Abnormal Ophiolite (Olivine/Pyroxene Rich) Sandstone NE Iraq: An Approach to the Origin and Tectonosedimentary Evolution of Zagros Foreland Basin

Abstract

Unusual Paleocene ophiolite sandstone rich in olivine/pyroxene identified in Zagros Thrust Belt (NZTB) in NE Iraq. NZTB is regionally extended from Iran to Alpen Belt. Kolosh sandstones are controlled by progressive thrusting during late Cretaceous-Paleocene. Zagros thrust sheets composed of ophiolites, oceanic crust, basaltic flows, and ash sequences. Kolosh sandstones reveal high percentages of fresh olivine-pyroxene grains accompanied by igneous intrusive and volcanic ultrabasic-basic fragments, which are reported for the first time in NE Iraq and along ZTB. Olivine, pyroxene, ultrabasic igneous altered, serpentine and chlorite fragments, heavy minerals (includes chrome spinal), anorthite, and labradorite all together composed about 70% of the mineralogical composition. Sanidine, anorthoclase, quartz and cristobalite, argillaceous, carbonate and chert fragments all together composed (12.25%), supported by argillaceous matrix (16.53%), which are derived from mantle and oceanic crust/ophiolite sequences from NE Iraq, emplaced during late Cretaceous with arc volcanism, which subjected to rapid submarine erosion and deposition. Intense wave action accelerated the erosion of beach rocks, and concentrate the heavy minerals insitue that slumped to deeper margins. Identified lithofacies types, grouped in four associations, slope/submarine channel, inner, outer fan, and hemipelagic/pelagic, respectively, represented progressive upward transgression from slope to basin plain systems controlled by progressive thrusting.

1. Introduction

The Kolosh formation is studied here for the first time in the Badelyan area in the NE extremities of the high-folded zone, very near to the suture of the NZTB (Figure 1) [1], which is widely distributed in NE Iraq. It consists of successive sandstones and mudstones interbeds [2]. The previous workers suggested no signs of turbidites and graded beddings in the formation and classified them as molasses sediment deposited in a narrow platform basin [3, 4, 5].

Figure 1.

The geological map of Dyana-Rawandoz-Sidakan-Rayat areas in the suture of the Zagros thrust belt NE Iraq shows the study area (rectangular) (Geol. map of Erbil and Mahabad GEOSURV, compiled by Sissakian et al. [1].

Ophiolite sandstones are very rare in the world, which was reported in Italy (Val Marecchia Nappe) composing high percentages of serpentine and ultrabasic fragments [6]. The ophiolite sequence is considered as the main source of intrusive ultrabasic/basic and basic volcanic grains are basically derived from mantle, oceanic crust, and from volcanic arcs. The green sand is composed of relatively high percentages of olivine grains that were recently recorded only in the Hawaii Islands, where the lava flows are composed of olivine basalt type [7]. High active storm and submarine waves made rapid erosion and deposition of the heavy fractions, for example, olivine grains in the shallow environment.

This chapter discusses sandstone units of the Kolosh formation, which contain high percentages of fresh olivine, pyroxene, and ultrabasic/basic igneous grains as an unusual case in Iraq and in Neotethys extension. It tries to interpret the potential source rocks of the olivine and pyroxene concentration related to tectonic thrusting and the mode and mechanism of deposition. The paper studied the facies and associations relative to tectonosedimentary evolution in response to thrusting tectonism and seismicity. Arrangement of the facies sequences of the Kolosh formation in the studied area refers to deep marine turbidites rejecting the previous suggestion of no turbidities and no graded beddings. The turbidites facies is supported by characteristic sedimentary structures of turbidity origin as discussed later.

2. Stratigraphic review

Previous studies described the Kolosh formation at the type section in the Kolosh village in Koi-Sanjaq area, NE Iraq. [8] were firstly described as the Paleocene-Lower Eocene deposits in the high-folded zone. Buday [4] said that the formation was composed of 400 m thick blue shales and green sandstones. The above upper 174 m is of Sinjar limestone formation underlying with 140 m thick of intertonguing Sinjar and Kolosh formations. In Derbendikhan area, a section of 1000 m thick was described by Ref. [9], including many conglomerate beds with mudstones, siltstones, argillaceous, and detrital limestone interbeds. The Kolosh formation is encountered in several oil fields of northern Iraq [3, 4]. The formation was deposited in a relatively narrow rapid sinking basin trending NW-SE and was superimposed on platform margins and separated from the geosyncline by uplifted emerged and eroded lines [3, 4, 9]. No turbidite and graded bedding signs were suggested by Ref. [4], which diminished to an open sea calcareous sequence.

Previous paleontological studies ascertained the middle Paleocene age [10]. According to Ref. [11], the lower contact in the type locality is unconformable and transgressive. However, the authors [4] ascertained conformable upper contact with Paleocene-Lower Eocene limestone. The firstly discovery of olivine/pyroxene-rich sandstones of the Kolosh formation in the Badelyan locality was carried out by the study of Ref. [2].

