Sedimentary facies and diagenetic features of the ...

Journal of African Earth Sciences 87 (2013) 59–70

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Sedimentary facies and diagenetic features of the Early Cretaceous Fahliyan Formation in the Zagros Fold-Thrust Belt, Iran Mohammad Sahraeyan a,⇑, Mohammad Bahrami b, Mohammad Hooshmand c, Shahid Ghazi d,e, Ali Ismail Al-Juboury f a

Department of Geology, Khorasgan (Esfahan) Branch, Islamic Azad University, Esfahan, Iran Department of Geology, Science and Research Branch, Islamic Azad University, Shiraz, Iran Department of Geology, North Tehran Branch, Islamic Azad University, Tehran, Iran d School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK e Institute of Geology, Punjab University, Quaid-e-Azam Campus, Lahore 54590, Pakistan f Geology Department, Mosul University, Mosul, Iraq b c

a r t i c l e

i n f o

Article history: Received 5 February 2013 Received in revised form 7 July 2013 Accepted 12 July 2013 Available online 23 July 2013 Keywords: Microfacies Carbonate ramp Calcareous algae Benthic foraminifera Fahliyan Formation

a b s t r a c t The Early Cretaceous Fahliyan Formation (middle part of the Khami Group), is one of the important reservoir rocks in the Zagros Fold-Thrust Belt. The Zagros Fold-Thrust Belt is located on the boundary between the Arabian and Eurasian lithospheric plates and formed from collision between Eurasia and advancing Arabia during the Cenozoic. In this study area, the Fahliyan Formation with a thickness of 325 m, consists of carbonate rocks (limestone and dolomite). This formation overlies the Late Jurassic Surmeh Formation unconformably and underlies the Early Cretaceous Gadvan Formation conformably at Gadvan Anticline. The formation was investigated by a detailed petrographic analysis to clarify the depositional facies, sedimentary environments and diagenetic features in the Gadvan Anticline. Petrographic studies led to recognition of the 12 microfacies that were deposited in four facies belts: tidal flat, lagoon, and shoal in inner ramp and shallow open marine in mid-ramp environments. The absence of turbidite deposits, reefal facies, and gradual facies changes show that the Fahliyan Formation was deposited on a carbonate ramp. Calcareous algae and benthic foraminifera are abundant in the shallow marine carbonates of the Fahliyan Formation. The diagenetic settings favored productioning a variety of features which include cements from early to late marine cements, micritization, dolomitization, compaction features, dissolution fabric, and pores. The diagenetic sequence can be roughly divided into three stages: (1) eugenic stage: marine diagenetic environment, (2) mesogenic stage: burial environment, and (3) telogenic stage: meteoric diagenetic environment. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The Mesozoic carbonate systems of the Arabian Plate, one of the richest hydrocarbon provinces of the world are mostly caused by the combination of their large-scale dimensions and source rock, reservoir rock, and seal rock in the same depositional system (Murris, 1980). The remarkable concentration of the fundamental ingredients of a petroleum system is a large extent, caused by the repeated formation of organic-rich shallow basins on the northeast of Arabian Plate. Their formation in large carbonate platform successions assured significant source rock deposition in immediate contact with potential reservoir facies (van Buchem et al., 2002). The type section of the Fahliyan Formation is situated on the southern flank of Kuh-e Dal, near the Fahliyan Village (about ⇑ Corresponding author. E-mail address: [email protected] (M. Sahraeyan). 1464-343X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jafrearsci.2013.07.004

10 km north of Nourabad Mamasani City), in the Zagros FoldThrust Belt. The Fahliyan Formation is widespread in the Zagros Fold-Thrust Belt (Fig. 1) and consists of massive oolitic to pelletic limestone with minor contemporaneous brecciation in the basal part of the type section (James and Wynd, 1965). Variation in relative sea level led to depositioning two, third-order sequences in the type section with sequence boundary types I and II in this formation with Neocomian age (Adabi et al., 2010). Limestone sequences of the Sulaiy and Yamama formations in Saudi Arabia/ Iraq, the Minagish Formation in Kuwait, and the Ratawi Formation in Kuwait/Iraq are the equivalents of the Fahliyan Formation (Christian, 1997). Since there are not any studies on the sedimentary facies and diagenetic features of the Fahliyan Formation in the Gadvan Anticline, the aim of this paper is to determine the main microfacies, interpret the depositional environments and diagenetic features of the Fahliyan Formation deposits.

