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Sherzad T. Al-Barzinjy

Published in: Iraqi Bulletin of Geology and Mining, Vol.4, No.1,2008, p 95-104.

Kometan Formation ( Ilam Formation in Iran), in most localities, contains sporadic and rare chert nodules, but in Dokan area, these nodules become widespread and associated with well-developed and high amplitude stylolites. They are mainly distributed along or around the bedding planes. The long axes of nodules and stylolite surfaces are aligned parallel to the bedding plane. Field observations and statistical analysis (rose diagram of long axes) showed that nodule elongation and stylolites peaks have no any relation with the known tectonic stress directions of the area. Therefore, the growth of the nodules and stylolites are attributed to deep burial diagenesis of the rocks of the formation under vertical lithostatic pressure and not due to tectonic stress. This result contradicts the previous studies, which supposed that stylolites peaks have direction of north- south and east- west.
Microscopic studies showed that nodules are developed by both displacement and replacement during deep burial. The microscopic criteria for emplacement are: 1- the limestone around nodules shows exceptional crowding of planktonic forams due to the dilatational relations with country rock. 2-They are associated with stylolites this mean that the nodules compressional environment is favored by nodules and stylolites on expense of host limestone. While the criterion for replacement include presence of partially dissolved and replaced forams directly at the contact between the chert and limestone. The local accretions of silica for replacement and displacement growth are assisted by diffusion and moving of watery solutions. The presence of widespread chert nodules and stylolites in Dokan area and their absence in other areas is attributed to exertion of the vertical lithostatic differential pressure at the Dokan area. While the surrounding areas were performed as pressure shadow or the pressure was hydrostatic type (equal in all direction). The load pressure is estimated indirectly by comparing with (CCD) pressure under which limestone dissolve and silica increase in deep oceans.

Upper cretaceous, Kometan Formation, is extensively exposed along both limbs and plunge areas of some major anticlines of northeastern Iraq, such as: Piramagroon, Sara, Azmir and Goizha Anticlines. According to Buday (1980), this formation was first described by Dunnington, 1953 at Kometan village near Endezah at the contact between the Imbricated and High Folded Zones. It consist of (200-100) m white weathering, light gray thin to thick-bedded limestone. Karim (2004) attributed the formation to the transgressive system tract of Upper Cretaceous. Petrographic analysis shows that it has fine grained texture with many planktonic forams, mainly including Oligostegina and Globigerina species. The present study is concerned with the area around Dokan Dam site (Fig.1 and 2.4) where the northwestern plunge of Sara Anticline is dissected by Lesser Zab River. In this area, spectacular large size and high frequent chert nodules occur with large–scale stylolite in the Kometan Formation. This type of occurrence is not observed elsewhere and is unique in the area of occurrence.

Fig.(1)Geological map of the studied area (modified from Sissakian, 2000).

In the studied area the lower and upper contacts of Kometan Formation are represented by glauconitic limestones (Bellen et al. 1959). The upper boundary, with Shiranish Formation, is noticeable where a bed of 1.5 thick exists between the two formations. Karim et al., (2000) studied this boundary in detail, a hardground and an omission surface was found. Qamchuqa and Shiranish Formations are conformably underlying and overlying the formation, respectively (Karim, et al., 2006). However, the nature and position of lower boundary is a little bit variable. Because near the Dokan dam site Gulneri Formation (1.5 m black bituminous shale) is underlying Kometan Formation while two km to the east of the dam site, the formation suddenly change to Qamchuqa Formation and the glauconite of the lower boundary in the studied area is not found.
The nodules are distributed through light gray limestone of Kometan Formation. They can be seen clearly on bedding surfaces and on planes cutting the beds vertically such as surface of joints, road cut and erosional cliffs (Plate3.4). They exist as oblate or irregular bodies and range in size from cobble to pebble. They are occasionally joined together laterally through neck-like connection. Their surfaces are light brown and generally smooth or may be knobby. The internal color is black or greenish gray and shows no internal organization. However the color becomes darker towards the center of the nodules. Their longitudinal axes are arranged parallel to bedding plane (Plate 3.1 and 3.2) but they have no preferred azimuthal direction (as measured in horizontal plain). Occasionally the nodules join together and form a net- like structures. Nodules are more frequently associated with stylolites and both generally concentrated along or near bedding plains (Plate 3.3 and 3.4). The stylolites are well developed and appear, in sections cut normally to bedding planes, as an irregular zigzag high amplitude lines but, in sections parallel to bedding (in this case can be seen in three dimensions), they are similar to rough file surface and their peaks are perpendicular to the bedding surface (Plate3.1).

