Bedded Gypsum in the Gercus Formation (Early-Middle Eocene) in Dohuk area

Geology of bedded Gypsum in the Gercus Formation (Early-Middle Eocene) in Dohuk area, Kurdistan Region, Northern Iraq.

Mushir Mustafa Qadir Baziany (mushirmustafa@geologist.com)

Department of Geology, College of Science, University of Sulaimani

Abstract
The Gercus Formation (Kashkan Formation in Iran) (Early Middle Eocene) crops out in north and northeastern Iraq within High Folded Zone. In the field, it is exposed as red siliciclastics successions of claystone and sandstone with occasional conglomerate, carbonate and evaporites lenses or tongues. In the studied area two main type of evaporites rocks are found. The first one is chemically deposited gypsum (or anhydrire) which has the granular alabasterine and fibrous satinspar texture, while the second type consists of fragmented and reworked gypsum (detrital gypsum), which is called as gypsarenite (gypsum sand-sized grains). The latter one is formed as an intraformational reworking of the previous deposited chemical gypsum by intense weathering in the cold and dry climate. The gypsarenite contain also angular detrital lignite (black bituminous).
In this area, the gypsum beds occur as centimeter thick nodular or laminated beds that interbedded with either red claystone or marl. These beds are arranged in three and nine parasequences in the upper and middle part of the formation in Dohuk dam and Bakrman village respectively. The cyclicity of lithologic signals of the gypsum beds are discussed in the view of the sea level change and systems tract subdivision. The field study showed that the position of each lithology on the sea level curve was unexpected result as concerned to the previous studies which, indicated the evaporites, carbonates (or marl) and clastics as low, transgressive and highstand system tract respectively. In contrary to this, the present study showed that the position of these lithologies (on the sea level curve) in a highstand, transgressive and lowstand systems tract respectively.
Accordingly, cycles begin with red claystone and gypsarenite as minor lowstand systems tract (or minor regression of sea level) and ends with evaporite as highstand systems tract (or transgression of sea level). These positions of the lithologies in the depositional system are based on the boundary conditions between the lithologies (beds). The regression and transgression (sea level fluctuations) are argued in term of relation with Milankovitch band of astronomical climate change which consists of eccentricity and precision of earth orbits around the sun and itself. During high eccentricity and precession orbits the sea level rises and the earth climate became warmer and both influx of seawater to the semi-coastal lake and evaporation increased, consequently evaporites are deposited in the peripheral lakes. But during combination of the low orbits the sea level fall occurred which accompanied with decrease of sea water influx and evaporation. This associated with dilution of the lake by fresh water from source areas, which deposited red claystone or gypsarenite. The construction and destruction interference of these orbital cycles generated many complete (ideal) and incomplete sedimentary cycles in the outcrop section of the basin periphery.