3. Materials and methods

This study is based on the stratigraphic and petrographic analysis of Paleocene sedimentary sequences of the well-exposed section in the Badelyan area NE Iraq (Figure 2). A total of 170 m thicknesses were studied and logged, which is located directly beneath the suture of the Zagros thrust belt. The field works cover the measurement of bed thicknesses and the total thickness of section. Moreover, litho and facies types, sedimentary structures, sedimentary sequences, and bed extensions are identified and measured. The field works of the measured section, lithological description, thickness variations, and stratigraphy of the sequences are studied and summarized in Figure 2. Descriptions of field features accompanied by the results of microscopic studies have been used for facies analysis and paleoenvironmental construction. For subsequent studies, a total of 50 hand specimens were collected. Sampling intervals were generally between 1 and 3 m. Additional data, including macroscopic rock descriptions and ichonological data, have been studied as well as trace fossils in the field work.

Figure 2.

The stratigraphic section shows different stratigraphic units of the Kolosh formation in Badelyan area, NE Iraq.

Petrographic facies analyses were carried out in 45 thin sections and reported all mineralogical constituents, matrix, and cement types. Detailed photomicrographs are picked up for all mineralogical details. Abbreviations for names of rock-forming minerals used to point to the types of minerals in the photomicrographs are followed [12]. The sandstone classification of Ref. [13] as well as the provenance and tectonic setting of Ref. [14] cannot be applied because of the high content cf. 25% of olivine and pyroxene grains.

4. New stratigraphic details

Location: the studied section lies 5 km N of Badelyan village, 150 km from Erbil Governorate, NE Iraq.

Thickness of the Kolosh formation in Badelyan section is about 170 m (Figure 2).

Lithology: the formation consists of mudstones/shales, muddy sandstones, pebbly sandstones, chaotic mudstone, fossiliferous sandstone, and thin pelagic carbonate beds. Very thin siliceous horizons are observed within the mudstone beds. All beds of shales and sandstones are arranged in graded turbidity cycles. Most beds are deformed, revealing characteristic sedimentary structures of turbidite origin.

Sedimentary structures are identified based on Refs. [15, 16, 17, 18, 19, 20] as main characteristic of turbidity origin cf. load and groove casts, ball and pillows, graded beddings, slump slide and convolute beds, scour, submarine channels, and wavy and undulated beds. Most of the sandstone and mudstone horizons are graded upwards of Bouma cycles. Most beds contained balls and pillow structures as well as load and scoured surfaces. Some mudstone beds show groove casts. Deformational structures are represented by slump and slide beds and sometimes convolute beddings.

Trace fossils are identified in sandstones, mudstones and muddy limestone beds.cf. burrows, borings, and trails.

Contacts: the lower is gradational with Tanjero formation and the upper part is conformable with Gercus formation.

Age: based on previous studies, the age of the Kolosh formation is Paleocene-lower Eocene.

Secondary features are (a) compaction of some mudstone beds and development of shale-like fissile, (b) development of large concretions, (c) bioturbation in parts, and (d) faulting.

5. Petrographic examination

Thirty-five thin-section slides are carefully studied to identify the mineralogical assemblages of the Kolosh sandstones. The modal analysis and average percentages are shown in Table 1. Petrographic identification of various mineralogical constituents is followed [21, 22, 23, 24, 25, 26, 27, 28].

Grains composition	Average modal %	Grains composition	Average modal %
Pyroxene	18.3	Alkali Feldspar	1.0
Olivine	8.3	Quartz	2.5
Igneous rock fragments	11.3	Cristobalite	0.5
Altered fragments (Fe-Mg minerals)	17.4	Chert and chalcedony	2.5
Plagioclase-feldspar	5.2	Argillaceous fragments	3.5
Serpentinite MRF	4.4	Carbonate rock fragments	2.25
Opaque minerals (include chromite and glauconite)	4.0	Argillaceous matrix	16.5
Chlorite	1.6	Carbonate cement	1.25
Sum	70%		30%

Table 1.

The average percentages of the mineralogical constituent in the studied sandstones of the Kolosh formation in Badelyan locality.

Olivine group: varieties of olivine minerals are identified, such as fayalite, forsterite, monticellite, and chondrodite, with an average percentage of 8.3% (Pl/1 A-E). They are mostly prismatic, angular to subangular, and are less common than pyroxene. Most olivine grains are fresh and few grains are uralitized and others have iron oxides and serpentine along the fractures.

Pyroxene group: Clino and orthopyroxenes are recognized based essentially on the extinction angle and petrographic properties. The average percentage of pyroxene minerals is 18.26%.

Clinopyroxene is of hedenbergite, aegirine-augite, diopside, spodumene, pigeonite, and jadeite minerals. These grains are mostly fine and angular to subangular and of prismatic shape (Pl/1 F and H, Pl/2 A-D). Most of the clinopyroxene grains are fresh and others are altered to iron oxide or uralite in the center and a rim of pyroxene is still fresh (Pl/3 G).

Orthopyroxene: hypersthene and enstatite are recognized (Pl/1 G, Pl/2 D), mostly prismatic, angular to subangular, fresh with few altered grains. Orthopyroxenes are less common than clinopyroxene grains.

Feldspar group: plagioclase and alkali-feldspars are identified such as Ca-plagioclase as major type mainly of anorthite and minor bytownite and labradorite composition. The alkali-feldspar is anorthoclase and sanidine (Pl/4 A-D). Feldspar grains are mostly prismatic, fine, angular to subangular and have about 6.17% altogether. Some grains show sericite or carbonate replacement but keep the original shape.