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M. Sahraeyan et al. / Journal of African Earth Sciences 87 (2013) 59–70

Fig. 1. Lithostratigraphic chart of the Mesozoic of the Zagros Basin (after James and Wynd, 1965).

2. Geological setting

energy index classification, and sedimentary data by comparison with modern environments (e.g., Flügel, 2010; Tucker and Wright, 1990; Wilson, 1975). Abundance of large benthic foraminifera, green algae, sponge spicules, mollusks, echinoderms, and non-skeletal grains (e.g., ooids, intraclasts, peloids, and aggregate grains) were considered. Sedimentologic textures and structures were considered qualitatively.

The Gadvan Anticline is located in Fars Salient in the south of Zagros Fold-Thrust Belt (ZFTB) (Fig. 2). The ZFTB in Iran forms the external part of the Zagros active orogenic wedge. It includes a heterogeneous sequence of the latest Neoproterozoic–Phanerozoic sedimentary cover strata, about 7–12 km thick (Alavi, 2007). The ZFTB is the deformed state of the Zagros Basin that is extended over the northeasternAfro-Arabian continental margin and is affected by the Early Cretaceous to present Zagros Orogeny. The ZFTB, as the external part of the Zagros Orogen (Alavi, 1980, 1994), extends southeast for nearly 2000 km from southeastern Turkey through northern Syria and northeastern Iraq to western and southern Iran. The northwestern boundary of the Zagros Fold-Thrust Belt is chosen to be the East Anatolian Strike-Slip Fault (EAF) in southeastern Turkey and the southeastern boundary of the Oman Line (Falcon, 1969). The Zagros Orogen consists of three distinctive parallel tectonic zones, which from the northeast to the southwest are: UrumiehDokhtar Magmatic Assemblage (UDMA), Zagros Imbricate Zone (ZIZ), and Zagros Fold-Thrust Belt (ZFTB). The ZFTB (the Zagros Simple Folded Zone of (Falcon, 1974), with an average width of 300 km, extends parallel and to the southwest of the ZIZ. It constitutes the external (hence less-strained) part of the orogen. In contrast to the ZIZ, in which exposed structures are predominantly thrust faults, the ZFTB is distinguished by its long (up to 150– 200 km), en echelon, and whaleback anticlines, which are spectacularly displayed on satellite images (Alavi, 2007). The salients and recesses of the ZFTB from southeast to northwest are as follow: Fars salient, Dezful recess (formerly Dezful Embayment), and Lorestan salient in Iran, and Kirkuk recess (Kirkuk Embayment) in Iraq (Alavi, 2004, 2007).

The Fahliyan Formation in the study area consists of five units. Unit 1 with a thickness of 55 m consists of thick-bedded or massive dark gray limestone (grainstone and packstone; Fig. 5). The rocks contain skeletal grains of various groups, including calcareous algae, foraminifers, brachiopods, mollusks, and echinoderms. Unit 2, with a thickness of 95 m is composed of dark gray limestone in the lower part and thin- to medium-bedded dolostone in the upper part. Most of the rocks were originally composed of coarse-grained carbonate. The lower limestone generally appears medium to thick bedded and coarse grained with a packstone texture. Common fossil constituents are calcareous algae, foraminifers, echinoderms, and mollusks. Unit 3, with a thickness of 50 m is composed of thin- to medium-bedded limestone and is rich in large fossils. Fossil constituents are calcareous algae, foraminifers, mollusks, and echinoderms. Unit 4, with a thickness of 92 m consists of very thick-bedded limestone with a packstone texture. Many horizons have facies abundant in peloids, which often have a microstructure resembling that of calcareous algae. Upward, the packstone facies tends to dominate, and fossil components decrease in both abundance and diversity. Unit 5, with a thickness of 33 m is composed of thin-bedded limestone (wackestone) with calcareous algae, foraminifers, mollusks, and radiolarians.

3. Study area and methodology

5. Facies analysis and depositional environment

The study area is located at the northeast of Gadvan Anticline, about 45 km northeast of Shiraz in the Zagros Fold-Trust Belt (Fig. 3). The Fahliyan Formation in Gadvan Anticline (29°370 1500 N, 52°580 4000 E) overlies the carbonates of the Late Jurassic Surmeh Formation with an unconformable contact (Fig. 4a) and underlies the Early Cretaceous Gadvan Formation conformably (Fig. 4b). This formation in the Gadvan Anticline consists of 325 m limestone and dolomite (Fig. 5). Field studies, petrography and microfacies analysis were carried out on Gadvan Anticline outcrop section and 125 samples were prepared. For petrographic analysis, the grain and matrix percentages were estimated using visual percentage charts (Flügel, 1982). (Dunham, 1962) classification was used for carbonate facies nomenclature. The (Wilson, 1975) and (Flügel, 2010) facies belts nomenclature and sedimentary models were applied. Facies types and depositional setting were interpreted based on analysis of matrix and grains, compositional, textural fabric, fossil content,

