High resolution cathodoluminescence spectroscopy of carbonate cementation in Khurmala Formation (Paleocene–L. Eocene) from Iraqi Kurdistan Region, Northern Iraq

Journal of African Earth Sciences 100 (2014) 243–258

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High resolution cathodoluminescence spectroscopy of carbonate cementation in Khurmala Formation (Paleocene–L. Eocene) from Iraqi Kurdistan Region, Northern Iraq Muhamed F. Omer ⇑,1, Dilshad Omer, Bahroz Gh. Zebari Department of Geology, College of Science, Salahaddin University, Erbil, Iraq

a r t i c l e

i n f o

Article history: Received 4 January 2014 Received in revised form 21 June 2014 Accepted 23 June 2014 Available online 5 July 2014 Keywords: Cathodoluminsecnce Dolomite texture Trace elements Carbonate cement sequence Khurmala Formation Northern Iraq

a b s t r a c t A combination of high resolution cathodoluminsecnce-spectroscopy (HRS-CL) with spatial electron microprobe analysis and optical microscopy is used to determine paragenesis and history of cementation in the limestones and dolostones of Khurmala Formation which is exposed in many parts of Northern Iraq. Khurmala Formation was subjected to different diagenetic processes such as micritization, compaction, dissolution, neomorphism, pyritization and cementation that occurred during marine to shallow burial stages and culminated during intermediate to deep burial later stages. Five dolomite textures are recognized and classified according to crystal size distribution and crystal-boundary shape. Dolomitization is closely associated with the development of secondary porosity that pre-and postdates dissolution and corrosion; meanwhile such porosity was not noticed in the associated limestones. Microprobe analysis revealed three types of cement, calcite, dolomite and ankerite which range in their luminescence from dull to bright. Cathodoluminescence study indicated four main texture generations. These are (1) unzoned microdolomite of planar and subhedral shape, with syntaxial rim cement of echinoderm that show dull to red luminescence, (2) equant calcite cements filling interparticle pores which shows dull luminescence and weak zonal growth, (3.1) homogenous intrinsic blue stoichiometric calcite with dull luminescence and without activators, (3.2) coarse blocky calcite cement with strong oscillatory zoning and bright orange luminescence which postdates other calcite cements, (4) ankerite cement with red to orange, non-luminescence growth zonation which is the last formed cement. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Khurmala Formation is a carbonate-dominated stratigraphic succession that was first described by Bellen et al. (1959) from K-114 well in northern Kirkuk structure as dolomite (pseudoolitic in parts) and fine recrystallized limestone. These carbonates interfinger with clastics of the Kolosh Formation and are locally anhydritic. The formation outcrops in north and northeastern Iraq overlying the Kolosh Formation (Sissakian and Youkhanna, 1979) (Fig. 1). The carbonate facies as well as the high pyrite content and the presence of gypsum and anhydrite beds indicate that the formation was deposited in a restricted lagoonal environment. Fossils, mostly dwarfed and obliterated by recrystallization, comprise miliolids, small valvulinids, clavulinids and alveolinids. The age of the formation was assigned as Paleocene–Early Eocene based on its ⇑ Corresponding author. Tel.: +964 770 134 83 22. 1

E-mail address: [email protected] (M.F. Omer). Current address: Warsaw University, Geology Faculty, Poland.

http://dx.doi.org/10.1016/j.jafrearsci.2014.06.016 1464-343X/Ó 2014 Elsevier Ltd. All rights reserved.

stratigraphic relationship to other dated formations (Bellen et al., 1959). The thickness of lagoonal Khurmala Formation in its type locality is 185 m (Buday, 1980). Various mechanisms for the formation of dolostone in lagoonal carbonates have been summarized by Hardie (1987) and Meister et al. (2013). Among the popularly accepted hypotheses are dolomitization related to hypersaline brines, mixed meteoric and sea water, and deep basinal brines. Since cathodoluminescence (CL) microscopy was first used in sedimentary petrology in the pioneering work of Sippel in the 1960s (Sippel, 1965, 1968; Sippel and Glover, 1965), the method became an important and widely applied tool for petrologists. CL application provided the first evidence for numerous new features in sedimentary rocks (e.g. CL-properties of carbonates diagenesis of sedimentary rocks, etc.) and was published in works by Marshall (1988), Baker and Kopp (1991), Pagel et al. (2000), Boggs and Krinsley (2006), and GÖtze (2012). Bruckschen and Richter (1994) indicated that cathodoluminescence zonation in dolomite is a worldwide phenomenon throughout Phanerozoic.

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Fig. 1. Location and geological map showing Khurmala Formation and other Cenozoic rocks (from Sissakian et al., 1995; Sissakian, 1997) GEOSURV. A: Gulley Duhok; B: Bakerman; C: Aqra; D: Sheraswar; E: Sorek.

The Depositional environments, stratigraphy and faunal assemblages of the Khurmala Formation have been studied by many researchers (e.g., Bellen et al.,1959; Ditmar et al., 1971; Jassim and Sissakian, 1978; Buday, 1980; Al-Eisa, 1983; Al-Barzanji, 1989; Al-Banna et al., 2006; Al-Sakry, 2006); however none of these studies involved cathodoluminescence nor other advanced techniques such as electron scanning microscopy and electron microprobe analysis. The goals of the present study are an integrated high resolution cathodoluminescence determination of texture and diagenetic history of dolomite as well as the paragenesis of carbonate cement of Khurmala Formation. In addition to CL, conventional mineralogical and petrographic techniques such scanning electron microscopy (SEM), plane polarized light microscopy, and electron microprobe analyses (EMPA) were used. 2. Geological setting and tectonic history Northern and northeastern Iraq is geologically part of the extensive Alpine Mountain Belt of the Near East, represented by TaurusZagros Fold Belt or the so called suture zone, which was developed

as a result of collision between the Afro-Arabian and the Eurasian continents (Sharland et al., 2001) (Fig. 2). The Iraqi part of this belt is located in the extreme northeastern part of the Arabian Plate, which is colliding with the Eurasian Plate (Iranian micro-plate). This collision has developed a foreland basin called the Zagros Foreland which classified into four zones: (1) Imbricate Zone, (2) High Folded Zone, (3) Low Folded Zone, and (4) Mesopotamian Foredeep (Fouad, 2010: Fig. 3). The low Folded Zone is much wider and consists of smaller and less disrupted folds (Ameen, 1992). The currently studied sections at Aqra, Bakerman and Gulley Duhok studied sections belongs to the High Folded Zone, which consists of a series of relatively large, mostly asymmetric anticlines, separated by narrow synclines. The other two sections, Sheraswar and Sorek are part of the Low Folded Zone, which consists of a series of relatively small and narrow anticlines separated by wide synclines (Fig. 3). Megasequence of the Arabian Plate AP10 during Paleocene–Eocene was deposited as a period of renewed subduction and volcanic arc activity associated with final closure of the Neo-Tethys (Sharland et al., 2001: Fig. 2). This led to uplift along the northeastern margin of the Arabian Plate with the formation of ridges and basins, generally of NW–SE trend in north and central

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Fig. 2. Palaeocene–eocence geodynamic development of the Arabian plate (Sharland et al., 2001).