Quartz attains average percentage of 2.05% of total grains, and has fine and subangular to subrounded shape (Pl/4 F).

Cristobalite attains average of 0.51%, less than quartz grains and is fine, angular to subangular grains (Pl/4 E).

Rock fragments: various types are recognized mostly igneous types with subordinate sedimentary and rare metamorphic fragments. The types of igneous fragments are ultrabasic, basic and few acidic, and intrusive and extrusive grains. While the types of sedimentary fragments are argillaceous, chert, chalcedony, and carbonate grains. The average percentage of various types of rock fragments is 15–20%.

Igneous rock fragments of intrusive and extrusive types, are composed of mainly ultrabasic, basic, and subordinate acidic fragments, attaining average of about 11.35%, these are:

• Intrusive igneous rock fragments varieties are ultrabasic igneous grins, including peridotite, heirzburgite, lherzolite, spilite, and pyroxenite (Pl/2 E-H, Pl/3 B-D). The basic intrusive fragments are composed of gabbroic grains (Pl/3 H). These grains are fine with few medium-grained, angular to subangular, and fresh with few altered grains. Some ultrabasic grains are serpentinized along the fractures or altered crystals in the fragment. Basaltic and andesitic grains of extrusive type are recognized also (Pl/3 E). These grains are angular to subangular and fine and mostly fresh and unaltered.

• Granitic fragments are acidic intrusive type, fresh, fine, and angular to subangular grains (Pl/3 F).

• Serpentinite grains attain average percentage of 4.42%, and are mostly fresh and are subangular to subrounded shape (Pl/3A).

• Chlorite grains attain average percentage of 1.56%, and are mostly of subrounded shape (Pl/4 G). Chlorite grains are fine-grained and have a dark green color.

• Altered grains are relatively of high percentages and attain an average of 17.4%. The grains of olivine, pyroxene, and ultrabasic and basic fragments show various degrees of alteration, mostly to iron oxides and/or uralitized pyroxene (Pl/4 A and C). The grains are altered to brown and dark brown in the center and the edges are still fresh. These are fine and subangular to subrounded grains.

• Sedimentary rock fragments are composed of limestone (includes fossils) and argillaceous fragments (Pl/3 H). The average percentage is 8.25% and is fine and subangular to rounded grains.

• Chert and chalcedony have an average percentage of 2.3%, are angular to subangular and composed of microcrystalline quartz (Pl/3 H). Chalcedony grains are rounded, composed of radiating silica, and sometimes filled with opal-CT. Radiolarian fossils are identified filled by chert and/or chalcedony.

• Opaque minerals include the majority of magnetite, chromite, chrome-spinel, and hematite. The average percentage is 4.0%, and are fine to very fine, angular to subangular in shape.

• Glauconite grains are scattered in the sandstone units of mostly rounded shape and green color.

• Cementing materials: Argillaceous matrix is the main type of cement with few carbonates. The percentages of the muddy matrix are more than 16.53% suggesting greywacke sandstone type [13], while carbonate is 1.2%.

The classification of sandstone cf. [13] and [14] provenance and tectonic setting diagram cannot applicate due to the high content of olivine, pyroxene, ultrabasic and basic, and altered fragments, which reach more than 50%.

6. Facies analysis

Classification of facies types in the Kolosh formation is followed [15, 16, 17, 18, 19, 20]. Sedimentological field studies combined with microscopic facies analysis of Paleocene siliciclastic sequences in the Badelyan area have resulted in the recognition of 11 sedimentary facies (Table 2, plate/5). Facies types are grouped into four main categories, based on their lithological characteristics and formation processes, comprised mainly of mudstones, sandstones, and minor carbonate and silicified facies (Table 2). The textural characteristics, grain and facies association and depositional settings of the studied rocks are summarized in Table 2.

Facies	Thick (m)	lithology	Characteristics	Process
F1	0.25–0.5	Laminated/fissile shale	Planner to undulated, laminated dark gray to black silty shale, 0.5–1.5 cm thick laminates	Hemipelagic
F2	0.25–0.5	Graded bedded muddy sandstone	Thin beds fining upwards, arranged in turbidity cycles, fine to very fine sandstones	Middle to outer submarine fan turbidite
F3	0.25–1	Calcareous mudstone	Dark gray, graded lower base and sharp upper base	Hemipelagic/pelagic mud-low energy turbidite
F4	1–10	Deformed slump and convolute sandstone/mudstone	Slump, slide, and convolute beds of sandstone and mudstone sometimes a mixture of dark gray to green color, scour and fill structure	Submarine fan channel levee; High-energy turbidity currents
F5	0.15–0.5	Undulated and wedge sandstone/mudstone	Wedge beds of dark green sandstone and gray mudstone, undulated beds	Edge of submarine fans, low energy turbidity currents
F6	1	Muddy pebbly sandstone	Dark green, coarse-grained turbidite, sharp, scour, and grooved surface	High concentration turbidite/ debrite
F7	0.5–1	Silicified mudstone/chert	Discontinuous or lenticular in part, pale gray, silicification	Pelagic mud to very low energy turbidity currents
F8	1–2	Chaotic mud-rich with hard sediments of muddy sandstone and calcareous mudstone	Disturbed, chaotic-hard mudstone and sandstone, deformed, load, scour surfaces, green to gray color	High energy turbidity currents, slope submarine fans-channels
F9	1–10	Submarine channel sandstone	Fine to very fine-grained, scoured, load and flute casted surfaces, dark green color	Slope submarine fan turbidite, low concentration turbidite
F10	1–1.5	Muddy pebbly bioclastic sandstone	Sharp base, scoured	Bioclastic rich turbidite
F11	0.1–0.2	Fossiliferous muddy wackestone	Continuous to discontinuous wispy beds with gradational contact	Pelagic/hemipelagic carbonate

Table 2.