Thickness of the Fahliyan Formation in the Gadvan Anticline is 325 m and consists of carbonate rocks with a high variety of skeletal and non-skeletal grains, calcite cements, micrite, and late diagenetic dolomites. Based on lithology, textures and fossil contents of outcrop samples from the study area, there are 12 microfacies types in the Fahliyan Formation. Four main facies belts were distinguished from distal to proximal, these are: shallow open-marine, shoal, lagoon, and tidal flat. These facies belts are described briefly below:

4. Lithology

5.1. Shallow open-marine facies belt This facies belt comprises mudstone, sponge spicule wackestone, sponge spicule-echinoderm wackestone, and bioclast-sponge spicule wackestone. Bioclasts are mainly composed of sponge spicules and echinoid/crinoid remains. Subordinate constituents are thin-shelled bivalves, benthic foraminifera, and dasycladacean algal


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Fig. 2. Location and geological map of the study area. (a) General map of Iran showing eight geologic provinces. The Gadvan Anticline is located in the folded Zagros, adapted from Lacombe et al. (2006) and Mobasher and Babaie (2008). (b) Subdivisions of the Zagros Mountains and Fars Sub-basin, after Motiei (1994), with situation of the study sections in Fars province.

fragments that are not in situ. This facies belt, from distal to proximal environments, consists of: 5.1.1. Mudstone This microfacies consists of micrite with less than 10% bioclasts (Fig. 6a). Bioclasts mainly composed of sponge spicules and echinoid/crinoid remains with subordinate bivalve, gastropod, small benthic foraminifers, green algae, and shell fragments. Large amount of micrite indicates a low-energy environment. This microfacies is equivalent to a fossiliferous micrite and comparable to SMF 3 and RMF 2 of (Wilson, 1975) and (Flügel, 2010), respectively. 5.1.2. Sponge spicule wackestone This microfacies is dominated by sponge spicules (20–40%) with less than 10% of other bioclasts (Fig. 6b). Subordinate

bioclasts mainly composed of echinoid/crinoids remains, thinshelled bivalves, gastropod, and algal fragments as well as benthic foraminifera such as Lenticulina sp. and Trocholina sp. Authigenic quartz and pyrite are the non-carbonate components, which present in this microfacies. This microfacies is comparable to SMF 8 and RMF 3 of Wilson (1975) and Flügel (2010), respectively.

5.1.3. Sponge spicule-echinoderm wackestone The main allochems of this microfacies are echinoderm (15%) and sponge spicules (10%) (Fig. 7a); a high content of shell fragments are also present. Bivalves, gastropods (Fig. 7b), green algae, and benthic foraminifera are present but rare in this microfacies. This microfacies is comparable to SMF 12 and RMF 7 of Wilson (1975) and Flügel (2010), respectively.

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M. Sahraeyan et al. / Journal of African Earth Sciences 87 (2013) 59–70 o

52 55 o / // 29 39 25

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53o00/ o / // 29 39 25

Feyzabad

ksv

kkz

Khuraki

kgd

kdr

ksv kkz kdr kgd kfa Jsm Jnz

Jnz

kfa Jsm

IRAN

Sarvak Formation Kazhdomi Formation Dariyan Formation Gadvan Formation Fahliyan Formation Surmeh Formation Neyriz Formation

ad G

Thrust

va

Fotuh Abad

n

Anticline showing plunge

ic nt A

Jsm

Stratigraphic section

e

lin kgd

kfa

kdr

ksv o

/

//

29 34 29 o / 52 55

o

0

/

29 34 29 o / 53 00

2km

//

Fig. 3. Geological map of the study area.

Sample No. Thickness (metres) Formation Gadvan

Age

Lithology

Description

Gray, thin-bedded limestone interbedded with marl

325

300 100

Light-gray to gray, thin-bedded limestone

275 90 250

Dark-gray, very thick-bedded to massive limestone

225 70 200

Fahliyan

Early Cretaceous

80

175

60

150 50 125 40 100

Dark-gray, medium to thick-bedded limestone Yellow, cream, thin-bedded limestone Dark-gray, medium to thick-bedded limestone and dolomitic limestone Dark-gray, medium to thick-bedded limestone Gray, thin to medium-bedded, limestone, interbedded with shale

30 Dark-gray, medium to thick-bedded limestone

75 20 50 25

Surmeh

Late Jurassic

Fig. 4. Field photographs showing: (a) A northwest view of the Gadvan Anticline (lower bounadry of Fahliyan Formation with Surmeh Formation). (b) A southeast view of the Gadvan Anticline (upper bounadry of the Fahliyan Formation with Gadvan Formation).

