSc thesis, Unpublished, University of Salahaddin, 126 p. (2010).

[10] Edilbi, A.N.F., Maleko, M.A., Mohamed, A.Y. and Bowden, S.A., "Evaluating of Baluti Formation at Sararu village, Ora Anticline, Iraqi Kurdistan: a stratigraphic and geochemical approach", Arabian Journal of Geoscience, Vol. 10. pp. 73, DOI 10.1007/s12517-017-2846-y (2017).

[11] Edilbi, A.N. and Sherwani G.H., "Petrographic and source rock potential of Chia Gara Formation (Late Jurassic- Early Cretaceous) in Northern Iraq and Kurdistan Region", Journal of Petroleum Exploration and Production Technology, pp.1801-1818 . (2019).

[12] Fagereng A, Sibson RH. "Mélange rheology and seismic style". Geology Vol. 38. pp. 751–754. (2010).

[13] Fagereng A "Geology of the seismogenic subduction thrust interface. In: Fagereng A, Toy VG, Rowland JV (eds) Geology of the earthquake source". A volume in honour of Rick Sibson. Geological Society, London, Special Publications, Vol. 359, pp 55–76. (2011).

[14] Hakimi, M.H., Mohialdeen, I.M.J., Abdullah, W.H., Wimbledon, W., Makeen, Y.M., and Mustapha, K.A., "Biomarkers and inorganic geochemical elements of Late Jurassic-Early Cretaceous limestone sediments from Banik Village in the Kurdistan Region, Northern Iraq: implications for origin of organic matter and depositional environment conditions", Arabian Journal of Geosciences, Vol. 8, Issue 11, pp. 9407- 9421. (2015).

[15] Hakimi M.H., Najaf A.A., Abdula R.A. and Mohyaldin I.M.J., "Generation and expulsion history of oil-source rock (Middle Jurassic Sargelu Formation) in the Kurdistan of north Iraq, Zagros folded belt: Implications from 1D basin modeling study", Journal of petroleum Science and engineering pp.1-21. (2018).

[16] Jassim, S.Z., and Goff, J.C. (Eds.) "Geology of Iraq". Dolin, Prague and Moravian Museum, Brno, 341 p. (2006).

[17] Karim. KH. "Origin of Ball and Pillow-like Structures in Tanjero and Kolosh Formations in Sulaimaniya  area, NE-Iraq, Kurdistan Academicians Journal part A, Vol. 4, No. 1. (2005).

[18] Kimura G, Yamaguchi A, Hojo M, Kitamura Y, Kameda J, Ujiie K, Hamada Y, Hamahashi M, Hina S "Tectonic mélange as fault rock of subduction plate boundary". Tectonophysics 568–569. pp.25–38 . (2012).

[19] Lloyd GE, Ferguson CC, Reading K. "A stress-transfer model for the development of extension fracture boudinage". J. Struct. Geol. Vol. 4. pp. 355–372. (1982).

[20] Mohialdeen I.M.J. "Stratigraphy and sedimentology of organic rich limestone of the Chia Gara Formation, Rania area, Sulaimani, Kurdistan Region, NE Iraq. Journal of Zankoy Sulaimani Part A, Vol 9, No. 1. pp.91-102. (2007).

[21] Mohialdeen I.M.J. "Source Rock Appraisal and Oil/Source Correlation for the Chia Gara Formation, Kurdistan-North Iraq", PhD Thesis (unpublished), University of Sulaimani, 130 p. (2008).

[22] Mohialdeen I.M.J., Hakimi M.H. and Al Beyati F.M., "Biomarker characteristics of certain crude oils and the oil source rock correlation for the Kurdistan oilfields. Northern Iraq". Arab. J. Geosci. Vol. 8, pp.507–523. (2013).

[23] Needham DT "Asymmetric extensional structures and their implications for the development of mélanges". Geol. Mag. Vol. 124, pp.311–318. (1987).

[24] Ross, D.J.K., Bustin, R.M., "The importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs". Marine and Petroleum Geology, Vol. 26, pp. 916–927. (2009).

[25] Sharland, P. R., Archer, R., Casey, D. M., Davies, R. B., Hall, S. H., Heward, A. P., Horbury, A. D., and  Simmons, M. D. "Arabian Plate Sequence Stratigraphy", Geo. Arabia, Special publication 2, Gulf  Petro Link, Bahrain, 372 p. (2001).

[26] Sibson RH, "Tensile overpressure compartments on low-angle thrust faults", Earth Planets and Space. Vol. 69, pp. 113. DOI 10.1186/s40623-017-0699y. (2017).

[27] Stach, E., Mackowsky, M-Th., Teichmuller, M., Taylor, G.H., Chandra, D., Teichmuller, R. (Eds.), "Coal Petrology". Gebruder Borntraeger, Berlin - Stuttgart, 535 p. (1982).

[28] Vannucchi, P., "Scaly fabric and slip within fault zones". Geosphere, Vol. 15, No. 2, pp. 342– 356, https:// doi .org /10 .1130 /GES01651.1. (2019).

[29] Wimbledon, W.A.P., Mohialdeen, I.M.J., Andreini, G., Rehakova, D., Stoykova, K., "The Jurassic /Cretaceous boundary beds in Kurdistan – a preliminary note on wider correlations", Journal of Zankoy Sulaimani, Special Issue, GeoKurdistan II, pp. 269-276. (2016).