Lithofacies types identified in the Kolosh formation of Badelyan locality.

Firstly, facies description and environmental interpretation of the facies are discussed. Further details on sedimentary environments and regional depositional settings are provided in the following;

Eleven lithofacies types are composed of interbedded thick mudstone, muddy sandstones, and pebbly sandstones arranged basically in fining upward Bouma turbidity cycles [29]. Bouma cycles are capped with pelagic limestone or silicified mudstones. The field observations of facies types and sedimentological features are compatible with deep water slope apron to basin plain depositional system. Specific facies types of turbidite origin are discussed and interpreted in Table 2.

6.1 Facies association

Lithofacies types of the Kolosh formation are grouped into four main facies associations, each one is referred to a certain depositional system, these are:

6.1.1 FA1, slope/submarine-channel fan

The lower part of the Kolosh formation in the studied area represents submarine channels developed in slope margins. Submarine erosion and deposition of the sediment is performed by gravity flows and/or turbidity currents as a result of migrated active channel floor [20, 30, 31, 32]. Debrites and developed turbidites comprise two stages of the sediment-gravity-flows. The debris flows of muddy pebbly sandstone are represented by suites of facies F4, F5, F6, F8, F9, and F10. The coarse grains and gravel are supported by a cohesive matrix of interstitial fluid and mud [20]. Sand and mud turbidites are deposited from suspended loads and graded upwards by fluid turbulence in the inner channel and on levee. Channel sediments commonly deposited the following facies, represent a spectrum of submarine mass-movement processes [20, 33, 34, 35, 36, 37, 38, 39].

1. Thick-bedded, amalgamated sandstones, and/or pebbly sandstones are deposited by collapse of high-density turbidity currents through traction reworking of bed load sediment (cf. F4, F6, F8).

2. Interbedded thin fine sandstones/mudstones are deposited from low-density turbidity currents (cf. F2, F5, F7).

3. Thin bedded and stratified mudstones/shales, deposited from dilute, low-density turbidity currents and subsequent suspension are deposits of mud between turbidity currents (cf. F1, F2).

4. Nongraded sandstone and/or chaotic mud-rich facies cf. F8, with harder sediment of F1/F2 facies within a mud-rich matrix, deposited from debris flows.

5. Deformed, contorted (overturned and/or offset stratification) heterolithic horizons deposited from slumps and/or slides (cf. F4, F5, F7) [39].

6.1.2 FA2, inner fan/channel levee

The sandstone/mudstone beds are arranged in turbidity Bouma cycles with or without basal division (Ta). The thickness of these beds ranges from 0.5 m for sandstones and 1.5 m for mudstones. Balls and pillows, disturbed beds, convolute beds, slump and slide beds, load, and scour and grooved surfaces are the main characteristic sedimentary structures.

Slump and/or slide of interbedded sandstone/mudstone horizons on both sides of the channel valley are filled and interfinger with channel and interchannel deposits that are interpreted here as levee/inner fan deposits cf. F2, F3, F4, F5, F7. Deformation of the slump is represented by recumbent folded horizons bounded by undeformed beds. The sandstone’s and mudstone’s balls and pillows are associated with slumping and/or sliding strata, and some are very large with a diameter of about 1 m.

6.1.3 FA3, outer fan

The outer fan deposits consist of thick mudstone/shale beds interbedded with very thin sandstone horizons graded upwards to reveal Bouma cycles without the basal divisions (Ta, Tb). The outer fan succession comprises a developed thick sequence of alternating non-channelized sandstone bodies and associated thin/thick bedded argillaceous mud sediments cf. F1, F2, F3, F5, F7, F11.

The middle part of the Kolosh succession cf. 45–90 m above the lower base permits detailed inspection of numerous thickening upward cycles. Each cycle consists of lower thin-bedded and fine-sandstone facies, transitionally overlain by thicker bedded/massive mudstones. These criteria and terminology are already discussed by Refs. [30, 31, 40, 41], which are interpreted as lobe-fringe and lobe-fan deposits, respectively. These two facies’ cycles are either symmetric or asymmetric, thus reflecting sudden or gradual shifting of the related feeder system as suggested by Refs. [20, 41].

Typical sedimentological features of thin-bedded lobe-fringe deposits are included cyclic vertical variations depicted by changes in the sand/shale ratio and in the sandstone bed thickness.

The lobe-fringe turbidite beds displayed base-missing of Bouma sequences, that is, Ta, Tb, and/or Tc. Typically, by far, the Tc-e subdivisions are predominant. In part, Tb-e cycles are displayed unusual thick beds, which are randomly scattered in the succession. The sandy beds’ division, which includes current-ripples is bounded by upper surface gradational with the overlying turbiditic mudstone.

