FACIES ANALYSIS OF THE EARLY CRETACEOUS ARABIAN PLATFORM, KURDISTAN REGION, NE- IRAQ
Kamal Haji Karim and Bakhtyar Mohammed Ameen
University of Sulaimani, college of science, Department of Geology
Published in: GRMENA 3
ABSTRACT
The lithofacies of the Arabian platform are studied in four outcrop sections during Early Cretaceous in the northeastern Iraq, Kurdistan Region directly to the southwest of Zagros Main Thrust Zone. In this area the platform is mainly represented by Qamchuqa Formation (Sarvak Formation in Iran) (about 600m thick) which consist of dolostone and both reefal and lagoonal limestone successions. The prevailing lagoonal facies are miliolid–bioclast mudstones, pelloidal wackstones to packstone. The reefal facies are: Ooid packstones to grainstones, stromatolites (sheetstone) and rudists bearing lithofacies (coral pillarstone), oncoids lithofacies (ballstone), branching coral bearing lithofacies (coral bafflestone). The forereef facies are coral-bearing sandy and marly Limestone (coral floatstone), orbitolina lime wackestone and Rudstone microfacies. The most common facies are dolostone, lime mudstone and wackstone. Therefore, the environment was relatively calm and warm with more or less restriction in circulation except in limited time and space as represented by high energy facies. The environment was consisted of barrier reef with associated fore and back reef (lagoon) environment. These facies also important tectonically as delineate the most northeastern extend (or boundary) of the Arabian Platform during Early Cretaceous.
INTRODUCTION
The Early Cretaceous Arabian platform is consisted of thick succession of dolomite and limestone. This succession covers, most of Iraq including the studied area and some part of southwestern Iran and Arabian Gulf and Saudi Arabia. The main unit of this platform is Early Cretaceous Qamchuqa, Shuaiba and Mauddud formations. These units are assumed as important reservoir for oil in Middle East (Buday, 1980, Alavi, 2004, Jassim and Goff, 2006). In the studied area among these units, only Qamchuqa Formation occurs which has relatively well exposed outcrops and the description of the formation is cited by Bellen et al, (1959). According to these authors it consists of about more than 750 meters of alternation of well-bedded, grey colored, dolomite and light grey limestone or dolomitic limestone. The name of Qamchuqa Formation is derived from the name of Qamchuqa village, which is located directly to the southwest of the type section, about 45 Km northwest of Sulaimani town in the High Folded Zone. According to Furst (1970 in Buday, 1980), the Upper and the Lower Qamchuqa Formations have been renamed Mauddud and Shuaiba formations, respectively in the middle and southern part of Iraq. The same author mentioned that, in Iran, the correlative formations are Dariyan (Aptian) and the Albian part of Sarvak Formation of the East Zagros Mountains in Iran.
GEOLOGICAL SETTING
The studied area is located within Sulaimani and Arbil governorates in northeastern Iraqwhich is bounded by latitude (350 51- 12= and 360 31- 51=) N and longitude (440 45- 49= and 450 12- 25 =) E. According to (Stocklin, 1968; Buday, 1980; Buday and Jassim 1987; Mc Quarrie 2004; Jassim and Goff, 2006), the studied area constitutes a part of Western Zagros Fold-Thrust belt, directly to the southwest of the main Zagros Suture Zone. Structurally, the area is located within the High Folded and Imbricated Zones (Buday, 1980 and Buday and Jassim, 1987) where the anticlines are in northwest–southeast direction. The formation crops out mostly in High Folded along the summit and sides (limbs) of many mountains (anticlines) such as Piramagroon, Sara, Asos, Asingaran, Zhelwan, Kosrat, Qarasard, Safeen, Kewa Rash, Karokh and Makok (Fig.1). In the same direction and between these mountains there are narrow or wide subsequent (strike) valleys, most of which coincide with synclines, while few of them are developed along the axes of anticlines by erosion of their core. Moreover, the mountain chains are dissected by many large or small consequent valleys, the largest ones are those in which the Little Zab and Great Zab Rivers flow. In all studied sections (as studied by Ameen, 2008), the Qamchuqa Formation is underlain by Sarmord Formation or it’s equivalent (Fig.2 and 3). Kometan and Bekhme formations are overlying the Qamchuqa Formation in the Sulaimanyia and Arbil Governorates, respectively.