Iraq and E–W trend in west Iraq. Significant lateral changes occurred across these tectonic features. Uplift of the eastern margin of the Arabian Plate during Early Paleocene and subsequent erosion explains the absence of the Danian sequence from most of the High Folded Zone and the Foothill Zone of Iraq (Jassim and Goff, 2006). The Arabian Plate sedimentary record developed through a series of major tectonic phases that consolidated the basement following the initial Precambrian Plate accretion. The sedimentary cover was deposited during a late Precambrian to mid Permian intra-cratonic phase followed by a Mesozoic passive margin phase and culminated by the development of Cenozoic active margin phase which was continued to the present. Periodically, major tectonic events such as uplift, inversion, rifting, flexing, and tilting created accommodation spaces across the plate, and resulted in major unconformities in the sedimentary record (Sharland et al., 2001). The Paleocene–Lower Eocene succession records foreland sedimentation development during Alpine Orogeny. The foreland basin in Iraq, between Arabian Craton and Alpine Orogen proper, is commonly called Folded Zone which is the Mesozoic and Tertiary successions, that consists of well exposed anticlinal and synclinal structures (Numan and Al-Azzawi, 1993).

3. Methods and materials Sixty-five samples were collected from five measured stratigraphic sections through the Khurmala Formation for this study (Fig. 1). Thin sections were prepared from the collected samples and stained with alizarin red-S solution to distinguish calcite from dolomite based on Friedman (1959). These thin sections were studied under standard polarized microscope to determine the petrographic and diagenetic characteristics. Twenty samples were studied under the hot Cathodoluminescence (CL) cathode CL-device at Bochum University-Germany (HC1-LM, Neuser, 1995) in combination with high resolution spectral analysis equipment (Neuser et al., 1996; Habermann et al., 1998). The CL instrument is linked to a Kappa DX30C video camera system for recoding digital images and with an EG & G triple grating spectrograph connected to a liquid-N2 cooled CCD detector allowing the documentation of very short-lived and dull luminescence phenomena. An acceleration potential of 14KV and beam current densities between 5 and 10 mA = mm2 were generally used for the CL measurements. Integration times for CL spectra were commonly between 10 and 60s. Electron microprobe analysis (EMPA) at GEOSURV Helsinki in Finland was used to detect the concentrations of trace elements in the calcite and dolomite

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Fig. 3. Main tectonic zones in Iraq (after Fouad, 2010).

cements crystals. Fe, Mg, Mn and Sr concentrations were determined using a Cameca SX100 (5WDS). Wave length dispersive analysis was conducted on carbon-coated thin sections; acceleration potential = 20 KV; beam current = 40 nA and beam diameter = 10 lm (Tables 2 and 3). 4. Petrography Bioclasts are the most abundant type of allochems in Khurmala Formation. They are represented by both intact and fragmented fossils. The size of these fossils varies from 0.05 mm to many centimeters. In all studied areas, skeletal grains are mainly represented by well-preserved hard parts of the fossil assemblages, but in some areas, these fossils were highly altered and obliterated by different diagenetic processes such as dolomitization, neomorphism, micritization, dissolution and compaction making them difficult to identify. The most common groups of skeletal grains are referred to benthic foraminifera, red algae,

Table 1 Depositional environments of Khurmala Formation by Al-Banna et al. (2006). Depositional environment

Microfacies

Supratidal (A)

1 2 3 4 5 6

Intertidal (B) Lagoon (C) Shoal (D)

– – – – – –

Lime mudstone Calcareous mudstone Sandstone lithofacies Argillaceous lime wackestone Benthonic lime packstone Oolitic lime grainstone

echinoderm and mollusk (Fig. 4a–c). Non-skeletal clasts are less abundant than skeletal grains, but locally they are the dominant grain type. Non-skeletal grains in Khurmala Formation includes peloids, ooids and lithoclast. Peloids are dominant in Sorek and Aqra sections (Fig. 4d). Ooids were it observed in Sheraswar and Bakerman sections but are less abundant than peloids (Fig. 6a). The ooids are mostly superficial and characterized by the presence of one or few lamina of various thicknesses around a large nucleus (Flügel, 1982). Intraclast grains were observed in a few carbonate units of the Sheraswar section. Micritization of allochems is common mainly bioclast but occasionally also other allochems. Many microfacies in these sections and all of micirtes are now neomorphosed either partially or completely or dolomitized and parts of sparry calcite are neomorphosed lime mud (micirte). The crystal size of dolomite in Khurmala Formation varies from very fine to coarse-crystalline (10–700 lm) or even larger in some samples. Very fine-grained dolomite replaces calcareous mud, peloids, and calcite cement veins. Medium to coarse dolomite replaces and engulfs the blocky calcite cement and larger carbonate constituents including skeletal grains. Thus, fine-grained dolomite occurs mostly in carbonate mud-rich facies such as lime mudstones and wackestones, whereas the medium to coarse dolomite occurs in the carbonate of mud-poor lithofacies such as packstone and grainstone. Skeletal elements are less frequently replaced by dolomite and are generally preserved as dolomite cement-filled molds. However, crinoids, foraminifera, corals and brachiopods are in some cases partially or completely replaced by dolomite. Crinoids are generally the only skeletal components to be syntaxially replaced (Fig. 4c), and if replaced, retain unit extinction.

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M.F. Omer et al. / Journal of African Earth Sciences 100 (2014) 243–258 Table 2 Microprobe analysis of selected samples of carbonate cements from Khurmala. Sample