The post-depositional and/or syndepositional plastic deformation and liquefaction are the causes of abundant convolute, slump, balls, pillows, and the small-scale features, such as pseudo-nodules observed, in the sandy and muddy portions of many beds. Occasionally thin veneers of bioturbated hemipelagic mudstone and/or lime mudstone are found at the top of some turbidity cycles.

6.1.4 FA4, pelagic/hemipelagic basin plane

Hemipelagic/pelagic basin plain sediments of the Kolosh formation are very monotonous sequences of alternating thin sandstone, siltstone, mudstone, and hemipelagic silicified mudstones. These beds are associated with numerous intercalations of thin turbidity lime mud beds and related slump units, which belong to quite distinct depositional settings [16, 30, 31, 42]. The carbonate mud/marl beds are derived from tectonically unstable flanking platforms (cf. around island arc) and include a variety of fine-grained distal turbidites facies rich in planktonic foraminifera and radiolarian fossils.

The basin plain sediments of the Kolosh formation are characterized by the following features:

The surfaces of the beds are even parallel regardless of the extent of exposures and turbidite beds are missing the base of Bouma cycles with predominant Tc-e divisions. The sandy and/or silty division of the turbidite beds are of thin parallel laminae (Td-division), which grades transitionally into turbiditic mudstone (Te-division) (Figures 3 and 4). Typically, the small-scale cross/wavy laminae of the current-ripple division (Tc) display low to very low angles and occur in well-developed climbing patterns. The basal portion (Tc) is fine to very fine sandstones. The turbidite beds are regularly alternating with thinner and bioturbated hemipelagic beds. Moreover, these beds contain distinctive assemblages of indicative benthic foraminifera of water depths in the range of 600–2000 m, as well as planktonic types [16].

Figure 3.

Depositional model of the Kolosh Formation discusses the slope and basin plan with anatomy of submarine fan and lithofacies types. Ta-b-c-d-e are subdivisions of Bouma turbidity cycle.

Figure 4.

Schematic anatomy model shows subdivisions of the submarine fan and Bouma turbidity cycle; A. Bouma cycles subdivisions, Ta to Te e.g. Ta= pebbly to coarse-grained sandstone, Tb= laminated sandstone, Tc= cross-bedded sandstone, Td=very fine sandstone to siltstone, Te=mudstone/shale and position in the fan. And subdivisions of submarine fan; 1) canyon, 2) Mobile channels with wings, 3) fan fringe and abyssal plan B. Facies types in the slope and submarine fan; slump deposits, debris flows and deep sea floor cf. abyssal plain (adopted from Mutti [16]).

7. Tectonosedimentary evolution

The depositional evolution of the Kolosh formation was mainly controlled by continuous subduction of continental and oceanic crust in the Zagros Tethyian basin and was indirectly related to the sea level changes. The Kolosh successions can be classified into four stratigraphic sequences, representing the certain depositional system. Successive facies associations are arranged from bottom to top cf. slope/submarine-channel fan FA1, inner fan/channel levee FA2, outer fan FA3, and pelagic/hemipelagic basin plane FA4 refer to progressive deepening from slope apron to the basin plain in the top of the formation (Figures 3 and 4).

7.1 Stage 1

The first tectonosedimentary stage comprises slope/submarine channels that flows down the slope and comprised of F4, F5, F6, F8, F9, and F10. These are evident from the bed geometry, sedimentary structures, and facies types. FA1 is characterized by muddy, pebbly coarse-grained turbidite sandstone F6, chaotic mud-rich facies, with harder horizons of F1/F2 facies in a mud-rich matrix F8, and muddy, pebbly, and bioclastic-sandstone F10. These facies are typically originated from relatively dense turbidity currents, in slope margins via the submarine channel.

7.2 Stage 2

Continuous thrusting developed subduction and consequently transgression and deepened into the forearc basin. These formed distributary channels in the inner margins of submarine fans, which are evident by deformational structures, such as slump, convolute, and channel levee beddings of F4 and F8, composed of thick/thin interbedded mudstones and sandstones.

7.3 Stage 3

Continuous subduction and transgression enhanced deepening and shifted the sediments inward the basin. Sedimentation was developed from the slope apron margin cf. FA1 and FA2 to the outer submarine fans in the basin plain. FA3 is composed of thin/thick-bedded argillaceous muddy sediments intercalated with thin beds of sandstone and deep marine carbonate mud rich in planktonic foraminifera and radiolarian fossils cf. F2, F3, F5, F7, and F11, thus refer to deepening in the Kolosh basin.

7.4 Stage 4

Continuous subduction and transgression shifted the sediments to the pelagic/hemipelagic apron, representing the last stage of tectonosedimentary evolution of the Kolosh forearc basin. These are evident from suite of facies types of alternative thin beds of sandstone, siltstone, mudstone, and hemipelagic carbonate mud/marlstone cf. F1, F2, F3, F7. F11. Moreover, it is evident from bed geometry, sedimentary structures, and microfossils assemblages, such as radiolaria, planktonic foraminifera, and hemipelagic benthic foraminifera. This margin is characterized by silicified mudstone and discontinuous silica horizons.