0

10

Dark-gray, at places red, very thickbedded to massive limestone Gray, thin to thick-bedded limestone Dark-gray, thick-bedded limestone Dark-gray, thin to mediumbedded limestone Dark grey, massive, highly weathered dolomitic limestone and dolomite

Fig. 5. Lithostratigraphic column of the Fahliyan Formation in the Gadvan Anticline.


Fig. 6. Photomicrographs showing: (a) Mudstone. (b) Sponge spicule wackestone.

5.1.4. Bioclast-sponge spicule wackestone This microfaciesis dominated by sponge spicules (20%) as well as subordinate thin-shelled bivalves and echinoid/crinoid fragments (Fig. 8a). Dasycladacean algae (Fig. 8b), Lithocodium aggregatum, benthic foraminifera, gastropods, and bivalves are also present. Bioturbation is common in this microfacies. This microfacies is comparable to SMF 8 and RMF 3 of Wilson (1975) and Flügel (2010), respectively. Caused by the lack of a rimmed margin in the Fahliyan Formation depositional environment, the movement of sediments was common and algal fragments were transported from lagoon to shallow open-marine environments.

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Fig. 7. Photomicrographs showing: (a) Sponge spicule-echinoderm wackestone. (b) Gastropod in the Sponge spicule-echinoderm wackestone.

good sorting and roundness with 0.4 mm size (Fig. 9b). Subordinate intraclasts, aggregate grains and bioclasts are also present in this microfacies. Bioclasts of this microfacies are composed of gastropods, echinoderms, benthic foraminifers, and dasycladacean algae. This microfacies is comparable to SMF 16 and RMF 29 of Wilson (1975) and Flügel (2010), respectively. 5.3. Lagoon facies belt Four microfacies are associated with the lagoon facies belt. Diverse skeletal grains with abundant calcareous green algae present in these microfacies.

5.2. Shoal facies belt The shoal facies belt is in the platform margin. Two microfacies are observed in this facies belt that are mainly composed of intraclasts, peloids, bioclasts, and aggregate grains. 5.2.1. Peloid-intraclast packstone/grainstone Intraclasts (30%) and peloids (25%) are the dominant components of this microfacies (Fig. 9a). Intraclasts are well rounded with 500 lm to 4 mm in size. Bioclast (5%) include green algae, benthic foraminifera, gastropods, echinoderms, and L. aggregatum. This microfacies is comparable to SMF 14 and RMF 11 of Wilson (1975) and Flügel (2010), respectively. 5.2.2. Peloid-ooid grainstone This microfacies is characterized by a high abundance of micritized small ooids (55%) and peloids (25%). Peloids have

5.3.1. Trocholina wackestone The main component of this microfacies is Trocholina sp. (20– 40%) (Fig. 10a). Molds of Trocholina sp. are filled by sparry calcite cement and show the unstable mineralogy (aragonite) of this foraminifer (Fig. 10a). Subordinate skeletal grains are sponge spicules, gastropods, bivalves, echinoid/crinoid remains, dasycladacean algae, benthic foraminifers, and shell fragments. In addition, about 2% authigenic quartz is present in some intervals. This microfacies is comparable to SMF 18 and RMF 20 of Wilson (1975) and Flügel (2010), respectively. 5.3.2. Pseudocyclammina wackestone The main component of this microfacies is Pseudocyclammina littus (20–30%) (Fig. 10b). Subordinate skeletal grains are sponge spicules, gastropods, bivalves, echinoid/crinoid debris, dasycladacean algae, benthic foraminifers, and shell fragments. This

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Fig. 8. Photomicrographs showing: (a) Bioclast-sponge spicule wackestone. (b) Dasycladacean algae in the Bioclast-sponge spicule wackestone.

Fig. 9. Photomicrographs showing: (a) Peloid-intraclast packstone/grainstone. (b) Peloid-ooid grainstone.

microfacies is comparable to SMF 18 and RMF 13 of Wilson (1975) and Flügel (2010), respectively.

are observed abundantly in this microfacies (Fig. 12a). Diagenetic processes have intensively influenced this facies. Comparing with modern carbonate environments such as Persian Gulf (Friedman, 1995), this microfacies has been deposited at the upper parts of tidal flats in a warm and arid region.