Introduction
The studied area is located within Dohuk Governorate, in the High Folded Zone in northern Iraq (Fig.1). The Gercus Formation was first described in southeastern Turkey in the Gercüs region by T. H. Maxson in 1936 (in Bellen et al., 1959). Gercus Formation is a Middle Eocene unit on the basis of the stratigraphic position which crops out in the High Folded Zone in northern Iraq. It stretches as narrow northwest-southeast belt from Dohuk to Darbandikhan area (Buday, 1980; Buday and Jassim, 1987; Jassim and Goff, 2006). The present study aimed to study the formation in Dohuk area where it contains bedded gypsums. The study concerned mainly with the relations between the three main lithologic constituents (clastics, carbonate and evaporite) as concerned to environment and sea level changes.
The Formation mainly consists of alternation of clastic rocks of claystone, sandstone, marl and calcareous shale with occurrence of conglomerate. Lenticles of gypsum, especially near the top. Rare lignite in sandstone near base, rock salt occurs sporadically (Bellen et al., 1959; Jassim et al., 1984). Bolton (1958) described Gercus Formation from Halabja area as Lower Eocene at base and Middle Eocene at top of the formation. Jassim et al (1975) indicated that block subsidence controlled sedimentation of Gercus Formation in Darbandikhan area. Al-Rawi (1980) studied the petrology and sedimentology of Gercus Formation in Shaqlawa and Darbandikhan areas. Al-Rawi (1983) studied the origin of red pigment and he mentioned that the Gercus Formation in northeastern Iraq consist a fluvial sequence of associated red and drab beds deposited under an arid to semi-arid climate. Basi (1984) studied the petrography of Gercus Formation from the Shikhan-Sersang area, near Swaratuka town and he mentioned that the formation consists of red claystone, siltstone, conglomerate, marls and limestone which deposited in shallow marine environment of relatively higher salinity. Al-Qayim and Al-Shaibani (1991) suggested that sediments in Gercus Formation are deposited in clastic dominated tidal flat. Ameen (1998) divided the Gercus Formation on the basis of main lithological distribution into three parts, lower, middle and upper parts. Tekin (2001) mentioned occurrence of laminated gypsum and celenite in a shallow inner-lagoonal environment in the Middle-Late Eocene Bozbel Formation in east-central Turkey. Al-Barzinjy (2005) combined the formation with the unit three (conglomerate) of the Red Bed Series and inferred that both deposited in one foreland basin in proximal and distal area of the basin respectively. Ameen (2006) studied the sequence stratigraphy of the Gercus Formation in Sulaimaniya area, NE. Iraq, and he mentioned that the whole formation consist of major lowstands system tract within stratigraphic record of Tertiary. Jassim and Goff (2006) mentioned that the Gercus Formation is a typical red molasse sequence derived from uplifted areas in the northern and northeastern.
In the peripheral area the share of sandstone, red claystone and limestone increase in expense of evaporites and green marls. The present study is concerned with the geology of lensoidal gypsum beds in the upper and middle part of Gercus Formation in Dohuk area. The study mainly depends on field observation in addition to thin section. This includes recording of the lateral and vertical lithologic changes and nature of the boundary between beds.

Geological Setting
The studied area is located within Dohuk Governorate, near the Dohuk dam with (GPS reading 36o 52- 43= N and 42o 59- 79= E) and extend to Bakirman Village at the intersection (36o 53- 52= N and 43o 40- 02= E) in northern Iraq (Fig.1). Two section are sampled both of which is located within the High Folded Zone to the south of North-Thrust Zone, in northwestern part of Zagros Fold-Thrust Belt. The first one selected at southeastern plunge of Chiayi Spi anticline which plunged near the Dohuk dam while the second is exposed at the northwestern plunge of Sharmin anticline. The Mid-Late Eocene the Gercus Formation was deposited in a strongly subsiding basin to the SW of an emergent uplift during the final phase of subduction and closure of the remnant Neo-Tethys ocean and in a relatively broad trough (foredeep) along the northeast margin of the Middle Eocene basin and it derived from uplifted areas in N and NE (Jassim and Goff, 2006). While Barzinjy (2005) assigned it as equivalent of unit three of Red Bed Series and deposited as coastal and fluvial sediments of the Zagros Foreland Basin.
The Gercus Formation comprises a mixed clastic, carbonate and evaporites sequences in the studied area especially at the upper and middle, but in general it consists of upwards fining cyclothems of carbonate-rich sandstone, siltstone, marls and conglomerates together with a few thin micrite carbonate beds and a lens of gypsum. It occurs along a relatively narrow NW-SE trending belt that extends from eastern Iran and extends to northwest wards into southeastern Turkey through northeast and northern Iraq. In the studied area the lower contact of the formation is not appear because of plunges into the dam. The upper contact of the formation is conformable by nearly 30m of a thick sequence of gypsum, gypsiferous marl and marl beds gradationally changes to thick limestone beds of overlain Pila Spi Formation.