8. Discussion

8.1 Mineralogy of the source rocks

At Badelyan locality, Kolosh and Tanjero formations are exposed at the suture boundary of the Iraqi thrust zone near Jabal Hassan Beg mountain. These are comprised of the accretionary prism lying directly beneath the emplacement sheets of thrust zone. Based on field observations, characteristic sedimentary structures and sedimentological features all together refer to deep marine sedimentation originated by turbidity currents.

Petrographic examination of the Kolosh sandstones reveals high percentages of fresh olivine, pyroxene, intrusive ultrabasic, and basic and extrusive basic igneous rock fragments, which are controlled by the provenance and tectonic evolution of the Zagros thrust belt and surrounding areas. The mineral assemblages reveal various potential tectonically-controlled source rock complexes (cf. mantle, oceanic crust, continental crust, and volcanic arcs). The presence of olivine, diopside, and augite accompanied with ultrabasic rock fragments (cf. dunite, peridotite tectonite, heirzburgite, lherzolite, and pyroxenite) suggest ophiolitic mantle origin [21, 23, 24, 25, 26, 28, 43].

The peridotites of the ophiolite belt are ranging in composition from lherzolite to dunite through heirzburgite. These rock types and mineral associations are similar to those of forearc peridotites, which are represented by fertile alpine mantel lherzolite to dunite and are depleted in tectonite heirzburgite [44]. It is suggested that these ultramafic bodies are huge fragments of supra subduction zone of residual mantle peridotites [45]. Serpentinization is observed in along cleavages and basal fractures of the Fe-Mg minerals [25], as complementary processes during the evolution of the ultramafic part of the ophiolite sequence adjacent to the foreland basin. Serpentinization of peridotite is thought to have taken place during the subduction stages before the collision of the Arabian plate with the Asian plate [21, 45].

Diopside-diallage is usually derived from coarse-grained gabbro and basalt and is usually accompanied by forstertic olivine [44]. Omphacite is similar to diopside, augite, and jadeite, which are found exclusively in eclogite cf. deep mantle of high T and P conditions. Hedenbergite is usually derived from peridotite and fayalite ferrogabbro [25].

Augite is essential mineral in peridotite, gabbro, basalt, and olivine gabbro. Pigeonite may be derived from basalt, diabase, and dolerite, whereas, aegirine-augite compose an essential member in the trachyte and basalt. Spodumene is a rare mineral, derived from Li-granite [25]. Enstatite occurs in all types of basic igneous rocks. Mg-rich orthopyroxene occurs in ultrabasic igneous rocks (cf. pyroxenite, heirzburgite, lherzolite, serpentinite, and picrate with Mg-olivine) [25]. Hypersthene is found in gabbro, norite, and andesite [21, 26, 27].

Mg-rich olivine (e.g., forsterite and monticellite) is an essential mineral in most ultrabasic igneous rocks (cf. dunites, peridotites, and picrites). Olivine forsterite and chrysolite are derived from peridotite, olivine gabbro, and basalt. More Fe-rich olivine (e.g., fayalite) is derived from alkali basalt, ferrogabbro, and trachyte [25]. Monticellite is derived from basalt, and chondrodite is derived from high-grade metamorphic rocks [21, 22, 23, 26, 27].

The altered grains show various degree of alteration and are found in relatively high percentages. It is composed mainly of uralite and iron oxides. Most of the altered grains are derived from basic and ultrabasic rock fragments as well as olivine and pyroxene.

Ca-plagioclase members An95-An55 cf. anorthite, bytownite, and labradorite, are characteristic minerals of ultrabasic and basic igneous rocks cf. peridotite, heirzburgite, lherzolite, gabbro, and basalt. Anorthite is derived from ultrabasic intrusive igneous rocks cf. peridotite, pyroxenite, heirzburgite, and lherzolite and from gabbro. Bytownite and labradorite are derived mainly from basalt and gabbro [23, 26, 43, 44]. The alkali-feldspar varieties are derived from acidic intrusive and extrusive igneous rocks. Orthoclase is derived from granites, while anorthoclase and sanidine with cristobalite are derived from acidic and intermediate extrusive igneous rocks, such as rhyolite, trachyite, and andesite [24, 25, 26, 43, 44, 46].

Varieties of rock fragments are important indicators of source rocks and provenance. The most important abundant types in the Kolosh sandstones were derived from intrusive ultrabasic and intrusive and extrusive basic igneous rocks (cf. pyroxenite, lherzolite, heirzburgite, peridotite, gabbro, and basalt). These rock types are derived from mantle and oceanic crust as an ophiolite sequence [21, 25, 26, 44]. The basalt and andesite fragments are derived from volcanic arcs [26, 44].

The sedimentary rock fragments were derived from various sources. Intrabasinal mudstone grains are most probably derived from mudstone beds by the effect of turbidity currents. The limestone fragments are extrabasinal and derived from Cretaceous carbonate formations. The chert and chalcedony fragments are suggested to derive from acidic volcanic rocks of arc and/or deep marine radiolarian beds of the ophiolite sequence [17, 18, 20, 44].