STUDIED SECTIONS
Four sections are chosen and sampled in the Sulaimani and Arbil Governorate for detailed study. The selection of these sections is based on the maximum obtainable information (facies change) and degree of the differences with that of the type section at Qamchuqa Gorge. However, the process of the selection has suffered from three constrains. The first is that the formation, in many places, form vertical erosional and fault cliffs, which cannot be sampled and even inspected. The second is that, due to high thickness, the lower boundary and even thick intervals of the lower part is not exposed in many places of studied area. The third is that the formation is effected by many thrust and reverse faults, especially in the Imbricated Zone, which most possibly have caused repetition of some intervals.
In addition to the four studied sections, the formation is inspected accurately in all available continuous and isolated outcrops. Most of the outcrops, when topography has permitted, are stepped on continuously along the dip and strike to see vertical and lateral facies changes. In opposite to other formations, the facies change is so rapid, in some places, that when stepping on, several different facies could be recognized in a distance of about 5kms.
The sampled sections are:
1-Yakhsamar Village Section
Located at the intersection of latitude and longitude 350 51- 12= N, 450 12- 25= E, directly to the northeast of Yakhsamar Village in Jafayati valley at 25 km to the east of Dokan Town.
2-Halladin Village Section
Located at the intersection of latitude and longitude 360 31- 51= N, 440 45- 5149= E about 4km to the northeast of Halladin Village in Jafayati valley at 20 km to the east of Dokan Town. The section consists of southwestern sides of Babo Mountain.
3-Type Section (Qamchuqa Gorge Section)
Located at the intersection of latitude and longitude 350 53- 53= N, 450 00- 53= E at about 10 kms to the southeast of Dokan Town, directly to the east of the ruins of Qamchuqa village, which is now transferred to near Dokan town.
4-Kewa Rash Section
This section is located directly to the northwest of Ranyia Town and about 10km to the west of Darband Ranyia Gorge. It begins from the base and ends at the top of the scarp slope of the mountain at the intersection of latitude 360 15- 37= and longitude 440 54- 38=.
Figure1: Geological map of northern Iraq (modified from Sissakian, 2000), showing location of the studied section and general stratigraphic column of the area (at the right side).
Lithofacies
The present study has utilized the classification of Dunham (1962) and its modified version by Embry and Klovan (1971) for studying the carbonate rocks of the Arabian Plaform (as represented by Qamchuqa Formation in the studied area). These authors noticed that the Dunham classification did not accommodated organic reefal limestone, and there was no size-scale for the different components. The modification was a very important step towards the simplification and generalization of the classification of all carbonate rocks in a single one. This is because the terms rudstone, bafflestone, bindstone and floatstone are most useful and specific in the description of reefal carbonates. Dunham’s classification has presented different facies types. The each facies indicates a specific environment or subenvironment. In this study, another classification is used for reefal carbonate rocks, it is that of Insalaco (1998). This classification is very useful for classification of reefal limestone, especially that of Qamchuqa Formation. It is depending on shape of growth, which is purely descriptive, not interpretative as that of Dunham.
Figure 2: The studied outcrop sections which consist of four limestone (L) and dolostone(D) successions(intervals).
The styles of growth fabrics (dominated by in situ growth of metazoan skeletons) are defined by Insalaco (1998), according to the dominate growths, from which the bulk of fabric constructed, he recognized the following facies or limestone structures (Table 1):
1. Platestone: dominated by platy to tabular skeletons such as tabular stromatolite (Fig.14).
2. Sheetstone: dominated by sheet like skeletons such as stromatolite and slender recumbent rudists, which are found in their living position (Fig.10).
3. Domestone: dominated by domal and irregular massive skeletons, such as domal stromatolite and solitary coral single colony (Fig.15).
4. Pillarstone: dominated by a vertical component of growth and relatively restricted lateral growth (branching, rod and tabular solitary forms) such as branching coral (Fig.15) and in situ growth of radiolitid rudists (Fig.10).
5. Mixstone: not dominated by one growth form and includes a variety of growth forms, such as coexistence of pillarstone and sheetstone as seen in the occurrence of columnar and lamellar stromatolite together (Figs.14).