Thin sections for petrographic study are prepared from the nodules and the host rock (limestone) in addition to the margins between the nodules and the limestone. These sections showed that there is clear relation between both, which indicate diagenetic origin of growth for the nodules. The diagentic growth occurred by two processes, as follows:

•Replacement growth
This type of growth is manifested by the following four features. First, the host rock is composed of calcareous test of planktonic forams embedded in micritic matrix (Fig.2.1). Second, the nodule,s interior is made up of very small radiating crystals of chalcedony, while dark materials (clay and iron oxides) increase near the outer boundary. Third, there is crowding of planktonic forams in the host rock, surrounding the nodules. Some of these tests exhibit breakage and deformation (Fig. 2.1). Fourth, boundary is not completely sharp but contains a thin layer or zone of transition between the limestone and the chalcedony. In this zone the forams are partially corroded and replaced by silica, therefore they appear as ghosts (Fig. 2.3 and 2.2).

•Displacive growth
This type of growth is indicated by the following two features. First, in addition to replacement, there are many evidences for displacive growth, including arrangement of long axes of forams parallel to nodules boundaries (Fig.3.1). Second, existence of very thin smooth solution seams surround and bend around the nodules exactly at the contact. According to Walness (1979), these seams are formed by pressure solution in impure limestone. Although Kometan Formation is relatively “pure limestone” but the nodules boundary is rich in clay. This clay is most possibly formed at the expense of dissolved limestone, when overburden stress produces a pressure which forces the large volume of limestone to dissolve. The soluble materials (dissolved CaCO3) are carried away by vertical or lateral movements of intrastratal solution.
Fig.(3) Four field photographs showing stylolites and chert nodules

Replacement and displacive origins of chert nodules are confirmed from the aforementioned five points. Both are diagenetic processes, the first one, is dissolution of a rock or mineral and concurrent deposition of other mineral of different chemical composition. The second one contains the process of nucleation and growing of a mineral by shouldering aside the host rock. This does not mean that there is no early diagenrtic chert nodules, as Al-Rawi (1988) found in his study of the Western Desert, clear thin section photos of
chert nodules, which contained distinctive nummulites. Unlike nodules of Kometan Formation, these are certainly of early diagenetic origin. Recently Sharp et al., (2002) studied the origin of isotope cyclicity in the bands (rings) in the chert nodules. They attributed this cyclicity to the thermal changes due to convection system.

Replacement and displacive growth of nodules, in present study, requires concentration of silica anions in certain locus within the limestone. The diffusion and movement of intrastratal solutions are the most important dynamic factor for transferring silica from direct or round about neighboring areas. The diffusion is the movement of silica ions in stationary pore solutions. While ions carried and transported by moving water are called metasomatism. High silica concentration sites are the reservoir for supplying silica for diffusion and metasomatism. The moving silica is in the form of H2SiO4 (Pettijohn, et al., 1972). This was the proximate source of silica, but what are the ultimate sources of silica in Kometan Formation? This question is answered through the following points:
- As Kometan Formation is environmentally open marine sea (Buday, 1980, and Jassim and Goff, 2006) so it may contain some biogenetic siliceous radiolarians and diatoms, which easily dissolve in presence of limestone prevalence. This dissolution is returned to high surface energy of these fine and disseminated silica grains as they are most stable and have less free energy, if these grains occur in clusters, hence aggregates will be formed during long times ( Ramberge 1952 in Pettijohn, 1975). McBride et al., (2006) showed that the chert nodules of Drunka Formation (Lower Eocene) in Egypt are mostly formed after moderate alteration of limestone by meteoric water and the replacement of carbonate mud by microcrystalline quartz was the dominant chertification process. They attributed the source of silica to secreting marine organisms.
-According to Blatt, et al., (1980) silica is precipitated in low (pH) environment and need concentration of silica more than 100ppm. In these circumstances primary silica precipitation was possible, when silica-rich cold upwelling current flow to the basin of Kometan Formation from deeper oceanic basin.
-According to Karim (2004), and Karim and Surdashy (2005) Qulqula Formation is compressed to accretionary prim during Conacian and Santonian in a basin in which Kometan Formation was deposited. Then the accretionary is uplifted at Campanian forming positive land in the area near Iran-Iraq border. Therefore, the basin included siliceous rock, so it is possible that the silica ions are carried by submarine currents (upwelling currents) and mixed with deposited lime mud, before lithification.
-According to Cecil (2004), eolian dust is an important source of chert in marine environments in the warm and arid climates. He referred to quartz as main constituent of eolian dust, which has high solubility due to high surface area and small grain size (less than (60 micron). In Kometan Formation, it is possible that received dust from terrestrial area that is located at the present position of Iraqi Western Desert (Rutba Uplift). The evidence for this is the occurrence of many quartzose sandstone units during Upper Cretaceous in this area (Jassim and Goff, 2006).