5.3.3. Bioclast wackestone/packstone The predominant skeletal grains are dasycladacean algae associated with benthic foraminifers (30–35%) (Fig. 11a). Other bioclast components are sponge spicules, L. aggregatum, gastropods, bivalves, echinoid/crinoid remains, and shell fragments. Non-carbonate grains such as authigenic quartz (2–5%) and pyrite (2– 10%) are observed. This microfacies is comparable to SMF 18 and RMF 20 of Wilson (1975) and Flügel (2010), respectively. 5.3.4. Lithocodium wackestone The predominant skeletal grain is Lithocodium aggregatum (40%). Subordinate biogenic components include gastropods, dasycladacean algae, echinoid/crinoid debris, benthic foraminifers, sponge spicules, and bivalves (Fig. 11b). Also, about 10% intraclast and authigenic quartz are observed in some intervals. This microfacies is comparable to SMF 18 and RMF 17 of Wilson (1975) and Flügel (2010), respectively. 5.4. Tidal flat facies belt Two microfacies are recognized in this facies belt. 5.4.1. Dolomitic mudstone Features such as fenestral fabric, evaporate molds, microbial filaments, mud cracks, and anhydrite nodules in a dolomicrite matrix

5.4.2. Intraclast-peloid wackestone/packstone This microfacies consists of intraclasts (10–15%) and peloids (25–40%). Pisoids are also present in this facies (10%) (Fig. 12b). Silt-sized quartz (10%) is observed in this facies. This microfacies has an extensive dissolution. This microfacies was deposited in the upper part of intertidal to supratidal sub-environment and is comparable to SMF 24 and RMF 24 of Wilson (1975) and Flügel (2010), respectively.

6. Paleoenvironmental model The Early Cretaceous Fahliyan Formation in the Zagros Basin was deposited on a gently dipping carbonate ramp (Hosseini and Conrad, 2008). Also, this formation is interpreted to have been deposited on a carbonate ramp with a very gentle slope in this study area (Fig. 13). The lack of any marginal reef development, the absence of major break of slope from shoreline into deeper water, no evidence of resedimentation (e.g., calciturbidites), and the presence of high-energy grainstone facies are consistent with the Fahliyan Formation have been deposited on a carbonate ramp (Wright, 1986).


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Fig. 10. Photomicrographs showing: (a) Trocholina wackestone. (b) Pseudocyclammina wackestone.

Fig. 11. Photomicrographs showing: (a) Bioclast wackestone/packstone. (b) Lithocodium wackestone.

According to (Burchette and Wright, 1992), carbonate ramp environments are separated into (1) the inner ramp, (2) the middle ramp, and (3) the outer ramp. In the studied section, the inner ramp depositional environment includes tidal flat, lagoon and shoal facies belts, and mid-ramp includes the shallow open-marine facies belt. In the shoal facies belt, grainstones facies are common. In the lagoon, the microfacies are mud-rich and dominated by benthic foraminifers and green algae. Sponge spicules are very common in the microfacies of the mid-ramp, associated with green algae and small benthic foraminifers in shallower parts.

serve to define and preserve the outline and morphology of the carbonate grains (skeletal and non-skeletal) (Fig. 9).

7. Diagenesis The following diagenetic features, developed in the studied area. A detailed account of the sequence of various diagenetic phases and their products depending on time hierarchy are presented here: 7.1. Micrite envelopes This is the first diagenetic phase, which takes place in the marine diagenesis of limestones (Adabi, 2009). Micrite envelopes develop around the skeletal grains, which are originally of aragonite composition. Such grains are very much prone to develop these envelopes. As aragonite is a metastable carbonate mineral, it is dissolved in the very early phase of diagenesis in the carbonate sediments and is subsequently replaced by calcite. These envelopes

7.2. Dissolution Sometimes the internal structure of the skeletal grains is destroyed and no relict structure is observed. However, the outline and morphology of these grains is preserved. 7.3. Cements Cement endows strength and stability to the carbonate microfacies and sediments. The well-developed cement always resists physical, as well as chemical compaction and fracturing episodes. Thus, the cementation of the carbonate sediments is believed to be an important diagenetic process. The early diagenetic cement precipitates as fibrous aragonite, while dog-tooth cement (Fig. 14a), dolomite cement, and drusy mosaic cement precipitate as the late diagenetic cements (Adabi, 2009). Intragranular cement is the next phase of the carbonate diagenesis. The sparry calcite cement with drusy mosaic of crystals is product of meteoric phreatic environment (Tucker, 1988). 7.4. Compaction 7.4.1. Fractures The brittle deformation of various grains results from the compaction of sediments. Dolomitic mudstones particularly undergo