So2

So2

So2

So2

So2

So2

So2

So2

So2

Sh1

Sh1

Sh1

Sh1

Sh1

Sh1

Sh1

CaCO3% MgCO3% (Fe + Mn)CO3

59.22 38.34 0.62

58.36 39.06 0.72

59.42 37.77 0.72

60.11 37.38 0.82

96.40 1.14 0.59

97.36 0.00 0.58

98.64 0.84 0.50

97.71 0.64 0.35

97.83 0.87 0.87

97.8 1.31 0.77

97.5 0.00 0.19

97.47 1.55 0.63

97.34 0.26 0.42

97.61 1.22 0.52

53.66 46.21 0.16

53.72 46.64 0.12

Sample CaCO3% MgCO3% (Fe + Mn)CO3

Kh1 53.60 44.00 0.92

Kh1 41.14 41.99 13.12

Kh1 41.18 44.08 13.02

Kh1 53.30 44.14 0.94

Kh1 53.18 43.83 1.14

So3 57.02 40.61 0.97

So3 59.82 37.66 0.81

So3 59.27 38.21 0.71

So3 58.85 38.13 0.83

So3 58.5 38.7 1.02

So3 96.1 0.74 0.49

So3 96.87 0.97 0.76

So3 97.28 0.37 0.35

So3 96.16 0.52 0.78

So3 96.78 0.00 0.53

GD5 97.97 1.26 0.00

Sample CaCO3% MgCO3% (Fe + Mn)CO3

GD5 97.43 1.01 0.04

GD5 98.53 0.66 0.00

GD5 98.32 0.94 0.00

GD5 97.46 0.62 0.09

GD5 97.48 1.33 0.00

GD5 35.70 44.17 17.99

GD5 38.87 41.34 16.90

GD5 43.01 43.73 10.99

GD5 45.24 39.83 11.97

GD5 41.7 43.5 12.2

GD5 53.1 42.8 1.37

GD5 53.80 44.43 1.35

GD5 95.98 1.45 0.05

GD5 96.43 0.90 0.06

GD5 96.71 0.73 0.03

GD5 95.22 2.25 0.01

Sample CaCO3% MgCO3% (Fe + Mn)CO3

GD5 97.19 1.01 0.10

GD5 95.75 1.32 0.03

GD5 96.15 1.18 0.07

GD5 97.28 0.54 0.10

GD5 96.95 0.88 0.00

GD5 97.31 0.89 0.00

GD5 96.87 0.67 0.04

GD5 95.23 0.93 0.09

GD5 53.40 43.98 1.33

So = Sorek; Sh = Sheraswar; Kh = Aqra; GD = Gulley Duhok.

Table 3 Trace elements concentration of carbonate cements measured by EMPA. Sample/comment

Mg (ppm)

Mn (ppm)

Fe (ppm)

Sr (ppm)

Fe/Mn

GD5_point1 A C2 GD5_point2 C2 GD5_point3 C2 GD5_point4 C2 GD5_point5 C2 GD5_point6 C2 GD5_point7 C2 GD5_point8 C2 GD5_point9 C2 GD5_point10 C2 GD5_point11 C2 GD5_point12 B C2 GD5_point1 (in dolomite) GD5_point2 C3 GD5_point3 C3 GD5_point4 C3 GD5_point5 C3 GD5-point 6 C3 GD5_point7 C3 GD5_point8 C3 GD5_point9 C3 GD5_point10 C3 GD5_point11 C3 GD5_point12 C3 GD5_point13 C3 GD5_point14 C3 GD5_point15 C3 GD5_point16 (in dolomite) GD5-dol. cem.1 GD5-rep. dol. GD5-rep. dol. GD5-dol.cem.2 GD5-dol.cem.2

4142 3423 3378 4006 3944 2458 3034 3654 4528 3199 4184 4171 14,200 2470 3165 2297 1975 2156 2720 1727 4689 3843 2284 1566 2002 3473 2490 12,870 14,880 13,970 13,720 13,100 14,990

106 98 131 152 130 118 146 26 196 101 196 161 1249 24 19 73 24 49 52 77 150 170 51 75 90 12 10 1079 1207 1078 1475 1707 1310

97 46 107 100 115 83 120 15 124 53 106 85 5694 5 6 7 29 5 4 7 2 31 17 3 14 17 11 5284 5385 5516 5869 6894 5397

268 54 220 48 136 164 73 135 77 174 290 193 76 462 199 131 247 251 172 154 240 238 192 228 209 273 429 b.d.l. b.d.l. 108 33 b.d.l. b.d.l.

0.90 0.46 0.81 0.65 0.88 0.70 0.82 0.57 0.63 0.52 0.54 0.52 4.50 0.23 0.31 0.09 1.20 0.10 0.07 0.09 0.01 0.18 0.33 0.04 0.15 1.41 1.10 4.80 4.40 5.11 3.97 4.00 4.10

C2 = Equant calcite cement. C3 = blocky calcite cement. dol. cem. = dolomite cement. rep. dol. = replacement dolomite. GD = Gulley Duhok. Detection limit of: Mg = 113; Mn = 48. Sr = 136; Fe = 41. b.d.l. = below detection limit.

5. Lithofacies and sedimentary environment A total seventy-five samples were collected from thick successive of which belong to Khurmala Formation in five sections.

Lithologically, sedimentary structures and macrofossils records were taken into account. The samples were collected at a maximum interval of about 0.5 m. Khurmala Formation in the Sorek section area is intertongue with the upper part of the Kolosh clastic

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Fig. 4. Photomicrographs showing different components in Khurmala Formation, (a) larger benthic foraminifera of Assilina granulosa. (Bakerman section, sample 8). (b) Coralline red algae (Lithothamnium spp.) (Bakerman section, sample 2). (c) Echinoderm spine showing replacement dolomite existence in shallow marine carbonates (arrow) (Aqra section, sample 9). (d) Inter-ooid spaces are filled with sparry calcite and micritic matrix, and most ooids are coarse grains (arrow) (Sorek section, sample12). Scale bars 0.2 mm, XPL.

Formation as a symmetrical wedge of limestone mixed with siliclastic deposits, and their thickness varies within a horizontal distance of a few hundred meters from northwest to southeast. The limestone in this section is thickly bedded, light grey in color, laterally changing to massive beds, which are very obvious phenomena in the field in this area. In addition small scale cross-bedding and horizontal lamination were recognized in the sandstones. The massive beds contain large voids filled partially or completely with organic matter which has been interpreted as a patchy reefs within ramp setting shelf (Al-Ehmeedy et al., 2012). The sand contents increases in the lower part of Khurmala Formation at Dokan section suggesting sea level transgression during deposition. The Khurmala Formation in Aqra section it consists of well bedded dolomitic limestones, pale brown to yellowish white in color and occasionally pale grey. The most appreciable feature is that the strata of the Khurmala Formation are occasionally overturned and show a difference in dip at the boundary between the two formations. The local thickness in Aqra section reaches 48.7 m. A very thin to thick beds of dolomitized marly limestones ranging in thickness from 1 cm to 40 cm and are separated mostly by very thin shale beds with a thickness of 0.5 cm to 20 cm in Sheraswar section. The Khurmala Formation in Bakerman section interfingers and intertongues with the upper part of Kolosh Formation. The lower part of Khurmala Formation in Gulley Duhok section consists of very thin to very thick beds of marly limestones and dolomitized limestones. Al-Banna et al. (2006) proposed two varieties of sedimentary facies in Khurmala Formation namely carbonates and clastics; the carbonates embraces four microfacies and the clastics two lithofacies. They suggested a tidally influenced shallow marine environment including supratidal for this section. The oolitic shoal bank environment is represented by oolite lime grainstone microfacies indicating high energy wave or tidal shallow marine environment (Table 1). The bank protected lagoonal bays and tidal bar environment are represented by packstone microfacies including