8.2 Potential ophiolites provenance

Anomalous concentrations of pyroxene, olivine, and ultrabasic/basic igneous grains in the Kolosh sandstones are suggested to derive from two sources: thrusted ophiolite sequences and lava flow basalt from island arc in the foreland basin. The emplacement of the Neotethyan oceanic crust took place during late Cretaceous (118–97 Ma), while the emplacement of Hasan Beg ophiolite complex was (106–92 Ma) ago [47]. Along the southern Neotethys suture, the outer Zagros orogenic belt (OZOB) crops out, including Rayat, Piranshahr, Kermanshah, Neyriz, and Haji-Abad Ophiolites resulted, of the late Cretaceous collision between the Sanandaj-Sirjan and the Arabian shield [47, 48].

Sedimentation of the qulqula group cf. deep-marine radiolarian chert, and carbonates of Arabian passive-margin is dated as Valanginian-Turonian (140–89 Ma), which constrains initial orogenesis and early subsidence of the Zagros foreland basin to about 90 Ma [49, 50, 51]. The provenance data of the clastics in the Tanjero formation (Maastrichtian), Kolosh formation, and Suwais red beds (Paleocene-Eocene) reveal partial derivation from basic and ultrabasic sources related to ophiolite emplacement [49, 50, 51]. In the studied area, several pre-tertiary tectonically-derived igneous complexes may have served as source rocks for the sandstones of the Kolosh formation.

Hasan Beg mountain successions is situated beside the Badelyan area and very near to Sidekhan province. Hasan Beg igneous complex comprises a late Cretaceous remnant of the ophiolite-arc system that developed within the Neotethys ocean. This igneous complex was subsequently accreted to the Arabian plate during the late Cretaceous to Paleocene [50]. It is predominantly consisting of calc-alkaline basaltic andesite to andesite cutting across by micro gabbro and diorite dikes indicating Albian-Cenomanian age (106–92 Ma) [47, 50]. Hasan Beg mountain rock units start at the bottom by pillow lavas with sheared and highly weathered intense deformed chlorite slate, which forms the contact between the lower pyroclastic metavolcanic rocks and the upper part of metasediments cf. slate, shale, and sandstone. The overlying metasediments are composed of highly fossiliferous black shale interbedded with 20 m thick sandstone lens. These rocks are overlain by radiolarian chert with a total thickness of about 1000–2000 m, and this variable is due to thrusting [52].

Qalander-sidekhan mountain sequence is located about 20 km from Badelyan locality and composed from bottom to top of Tanjero and Aqra formations (late Cretaceous) separated by tectonic contact from the overlain govanda limestone (early-middle Miocene age), succeeded by red bed sandstones (middle-late Miocene), which overlies by a tectonic slice of naopurdan metavolcanic rocks [52]. In the Qalander locality the naopurdan rocks are divided into three parts: 1) The lower metavolcanic unit is composed mainly of basalt flows, 2) The middle unit is variable in thicknesses averaging about 50 m and composed of pillow lavas, brecciated lavas, and inter-pillow ash. Many dolerite dykes are cutting across the Qalander successions, in which pillow and flow lavas are about 450 m thick. The upper unit contains sequences of sandstones grades upward to conglomerates, containing pebbles of basalt, serpentinite, and marble, which have been derived from erosion and rapid deposition of the igneous rocks, probably from naopurdan and walash metavolcanic successions [45, 53].

The Sidekhan mountain, located NW of Qalander mountain has the same successions as the Qalander mountain with govanda formation and naopurdan successions, with tectonic breccia.

Rayat ophiolite mantle sequence lies in the NE corner of Iraq (30 Km from Badelyan area) and consists of serpentinized peridotites, serpentinite, tectonite heirzburgite, lherzolite, and dunite with Cr-spinel [27]. They are mantle residues with distinct geochemical signatures of affinities of both the mid-ocean ridge and supra subduction zone. The crustal succession includes gabbro, dibasic dikes, rare pillow basalts, and radiolarite overlain by late Cretaceous pelagic limestone. Rayat ophiolite is extended to Piranshahr area in NW Iran and to the Cilo ophiolite in SE Turkey [44, 46]. The Rayat ophiolitic mélange is mainly composed of peridotite and sheared serpentinite with heirzburgite [45]. The chromitite in the Rayat peridotite is similar to the mantle chromitite and Moho transition zone chromitite (upper mantle zone) of the Tethyan ophiolites [45].

Galalah-choman ultramafic/mafic rocks are situated about 25–28 km from Badelyan area. Galalah area consists of 50 m brecciated and pillow lavas alternating with ultramafic/mafic volcanoclastic rocks, and above with 40 m massive serpentinite and mélange marble of heterogeneous composition (mixture of calcite, chlorite, and serpentinite). This succession is overlain by a thin layer of radiolarian chert and red beds, which are tectonically separated by crush zone [54]. Choman successions are variable in thicknesses of about 200 m of pillow lava, interbedded with volcanic ash, tuff, and breccia [45]. Generally, galalah and choman rocks consist of basaltic flow lava interbedded with ultramafic/mafic rocks, overlying serpentinite rocks. They can be suggested as other sources of pyroxene, olivine and ultrabasic, and basic intrusive and extrusive basaltic igneous grains in the sandstone units.

