*Abnormal Ophiolite (Olivine/Pyroxene Rich) Sandstone NE Iraq: An Approach to the Origin… DOI: http://dx.doi.org/10.5772/intechopen.108333*

the lobes. These turbidites are characterized by the down current areas from those reached by the lobes, which are discussed as fan lobe fringe.

Thin-bedded horizons of the outer fan and fan-fringe margins are undergoing alternative phases of high and low rates of sedimentation. The high rate is discussed by the active progradation of relatively sand-rich deposits, which is evident by the abundance of load and scour structures, particularly pseudo-nodules. These structures require soft pelitic sediments, which are rapidly overlain by denser sand beds. The latter horizons are represented by thin veneers of highly bioturbated hemipelagic mudstone/marl, which are found particularly in the thinnest and mostly shale facies of the lobe-fringe and fan-fringe deposits.

All features of the thin-bedded horizons in the Kolosh basin plain are result from the transport and sedimentation of sand, silt, and mud across the flat basin floor. Sedimentation of thin-bedded deposits is carried out by vanning, dilute/low-density turbidity currents that gradually lose the suspended load with distance. The rate of hemipelagic deposition between successive turbidite layers is apparently controlled by the distance from the source of the turbidity currents or, in other words, by the rate of turbidite sedimentation. Thus, hemipelagic mud interbeds appear to be increasingly thicker in a down-current direction compared to the turbidite dispersal system.

#### **8.5 Tectonosedimentary evolution**

Depositional evolution in the Kolosh foreland basin is extremely controlled by tectonic activity. The collision between Arabian and Eurasian Plates is started in late Cretaceous age accompanied by volcanic arc eruptions (**Figures 5** and **6**).

Continuous collision formed subsidence and deepening to create a trench basin and consequently transgression. The lower part of the Kolosh formation is deposited in the marine slope margin, which is evident from the sedimentological features supported by sedimentary structures and facies types. The identified sedimentary structures and related facies types in the middle part of the Kolosh formation suggest progressive development of the environment to inner submarine fan and channel

#### **Figure 6.**

*Schematic diagram of foreland basin with forearc and back arc divisions and subduction zone. The diagram shows the site of deposition of the Kolosh Fm. In the accretionary wedge.*

levee margins. Continuously thrusted evolution progresses the deposition to outer fan margins. The later stage of sedimentary evolution represents progress into basin plain margins and continuous collision of continental and oceanic crust creating a trench basin, where Kolosh formation was deposited.
