Types of recent microbailite in slightly acidic spring in Ranyia Area, Kurdistan, NE-Iraq

 Types of recent microbailite in slightly acidic spring in Ranyia Area, Kurdistan, NE-Iraq

Kamal H. Karim,   Polla A. Khanaqa,    BAkhtiar  M. Ameen

   Published In:  Iraqi Bulletin of Geology and mining, V.7, No.2. pp.27-40.


The role of the microbes in precipitation of stromatolitic limestone and lime sand in water of recent spring in the Kurdistan Region, Northeast Iraq is discussed. The spring mouth is forming a small circular lake (350m in diameters) and with slightly acidic water and saturated with dissolved CaCO3. Due to effect of the microbes and wind direction (and its related currents), the limestone and lime sand are deposited mainly on the southwest bank of the lake (spring mouth). The roles of microbes can be seen in forming many types of the microbial limestones (microbialites), which consist of finely laminated (stromatolite) or clotted structures (thrombolite) or both. The existed stromatolite forms well developed different sizes of oncoids, and micro-oncoids (ooids) which have diameter of 1mm to 20cm. The thrombolite   consists of irregular clusters (clotted) of ooids (or micro-oncoids) encrusted by micrbial micritic layers and separated by voids. On the lake bank the small oncoids and thrombolite which forms clotted clusters of ooids mass exposed on the shore while large oncoids are existing away from the shore on the periodically flooded areas. Both types of limestone and lime sand show the effect of wind energy in developing the   rock with certain types of structures and texture within examined rocks which is analogous with Arabian Gulf.

Keywords: Recent lake sediments, large oncoid, microbialite, stromatolite, thrombolite, ooids, Rasha Baa wind, Arabian Gulf

أنواع  حديثة من  الميكروبيلايت  في ينبوع  قليل الحمضية  في منطقة  رانية،  اقليم  كردستان، شمال شرق العراق

 كمال  حاجي كريم    و  بولا ازاد خانقاه  و  بختيار محمد امين

  نقش  في هذه الدراسة  دور الميكروبات في ترسيب  الحجر  الجيري الستروماتوليتي و الرمل الجيري  في  مياه ينبوع  حديث  في  إقليم كوردستان، شمال شرق العراق. فوهة  الينبوع  تشكل بحيرة دائرية صغيرة (ذات قطر 350مترا) و ذات  مياه قليل الحمضية و مشبع بايون الكاليسيوم. نظراً لتأثير اتجاه الرياح والميكروبات ، تكون  الحجر والرمل الجيري أساسا على الضفة جنوب غربي لالبحيرة. يمكن ملاحظة دور الميكروب في ترسيب أشكال عديدة من الحجرالكلسي حيث يظهر بشكل ستروماتوليتي (تركيب الرقائقي) وثرومبوليتي (تركيب التخثري). يظهر الستروماتوليت باحجام مختلفة وتكون بشكل الأونكويد و السرئيات التي يبلغ قطرها من 1 إلى 20 سم. الثرومبوليت  تتألف من  التجموعات الملتحمة وغير نظامية من أونكويد الدقيق و السرئيات التخثرية و هذه المجموعات مغطاة بطبقات ميكريتية و مفصولة بفراغات. يقع الأونكويد  الصغيرة وثرومبوليت على  شاطيء البحيرة  بينما تقع الأونكويد الكبيرة بعيداً عن الشاطئ على المناطق التي تجتاحها الفيضانات دوريا على جوانب البحيرة.  وتظهر كل الرمال و الحجر الجيري تأثيرالطاقة الريحية في تزويد الصخور بأنواع معينة من التراكيب و الانسجة بحيث يشبة ماموجود في الخليج العربي من حيث   الطاقة والمكونات.


