Facies Analysis and Depositional Environments of the Euphrates Formation Between Fuhaimi and Al-Qaim Valleys, in Western Desert-Iraq

The study involves the Euphrates Formation in the area between Fuhaimi and AlQaim valleys in the western desert of Iraq. The Formation was divided into six informal units depending on various lithological properties. Facies analysis reveals the presence of the following microfacies, from bottom to top respectively; pelletal lime packstone, pellitoidal lime grainstone, oolitic lime grainstone, benthonic lime packstone and dolomitized lime mudstone microfacies. These microfacies, in addition to some geochemical properties were used to interpret the depositional environments, which are ranging from shelf, low water energy restricted and lagoonal environment to relatively high water energy open marine. ــــــــــــــــــــــــــــــــــــــــــــــــــــــ يف مئاقلاو يميحفلا ييداو نيب تارفلا نيوكتل ةيبيسرتلا تائيبلاو ينحسلا ليلحتلا ةيبرغلا ءارحصلا قارعلا


INTRODUCTION
The study area extends from Fuhaimi Valley East of Anah town to Al-Qaim valley (Fig. 1).Geomorphologically, the area is regarded as hilly topographic terrain forming continuous ridges of relatively low relief.Many valleys with various sizes dissected the area in NE-SW direction, oriented towards the Euphrates River.
Fuhaimi valley has a length of about (14Km) and (4Km) average width, whereas Al-Qaim valley has about (9Km) length and (2-3Km) width range, although it becomes narrow near the General Establishment of Phosphate.The two studied valleys form dendritic drainage pattern.The main objective of this work is to interpret the depositional environment of the Euphrates Formation in the studied area.

STRATIGRAPHY AND SEDIMENTATION
Very excellent work about the Euphrates Formation (Lower Miocene) in the studied area was carried out through the detailed geological mapping by Fouad et al. (1986).They delt with this formation from structural, stratigraphical, biostratigraphical, paleontological and petrographical points of view.Al-Abbasi (1994) made a sedimentological study of the Euphrates Formation at eastern Butma area northwest Iraq.He constructed a depositional model for the Euphrates Formation, where most of the microfacies are proved to be deposited in lagoonal and back reef environments.In addition of these workers, other works were performed, among them; Hopkins (1958), Al-Mubarak (1971), Ctyroky and Karim (1971) and Tyracke and Younan (1975).
Unconformable contact exists between the Euphrates Formation and the underlying Anah Formation (Oligocene) in Anah area.Euphrates Formation was deposited during the Early Miocene transgression that followed the continental erosional period during the most Late Oligocene and Early Miocene (Fouad et al., 1986).The later subdivided the formation into six informal units: conglomeritic, shelly (lower and upper), lower chalky, upper chalky, brecciated and undulated units.In addition to their description, the present work adds other physical descriptive features in Fuhaimi valley like lateral facies change and all the sedimentary structures which have been documented in the present work, (Fig. 2).
The conglomeritic unit, (1.5-5 m), thick is represented by subrounded and rounded pebbles and angular to subangular breccia fragments of older Oligocene carbonates.The fragments range in size from a few millimetres to about two centimetres in diameter.In Fuhaimi valley, the brecciated fragments are composed of light grey to grey colour, tough, fine crystalline carbonate fragments and occasionally containing coral reef clasts.
The shelly unit, (12-20 m), dark grey in colour, fine to medium crystalline, tough and fossiliferous dolomitic limestone.It shows lateral facies changes of arenaceous and finely crystalline dolostones.Cross bedding and ripple marks occur frequently.The fossils are gastropods, pelecypods, oysters and echinoids.
The lower chalky unit (16-22 m), white to pale grey, finely cross laminated, medium tough, thick bedded, fossiliferous dolostone, boring and burrowing are well known, some beds are coquina and comprised by echinoids, pelecypods and gastropods.Some oolitic beds occur as lenses.
The upper chalky unit, (15 m), pale grey, cross laminated, tough and cross bedded oolitic dolostones overlain by medium tough, thin bedded and fossiliferous dolostone.These beds are overlain by grey, fine cross laminated, tough, thickly bedded dolostone and marly dolostone interbeds with thin green marl.
The brecciated unit, (8-16.5 m) is characterized by green friable alternating with thick bedded white dolostone, marly dolostone and dolomitic limestone at the lower part, whereas the upper part is represented by alternations of pale yellowish green, thick and thin beds of marl and marly dolostones.Slumping and syndepositional folding and faulting occur occasionally.Brecciation, load cast and flow structures are also present.
The undulated unit, (0.5-5.5 m), pale grey, cross laminated, thin bedded, fossiliferous limestone with scarce stromatolites.The undulations look like dish or tepee structure.Conformable contact with the overlying Nfayil unit is observed.
Exposures of the Euphrates Formation in Al-Qaim valley (Fig. 3) reflects alternation of various arenaceous lithologic associations; arenaceous limestone, arenaceous marly limestone, arenaceous marl …etc.They have various colours ranging from grey, pale grey to yellowish grey.Many sedimentary structures which reflects supratidal zone environment have been documented, like crystal mold and relicts of original gypsum within calcite nodules, i.e. calcite after gypsum.This habit of evaporite has been considered as a diagnostic feature of hypersaline lagoonal tidal flat associations (Wood and Wolf, 1969;Phleger, 1969;Bosellini and Hardie, 1973;West, et al., 1980).Although may occurs frequently in the outer bar or shoal facies (Al-Naqib et al., 1986).Modern analogue of such feature and apart from typical association of evaporite and dolomite in the sabkha of the Arabian Gulf, evaporite crystals few millimeters in length occur in the upper intertidal zone (Leeder, 1982).Al-Hashimi and Amer (1985) stated that, the formation according to its wide distribution and relatively long range (whole Early Miocene) has been deposited under various environmental conditions, with diverse microfacies and faunal assemblages.At the early Lower Miocene, the base of the formation at Fuhaimi valley reflects typical coral reef facies, followed by restricted environment (backreef facies).Though at Al-Qaim area the formation may represent the shallower part of its depositional basin, where at Fuhaimi area, the formation appears to have been deposited under various environmental setting reflected by its numerous microfacies and various sedimentary structures.