Stratigraphy and sedimentology
In the studied area the upper part of the formation consists of alternation of red claystone (or gypsarenite), white marl (occasionally gypsiferous) and gypsum (or anhydrite). In general, the alternation of gypsum and red claystone are represented by both lamination and bedding, the thickness of lamina ranges from 2mm to 1cm and beds is reach 20cm (Fig.2 and 3). The gypsum beds are occasionally nodular (Fig.2C) and in some place contain pockets of small (2-5cm) convex downward lens and streaks of red claystone (Fig.2A). The gypsum beds are accompanied in some place with the secondary stainspar gypsum (fibrous gypsum) (Fig.2B and 4C). The most surprising and important event during this study is finding, for the first time in the Iraq, both detrital gypsum (as gypsarenite) and detrital lignite at the base of the upper part of the formation which alternate as lamine together and forming a bed about 1.3m thick (Fig.2 and 4).


Fig.1: Location map of the studied area showing outcrops distribution of the Gercus Formation(Google Earth, 2008).

Bates and Jackson (1980) defined gypsarenite as a sandstone composed of discrete, wind-drifted particles of gypsum. Robertson (1998) mentioned that it is composed of coarse gypsum together with sand-sized detrital grains. Paz and Rossetti (2006) mentioned that it consists of gypsum that occurs as clasts ranging from 1mm to 1cm in diameter and it is intraformational reworking of previously deposited evaporites. Orti, et al. (2007) mentioned that the gypsarenite derive from the synsedimentary erosion and resedimentation of the gypsiferous units. On the basis of the above citations, it is clear that before the deposition of the gypsum of the present study there was another deposited gypsum beds (These beds are called old gypsum in the present study) from which the gypsum clasts are derived by erosion and transported to the present final location and deposited as gypsarenite. It is possible that the old gypsum was deposited during high sea level (highstand systems tract). After sea level fall, the gypsum was exposed and eroded as lowstand sediments. The gypsarenite contain cross lamination which shows southward paleocurrent direction. The petrographic study of this gypsarenite bed shows that the grains have subangular-subrounded shape (Fig.4A and B). The gysarenite contain black clasts of bitumen which concentrated in certain lamina and alternate with gypsarenite laminae. The bitumen clasts are also eroded from the source area.
The thickness of the formation is very variable due to basin configuration which was relatively deep in the central part and shallow at the peripheral area. According to Buday (1980) the thickness of the formation in Dohuk area amount 850m, while Bellen et al. (1959) recorded 838m for the formation at the type section. The previous author mentioned that this thickness decreasing towards the southeast, near the Iranian border on the Diyala (Sirwan) River, it seldom reaches 100m. In the studied area the thickness of selected section for this study is nearly 85 m.
In the basin periphery at Dohuk area the strata of the formation are onlapping unconformably on the Kolosh Formation and overlaying by Pila Spi Formation, but a lens of gypsum separates the tow formations (Bellen et al., 1959). Jassim and Goff (2006) mentioned that in Dohuk area the Gercus Formation overlies the Lower Eocene Khurmala Formation and interfingers with the Avanah Formation and overlain by the Pila Spi Formation. In the studied area the lower contact of the formation is not appear because of plunges into the Dohuk Lake and dam. The upper contact of the formation is conformable by nearly 30m of a thick sequence of gypsum, gypsiferous marl and marl beds gradationally changes to thick limestone beds of overlain Pila Spi Formation (Fig.5).


Fig.2: Left) Gypsum bed, contain pocket of lensoidal red claystone (concave downward), at the base composed of coarse lamina and finely laminated at the top, which underlain and overlain by sharp contact with red claystone. B) Thick gypsarenite (or brecciated gypsum) bed which contain lamina of black bituminous and above this bed have fibrous secondary gypsum (stain spar). C) Nodular gypsum formed by diagenesis due to spontaneous deposition of gypsum and red claystone (the red claystone appear as a streak (irregular line).