   Burne and Moore (1987) introduced the term microbialite for organosedimentary deposits of benthic microbial communities. According to this definition, stromatolite can be seen as a type of microbialite showing lamination as a specific feature (Dupraz et al, 2006). In geology, the science that explains the role of microbes (Cyanobacteria, algae, and fungi) and classifies them is called microbialite. Therefore the science of microbialites is a new science, which is very useful for studying of recent and ancient carbonate rocks in addition to   marine and non-marine (springs and lake) carbonates of Iraq. According to Flugel (2004), the microbes have played a major role in the formation of carbonate deposits at geological scale, during the Precambrian and the later ages. He added that their role is more prominent, in non-marine environments, in the areas where CO2 release is a faster process, such as springs and waterfalls, or where evaporation is more intense such as lagoons.

Microbialites are produced by a microbial community as a biofilm which is generated by the activity of the microbes(Chafetz and Buczynski, 992).They added that, the microbes, in general, are mainly cyanobacteria and other bacteria (including heterotrophic bacteria), but other organisms such as algae or fungi may also contribute to the biofilm. Under certain conditions, the microbes may induce carbonate precipitation.   Schmid (1996) proposed a refined classification and nomenclature of microbialites, based on a combination of both microstructure (peloidal crusts) and macrostructure (e.g. thrombolites). He has suggested that growth forms depend mainly on sedimentation rate and water energy; for example, microbialites develop dendroid forms as a reaction to slightly elevated rate of the sedimentation at low-energy conditions (Schmid, 1996).  In macroscopical scale, three types of microbialites can be distinguished: Thrombolites, stromatolites and leiolites (Braga et al. 1995), the latter being characterized by dense and non-laminated structures.

The present study deals with microbialite of recent acidic lake. This lake consists of a spring mouth which is located at 8km to the southeast of Ranyia town within Sulaimani governorates in northeastern Iraq. It located at the intersection of latitude (N: 350 51 12=) and longitude (E: 440 45 49=) directly to the north of the Dokan Lake.  This area constitutes a part of Zagros mountain belt, where the high mountain chains are in northwest–southeast direction (Fig.1 and 2). 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 such the Bingird and Ranyia valleys. The acidic spring (Ganau Spring) is issuing from the bottom of the latter valley and surplus water flow into the Dokan Lake (Fig.3 and 4A).

Geological Setting

The study area is located in the Zagros Fold-Thrust Belt, directly to the southwest of the main Zagros Suture Zone. Structurally, the area is located at the boundary between High Folded and Imbricated Zones (Buday, 1980, Buday and Jassim, 1987 and Jassim and Goff, 2006). The area mainly consists of high amplitude anticlines and synclines which have same elongation (northwest–southeast) of the mountains. Many of the anticlines are asymmetrical with the southwestern limbs steeper than the northeastern ones. The area around the Ganau spring is covered by alluvial deposits (clay, sand and gravels) which are underlain by the Jurassic formations in the Ranyia valley bottom.   These formations (as limestones and dolomites) include Sargelo, Barsarine and Naokalekan. In the valley, the Cretaceous formations are located in the high elevation along the valley sides such Sarmord (marl) and Qamchuqa Formations (dolomitic limestone). All these formation are described by Bellen, et al (1959) in detail.
Fig.1: Geological map of northern Iraq (modified from Sissakian, 2000),showing location of the studied section.

 Fig.2: top) Schematic geologic cross-section of the studied area passing through Ranyia (Kewa Rash) and part of the Kosrat anticlines. Bottom) Ganau spring cross section (modified from Manmi, 2008).