benthonic lime packstone microfacies and lime wackestone microfacies that were deposited at water depth of less than 40 m. Al-Qayim (1995) concluded that the lateral and vertical distribution of these facies are strongly influenced by paleographic configuration, local tectonic disturbances, the amount of clastic influx entering the lagoonal belt, as well as global sea level fluctuations. 6. Diagenesis of Khurmala Formation Diagenesis of carbonate rocks is more varied than that of clastic rocks particularly, because of the metastable nature of carbonate minerals (Bathurst, 1972). Diagenetic processes include physical, chemical and biological changes affecting sediments after deposition, then lithification and emergence on ground surface under normal pressure and/or temperature (Larsen and Chilingar, 1979). The diagenetic history of recrystallized limestone in the Khurmala Formation of Northern Iraq includes multiple episodes of calcite, dolomite and less commonly ankerite cementation, in addition to micritization, neomorphism, mechanical and chemical compaction, and dissolution. Theses diagenetic processes occurred during marine to shallow burial stages and culminated during intermediate to late stage deep burial conditions. Many of these diagenetic processes are facies-controlled and were only found in certain depositional units. The characterization of diagenetic history is based on petrographic observations, high resolution cathodoluminescence spectroscopy and geochemistry (Fig. 5). The descriptions of the most important processes are herein cited. 6.1. Micritization Micritization of bioclast is common of all rocks Khurmala Formation. However, in some case other allochems are micritized leaving

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Diagenetic events

Diagenetic setting Early (marine meteoric) Middle

Late

-Micritization -Compaction -Syntaxial cement -Neomorphism -Dissolution -V. fine to fine crystalline dolomite planar-S-M .crystalline rhomb dolomite planar-S-Replacement dolomite -Early dolomite cement -Late dolomite cement -Equant calcite cement -Blocky calcite cement -Pyritization -Dolomite-ankerite cement

Fig. 5. General paragenetic sequence constructed for Khurmala Formation with multiple episodes.

behind miciritc lumps. Generally, micritization also micritized leaving behind micritic lumps. Micritization occurs where endolithic colonies (cyanobacteria)thrive in shallow marine low energy environments. These cyanobacteria initially bore around the margins of the allochems and the bores are filled with micrite forming a micritic rim around the allochems. In some allochems the alternate brownish and light coloured microcrystalline aragonite layers are dissolved (Fig. 6a). This type of micritization is attributed to borings by endolithic cyanobacteria which are infilled by aragonite (e.g. Harris et al., 1979). Micritized grains also form due to syndepositional recrystallization of skeletal carbonate to the equant micritic fabric (Reid et al., 1992; Macintryre and Reid, 1998). Evidence of little sediment transport, the presence of ubiquitous microbes and little cementation suggest micritization by endolithic algae in the present case. This type of micritization is prevalent within the restricted lagoon environments containing the stagnant marine phreatic zone environment (Tucker and Wright, 1990).

(1) Blocky calcite cement which consists of relatively large anhedral to subhedral calcite crystals usually formed after lithification and compaction of sediments which is synonymous to granular cement. This type of cement is the most common type cement in all studied sections (Fig. 6e). (2) Syntaxial rim cement is the most common cement fabric in the upper part of Sheraswar and Bakerman successions representing the change from marine phreatic to fresh water phreatic (Fig. 6f). (3) Cementation by dolomite which is common type in all sections, especially in the middle and upper parts of Gulley Duhok where fine to medium rhombohedral and subhedral clear dolomite crystals are found as fracture- and vug-fillings in most of the dolomite rocks (Fig. 6g). Dolomite occurs as pore filling cement indicating precipitation in shallow subsurface burial diagenesis. 6.5. Compaction

6.2. Dissolution

Neomorphism includes all in-situ transformations, by solution re-precipitation, between one mineral and itself or a polymorph (El-Saiy and Jordan, 2007). These transformations involve inversion of aragonite to calcite and mainly calcite to calcite by recrystalization (Bathurst, 1983). This process either partially or wholly affects the dolomite rocks. Most of the Gulley Duhok and Sheraswar sections are affected by this process (Fig. 6d).

The carbonates of most studied sections were subjected to both chemical and physical compaction. The criteria used to estimate the effect of physical compaction is evidenced by breakage of grains and crushed skeletal grains; the victims of physical compaction were mainly caused by the contact of bioclasts and lithoclasts (Fig. 6h). The chemical compaction is represented by pressure solution structures such as microstylolites. The main kinds of microstylolites recognized in this study based on classification of Logan and Semeniuk (1976) are horizontal, irregular high peak amplitude, stylobreccia and parallel sets of horizontal stylolites (Fig. 7a,b). The microstylolite planes are filled with bituminous materials and generally regarded as the result of pressure solution involving dissolution around points of contact between adjacent grains in response to pressure (usually the weight of the overburden) and also affected by tectonic pressure as most of these rocks contain many veinlets filled with secondary calcite (Walness, 1979).

6.4. Cementation

6.6. Pyritization

The following cement fabrics were observed in the studied carbonate rocks of Khurmala Formation:

Pyrite is frequently observed as a diagentic processes in Khurmala Formation, especially in Sheraswar section where it

Dissolution occurs on both micro- and macro-scales and is associated with restricted rock units in the Khurmala Formation. This process is regarded as a main factor responsible for the developments of fenestral, bird-eyes, moldic, interstitial, vugy and channel porosities (Fig. 6b and c). It has controlled the development of secondary porosity. 6.3. Neomorphism

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Fig. 6. Photomicrographs showing diagenetic processes in Khurmala Formation, (a) Alternate brownish and light coloured microcrystalline carbonate layers represent micritized and dissolved portions of the bioclasts and ooids. (Sheraswar section, sample 6) X.P.L. (b) Vugs within interlocking dolomite mosaic. These open, interconnected vugs are found within a mosaic of tightly interlocking dolomite crystals. (Aqra section, sample 8) P.P.L. (c) Moldic pores-secondary vuggy pores formed by the selective, complete or partial dissolution and recrystallization of grains or crystals (red arrow). (Aqra section, sample 9) X.P.L. (d) Neomorphosmed dolomitized with porous intraparticle porosity within grains of fossils (white arrow) (Sheraswar section, sample 2) X.P.L. (e) Isochemical diagenesis of blocky calcite cements occludes primary porosity (Aqra section, sample 14) X.P.L. (f) Syntaxial rim cement developed around echinoderm. (Bekerman section, sample 10) X.P.L. (g) Dolomite texture of unimodal very fine to fine crystalline plannar-s (subhedral) mosaic dolomite (Aqra section, sample 25) P.P.L. (h) Crushed skeletal grains, the victim of physical compaction caused by bioclast to lithoclast contact. (Aqra section, sample 10) X.P.L. Scale bar = 0.2 mm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

has developed in the lime mud matrix or in the intergranular porosities (Fig. 7c). The pyrite crystals are commonly influenced by alteration which is reflected as a reddish-brown rim of goethite. During early diagenesis, the majority of pyrite precipitates as a spherical framboids formed by aggregation of submicron

size individual particle, or as single or clustered euhedral crystals (Wilkin and Barnes, 1997a). Framboidal typically form in an euxinic water column and/or during early diagenesis in a sediment (Wilkin and Barnes, 1997a,b; Sawlowicz, 2000).