Signal of cyclicity and ideal cycle
The signals of cyclicity are very strong and clear in the Gercus Formation especially in the studied area. This area is constituent the proximal (or periphery) area of the basin of the formation during Middle Eocene. The signals consist of regular repetition of the packages of lithologies for several times in outcrop section. Each package consists of red claystone (or gypsarenite), marl and gypsum (Fig.3 and 6). Dohuk outcrop section has the thickness of 85m which end with the thick limestone beds of the Pila Spi Formation, while in Bakrman village outcrop section has 100m thickness which in the middle to upper part of the formation.
Al-Rawi (1980 and 1983) mentioned that 23 and 22 cycles are exposed in the Gercus Formation in Derbandikhan and Shaqlawa area respectively, and the upper part of the cycles consists of marl (limestone) and siltstone (sandstone). Karim (1988), Karim and Al-Rawi (1992) mentioned that the cycles of the Lower Fars consist of sandstone, green marl with laminated gypsum or anhydrite from the bottom to the top respectively. The cycle mentioned by them is ideal cycle which belongs to upper part of Fatha Formation in the Mosul area. Ameen (2007, inpress) mentioned that the ideal cycle of the Lower Fars Formation consist of red claystone (or sandstone), green marl and gypsum (or limestone) in the Sulaimanyia area.
In the present study the ideal cycle of the Gercus Formation nearly same as later author, means the ideal cycle of the upper and middle part of the Gercus Formation consist of red claystone (or gypsarenite), marl (or gypsiferous marl) and gypsum(Fig.7). This ideal cycle can be seen in many places but in most case one can see many incomplete cycles which only consist of two lithologies such as red claystone (or gypsarenite)- marl (white marl), marl-gypsum and red claystone (or gypsarenite)-gypsum (Fig.8 and 9).
Fig.3: Left) Close up photo of alternation of gypsum and red claystone as deposit (signals) of astronomical one cycle. Right) General view of outcrop of Gercus formation at northwestern plunge of Sharmin anticline near Bakrman village, NW Akre town.

Fig.4: A & B) Powder of gypsarenite bed under binocular microscope (A is half-powder and B is full-powder, both 16X) in which white and orange grains are gypsum and black grains are lignite. C) Thin section of secondary gypsum (stain spar-fibrous gypsum) under polarize microscope, 20X. D) Polish section of gypsarenite bed under binocular microscope, in which white layers are gypsarenite and black layers are lignite (black bitumen). E & F) Thin section of gypsarenite bed under polarize microscope, 20X, in which shows sub-angular detrital lignite(black grains).

Fig.5: Deposition of mixed clastic, carbonate and evaporite sediment as cyclicity in the upper part of Gercus Formation which overlain by massive bedded limestone of Pila Spi Formation at left side of Dohuk lake.
Fig.6: Three ideal cycles (parasequences) of red claystone (or gypsarenite) (R.C), marl (M), gypsiferous marl (GM) and gypsum (Gy) at southeastern plunge of Chiayi Spi anticline, NW Dohuk dam.

Reason of cyclicity and time relation between lithologies
The clear and strong signals of cyclicity can be attributed to Milankovitch astronomical theory (or Milankovitch band) for interpreting and classification of layering of sedimentary rocks. The existed, cyclicity of the formation is resulted from earth orbital cyclicity (orbital signals). Orbital cyclicity, in tern, generate alternated warm and cold climate of long and short duration (repeated climatic fluctuations) which reflected by deposition of different lithologies in the sedimentary basins. The climatic variations affect ice accumulation in the poles and sea level change (rise and fall). Milankovitch band consist of three types of wave lengths which have the duration of 106, 41, 19 Ka (kilo anus). These wave lengths (earth orbits) are called eccentricity, obliquity and precession cycles respectively which are resulted from different orbital of earth around its self and sun (for more detail see Haq, 1991; De Boer, 1991; De Boer and Smith, (1994); Gale, 1995; Fisher, 1995; Holland, 1998; Van Vugt et al., 2001 and James et. al., 2001).
The cyclicity of the upper and middle part of the Gercus Formation is most possibly located in the Milankovitch band especially the wave length (sea level change duration) of 100 Ka (eccentricity) which modulated by precession (20 Ka) and obliquity. This is because the total duration of the formation is about 5 Ma (Bellen, 1959; Buday, 1980; Jassim and Goff, 2006). The field study shows that the formation contains more than 10 complete or incomplete sedimentary cycles. When the total duration of the formation, in the basin center, is divided by the numbers of the cycles we get 100 Ka. The deviation of the many cycles from ideal one is most probably returned to modulating of the eccentricity by obliquity and precession cycles together and singularly. The gypsum which present within the upper and middle part of the Gercus Formation is deposited during high eccentricity and precession which is meaning the nearness of the earth from the sun and tilting of earth axis to it is orbit (Fig.10A).
During high eccentricity and precession, the claystone (or gypsarenite) is deposited during cold duration when the earth at long distant from the sun and the earth at low tilt angle of its orbit. But gypsum is deposited in opposite situation to that of claystone (during closing of the sun from the earth). The marl and limestone are deposited at the intermediated distance.