In the Qamchuqa Formation, the above mentioned reefal limestone (or facies) exists, but there is a very important type of reefal limestone, which is not mentioned in the classification of Insalaco (1998). This type of limestone is oncoidal limestone (or oncolite).
Therefore, another type is added to the above-mentioned classification, as the sixth type under the name of Ballstone.
6. Ballstone: Dominated by ball-type or globular structures such as oncolite (Figs.13).
Table (1) Comparison of Dunham (1962) classification (modified by Emery and Klovan, 1971) with that of Insalaco (1998).
Dolomitized lithofacies
This facies consists mainly of dolostone, which contains both coarse and fine grain dolomites successions. It is commonest facies in the studied area which constituent about 55% of the total thickness of the formation. This facies is more common in the middle and upper parts (D2, D3 and D4) (Ameen, 2008). In most places, they are fractured and even brecciated; the fractures are filled with white coarse crystalline dolomite. Under microscopes, they show high porosity with moldic, vugy and intra crystals type, which acts as an excellent hydrocarbon bearing beds (reservoirs). The origin and depositional environment of this facies is most problematic in the study of Qamchuqa Formation. This is because nearly all original components are destroyed by pervasive dolomitization, therefore tens of outcrop sections are inspected to find if some relict of the original component can be seen.
In some of these sections, the following four original components are found. The first one is a large elongated gastropods (3cm wide and 6cm long), which are found north of Zewe village on the Piramagroon mountain, with some bioclastics. The second occurrence of regular and continuous millimeteric laminae in some thick succession south of Sargelo and Smaqully gorges in addition to Zhelwan and Piramagroon mountains. In these localities, the lamination consists of alternation of white coarse crystalline dolomite and grey or dark brown dolomite (Fig.3). The third original found component is wavy and patchy laminations with ragged boundary that are recorded north of Marga village and on the southeastern sides of Piramagroon and Kewa Rash mountains. These laminations are not continuous (extend laterally no more 40cm) and terminated from left and right sides with ragged boundary. These types are similar to those found in the lower limestone succession (L1 or L3), except for their large sizes, as compared to those found in the limestone units. The fourth type is dolostone with relict of coarse bioclastic, which recorded in Kewa Rash Section and Zhelwan anticline (5kms northeast of Halladin section).
From the aforementioned four points, it could be deduced that the most of the dolomite successions are not primary (depositional) in origin, but most possibly altered by early or late digenetic processes (pervasive dolomitization). It is possible that the dolomitization is assisted by high Mg contents of the basin of the Qamchuqa Formation during deposition due to high temperature environment of the upper part (see depositional environment in Ameen, 2008). In this concern, Millman and Muller (1973) and Sartori (1974 in Reading p.360) mentioned that High-Mg calcite was formed during high-elevated temperature. Consequently, the micrite and most allochems are transformed to medium or fine rhombs of dolomite (Fig, 3). The low diversity of fossil and high dolomite content of this facies suggests some degrees of severity in the environment (hot and dry climate) in which opportunist animals survived. This environment was most probably consisted of sand flat behind the reef. After deposition, these sediments are covered by high mg lagoonal water due to progradation of lagoonal sediments over the reefal limestone. By this progradation the reefal and lagoonal environments have changed location geographically (specially) or vertically (temporary) with time. Consequently, the porous reefal and sand flat limestones are overlain by lagoonal high Mg water. Then, late dolomitization occurred by percolation of lagoonal water in the rocks.
In this basin the components were in direct contact with Mg ions after deposition for early dolomitization, which aided by reworking of the components by bioturbating organisms. The dolomitization was supported by addition of Mg ions from the seawater during the reworking of sediments. It is possible that bioturbations, which are very common, were exposing the sediments several times to seawater during which the introduction of Mg+2 ions were occurring for dolomitization. This process is mentioned by Allen and Allen(1993, p.325), they stated that the burrowing metazoans churn up the sediment to a depth of 5 to 30 cm, allowing the penetration of oxygen and sulphates into the sediment column, thus promoting bacterial degradation. In our opinion, this is also true for adding Mg ions into the sediments. The limestones Units (L1, L2, L3 and L4) also contain localized dolomite as irregular spots, these patches are nothing except the burrows of organism, which attached by selective dolomitization (see Ameen, 2008).