MICROFACIES ANALYSIS
According to the classification of Dunham (1962), thirty thin sections from Fuhaimi and Al-Qaim valleys have been petrographically described in details.Five microfacies have been determined:

Pelletal lime packstone microfacies
This microfacies consists primarily of fecal pellets and the ground mass is composed of microspar and the micrite.The pellet is fine micritic grain, less than (0.1 mm) in diameter, rounded to oval in shape, black in colour and generally well sorted.
It can be interpreted that the mud feeding animals in the shallower parts produced the fecal pellets in rounded and oval shape (Beales, 1965).The fecal pellets are concentrated in the peritidal zones (quite water) (Flugel, 1982).

Pellitoidal lime grainstone microfacies
Pellitoid can be defined according to Blatt et al., (1972) as a by-product grain which was produced from micritization and rolling of the bioclasts.They are characterized by subrounded shapes and definit ornamentation of original fossils, in addition to its relatively large grain size in comparison with fecal pellet grains.
It is obvious that these grains were passed through two stages: decay and recrystallization, before they became rounded and structurless (Wilson, 1975).Among the evidence, which support this idea, in this microfacies, is the presence of micritized fossils, which is enveloped by thin alternated laminae of black colour micrite.Additionally, the presence of fossil ghosts and remnants of fossil ornamentation's on some pellitoid grains.Accordingly and in agreement with Wilson (1975) and Flugel (1982), this facies could be compared with the (SMF-17) which was deposited in the facies zone (FZ-8) (restricted platforms) where shelf and tidal flats with restricted circulation.

Oolitic lime grainstone microfacies
This microfacies consists dominantly of ooids in addition to the presence of bioclasts and some planktonic fauna.The ooides can be defined here as rounded and oval shape grain, which have less than (2mm) in diameter.The ooides in this facies is regarded as a normal individual ooides.It is noticed that its nucleous could be a fragment of fossil or dolomite grain.The nucleus is surrounded by more than one envelope, tangential or radiated or both.The groundmass of this facies consists dominantly of sparite.
Most of workers indicated that the presence of ooides must be confined to the disturbed shallow high energy water, In addition to its high calcium and carbonate enrichments (Reeckman and Friedman, 1982;Flugel, 1982).This microfacies could be compared with (SMF -15) as suggested by Flugel (1982) and Wilson (1975).It is confined to facies zone (FZ -6) (winnowed edge platform sands) which is characterized by shallow high-energy water.

Benthonic lime packstone microfacies
This microfacies consists oftenly of benthonic fauna, particularily milliolids, peneroplidae and rotalids, in addition to ostracods and fragments of algal crusts.The grains in this facies are tangential and compacted, whereas the groundmass is mainly micritic and microspare.This microfacies was affected by dolomitization.It regards as dominant facies among the studied facies.Its presence is confined to the middle part of Euphrates Formation.The abovementioned fossils refer to high diversity in its persistence within the formation.According to Brasier (1975) and Gischler and Lomando (1999) this phenomena could be used as strong clue about lagoon or back reef shallow warm zones with normal salinity environment.
On other hand, Moor (1957) found that these fauna could be found in the back reef area and within (15m) depth.Additionally, Bandy (1964) and Murray (1968) found that these fauna could live attached to the shelf marine benthonic grasses which is characterized by relatively low water energy of deposition.
In comparison with the standard microfacies model of Wilson (1975) and Flugel (1982) it can be estimated that this microfacies is a match to the (SMF-18) which is localized between (FZ-7) and  represented by the open and restricted platform respectively.