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Fig. 12. Photomicrographs showing: (a) Dolomitic mudstone. (b) Intraclast-peloid wackestone/packstone.

fractures, which spread all parts of the rock body. Various phases of fracturing, sometimes along the stylolites, are observed in these deposits (Fig. 14b). In addition, Fractures are filled by calcite cement (Fig. 12b). The features are in conformity with the action of pronounced tectonic stresses, which is confirmed by folding and faulting, from small scale to regional scale, in the area hosting studied formation.

7.4.2. Stylolitization The signatures of stylolitization have been recognized as close packing and deformed morphology of the component grains in various microfacies. Increased tectonic stresses and thousands meters overburden pressure (commonly more than 2000 m) produce ultimately pressure solution seams, known as microstylolites in the

Fig. 14. Photomicrographs showing: (a) Dog-tooth cement in the deposits of Fahliyan Formation. (b) Fractures associated with stylolites in the mudstone microfacies.

thin sections. The microstylolites recorded in the Fahliyan Formation deposits are low amplitude. The frequency and presence of these microstylolites is noted relatively low. The microstylolites are found mostly in mudstones microfacies (Fig. 14b); however, these are also present in other microfacies (Fig. 15). The low amplitude of stylolites developed in the dolomitic mudstone facies has disrupted the calcite filled fractures. 7.5. Dolomitization The dolomitization of limestones during diagenetic processes is a common feature of the carbonate sediments. It is such a diagenetic process whereby the incorporation of Mg ions in calcite from the pore fluids charged with these ions results in the formation of

inner ramp Middle ramp MF 1-2 Tida l fla t

MF 3-6 Lag oon

Sea level MF 7-8 Sho al

MF: Microfacies FWWB: Fair Weather Wave Base

MF

9-12 O pe nm

FWWB arin e

Fig. 13. Depositional model of the Early Cretaceous Fahliyan Formation in the Gadvan Anticline.

Outer ramp


67

and are the most significant contributors to overall pore volume in the field. 7.6.2. Solution enhanced inter-granular Solution enhanced inter-granular pores form as a result of dissolution of the cement between carbonate grains and this pores is enlarged by dissolution processes and cementation between grains is further removed. 7.6.3. Solution enhanced intramatrix Solution enhanced intramatrix porosity is present in muddier facies and nongrain dominated rocks. The carbonate mud matrix undergoes stabilization to neomorphic microspar. Pores form as a result of dissolution of the carbonate mud matrix. Where these pores are well developed, porosity occurs as spotty, splotchy areas of dissolution (Fig. 17a). This is the precursor stage to vuggy porosity. 7.6.4. Vuggy Vuggy pores occur as enlarged solution enhanced intra-matrix pores (Fig. 17b). Pore outlines are irregular and pore shapes are usually blocky in nature. For this study, vugs are fabric selective and do not resemble preexisting component grain shapes or sizes. Vugs may contribute to overall effective porosity when interconnected by sufficient intra-matrix porosity. 7.6.5. Intraparticle Intraparticle porosity exists in the field in the form of intraskeletal pores. The nature of these pores is depositional that they

Fig. 15. Photomicrographs showing stylolites in the bioclast wackestone/packstone microfacies.

secondary nature of dolomite. In the Fahliyan Formation deposits, dolomitization is extensive and developed at various levels. Such dolomitization is not texture selective and attacks the whole fabric of rock as a result of which the wholly or partially rock gets dolomitized. The pervasive dolomitization has been recorded in the studied thin sections mainly in the form of microdolomite (Fig. 12a), in which the crystals of dolomite develop in a very small size and sometimes larger magnification is required to observe these crystals. 7.6. Pore types Several classification systems for carbonate porosity have been developed, such as those developed by (Archie, 1952) and (Choquette and Pray, 1970). The Archie (1952) classification system is based on the texture of rock matrix, visible pore structure, and typical petrophysical behavior that would be associated with the rock. The system developed by Choquette and Pray (1970) classifies pores based on whether they are fabric selective or not? It includes subsidiary descriptive sections that relate to the diagenetic alteration as well as additional size modifiers. 7.6.1. Grain moldic and incomplete moldic Molds and incomplete molds are formed from the diagenetic dissolution of metastable grains. Moldic pores show sharp and distinctive outlines of leached grains, while incomplete moldic pore boundaries are less distinctive and adjacent to recrystallized remnants of the original grain. Moldic and incomplete moldic pores (Fig. 16) dominate the skeletal packstone/grainstone microfacies

Fig. 16. Photomicrographs showing: (a) Grain moldic pore. (b) Incomplete moldic pore.