 Ganau Spring Chemistry    

The spring mouth is wide and forms a lake which is nearly circular with a diameter of about 350meters and about 20meter deep at its central part. The water comes out below the lake and has about 12 liters per second of overflow in summer  which discharged through small channel to the Little Zab River which is  partialy covered by Dokan lake (Fig. 1 and 2A).The water  temperature is nearly constant  (25 Co) due to  deepness of the  aquifers. The flooding area (during spring)of the southwestern bank of the lake is covered by microbialites (micro-oncoids and macro-oncoids) while the subtidal and intertidal (sea terminology is used) are covered by ooids cross bedded microbial limestones (Fig.3B and 4). Below the water the microbialite is covered by mixture of the lime sand   mud in some places. The water of the lake is not affected by tide but spring flood and summer drought may act as tide as concerned to low and high water level in the lake. The aquifers of the spring are consisting of highly bituminous Jurassic rocks (limestone, dolomite and calcareous shales) that are located to the north and northwest of the spring (Fig.2) (Manmi, 2008).  According to this author, the acidity of the spring is attributed to the organic matters and pyrite content of the above rocks although the water is slightly acidic (pH: 6.7) and sulfuric, yet it is used for irrigation and livestock drinking. The chemical composition of the spring water is shown in the table (1) as compared with normal sea water.

                                                    Fig.3:  Location of the Ganau Lake as seen in the Google Earth Images


Table (1) Comparison between   chemical analysis of Ganau Acidic spring and normal marine water

Chemical characteristics Ganau Acidic spring (Manmi, 2008) Normal marine water (Blatt et al, 1980)
pH 6.7 7.9-8.2
Ca++   (ppm) 247ppm 400
H CO3 (ppm) 402 140
Cl(ppm) 305 18980
SO4(ppm) 257 2649
Mg++(ppm) 29 1272


  Fig.4: A lateral view of the Ganau spring (or lake) as seen from the Kewa Rash Mountain from northeast showing area of the exposed microbialite(irregular black line) and prevailing wind direction.

Fig.5: A) Lateral view of the area around Ganau spring as seen from the Kewa Rash Mountain  from northeast. B) Western bank of the lake showing outcrops of microbialite and deposits.


     In the field, the lake associated with many types of the limestone especially on the southwestern bank. These limestones are all milky in color and their structures change away from the lake shore (beach). On the beach it generally consists of sandy limestone and shows both parallel and cross lamination. Few meters from the beach on peripheral zone, the limestone become nodular and the number of the nodules decreases while their size increases (Fig.5b and 6). The sandy limestone contains occasional few imbtricated flat pebbles. When these limestones are inspected by hand lenses and binocular microscopes, the following types of the microbialite are found:

 Types of Microbialite

This type of stromatolite is the commonest type microbialite that can be seen around the spring on the area of the periodical flooding (equivalent to sea supratidal). The shape of the stromatolites is   globular or discoidal which have well developed concentric dark and light milimetric laminations. The laminae consist of alternating micritic laminae that are concentrically arranged around a nucleous. The nuclei mostly consist of fragment of clotted oolitic limestone (fragment of thrombolite) or smaller oncoids. The sizes of the oncoids are variable which range from 2mm (sand size) to about 20cm (saucer or boulder size) in diameter (Fig.6 and 7).There are micro-oncoids (Flugel, 2004)  which are smaller than 2mm (Fig.8).  The shapes of large oncoids are double convex and lensoidal and some of them have width more than twice longer than height. The smaller ones have more or less the shape of egg.  Alternations of thin laminae (crypalgal laminae) of light and dark colored are arranged concentrically around the nucleous.  The deposition is attributed to the processes of trapping and binding of sediment by microbial organism during annual seasonal change. The organisms such as primitive blue green algae (Cyanaobacteria or blue-green bacteria) and fungi participate in the deposition with the aid of direct precipitation of carbonate minerals (Hoffman, 1976; Pettijohn, 1975; Blatt et al., 1980 and Selley, 1988).

In the present study, there is a powerful evidence that proves microbial origin of the oncoids and ooids laminae. This evidence is that the shape of the oncoids (especially large ones) is discoidal (Fig.6). The discoidal large oncoids cannot rolls to precipitated physically concentric and continuous laminae around the nuclei.  On the discodal shapes, the direct precipitation of the carbonate as micrite, occur physically on the upper surface only. The precipitation on one side of the oncoids is not observed in the lake but it is observed that the laminae exist around the oncoids (Fig.7) which proves that they are formed by microbes.