Lithologic relations within the systems tracts
The whole formation consists of major lowstands systems tract on the basis of lithologic characters within stratigraphic record of Tertiary which belongs 2nd order sea level change (Ameen, 2005). This author studied the Sequence stratigraphy of Gercus Formation in detail and divided the whole formation into two depositional sequences, named upper (encompass upper part) and lower (encompass lower and middle parts of the formation) sequences which are modulated by 3rd order sea level change (Milankovitch band or minor regression and transgression). Tekin (2001) cited that laminated gypsum and claystone are deposited during minor transgression.
The Gercus Formation as a main lowstands systems tract consists of many small depositional cycles. Each small cycle consists of one ideal cycle which has duration of 100 Ka. Main problem with the cycles in the Gercus Formation is where to indicate the each lithology on the sea level curve. This problem includes what lithology to be assigned as transgressive, highstand and lowstand system tracts. In the literature, Einsele (1998) discussed in detail the sequence stratigraphy of carbonate-evaporites successions (or systems). He assigned the evaporites as deposits of lowstand systems tract and carbonate as highstand systems tract (Fig.11). Einsele (1998); Babel (2004); Gurbuz and Gul (2005) are referred to accumulation of gypsum on the slope during the sea level fall and referred to deposition of carbonate during sea level rise. In contrast to latter studied Ameen (2006), inferred that gypsum, in basin periphery, of the Fatha Formation, was deposited during sea level rise.
In Gercus Formation, we tried by field work to find in what type of sea level fluctuation( systems tract) the gypsum of Gercus Formation were deposited when field relation and the above ideas are considered. The field relation of the lithologies of the formation, in proximal area (studied area), showed that the gypsum bed are deposited during highstand system tract (during sea level rise or transgression). The red claystone deposited as lowstand system tracts. The evidence for these assignments of system tracts as following:
A- In the field the cycles are associated with red claystone; this rock represents the shallowest deposits in the basin which is representing the deposits of delta plain and distributaries channels. The marl is located above the red claystone which represent sediment of deeper water and above the later lithology comes gypsum or limestone (Fig.8 and 9). Therefore, it is more convenient to assign red claystone as sediment of LST which deposited during sea level fall (Fig.10).
B- The most power full evidence for deposition of the gypsum during sea level rise is existence of gypsarenite which, as a type of coarsest clastic in the both studied sections, is deposited during regression (lowstand system tract when the citations of Emery and Myers, 1996 and Ensile, 2000) are considered. Therefore the gypsum beds are deposited during sea level rise which later eroded during sea level fall.
C-No erosion surface was found under the gypsum beds as mentioned by Einsele (1998). Conversely the contact of gypsum with marl is gradational in some cases which are represented by gypsiferous marl or marly gypsum rock at the contact.
D- The contact between gypsum beds and overlying red claystone (or gypsarenite) is sharp which refers to erosional surface (Fig.11A). This erosional surface may be attributed to shallowness of the water which induced the erosion by current activity. The erosion surface most possibly equivalent to type-2 sequence boundary.
E- In the studied area as observed by the present author and in the Mosul area as observed by Karim (1988) and in the Sulaimanyia area as observed by Ameen (2007, in press) the most common rock that associated with gypsum, in Fatha Formation is marl. In many cases both rock make laminated beds which consist of regular alternation of millimeteric lamine of both lithologies (Fig.2A). When little marl is deposited spontaneously with gypsum, a nodular bed of gypsum is formed by pressure and deformation (Fig.11C).