Figure 3: 1: Alternation of dolostone and spary white dolomite a) laminae in the D2 of the Yakhsamar section. 2) Spreading out of dolomite crystals (selective dolomitization) from the burrows in the Kewa Rash section. 3 and 4) The contact between a borrow and host rock at the left showing selective dolomitization with sutured (3) and straight contact ( 4), Kewa Rash section . 15X, N.L.
Figure 4: Orbitulina lime wackstone which can be observed in the L4 of the Piramagron and Qarasard anticlines.
Marly limestone lithofacies
This facies is dominant in the top of the lower, middle and upper parts of the sections (Figs.2), in addition to transition zone between Qamchuqa and Sarmord formations. This facies not make up more than 5% of the total thickness of the sections. This facies is interbedded occasionally with highly fossiliferous limestone beds, which contain rudist whole skeletons, pelecypods bioclasts and orbitoid forams. It is possible that they are formed during the sea level rise (Local drowning of the Arabian platform) in the lagoon or on the reef when high amount of erosion is happened by stormy episode, and the product is deposited in more oxygenated and less saline lagoon in which organisms survived.
Lime mudstone and wackstone microfacies
This facies is very most common facies after the dolostone facies and makes up more than 60% of the limestone units and about 30% of the total thickness of the formation. The matrix consists of dark brown micrite with some silt and sand sized bioclasts of algae, mollusk and few benthonic forams such as miliolids. This microfacies are common, especially in the lower part of the sections, in the L1. L2. In this facies, the grains of wackestone usually range between (10–15) percent in a micritic matrix. Skeletal grains include; bryozoans, echinoderms and algae. Non-skeletal grains include intraclasts and pelloids (Fig.7).
Orbitolina Lime Wackestone microfacies
This microfacies is characterized by a high content of large benthonic foraminifera (orbitolina), which is found with light color, slightly argillaceous mudstone (marly limestone) (Fig.4) and associated with bioclasts pelecypod. This submicrofacies is found in some beds in the upper part of all sections (L4), in addition to transition zone of the type section(Fig.4 and 5.2).This submicrofacies is a typical for open marine of circulation conditions (Wilson,1975).
Orbitolina has been described from the undolomitized horizons, in the type locality of the formation by Dunnington (1958), and has been used as a stratigraphic tool. At the same location, Qaradaghly (2007) recorded this fossil in the upper part of Sarmord Formation also which is assumed as transition zone in this study. The Orbitolina bed is present both in the neritic zone and in the fore slope sediments (Henson, 1940 unpublished report). The same author mentioned that Orbitolina has been described from Iran, Oman, Sudia Arabia, Syria and Lebanon. Orbitolina species have been described from many localities of the Middle East and Tethys Sea and have been considered an important fossil in the Early Cretaceous deposits in the region. Al-Sharhan, (1995) has recorded orbitolina-rich lime mudstone and wackstone in the Middle Cretaceous rudist bearing carbonate in Arabian Gulf. These facies (rocks) are interbedded with thin partings and beds of buff to gray shale. According to Pittet, et al., (2002) discoidal orbitolinids and calcareous algae were deposited during early transgression.
Chert Nodules- bearing Lithofacies
The cherty facies consists of decimeter-scale beds, which contain large chert nodules, distributed on the bedding planes (Fig.5). This facies occurs in the lower and middle parts of Qamchuqa Formation in L1, L 2 and L3 (Fig. 2). Philip et al. (1995) have observed, in this facies, presence of sponge spicules, echinoids, red algae, annelids, and small benthic foraminifera. In this study, the chert nodules found in limestone units which has characteristics of lagoon and forereef, therefore their development most possibly attributed to the episode of slow sedimentation during sea level rise (local drowning of the Arabian platform) with a paleo water depth just above or around storm wave base.
Figure 5: 1: Chert nodules on the top of a limestone bed in the L1 of the Qamchuqa Gorge section. 2: Orbitolina wackstone submicrofacies, Orbitolina (a), Piramagroon Mountain, X20, P.L. 4).
Figure 6:1: Bioclast limemudstone microfacies, cyclaminal (black arrow), bioclast (white arrow) Kewa Rash Section. X40, P.L. 2: Bioclast lime mudstone microfacies.
Figure: 7: 1: Peloidal Packstone – Grainstone microfacies, Halladin, Section, X20, P.L. 2: Peloidal lime packstone microfacies, peloids (P). Kewa Rash section, X20, P.L.