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Fig. 7. Photomicrographs showing, (a) bedding-parallel irregular anastomozing sutured seams (microstylolite set) in lime mudstone (arrow) (Aqra section, sample 3) X.P.L. (b) Smooth microstylolite cut cross over very fine to fine-crystalline planar-s of dolomite texture (Aqra section, sample 6) P.P.L. (c) Single cubic isotropic pyrite (Sheraswar section, sample 5) P.P.L. (d) Intercrystalline porosity – the porosity between crystals that may be of either primary or secondary origin (arrow) (Aqra section, sample 11) X.P.L. (e)Very fine to fine-crystalline planar-s (subhedral) mosaic dolomite contain foraminifera fossils chamber of interagranular porosity formed by dissolution of dolomite and relics porous (arrow) (Aqra section, sample 29) X.P.L. (f) CL-Image of destroyed vuggy porosity within the dolomite crystals of red to orange luminescence growth crystals (white arrow) (Gulley Duhok section, sample 2). (g) Dolomite texture of unimodal fine to medium crystalline plannar-s (subhedral) mosaic dolomite, intercrystalline porosity (arrow) (Aqra section, sample 2) P.P.L. (h) Dolomite-rock texture unimodal medium to coarse-crystalline planar-s texture (Gulley Duhok section, sample 8) P.P.L. Scale bar 0.125 mm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

6.7. Leaching and porosity development Pore systems in carbonates are much more complex than in siliciclastics rocks (Choquette and Pray, 1970; Lucia, 1995, 2007). This complexity is a result of the overwhelming biological origin of carbonate sediments and their chemical reactivity. In carbonate sediments, the shape of the grains and the presence of

intragranular porosity as well as sorting have a large effect on porosity. The presence of pore spaces within shells and peloids that make up the grains of carbonate sediments increases the porosity over what would be expected from intergranular porosity alone (Dunham, 1962). Although there is no simple relation between porosity and fabric, it is apparent by inspection that intergranular pore size decreases with smaller grain size and with closer grain

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packing. Porosity also decreases with closer packing, resulting in intergranular pore size being related to grain size, sorting, and intergranular porosity. The size and volume of intragranular pore spaces are related to the type of sediment and its postdepositional history (Lucia, 2007). The following types of secondary porosity are recognized in Khurmala Formation: (1) Intercrystalline porosity: This type of porosity is indicated by the presence of rhombic pores randomly scattered throughout the groundmass (Fig. 7d and g). It is the porosity between crystals that may be of either primary or secondary origin (Lønøy, 2006). If it is of secondary origin, intercrystalline pores are associated with calcite recrystalization or dolomitization. Microscopic investigation reveals that it is mostly of secondary origin. (2) Intraparticle porosity: It occurs within individual particles or grains, particularly within skeletal grains. This is caused by partial solution of fossils. It is mainly observed in grainstone facies (Fig. 7e). (3) Vug porosity: It is secondary solution pores that are not fabric selective (Choquette and Pray, 1970; Lønøy, 2006). This type of porosity is common in Sheraswar and Gulley Duhok sections (Fig. 7f). (4) Moldic porosity: This type of secondary vuggy pores formed by the selective, complete or partial dissolution and recrystallization of grains or crystals (Lønøy, 2006). It is common in Aqra and Sheraswar sections (Fig. 6c). 7. Petrography of dolomite texture The most important controls affecting dolomite texture are: mineralogy of the material being replaced, whether or not the dolomitizing solution is saturated with respect to the replaced mineral and the availability of the nucleation sites (Sibley, 1982). Five dolomitic textures were recognized and classified according to crystal size distributions (unimodal or polymodal) and crystal boundary shape (planar or non-planar) using the classification scheme of Sibley and Gregg (1987). This classification is based on petrographic observations, support by analysis of CL images. The crystal sizes were subdivided using size scale Folk (1962). 7.1. Dolomite texture-1: unimodal, very fine to fine-crystalline planars (subhedral) mosaic dolomite This texture is common in the lower parts of both Aqra and Bakerman sections which forms dense, dark mosaics of interlocking sub-to planar-s crystals (10–60 lm; Fig. 7e). The dense mosaics show no recognizable allochmes and are still associated with organic matter. Microstylolites are associated with this type of textures (Fig. 7b). Intercrystalline and interaparticle porosities were determined in these fine-grained dolomites by SEM studies. 7.2. Dolomite texture-2: unimodal, fine to medium crystalline planar-s (subhedral) mosaic dolomite

non-mimetic replacement of allochems (ooids, peloids, intraclasts and bioclasts). These allochems can be recognized as ghost textures. Besides coarse-crystalline, planar-e dolomite rhombs floating in micrite and dolomicritic matrix, stylolitic porosity was also determined in this type of dolomites by SEM studies (Fig. 8). 7.4. Dolomite texture-4: medium to coarse crystalline planar-e (euhedral) replacement dolomite This type of replacive dolomite has a planar-e texture. Typically the dolomites consist of scattered rhombs of 150–700 lm diameter floating in a micritic and dolomicritic matrix (Fig. 9a). The cores of the rhombs are cloudy and they have a clear outer zone. Some of the rhombs show intercrystalline truncation features. Fractures filled by dolomite are cutting across texture-4 dolomite. 7.5. Dolomite texture-5 medium to coarse crystalline, non-planar-a (anherdral) dolomite Dense tightly packed mosaics of medium to coarse-crystalline, non-planar-a dolomite comprise this type (Fig. 9b). The crystals have irregular, serrated, curved or otherwise indistinct boundaries. They are showing vague non-mimetic replacement. Preserved crystal faces are rare or absent. This type of texture is less common compared with planar-s and planar-e, mainly restricted to the upper part of Aqra section. Based on the classification of Sibley and Gregg (1987), the shapes of dolomite crystals in the Khurmala Formation of the five studied sections can be grouped into three classes. These are (1) very fine to coarse-crystalline planar-s (subhedral), (2) medium to coarse crystalline planar-e (euhedral) replacement dolomite and (3) medium to coarse crystalline, nonplanar-a (anhedral) dolomite (Figs. 6–9). The most abundant one is planar-s and -e dolomite and the less abundant one are non-planar-a (anhedral) dolomite. 8. Cathodoluminescence study of cementation styles Cathodoluminescence has been used to provide insights into the chemical discrepancies between preserved remnants of depositional components that results from various diagenetic processes in carbonate rocks as recognized from thin section petrography. CL provides visual information on the spatial distribution of certain trace elements, especially manganese (Mn2+) and iron (Fe2+) in calcites and dolomites (Machel and Burton, 1991; Scholle and UlmerScholle, 2003). An integration of cathodoluminescence and EMPA analysis revealed that the three groups of carbonate cementation occurred in Khurmala Formation have nearly pure end-member composition, corresponding to 46.64% MgCO3 and 98.64% CaCO3, with trace amounts of Mn, Fe, and Sr (Tables 2 and 3). The three types are (1) Pure calcite cement that is composed mainly of high-Mg calcite, where it occludes primary and secondary porosity (Fig. 6e), (2) dolomite cement, and (3) ankerite cement (Fig. 10). 8.1. Calcite cement