Effect of tectonism on the basin of Gercus Formation
Although parasquence and minor cycles are showing strong astronomical signals as discussed before, the sequences and basin configuration is mainly controlled by tectonism of the northern Iraq. Karim et. al., (2008) discussed in detail the effect of tectonism in generation local basins (intermontine basins) in the northeastern Iraq. He showed by diagram that regularities was generated Lower Eocene and transformed into intermontone basin during Middle Eocene (Fig.12). It is possible that during latter age the local shallow basin separated from the main foreland basin in which the gypsum is deposited. The main sea is supplied intermittently the local basin with dilution by river water. The absence of carbonate, in the Gercus Formation in the studied area, is most possibly attributed to clay influx and to the fact the main foreland basin was organism rich by which the carbonate is extracted.

Reasons for deposition of gypsum during HST (sea level rise)
During warm and humid climate the sea level rise occurs due to poles ice cap melting while during cold and dry times the sea level fall due to ice accumulation. The important processes that enhanced deposition of gypsum during HST is four points, the first is that during warm intervals both evaporation and salinity are increased. In upper sequence of the Gercus Formation, the evaporation is increased due to nearness of the earth from the sun with high precession while salinity is increased by flooding of marine water over the sill that separated the basin of formation from normal marine water (Fig. 10).
The second point is that during HST the precipitation on the basin and source areas decrease, this prevents the dilution of the salinity of water of the basin by fresh water, at least, in the proximal area. The warmness was the highest when the high tilt angles of precession and obliquity are associated with eccentricity. The third point is that during remoteness of the earth from the sun with low tilt angle, the influx of marine water over the sill is stopped. In other side, the fresh water supply from the source area increased to a point that the water of the basin is diluted in the peripheral areas (at the studied area) this associated with decrease of evaporation. In these cases the sea level fall occurred and LST is deposited which represented by red claystone and sandstone (or gysarenite and detrrital lignite). The fourth point is that the deposition of HST is has long duration as compared to TST and undergo some shallowing after main deepening. This shallowing is resulted from sediments fill and evaporation (Vail et al, 1977, Van Wagoner, et al, 1988 and 1990, Haq, 1991, Emery and Myers, 1996).


Fig.7: An ideal cycle at the middle of the Dohuk dam section for which the lithologies and sequence stratigraphy are shown.
Fig.8: Stratigraphic column of Dohuk dam section, which shows the type of cycles and their interpretation, and sea level curve.

Fig.9: Stratigraphic column of Bakrman Village section, which shows the type of cycles and their interpretation, and sea level curve.

Fig.10: Effect of eccentricity and precession of climatic change in the northern hemisphere during Middle Eocene. A: High eccentricity and precession generate HST and high evaporation in which gypsum deposited. B: Low eccentricity and precession generate LST and influx of fresh water to the closed lagoon.

Fig.11: Systems tract of carbonate-evaporites system of Einsele (1998) which shows evaporite as LST and carbonate as TST and HST.

Fig.12: Tectonic and paleogeographic setting of the northeastern Iraq during Upper Cretaceous-Middle Eocene. In the Lower Eocene (B) many irregularities are generated in the basin which possibly formed local evaporitic coastal basin (Karim et al,2008).

Conclusions
This paper has the following conclusions:
1-For the first time the most surprising and important event during this study is finding both detrital gypsum (gypsarenite) and detrital lignite in the Iraq.
2-The Gercus Formation as a whole consists of lowstand systems which contain many cycles of sea level fluctuation in the range of Milankovitch bands.
3-This system tract contains tens of packages of lithologies which repeated regularly in outcrop section in the basin periphery.
4-Each package consists of red claystone (or gypsarenite), marl or gypsum as complete or (ideal) cycle which makes repeated cycles.
5-The relation of these lithologies as concerned to systems tracts are opposite to the previous studies as red claystone, marl and gypsum are deposited during time of LST, TST and HST respectively.
6-The timing relations of deposition of these lithologies are indicated on the curve of eustatic sea change by sequence stratigraphy.
7-The deposition of each lithology during this system tract attributed to Milankovitch band which include eccentricity, precision and obliquity orbits of earth around sun and itself.
8-The interference of these orbits generate cold and warm, time intervals which lead to deposition of red claystone and gypsum respectively.

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