The kata rash igneous complex comprises Cretaceous remnant arc and is broadly similar in age to those of late Cretaceous peri-Arabian ophiolite belt in other countries as well as other late Cretaceous Zagros supra subduction zone assemblages [53, 55]. This refers to the great lateral extent of late Cretaceous arc systems in the consumption of the Neotethys ocean.

8.3 Sedimentary mechanism of olivine/pyroxene concentration

Ophiolite successions are suggested as the main source of pyroxene and olivine grains in the Kolosh sandstones. It consists of rocks of various pyroxene/olivine-rich assemblages, including peridotite tectonite, lherzolite, serpentinized spinel peridotite, pyroxenite, serpentinite chromitite, heirzburgite, gabbro, and basalt lavas.

Later, slumps of these pyroxene/olivine-rich sands by turbidity currents are transported deeper in the basin to deposit finally as a turbidity sequence. Tectonic activity and seismicity in the foreland margin usually create turbidity currents and slumping of accumulated heavy sarends in the shallow zone to deeper margins [7, 16, 56].

Zagros Cretaceous ophiolite complexes are exposed in the early tertiary and are amenable to subaerial and shallow submarine erosion along the shorelines. Intense subaerial erosion enhanced the rapid mechanical disintegration of the ophiolites evidenced by the fresh olivine and pyroxene grains in the studied sandstones. Submarine wave action in shallow margins induced concentration and accumulation of the heavier fraction of the detrital rich in pyroxene and olivine grains according to the model discussed by [57] (Figure 5).

Figure 5.

A model discusses the Late Cretaceous geodynamic evolution of subduction Arabian and Asian plates to form Zagros foreland basin, in Late Cretaceous-Paleocene age A) Various intrusive magmatic bodies with arc-related basaltic flows, B) continuous subduction and subsides of the Kolosh basin (trench) and subsequent erosion of ophiolitic; ultrabasic, gabbro, to deposit the Kolosh Formation in the continental slope and/or deep marine trench (modified after Calvo [57]).

The Kolosh formation is suggested to deposit in the deeper Zagros trench subduction margin. Intense tectonic activity and/or earthquake seismicity are responsible for successive episodic turbidity currents that deposited the Kolosh successions.

8.4 Depositional environment and evolution

Grain sizes, sedimentary structures, and facies types indicate tectonosedimentary evolution of deep marine environment (Figure 3). The facies types and associations refer to slope submarine channels advanced to submarine fans in deeper margins. The facies types accompanied by sedimentary structures refer to a debris flow in the inner fan advanced to outer fan and later to basin plane pelagic/hemipelagic margins (Figures 4 and 5). Submarine fans include lobe-fans and lobe-fringe produced by advanced turbidity currents toward the basin plane. Thin-bedded successions of the lobe-fringe are products of waning and relatively dilute turbidity currents.

The field observations show that the vertical and lateral extent of thin-bedded lobe-fringe sediments are the distal equivalents in both down-current and cross-current directions of the fine-grained and thin-bedded sandstone horizons that comprise the depositional lobes of the end of the outer fan margin.

Thus, the heavily concentrated turbidity currents, which reached the outer fan are dropped to the coarsest suspended load firstly in the inner fan margins. Through successive events, sediments are formed lobs of the coarse-grained and thick-bedded sandstone bodies, which are most probably deposited in and around the channel levee in inner fans. Farther downslope, the same current became more dilute suspensions carrying the finer-grained material to accumulate as peripheral fringes around 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).

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.

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 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.

9. Conclusions

The anomalous concentrations of fresh olivine, pyroxene, and ultrabasic/basic rock fragments grains in the Kolosh sandstones were derived from wide varieties of ultrabasic/basic intrusive and extrusive igneous rocks of pre-tertiary ophiolites from Zagros thrust zone. Olivine/pyroxene-rich sandstones of the Kolosh formation are important to study the regional correlation of the ophiolite’s composition and the differences along the Neotethyan ophiolite belts of the Neotethys ocean.

Olivine and pyroxene grains were derived from varieties of rocks, such as tectonite peridotite, pyroxenite, lherzolite, heirzburgite, basalt, and dolerites, basically derived from Rayat and Hasan Beg ophiolites, the nearest to Badelyan study area. The radiolarian chert and carbonate formations with ophiolite rocks in Rayat, Choman, Galalah, and Hasan beg are suggested as sources of chert and carbonate rock fragments in the Kolosh sandstones.

The fresh olivine and pyroxenes grains are the products of subaerial and submarine erosion of the thrusted mantle and oceanic crust sheets. Intense wave action accelerated erosion on the beach and concentrate the heavy olivine and pyroxene grains, which were slumped to a deeper trench basin by the tectonically-induced turbidity currents.

Facies analysis and related sedimentary structures suggest progressive development in the Kolosh basin starting from slope marine margin in the lower part up to the basin plain in the upper part of the formation passing through the submarine fan environment.

The Neotethys foreland basin is tectonically controlled by thrusting and the Kolosh sediments are deposited in progressively deepening upward setting. According to the field observations, facies and associations and stratigraphic successions suggest four stages of tectonosedimentry development, these are slope channel, inner fan, outer fan, and hemipelagic/pelagic apron. These stages are controlled directly by tectonism and seismicity created by continuous thrusting.