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environment, (2) mesogenic stage: burial environment, and (3) telogenic stage: meteoric diagenetic environment (Fig. 18). The transitions between stages are gradual and some diagenetic processes, such as pressure solution, may occur in several stages. 8.1. Eugenic stage: marine diagenetic environment In the eugenic stage, diagenetic alterations of these deposits are related to marine and early meteoric processes. Submarine lithification/cementation includes occurrence of isopachous, fibrous and micritic calcite cement rimmed peloids, and skeletal fragments. Solution channels and vugs are elongate, sub-parallel to bedding, and truncate laminations (Fu et al., 2004). Such vugs are lined with a rind of dolomite crystals. All the calcrete features were dolomitized, indicating that early massive dolomitization postdated subaerial diagenesis. In addition, karstification occurred during subaerial exposure (Fu et al., 2008). 8.2. Mesogenic stage: burial environment Physical compaction begins in the first several meters of burial environment (Choquette and James, 1990; Shinn and Robbin, 1983). Early generations of stylolitization are interpreted to occur in the Early Cretaceous when the Fahliyan Formation deposits were buried to a depth of between about 500 and 1500 m. The onset of stylolitization in limestone defines intermediate burial environment and occurs at 500–1000 m depth (Nicolaides and Wallace, 1997). The depth of stylolitization in dolomitic mudstone is deeper because of the increased resistance to pressure solution. The early crystallization of microcrystalline dolomite and the late-stage dolomitization are interpreted to have occurred during Early Cretaceous time. 8.3. Telogenic stage: meteoric diagenetic environment Fig. 17. Photomicrographs showing: (a) Solution enhanced intramatrix pores in the carbonate mud matrix. (b) Vuggy pores.

Pressure solution (stylolitization) continued into the telogenic stage. Fractures, truncate stylolites and most diagenetic fabrics are probably formed in response to the tectonism associated with the Cimmerian Orogeny. Calcite cement in fractures and solution vugs postdates the Cimmerian Orogeny because calcite cannot survive burial temperatures (Fu et al., 2008).

8. Diagenetic sequence

9. Discussions and conclusions

The Fahliyan Formation deposits have been extensively undergoing diagenesis. The diagenetic sequence can be roughly divided into three stages: (1) eugenic stage: marine diagenetic

The Arabian Plate was covered by shallow-water carbonates of the Yamama Formation during the Berriasian to Valanginian (Ziegler, 2001). In addition, (Sadooni, 1997) proposed sedimentation

Diagenetic features

existed before any type of diagenetic alteration took place. It is most common in gastropods (Fig. 7b) and within L. aggregatum mesopores (Fig. 11b) and is insignificant in contribution to overall porosity.

Early fracturing Early stage of dolomitization Karstification Micrite envelopes Isopachous calcite cement Marine phreatic

Marine, meteoric Mixed zone

Late stage of dolomitization Recrystallization of dolomite Dissolution and leaching Syntexial overgrowth

Meteoric

Late fracturing Stylolitization Late stage of dolomitization (?) Cementation

Burial

Increasing diagenetic levels with increasing time Increase of burial Fig. 18. Diagenetic sequence for the Fahliyan Formation illustrating the relative timing of the major diagenetic phases.