This type forms mosaics of subhedral to anherdal planar-s crystals (70–500 lm) that are milky white, clear, or have a cloudy appearance. One main characteristic of this type is the presence of intercrystalline porosity (Fig. 7g). 7.3. Dolomite texture-3: unimodal, medium to coarse crystalline planar-s (subhedral) mosaic dolomite This type forms dense mosaics of subhedral to anherdral planar-s crystals (7–600 lm), that are milky white, or have a cloudy appearance (Fig. 7h). Most characteristics of this type is

Three types of calcite cements were recognized, in pores, fractures and veins. They are (1) non-ferroan syntaxial calcite around echinoderm (C1) that have varying size based on the size of echinoderm (Fig. 6f); (2) weakly-zoned equant calcite (C2) of dullluminescence with typical homogenous red color, that commonly fills interparticle, interskeletal and biomoldic pore spaces and is surrounded by very fine to fine crystalline dolomite with a size range 22–57 lm (Fig. 11a); (3) oscillatory zoned, coarse blocky calcite cement (C3) with characteristic bright orange luminescence and tight zonation (Fig. 11b) that reduces secondary porosity and

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Fig. 8. Coarse-crystalline, planar-e dolomite rhombs floating in micrite and dolomicritic matrix, and porosity of stylolitic cracks (SEM) and energy dispersive X-ray Spectrum (EDX).

Fig. 9. Photomicrographs showing: (a) Dolomite texture – 4 medium to coarse crystalline planar-s (euhedral) replacement dolomite (Sheraswar section, sample 19) P.P.L. Scale bar 300 lm. Xenotopic dolomite texture5 of non-planar (anhedral) dolomite (Aqra section, sample 12) X.P.L. Scale bar 0.2 mm.

veins; it postdates other calcite cements and can be divided into two different generations (1 and 2) (Fig. 11b) ranging in size from 84–333 lm. The high resolution spectral analysis of emission (HRS-CL) and EMPA analysis for C2 and C3 in Khurmala Formation revealed low trace elements of Mn content range 26–196 ppm C2, while in C3 range between 10–150 ppm and low content of Fe with variable content of Sr (Table 3) is similar to the low Fe+2 calcite type studied by Habermann et al. (1998).

(Fe+Mn)CO3 100 10 90

So2 Sh1 Kh2 So3 GD5 GD5AB

20 80 30 70 40 60 50

8.2. Dolomite cement

50 60

Cathodoluminescence (CL) microscopy provides additional information to aid in the recognition of dolomite cement and paragenetic relations with their host rock (Marshall, 1988; Baker and Kopp, 1991; Pagel et al., 2000; Gaft et al., 2005; Boggs and Krinsley, 2006). The incorporation of Mn+2 in calcite results in a red to orange luminescence with a maximum broad band peak at 605–620 nm; while the red luminescencing dolomite has a peak around 656 nm (Richter et al., 2003). Two main types of dolomite were petrographically recognized in the Khurmala Formation, the ‘‘replacement dolomite’’ is common in lime-mudstone and wackestone facies while ‘‘dolomite cement’’ primarily is observed in packestone and grainstone facies. The majority of dolomite in the studied sequence is formed by the replacement of lime mud. Crystal size of the replacement dolomite is controlled, in part, by the depositional texture of the sedimentary facies that was subsequently dolomitized. Under hot cathodoluminescence, dolomitized lime mud and shelled fossils have identical uniform orange-red luminescence (Fig. 12a and b). Cander (1994) suggested that a dark nucleus can be interpreted as a replacement dolomites produced in marine pore waters and the lighter-luminescing overgrowths as a

40 70 30

C

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80 90

10

100

CaCO3

A 90

100

B 80

70

60

50

40

30

20

10

MgCO3

Fig. 10. (Fe, Mn)CO3–CaCO3–MgCO3 ternary diagram showing cementation of ankerite in Khurmala Formation.

product of cement precipitation from mixed marine-meteoric ground waters. Dolomite cement occurs in two different types which were observed with different paragenetic events. The first type is the earliest dolomite cement generation that fills primary, moldic and fracture porosity (Fig. 12c). Numerous micro-inclusions which are most probably clays give the cement a cloudy appearance.

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red colors that occur as bands filling most cavities. Precipitation of this cement usually postdates the formation of all secondary porosity. Both cements consist of very fine to fine crystalline dolomite. Replacement and dolomite cements of 1 and 2 types are characterized by a high content of Fe (>2000 ppm, Table 3), resulting in quenching of Mn-induced luminescence (Gillhaus et al., 2001). Baker and Burns (1985) estimated that strontium in modern marine dolomite ranges from 245 ppm up to 600 ppm. In this study that the replacement dolomite is characterized by low content of trace element of strontium ranging between 33–108 ppm (Table 3). This low concentration is quite common in ancient dolomite sequences (Weber, 1964; Tucker, 1983). Veizer et al. (1978) proposed that dolomitization in a system with low water/rock ratios can account for the low strontium content of ancient dolomites.