Peloidal packstone-grainstone microfacies
Peloids are structureless oval or spherical grains (0.2—2mm), but may be irregular due to crystallization and dolomitization of micritic composition, which constitute major components of this facies. In this facies some intraclasts, few milliolids and echinoderms occur too (Fig.7.1). The presence of this facies is limited within the lower part of the sections in the Qamchuqa Formation, and in the transition zone with Sarmord Formation in Halladin section (Fig.2). The peloidal packstone microfacies may be deposited in shallow warm waters with moderate circulation (Wilson, 1975) possibly in lagoonal environment.
Oolitic Packstone–Grainstone microfacies
The oolids in this study are characterized by presence of one or few thin laminae surrounded large nuclei tagentially, which belongs to superficial types and contains some composite ooids that consist of more than one ooids (Fig.9.2), which are encircled by the laminae. This facies is located in the top of L1 in the Halladin section; it has thickness of about 5m and is associated with pale green sandy marl (Fig.8). This facies, laterally changes to pale yellow, crystallized and slightly dolomitized limestone. Obed and El-Hiyari (1986) have found in Cenomanian gypsum bearing horizon in Jordan ooids which are nearly similar to those of Qamchuqa Formation. The occurrence of superfacial ooids may provide a distinct delineation on paleoenvironmental condition, which is closely associated with relatively low-energy environments.
Oolids are smaller than oncolites (0.5-2mm) and the lamina are formed by chemical precipitation of calcium carbonate. However, it is possible to be formed by process of organic accretion as mentioned by Tucker (1991) about the possibility of development of oolids by organic precipitation and he called them micro-oncoids. Blomeier and Reijmer (1999) found ooids similar to those of the present study; they called them tagential–structure ooids.
Rudist-bearing lithofacies (Pillar and Sheetstone)
This facies consists whole or part of Radiolitids rudist skeletons of medium size in the Qamchuqa Formation (Fig.10). It also contains other faunas such as gastropods and pelecypods. The rudists of the Qamchuqa Formation are small and thin shelled as compared to those of Aqra Formation as shown by Karim, 2006; Karim and Surdashy (2006). In Qamchuqa Formation, the rudists are found in their living position, erected normal to bedding plane and some of them slightly inclined, as those found directly to the northeast of Yakhsamar village on the paved road. Castro et al. (2001) have used the term rudist float or rudstone for this facies. Al-Sharhan (1995) has recorded rudist conglomeritic floatstone in the Mishrif Formation with lime mudstone and wackstone in the Middle Cretaceous rudist bearing carbonate in Arabian Gulf. These facies (rocks) are interbedded with thin partings and beds of buff to gray shale. Polmar et al. (2005) found rudist pillarstone of slender rudist of Hippuritid types.
Rudist association is very similar on a global scale until the Aptian (Skelton, 1982). According to Coogan (1977), Radiolitids (Fig.10) tolerate a wider range of conditions from back-reef to shelf margin to fore-slope. Pomar et al. (2004) mentioned that the rudist buildups consist of a rudist and coral belt at the platform margin, passing landward into a slender hippuritid lithosome, locally overlain by a bioclastic blanket that passes basinward, into bioclastic apron like clinobeds and into fine-grained packstone-wackestone. Rudist association is very similar on a global scale until the Aptian (Skelton, 1982). According to Coogan (1977), Radiolitids rudist (Fig.10 A nab B) tolerate a wider range of conditions from back-reef to shelf margin to fore-slope. Pomar et al. (2004) mentioned that the rudist buildups consist of a rudist and coral belt at the platform margin, passing landward into a slender hippuritid lithosome, locally overlain by a bioclastic blanket that passes basinward, into bioclastic apron like clinobeds and into fine-grained packstone-wackestone. Rudists are also described in deep wells of Kirkuk and Bai Hassan fields (Al Shakry, 1977). The Cretaceous Rudist bearing carbonates of the Arabian Gulf included the main type of the rudist in Shuaiba (Early Cretaceous) Formation mainly of caprinids, with a lesser number of caprotinids, monopleurids (Al-Sharhan, 1995, P.531).
Figure 8: Halladin section (Babo Mountain) showing the interval that contains ooid packstone–grainstone.