Dolomitized lime mudstone microfacies
It consists primarily of dolomitic micrite and less than (10%) skeletal bioclasts.This facies was suffered from severe dolomitization, which changes most, if not all, the primary features of the constituting grains.Dolomite crystals are characterized by rhomboidal shapes, small uniform size and homogenouse internal structure.
This microfacies is very common in the Euphrates Formation, especially in its lower parts.It could be compared with the (SMF-22) which is localized within the facies zone (FZ-8), which defines a restricted platform (Wilson. 1975;Flugel, 1982).The Euphrates Formation constitutes varied carbonates; dolostone, dolomitic limestone, limestone and marl.The major oxides represent their main components, which are present in various mineral phases; calcite, dolomite as well as insoluble residue (clay minerals and silica).

SOME GEOCHEMICAL ASPECTS
Table (1) shows the oxide concentration in the units of the Euphrates Formation.shelly, lower chalky and upper chalky units constitute CaO between (23-37)%.These concentrations are influenced by the composition of dolostone mainly.The variation in dolomitization among the above three units reflects a range of MgO concentration (13-23)%.
Al 2 O 3 , Fe 2 O 3 , Na 2 O and K 2 O represent the clay minerals contents (Weaver and Pollard, 1975).As well as, Fe 2 O 3 found as iron oxides phases and/or replacing MgO in dolomite (Scott, 1986).In spite of relatively low concentration of both aluminum and iron in the carbonates, they may be assigned for the effect of the depositional environment (Naida et al., 1971).Na 2 O may be found as a secondary mineral or adsorbed on the clay mineral surfaces (Bear, 1965).Insoluble residue represents the clay minerals and silica as fine grain quartz.
There is a similarity in the covariation of Al 2 O 3 , Fe 2 O 3 , Na 2 O, K 2 O and insoluble residue among the three units.It is clear that the concentrations have lower percentages than the brecciated and undulated units.These chemical characterization which reflect high MgO and low content of clay minerals and detritus may be related to the conditions of the depositional environments of the shelly, lower chalky and upper chalky units.They were quiet shallow environments with restricted circulation.CaO concentration in the brecciated unit is similar to the above three units while MgO is lower.All the rest oxides and insoluble residue show higher concentrations than the previous three units due to the presence of marl alternated with dolostone.The chemical composition of the brecciated unit reflects an increase in fine clastic materials.
It appears that the undulated unit is limestone, so the CaO concentration is higher, whereas MgO concentration is lower.Its insoluble residue content is relatively high which indicates higher proportion of detritus with concomitant decrease of MgO concentration.
Table (2) shows the R-mode correlation coefficients among the major oxides.CaO and MgO covary together with positive correlation due to their presence in the carbonate minerals, although they present in clay minerals but the later is very low in the studied units except the undulated unit.Other oxides (Al 2 O 3 , Fe 2 O 3 , Na 2 O, K 2 O) are correlated together positively, which reflect mostly their presence in clay minerals (Millot, 1970).It is clear that the positive correlation clustered into two groups; the first group is CaO and MgO (the carbonates), although they present in clay minerals, and the second group is the detritus (insoluble residue).

DEPOSITIONAL HISTORY
The basal thickly bedded conglomerate in the lower part of the Euphrates Formation is inherited from the erosion of various Oligocene sediments which had been redeposited due to marine transgression and the beginning of the new sedimentary cycle at the Early Miocene.Hence, the fossils content of the reworked pebbles belongs to the Oligocene formations, whereas, the fossils which had been identified within the cementing material represent the early Miocene period.
During the Early Miocene, and soon after the deposition of the basal conglomerate, the marine environment became relatively quite with warm and shallow water.Therefor, pelletal lime packstone facies was deposited in open platform zone (shelf lagoon), where the wave enargy was relatively intermediate.Then, the pelletoidal lime grainstone facies was followed reflecting relative restricted platform zone environment.The high MgO concentration and low content of clay minerals and detritus also assure this.
After that, oolitic lime grainstone facies was deposited, which indicates shallow, warm and high water energy marine environment.The fourth facies is represented by bentonic lime packston facies, which assigned to the sediments of shallow warm lagoon environment.This environment could be alternated between open platform and restricted platform respectively.The final phase of the later microfacies represented in the study area by restricted platform environment, which reflected by dolomitized lime mudstone facies.

CONCLUSION
In conclusion, the Euphrates Formation in the studied area was deposited in various depositional environment ranging from the shelf, low water energy, restricted and lagoonal environment to the relatively high water energy open marine environments.The variations are detected by both microfacies and geochemical analysis.

Fig. 1 :
Fig. 1: Location map of the studied area.

Fig. 3 :
Fig. 3: Geological section of the Euphrates Formation in Al-Qaim valley.
Twenty-four samples were analyzed chemically for major oxides in the Laboatory of Research Center for Environment and Water Resources.CaO and MgO analyzed by titration with EDTA, Al 2 O 3 and Fe 2 O 3 by colourimetery, I.R. by gravimetery and Na 2 O and K 2 O by flamephotometer.

Table 1 :
Major oxide and insoluble residue contents of the Euphrates units (range and average, wt.%).