8.3. Ankerite cement Petrographic investigations of Khurmala Formation in the five studied sections revealed that dolomite-ankerite cement occurs in the form of minute rhombs (20–100 lm) scattered in the primary pore space of dolomite-rocks within Aqra and Gulley Duhok sections (Figs. 12e,f). They are commonly lining skeletal molds but also fill fractures and unfilled primary porosity (Fig. 12c). Microprobe analysis revealed variable amounts of (Fe + Mn)CO3 in Aqra section ranging from 0.92–13.12 wt%, while their ranges in Gulley Duhok were 0.00–17.99 wt% (Table 2). Strongly ferroan zones are common in generation fourth of cement. In cathodoluminescence, the cement is non-luminescent and shows dark color inner zone and orangish red colors of the outer zone (Fig. 12e). Cathodoluminescence investigations revealed that dolomite-ankerite post-dates both authigenic pyrite and void filling calcite, though the later shows ubiquitous recrystallization to form interlocking blocky calcite. Most of the rhombs exhibit zonation in CL and BSE images (Fig. 12e). Milliken (2002) proposed that ankerite in carbonate rocks, similar to other volumetrically important authigenic minerals, manifests a special distribution at small scales that reflects a difficulty with nucleation, suggesting that these petrographic forms of ankerite were precipitated from the same fluids and record a common history of fluid-rock interaction. The Fe-Mn rich ankerite cement that belong to group (C) of carbonate cements in Khurmala Formation was precipitated from reducing pore water.

9. Cement sequence Fig. 11. (a) CL-image of weakly-zoned calcite cement showing a dull luminescence (C2), and an EMPA scan through the A–B section showing the fluctuation in trace element concentrations across the traverse; (b) the vugs are filled by a zoned blocky calcite cement (C3). The zonation consists of two generations 1.a calcite with intrinsic blue CL (stoichiometric calcite without activators); 2.a heavily zoned calcite cement with bright orange luminscence, and an EMPA scan through A–B section showing variation in trace element concentrations along the direction of zoned blocky calcite cement. (Gulley Duhok, sample 5).

Cathodoluminescence of an area displaying micro-boxwork dolomite and early fibrous marine cements is shown in Fig. 12c. The luminescence pattern varies from dull red to bright red, and orange red throughout this dense, tight dolomite (Fig. 12c). Most of the original carbonate fabric associated with carbonate sediment and early marine cements can be seen in the dull and luminescence red patterns. However, this feature of dolomite cements is not visible under crossed polars (Fig. 12d). The second type is the last formed dolomite cement and is characterized by bright orange

The broad variations in properties of calcite and dolomite cements are advantageous for stratigraphic investigations, since in an ideal case, all cement generations can occur in a single crystal (Fig. 13). Correlation of a single CL zones between different samples is not possible because some zones may be missing or because a pore can be filled by one single enlarged zone. For this reason, only characteristics CL criteria for cement sequence (e.g. dull to red CL, microdolomite and echinoderm cement, dull CL, weakly zoned equant calcite, blue CL, homogenous stoichometric calcite, bright orange zonation CL, blocky calcite cement, non-luminescent red to orange zoned CL, and ankerite cement) were utilized for differentiation and correlation of cements. In the last three decades various authors have reported a detailed chemical zonation in carbonate cements by means of CL-microscopy, especially in hoxomoaxial rim cements crystallized on echinoderm or in radiaxial fibrous cement (Kim and Lee, 2003). The CL properties of carbonate minerals reflect the spatial distribution of Fe (quencher) and Mn (activator) ions (Bruhn et al., 1995).

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a

b vf m

R

P

c

d 1 2 2

1

e

f

A

A

0.3mm

100µm

Fig. 12. Paired plane-polarized light and cathodoluminescence photomicrographs of the diagenetic phases of Khurmala Formation showing: (a) replacement dolomite of dolomitized gastropod wackstone showing two types of replacement, very fine (vf) to medium (m) and pores (p); the dashed line delineate the boundary of the skeleton (Sorek section; sample 2; XPL); (b) CL image of the area shown in Fig. 12a; the replacement dolomite shows a homogenous red orange (R) luminescence; (c) CL image of the earlier dolomite cement(1) dull luminescence showing sharp contacts with the later dolomite cement of bright orangish red color growth bands (Sorek section; sample 2); (d) the same image of (c) under plane-polarized light indicating only the outlines of larger dolomite crystals; the early and late dolomites cannot be recognized from each other; (e) CL image of dolomite-ankerite rhombs (A) occurring as cement with zoned internal structures that postdated both, the early and late dolomite cement (Gulley Duhok section; sample 7); (f) EMPA-BSE image showing the paragenetic sequence of cements in Khurmala Formation indicating voids filled by blocky calcite (c), dolomite (d), ankerite zoned crystals (a), quartz (q) (Aqra section; sample 24). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Cathodoluminescence investigation of carbonate cements in the present study shows that the Khurmala Formation has been undergone different generations of cementation with variable luminescence. The early diagenetic cements of generation 1 occur in the lower part of the formation which has dull to red luminescence (Fig. 12c). Other characteristic features are mainly composed of very fine to fine microdolomite texture planar-s, with syntaxial rim cements on echinoderms. Similar CL characteristics were reported from early calcitic cements of Cretaceous sediments of Texas (Modovanyi and Lohmann, 1984). EMPA analysis of trace elements for generation 1 dolomite shows a high percentage of Fe/Mn with a range of 3.97–5.11 (Table 3). Generation 2 is characterized by dull luminescence of weakly zoned growth equant calcite cement filling interparticle pores (Fig. 13). Cement generation 3 with the help of recurrent CL sequences, can further be subdivided into the cement sequences 3.1 and 3.2. Subgeneration 3.1 is generally composed of homogenous intrinsic blue (CL) stoichometric calcite without activators (Fig. 11a). Occasionally

this type of cement reflects fluid stability under oxidized conditions with no much fluctuation. It is generally accepted that Mn+2 and trivalent REE-ions are the most important activators of extrinsic (CL) in carbonate mineral, while Fe+2 is a quencher of CL (Marshall, 1988; Pagel et al., 2000). Subgenerations 3.2 is characterized by bright orange luminescence with abundant sector zoning (Reeder and Paquette, 1989). A similar feature has been reported by Bruckschen et al. (1992) (Fig. 11b). Heavily zoned calcite cement (C3) is documenting unstable pore water conditions with a frequent change in Mn-concentrations. This phase reflects changing redox conditions as this overgrowth are zones added during crystal growth. The matrix are present in red CL which also affected by later fluids that may have caused recrystalization of the original mineralogy. The cementation history ends with generation 4. This cement is rich in Fe-Mn content and called ankerite cement. A main characteristic of this cement is the bright orange red, non-luminescence growth zonation (Fig. 12e). Generation 4 is considered the last

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Fig. 13. Schematic section through the Paleocene–L. Eocene Khurmala Formation showing the CL pattern of the cement generations.