Molusks bioclast-bearing lithofacies (Gstropo-pelecypod coquina)
This facies exists in all sections and in limestone and some dolostone members but is more common in the north of the studied area (area of fore reef) where Qamchuqa Formation is change to Balambo Formation. The lithofacies exists either as completely skeleton or as worn bioclasts, which give in some place, brown color to the beds related to this lithofacies (Fig. 11 and 12.3). The elongate shells appear as parallelly arranged to direction of reworking current. Obed and El-Hiyari (1986) have found this facies in Cenomanian gypsum, bearing horizon and called it Coquina, and assigned the related environment as high energy. This facies is commonly highly bioturbated and contains patches of dolomite.
Stromatolite Bindstone Lithofacies
This type of facies contains two types of structures formed by accretion of biofilm and binding processes. These structures, in the Qamchuqa Formation, are stratiform (flat, wavy and vertical) and circular stromatolite: oncoids) (Fig.13).This facies acts as reef material binder and recorded mostly in the Halladin and rarely in the Qamchuqa sections.
Coral-bearing Sandy and Marly Limestone Facies
This facies is found in the transition zone between Qamchuqa and Sarmord formations in the Qamchuqa Gorge section and north of Bardashan village (Fig.14.1). This facies consists of bioturbated sandy marl and marly limestone, which contain in situ (in the life position) solitary coral colony of scleractinian corals (Genus Isastrea). The sizes of these colonies range from (2–7) cm with well-preserved micro-and macro-structures which shows radial oriented septae and central clumella (Fig.15). In addition to sandy marl bed, the transition boundary contains thick competent bioclastic and bioturbated limestone beds (0.2–3 m thick), with make well-exposed outcrops, while the marl and marly limestone that contain the coral are incompetent and commonly covered. Blomeier and Reijemer (1999) found the bioclasts of this type of coral in slope environment and they mentioned that they derived from platform margin. From the stratigraphic position and its overlying reefal limestone, it can be assumed that these facies is ascribed to the stage of stabilization of reef community. Most possibly, this facies was part of the fore reef facies, when the environment was not so much stormy or turbid to prevent survival of opportunist corals. From this facies it can be inferred that the Qamchuqa Formation was prograding northeastwards on the Sarmord Formation, during Valanginain.
Figure 9: 1: Wackstone Submicrofacies, Milliolid (arrow), Textularia (t)Yakhsamar sectionX40, P. L. 2: Oolitic Packstone – Grainstone Submicrofacies, Halladin section, X20, P.L.
Figure 10: A: Longitudinal sections of Radiotids rudist arranged vertically on bedding plane in their life position (pillarstone) Yakhsamar. B) Longitudinal and cross sections Radiolitids rudist, in Yakhsamar section.
Figure 11: Different species of gastropods with their bioclast in the lower part of Halladin section.
Figure 12: 1: Accumulation of shell of large bivalve replaced by calcite in Yakhsamar section. a: weathered surface, b: fresh surface. 2: Burrowing surface (a), high content of large bivalve fossils (b). Yakhsamar section. 3: Accumulation of large pelecypod shells on the bed surface (a). Qamchuqa Gorge section. 4: Large pelecypods above the burrowing surface. Challawa area.
Figure 13: 1: Large oncoids (arrows) inter space between the coral branching highly Brunched, Halladin section. 2: Stratiform stromatolite, Qamchuqa Gorge section. 3: Large oncoid (arrow) in lime wackestone, Halladin section. 4: Stratiform stromatolite, Halladin section, X40, P.P. part of Qamchuqa Formation, in the southwestern limb of Piramagroon anticline.
Branching coral lithofacies (Bafflestone)
This facies contains large and small branching roguse coral colonies, which can be seen in outcrops and under microscopes. The length of some branches is more than one meter, as it is found in the Halladin section (Babo mountain) in the middle of the lower part of the formation (L1) (Fig.14.2, 3and4). Unlike the solitary coral, the structure of this type is mainly destroyed by recrystallization; consequently, the fine texture is replaced by spary calcite. In literature, this facies is called baffle stone ( Emery and Klovan, 1971), because it has dendritic form and acts as sediment collector from the nutrient bearing current and waves by filtering and trapping sediments. The spaces between the branches are mainly filled by micrite (mudstone or wackstone) with some bioclast and rare oncoids. Walker and James (1992) have put this facies in the colonization stage of the reef structure. The presence of corals indicates normal marine salinity (Riding and Tomas, 2006).