As mentioned, the relations of bedding with nodules and stylolite surfaces are parallel arrangement of both nodules and stylolite surfaces. According to these relations, it is evident that the lithostatic (overburden) pressure is the only stress for development of stylolites and parallel arrangement of nodules long axes. Relative degree of vertical stress can be known from the size of stylolites. Large-scale stylolite in Dokan area reveals relatively high stress. But how can the absolute values of this pressure be measured?. To answer this question, one must return to value of pressure under which calcite (limestone) dissolves and silica precipitates. In present deep oceans this value is 300kg/cm2 and under this pressure limestone partially dissolved while under more than 400kg/cm2 it totally dissolves (Nichols, 1999). The latter pressure is equivalent to 4000m depths, which is called calcite compensation depth (CCD). Below (CCD) the skeleton of radiolaria can form the main biogenetic component of the pelagic sediment (Stow et al., 1996). Finally a question may arise, why these pressures are mainly restricted only to Dokan area?. The answer of this question needs the following argument:-
The presence of widespread chert nodules and stylolites in Dokan area and their absence in other areas is attributed to exertion of the vertical differential lithostatic pressure in Dokan area. While the surrounding areas were performed as pressure shadow or the pressure was, possibly, of hydrostatic type (equal in all directions). When the strain diagram is drawn, it consisted of oblate spheroid in which maximum principle stress (σ1) acts in vertical direction and other two axies are equal (σ2 and σ3) and act in horizontal direction (Fig.4).
In Dokan area, the lithostatic pressure of Maastrichtian and Tertiary formations were exerted on the Kometan Formation before total lithification. This exertion may be due to presence of a fault, which removed the lateral support for the lithologic column of the Dokan area. The total thickness of this column (all post Campanian formations) in the Dokan area and surroundings exceeds 2500m (considering the removed intervals), which might have been responsible for stress more than 5000 kg/cm2.

The directions of 324 (elongate) chert nodules are measured randomly at different localities using Brinton Compass. The original quadrant compass readings are converted to their equivalent azimuthal reading (Table1). Then the data was used for drawing rose diagram through Window- based RockWare program. The options asked by the program are accurately selected according to the characteristics of the nodules. The most important options are Bidirectional, Full rose and activated filters options. These options are critical for accurate drawing of the rose diagram as they belong to the nature of nodules. The drawn diagram by the program shows nearly random azimuthal alignment distribution of the elongated nodules (Fig.2A).
This proves that the nodules are developed by diagenetic processes during deep burial before the folding of the area .The diagram shows slight polarity in the direction of northeast-southwest and northwest-southwest, this possibly is attributed to very late deformation of nodules (stretching) by tectonic stresses. The random distribution of the stylolites peaks (as seen in three dimensions) can be seen in the studied area (Fig.1.2). So both stylolites and chert nodules are developed (grown) together in a late diagenetic environment of high load pressure (lithostatic pressure) before folding of the area and devote any shear (directional) stress in their original growth. But Taha et al. (1995) have recorded opposite conclusion to the present one. They found that stylolites peaks are directed nearly toward north south and east west. In our opinion, the authors work was very local, while the present study is associated with extensive field in larger area.
Table (1) Compass azimuth readings of 324 elongated chert nodules in Kometan Formation, Dokan area. The readings are arranged in seven columns as originally measured in the field and feed in to the P.C.

Fig.(2) A) rose diagram of the elongated chert nodules in Kometan Formation in Dokan Area. B) Strain ellipsoid of the rock of formation as affected by overburden pressure forming high amplitude stylolites and chert nodules.

This study has the following conclusions:

•The association of common chert nodules and well-developed relatively large-scale stylolites are recorded in Dokan area.
•The stylolite surfaces and long axes of nodules are parallel to bedding surface while stylolite peaks are normal to beddings.
•The above two conclusions suggest, that the effect of load pressure (lithostatic pressure) was responsible for development of styloites and chert nodules before folding of the area.
• The concentration of this pressure on Dokan area during Tertiary is attributed to creation of a paleo-swell during Upper Cretaceous.
•Thin section and field studies proved that the nodules are formed by both replacement and displacement.
•The source of silica is attributed to radiolaria and possible upwelling currents.
•Both field observations and rose diagram showed that chert nodule and stylolites have random distribution and have no preferred direction. They are developed under high load pressure by diagenetic processes.

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Professor at Department of Geology, University of Sulaimani, Kurdistan Region, Iraq

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