The comparison of the present oncoids with those of Triassic, Jurassic and   Lower Cretaceous of Iraq (see fig.9 and 10 as taken from Karim, 2006, Ameen, 2008 and Daod and Karim, 2010)   showed that the present oncoids have nearly similar characteristics of those cited in literature but with large size (ten times larger than those mentioned previously in Iraq), and with better developed laminae.  This is may be attributed to the absence of effective grazing pelecypods and gastropods  due to acidity of the lake in addition to  calmness of environment and availability of dissolved calcium carbonate. The shapes of the oncoids depend on the types of nuclei, so that wherever the nuclei are elongated, then oncoids maintain nearly the same form. According to Lehrmann et al. (1998), oncoids indicate occasional wave agitation, probably near wave base, while Gunatilaka (1977) mentioned that oncoids are very likely a morphological and ecological adaptation of the mate forming communities to specific energy conditions.

Fig.6: Discoidal oncoids on the southwestern side of the Ganau spring. A) Complete top  view. B) Equatorial view   as cut by erosion which show that nucleous consist of thrombolite.

Fig.7: A) Large saucer-sized oncoids on the periodically flooding side of the lake. B)Polished surface (A) after cutting into two equal halves showing internal structure of the left side which shows development of a lager oncoids around an older smaller one.

Fig.8:  Ooids  of the Ganau Spring as seen,  A) in  thin section  showing concentric microfabric   B) as  polished slab of the  lime sand of the lake shore.

Fig.9:  Oncoids of Kurdistan Region, Northeast Iraq (taken from Karim, 2006 and Ameen, 2008).   A) Triassic (Avroman Limestone Formation) and B) Lower Cretaceous (Qamchuqa Formation).

Fig.10: A) Several oncoids developed around irregular rip up clasts (white) in  Jurassic Barsarine Formation (Barzinja Area). On the oncoids a columnar stromatolite are developed too. B) One of the oncoidenlarged to show growth of the columnar stromatolite (Daud and Karim, 2010).


    The term   thrombolites has been introduced by Aitken (1967) for that type of microbialite, which consists of clotted fabric and more or less rounded small bodies with general obscured internal structures. The laminations are disturbed and hard to be identified, but they contain a network of small-coated fenestrae and coated grains (Aitken, 1967 and Shpiro, 2000). It is formed, like stromatolite, by products of trapping and binding of grains, by filamentous cyanobacteria or they were an extensively burrowed form of stromatolite. Thrombolites are clotted algal or cyanaobacterial mats, can be found in a range of aquatic environments: fresh water, marine and hypersaline. Feldmann(1998) mentioned that the thrombolites formed by microbes, algae and metazoans, and also mentioned that Phanerozoic thrombolites have been interpreted as unlaminated stromatolites constructed by cyanobacteria. Environmental conditions must favor growth of thrombolite to accumulate thickly enough, to be recognized in the fossil record. Such conditions may include a supersaturation of calcium carbonate in the water, slow rates of sediment accumulation, or elevated salinity and temperature conditions. Modern thrombolites are found in a variety of environments including hypersaline lagoons, tidal channels and fresh water lakes.

When the above characteristics are considered, the thrombailite in the present study show some similar and different characteristics. The similarities are manifested by   the fact that a part of the limestone of the spring has porous (coated fenestrae) and structureless (with obscured laminae) bodies of different size which binded by micritic material of microbe origins (Fig.11, 12A and 13).

 Fig. 11: A) Clotted clustered oolids that are developed as thrombolite (Bound and covered by microbal crust see fig.13C). B)  Image of scanning electronic microscope shows fibrous biogenic micrite of the thrombolite.