Figure 14: 1: Solitary coral colonies (Isasteria or Hexagonaria), Bardashan village, and Raniya area. 2: Cross section of branching corals under stereoscope microscope, X10, N.L. 3: Base of brunched coral colony, Halladin section. 4) Coral highly brunched, Halladin section.
Rudstone Facies (Reef talus facies)
This facies is introduced into Dunham (1962) classification by Emery and Klovan (1971), it consists of self-supporting large allochems (more than 2mm thick) bounded by mudstone (micrite) (Fig.15A). This facies can be seen in all sections, more common to the north and northeast of the Halladin and Kewa Rash sections, which coincide with the lateral transition zone between Qamchuqa and Balambo formations (fore-reef area) in the extreme northern extent of the formation in the Zhilwan and Karogh anticlines. This facies mostly consists of reworked intraclasts (lithoclasts of limestone) and bioclasts of coral, bryozoans, gastropod and pelecypods or clasts of micritic limestone. In some cases, this facies grades to conglomeratic grainstone, as milky and spotty appearance and cemented by spary calcite or dolomite.
According to Wilson (1975), this facies is deposited in forereef environment where the strong wave and current action are prevalent. According to the above author, the allochems of this facies must be derived from the reef, but many authors have included the non-reefal allochems in this facies, such as Al-Sadooni and Al-Sharhan, (2003), they have assigned the orbitolina bearing limestone as orbitolina rudstone. Kenter et al. (2005) found boundstone breccia in the forereef area, which formed gravity.
Floatstone Facies
This facies is rare and consists of grains larger than sand size, which are floated or embedded in fine matrix of lime mudstone. The grains are generally derived from the fragmentation of reef builder skeletons, such as coral or rudist (Fig.15B). The oncoids may also make up this type of facies, when showing some transportation and floating in fine matrix of bacteria or algal origins.
In all sections there are beds and horizons rich in large bioclasts, of gravels and granules sizes, of rudist, pelecypods embedded in the fine grain grey matrix and form gravely wackstones and packstones. If these rocks included in floatstone facies, then they become common, but the problem is that it is not known whether they are derived from the reef or from fossiliferous beds (biolithosome). Therefore, the very coarse bioclastic limestones are discussed under the name of other facies such as peleypods, gastropod wackstone and packstone facies.
Figure15: A: Fragments of coral and rudists in fine matrix (rudstone) D3 of Piramagroon anticline. B: Spars fragments of coral embedded in fine matrix (floatstone) L1 of Qamchuqa section.
Environment of Deposition
Although the studied area is makes up only less than 10% of the geographic distribution of the formation but the studied facies are mostly new and gives strong evidences for the depositional environment of the whole Arabian Platform. According to these facies the rock body of the platform is deposited in barrier reef with associated with back reef and fore reef which alternated several time during Early Cretaceous (fig. 16 and 17). The environment was relatively calm and warm with some current restriction (see Ameen, 2008) for more detail l about this issue).
Figure16: Depositional model for Arabian platform during Early Cretaceous (Green: Qamchuqa Formation (Patform limestone and dolostone), Yellow: Balambo Formation (Pelagite), White: Nahr Omer Formation (mainly siliclastics).
Figure17: Position different depositional environments of Qamchuqa Formation, during Aptian( (Amen, 2008)
CONCLUSIONS
1-The low energy (lagoonal) facies of the Early Cretaceous Arabian platform are mliolid–bioclast mudstones, stromatolite sheetstone (or boundstone).
2- The high energy reefal facies includes ooid packstones to grainstone and pelloidal packstone to grainstone, Rudstone microfacies, coral pillarstone and floatstone.
3- Oncoids lithofacies (ballstone), rudist pillarstone, orbitulina bearing lithofacies, chert bearing lithofacies and peloid wackstone to packstone are deposited in deposited in intermediate energy environment which typified by more or less agitation.
4-The most abundant lithofacies is dolostone which most possible deposited on the sand flat directly behind the reef body.
5-These facies indicates the most northwestern boundary of the Arabian Platform during Early Cretaceous.
6- The environment was consisted of barrier reef with associated fore and back reef (lagoon) environment which was relatively cam and warm with more or less restriction in circulation.
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