carbonate cement generation created in Khurmala Formation which could be took place during Lower Eocene. 10. Discussion The main diagenetic features observed within carbonate dominated facies include micritization, compaction, neomorphism, dissolution, cementation representing early or syn-depostional and post-depositional changes. Earliest micritization of bioclasts is common, but ooids are also micritized. Micritization is mainly caused by endolithic cyanobacteria in environments with low concentrate rate, and the presence of ubiquitous microbes (Bathurst, 1972). Such conditions are prevalent within the restricted lagoonal environments which are interpreted for the Khurmala Formation (AlBanna et al., 2006). Based on petrographic interpretation and geochemical analyses of the carbonate rocks in the Khurmala Formation it is suggested that, dolomitization occurred in deep burial diagenetic environment. This interpretation is based on the following observations. Shape and size of replacement dolomite crystals is mostly planar-s forming mosaic of subhedral to anhedral crystals that commonly emulate the former crystal sizes of dolomitized carbonate facies. Lime mudstone facies were commonly transferred to finegrained dolostones whereas coarser facies such as wackestone or packestone facies were transferred to coarse-grained dolostones. The deposition of lime mudstone, argillaceous lime wackestone and sandy lithofacies in Khurmala Formation is interpreted as intertidal and supratidal environment as suggested by AlBanna et al. (2006). The fine crystal size of replacement dolomite in these lithologies may be result of an early replacement of precursor peritidal lime mudstones or of neomorphism of pencontemporaneous or early diagenetic dolomite (Zenger, 1983; Amthor and Friedman, 1991). Type 3 dolomite texture of the studied Khurmala Formation is interpreted to represent an intermediate to late diagenetic replacement. The preservation of original depositional textures and the coarse crystal sizes suggest a

major, probably long lasting, dolomitization event. This type is comparable to type 4 of Amthor and Friedman (1991) which was interpreted by them as of late burial origin. However, the characteristic cloudy appearance of dolomite of type 3 is common in rocks of all ages (Amthor and Friedman, 1991), as well as nonmimetic replacement of allochems. Dolomite associated with the chemical compaction structures, such as dissolution seams, argillaceous lamination microstylolites (Fig. 7a,b), was possibly formed by the same mechanism and perhaps at the same time as the replacive dolomite. After mechanical compaction which compressed the surrounding shales, chemical compaction followed, producing structures which provided conduits for expelled Mg+2 to travel along and thus provided suitable environment for the formation of small euhedral dolomite crystals. The typical absence of crystal zoning observed in the scanning electron microscopy and cathodoluminescence studies of Khurmala Formation dolomite suggest that each phase of dolomitization most probably occurred as a single event rather than by steps. Absence of crystal zoning indicates that either the chemistry of the diagenetic fluid remined constant during the formation of the dolomite (Reeder, 1981; Tucker and Wright, 1990; Walker and Burley, 1991; Allan and Wiggins, 1993; Warren, 2000) or that no changes in the precipitation rate of the crystals occurred (Tucker and Wright, 1990). According to those observations, the replacement dolomite apparently was not affected by significant recrystallization (Land, 1985; Kupecz et al., 1993). The high concentration of Fe (up to 3.92 ppm) also indicates deep burial conditions for the formation of dolomite (Dickson and Coleman, 1980; Tucker and Wright, 1990; Warren, 2000). Iron and manganese are redox sensitive and the high contents of these elements in both dolomite types recognized in Khurmala Formation, the replacement dolomite and the dolomite cement indicate that the dolomitization fluids were reduced (Table 3). The timing of the cementation pathways (Fig. 13) was as follows: The CL characteristics of cement generation-1 suggest that this cement originally consisted of high Mg-calcite (HMC) of marine

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origin (Lohmann and Meyers, 1977; De Wet, 1987); the patchy CL probably indicates varying Mg concentrations in different crystal domains. Transformation into LMC commenced probably in Mgrich domains and was not coeval within all domains of cement generation 1 (Richter, 1984). Simultaneously the Mn/Fe ratio or the Mn content of pore fluid, or the fractionation factor for Mn+2/Fe+2 into calcite, may have varied as a consequence of changing precipitation rates, temperature, Mg content of the pore water or other phenomena (Dromgoole and Walter, 1990). This may have created patchy luminescence (Neuser and Ritcher, 1986). In addition, microdolomite inclusions indicate that the Mg-rich precursor cement was transformed into LMC and dolomite in a closed or semi-closed system (Ritcher, 1974; Richter et al., 1986). The weakly zoned equant calcite of cement generation-2 is usually thought to be the result of intermediate to late stages of burial diagenesis. It appears that elements other than Mn and Fe do not have any appreciable effect in enhancing or reducing luminescence (Budd et al., 2000). Because of its typical CL features, the cement subgeneration 3.1 has dark blue intrinsic or non-luminescence pattern (Fig. 11b) which can be interpreted as a precipitate that was influenced by oxidizing pore waters of a meteoric phreatic lens (Emery and Dickson, 1989; Horbury and Adams, 1989). The bright orange CL colors and the zonation in cement subgeneration-3.2 perhaps reflect changes in pore water chemistry and slight differences in redox conditions (Fig. 13) that was precipitated under shallow burial conditions in a phreatic environment during an incipient stage of late diagenesis. Based on its CL characteristics of generation-4 and the fact that this generation has orangish red non-luminescence zonation (Fig. 12e and f) suggests that has postdated other carbonate cements and that pyrite was precipitated under reduced environments during Late Eocene time (Fig. 13). 11. Conclusions The main conclusions of this study are as follows: 1. Paleocene–L. Eocene Khurmala Formation was subjected to different diagenetic processes. Micritization was the early process which occurred under marine meteoric conditions, besides dissolution, neomorphism compaction, cementation and pyritization. 2. The xenotopic form of replacement dolomite and the presence of dolomite cements suggest dolomitization at elevated temperatures. 3. Unimodal very fine to fine crystalline planar s-(subhedral) mosaic dolomite is interpreted as of early dolomitization origin. 4. Unimodal medium to coarse crystalline, planar s-(subhedral) mosaic dolomite is interpreted as an intermediate to late diagenetic replacement occurrence. 5. Coarse to medium crystalline non-planar a-(anhedral) dolomite occurred as replacement of a precursor limestone or dolostone. 6. Electron microprobe analysis revealed three groups of carbonate cements; calcite, dolomite and ankerite. Ankerite cements postdated other cements and pyrite. 7. The Khurmala Formation dolomite is slightly grey in color and consists mainly of dolomite that was controlled, in part by the former depositional fabrics of the calcareous facies. Finegrained dolomite occurs mainly in carbonate mud-rich facies and coarser dolomite is found mostly in carbonate mud-poor lithofacies. Replacement dolomite has uniform orangish red luminescence. Earlier dolomite cement is characterized by dull to red luminescence; while the later dolomite cement is bright orangish red in color. Both replacement and dolomite cement lack zoning. In contrast, the blocky calcite cement shows concentric zoning of bright, dull and non-luminescent bands.

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8. The characteristics of luminescence features of carbonate cement sequences in the Khurmala Formation helped in distinguishing four cement generations with different non-luminescent, dull luminescent and bright luminescent. These cement generations (1; 2; 3-1; 3-2; 4) occurred under reduced conditions, whereas cement generation 3-1 occurred under oxidizing environments.

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