The bodies consist of clusters of well sorted micro-oncoids (small-coated grain of Aitken, 1967). The only difference is that the clotted grains, in the present study, are not pellet but micro-oncoid or ooids. The association of the ooids and microbialite  (as oolitic cryptalgal laminate) is mentioned by Lehrmann et al (1998) from Great Bank of Guizhou from South Chaina. The location of the thrombolites notifies that deposited in higher energy than the large and medium size oncoids. The high energy and association of the thrombolite with cross bedding is mentioned by Grotzinger, et al (2000) in Nama Group of Namibia. The thrombolite of the present study is similar to that mentioned by Riding (2000) which consist of calcified microbal thrombolite. It is possible that gradation   between stromatolite and thrombolite exist in the sediment of the lake so that the figure of the 14 can be applied to the lake. According these authors, it displays well defined clots which consist of coarse agglutinated thrombolites and incorporated sand and even gravel-sized sediments (Fig.13).

Fig.12: A) details of large oncoids that obtained it,s shape from the nucleus (thrombolite fragment).B) Well developed concentric stromatolite laminae around thrombolite nucleus.

Fig.13: A) clotted ooids as thrombolite surrounded fragment by oolitic limestone.  B) Well developed micritic laminae (stromatolite) and spongy laminae (thrombolite) in medium size oncoids. C) Clotted texture of oncoids arranged as clotted texture (thrombolite)  with coated voids under normal light.

Fig.14: Types of microbiolite in the Ganau spring (Orange circle) within microbialite classification of Schmid (1996).


    In the area of the lake, the wind has many directions, such as southwest, northeast. The most prevailing, powerful and turbulence one is the latter one which is   cold in the winter and warm in the summer. Because of these negative proporties, it called locally “Rash Baa“which means black wind and blow from northeast.  Around the lake, the role of the wind, as main energy input, can be seen in three features. The first is lime sand (with some lime mud), limestone and oncoids are accumulated only on the southwestern part of lake (Fig.15). This part of the bank is located in the far side and in the direction of southwestern wind (Rasha Baa). By this wind the lime sand is transported to the mentioned bank and accumulated to form limestone after binding by cementation with the aid of the microbes. The second is that the current direction (as measured by cross bedding) is toward southwest. These cross beddings are found in the oolitic limestone   on the southwest lake bank (Fig.16).

The third is that the large oncoids are developed on the supertidal area which is separated by cross bedded oololitic sand bars. These oncoids   show some degrees of asymmetry which have longer diameter parallel to direction of the wind. Even the stromatolitic layers are thinner and show smaller angle of curvature in the side of the wind (Fig.6A).  Therefore the effect wind energy is very clear in localizing the limestones grains on the lake bank and giving certain type of directional structures. This consequence of wind direction and velocity is similar to the stromatolite and lime sand that exist in Arabian Gulf in the far side in the direction of the wind (on the coast of United Arab Emirates) which accumulated on the coast of United Arab Emirates where the Shimal wind (blows from northwest) (see Purser, 1973 and Reading, 1986) and has same the role of the Rasha Baa in localizing and accumulation of lime sand and stromatolite on the Ganau lake.

 Fig.15: A) Cross bedded ooids (micro-oncoids) on the southwestern bank of the lake. B)  Bedded and laminated ooids limestone, contain both stromatolite and thrombolites.


 Fig.16: Field examination of the recent microbialite on the southwestern beach of the Ganau Lake


  • Both micro-oncoids (less than 2mm) and large oncoids (2mm-20cm in diameter) are found on the bank of the slightly acidic lake that formed around a mouth of a vertically issuing spring.
  • The lake is saturated with dissolved CaCO3 and precipitated the microbically induced micrite as a stromatolite and as binder of thrombolite.
  • The lime sand and thrombolite occur near the shore while the large oncoids exist on the periodically flooded area around the lake.
  • The wind direction and agitation is nearly similar to that of Arabian Gulf in precipitation of annual varve of stromatolite and laminae of lime sand in addition to transporting of sand to the southwestern bank of the lake that are located in the far side in the direction of the wind.



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Post Author: Professor Kamal Haji Karim

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