Journal of Earth Sciences and Geotechnical Engineering

Old Alluvial Fan Relics in North and Northeast Iraq

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• Abstract

   

  The southern part of Sulaimaniyah Governorate and northern parts of Erbil, Kirkuk and Diyala Governorates are mountainous and hilly areas, the relief difference ranges from (50 – 500) m. Tectonically, they represent the contact between the Low Folded and High Folded Zones, which is marked by an outstanding geomorphological feature that is a continuous limestone ridge represented by the Pila Spi Formation (Late Eocene). The ridge form the southwestern limb of tens of anticlines that have NW – SE trend. The continuous ridge faces; south and southwest ward a highly dissected plain, which is covered by clastics of Injana, Mukdadiya and Bai Hassan formations, the plain forms typical badland morphology. Moreover, the Pila Spi Formation is overlain mainly by the Fatha Formation (Middle Miocene), it consists of clastics alternated with rarelimestone and gypsum. However, locally, the Pila Spi Formation is overlain by Oligocene and Early Miocene rocks, mainly of carbonate. During Pleistocene, due to wet phases, the flat plain was a favorite depositional basin for the flowing rivers, streams and valleys from the north and northeast to deposit their carried loads after gradient changes. Accordingly, huge amount of sediments were laid down in form of alluvial fans that are usually capped by calcrete. The developed fans were covering vast areas that extend southwards; about 45 Km from the main ridge of the Pila Spi Formation. The thickness of the alluvial fans is highly variable, it ranges from (3 – 15) m. The main constituents are also variable; either consists mainly of carbonates that are derived from the Pila Spi Formation cemented by calcareous and sandy materials, or consists of pebbles; derived mainly from the Bai Hassan Formation, which are carbonates and silicates, with subordinate amounts of igneous and metamorphic rocks cemented by calcareous and sandy materials with rare gypsuous materials too. The size of the clasts is also variable, in the former case they reach up to 50 cm, whereas in the latter case, they reach up to 30 cm. During Holocene, most probably late Holocene, the alluvial fans have suffered from intense erosion, consequently large parts were eroded and washed out by the developed dense drainage system. The remaining parts nowadays, are in form of relics capping folded rocks of Fatha, Injana, Mukdadiya and Bai Hassan formations. Those which are nearby the main ridge form plateaus of different sizes, with gentle inclination manifesting the paleo-relief. Whereas, those which are far from the main ridge; form almost flat areas, occasionally are occupied as agricultural fields.