of Yamama Formation in SE Iraq on the gentle slope of the leeward ramp on the Arabian Plate. The Berriasian–Valanginian Minagish Formation in the north of Kuwait was deposited on a homoclinal carbonate ramp (Davis et al., 1997). During the Neocomian-Aptian, shallow-water carbonate sediments, containing a rich assemblage of foraminifers and calcareous algae were deposited in the Zagros Basin (Parvaneh Nejad Shirazi, 2008). At the beginning of the Cretaceous, the southwest of Iran was located to the north of the equator (less than 5°). The large-scale basin configuration had just changed from one of a differentiated passive-margin of shallow and deep shelves and intra-shelf basins, which characterized the Jurassic strata (Murris, 1980) to that of a very low relief passive-margin ramp setting (Al-Fares et al., 1998). In the Gadvan Anticline (Zagros Fold-Thrust Belt), the Fahliyan Formation is generally represented by shallow-water limestone. It overlies the carbonate sediments of the Late Jurassic Surmeh Formation unconformably and is overlain conformably by dark shale and argillaceous limestone of the Early Cretaceous Gadvan Formation. In the Fahliyan Formation deposits, four facies belts were recognized from the distal to the proximal part of the platform include open-marine, lagoon, shoal, and tidal flat. In the studied area, the Fahliyan Formation consists of 12 microfacies which deposited in the open-marine (mudstone, sponge spicule wackestone, sponge spicule-echinoderm wackestone, and bioclast-sponge spicule wackestone), shoal (peloid-intraclast packstone/grainstone and peloid-ooid grainstone), lagoon (Trocholina wackestone, Pseudocyclammina wackestone, bioclast wackestone/ packstone, and Lithocodium wackestone), and tidal flat (dolomitic mudstone and intraclast-peloid wackestone/packstone) facies belts. The present study indicates that the Fahliyan Formation was deposited on a homoclinal carbonate ramp with a gentle slope. The abundance of dasycladacean algae and benthic foraminifers in these deposits indicate a shallow-water depositional environment (inner to mid-ramp) within the zone of effective light penetration. Based on the paleoecological analysis, dasycladacean algae and benthic foraminifers of the studied formation show both normal marine and restricted environments. Due to the lack of reefal facies in the Fahliyan Formation, algal fragments were transported from lagoon to the shallow open marine environment (Flügel, 2010). The dominance of sponge spicules and micrite, scarcity of benthic foraminifera and dasycladacean algae, and pyritization suggest a shallow open-marine with low energy depositional environment (Adabi et al., 2010). The shallow open-marine facies belt was developed at the seaward part of the platform margin. Sponge spicules-bioclast wackestones were deposited in shallower parts, whereas subordinate constituents have been transported from lagoonal environment (e.g., some benthic foraminifera and dasycladacean algae). Mudstone to fossiliferous mudstone facies indicates the deeper parts of the basin. The abundance of allochems, grain-supported texture, well sorted and well rounded grain sand, and subordinate mud content in the microfacies of this facies belt indicate high-energy conditions (Adabi et al., 2010; Lucia, 1999; Palma et al., 2007; Reolid et al., 2007). Based on faunal and lithological composition, this facies belt developed within an inner ramp environment (lower parts of the measured section). The extension of carbonate shoals is one of the factors that characterize ramp environment (Elrick and Read, 1991). Peloids present in this facies belt show a transition from low- to high-energy conditions (Adabi et al., 2010). Some petrographic evidence such as the abundance of aragonite skeletal and non-skeletal components calls for a sub-tropical environment with original aragonite mineralogy (Adabi et al., 2010). Some components of the shallow open-marine facies belt were transported by storm wave into the lagoon (e.g., sponge spicules). Also, Calcareous sponge spicules are present in low-energy

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environments with normal salinity and are associated with green algae fragments. In the lagoon facies belt, bioclasts are diversity and are associated with peloids (Adabi et al., 2010; Bachmann and Hirsch, 2006). Peloids are common in some microfacies of this facies belt. Bioclastic packstones suggest higher energy conditions in a shallow subtidal setting (Flügel, 2010). The wackestone and packstone textures indicate deposition in the proximal part of a subtidal lagoon (Lasemi et al., 2008). Strong dissolution processes and abundant vuggy porosity, lamination, presence of silt, and sand-sized quartz and fenestral fabric suggest deposition in supratidal to intertidal environment (Adabi et al., 2010). Low diversity of faunal assemblages imply stressed paleoecological conditions such as elevated salinities (Adabi, 2009). These criteria and association with shallow marine facies suggest a peritidal sub-environment (Adabi, 2009; Bodzioch, 2003). During the Early Cretaceous, a shallow-water platform formed in the studied area. This platform was flooded and the Fahliyan platform started to grow. The seaward margin of this platform was dominated mainly by grainstone microfacies, and lagoon facies belt is muddy and algal and benthic foraminifers dominated. Mud-dominated skeletal limestone is the dominant microfacies of the carbonate beds in the upper parts of the Fahliyan and Gadvan Formations, which indicates deeper marine conditions. The Fahliyan and Gadvan time interval was a period of important clay supply in the Gadvan Anticline. The diagenetic settings favored the production of a variety of features, which include cements from early marine to late cements, micritization, dolomitization, compaction features, dissolution fabric, and pores. The diagenetic sequences of the Fahliyan Formation can be roughly divided into three stages: eugenic stage, mesogenic stage and telogenic stage. Acknowledgements The authors appreciate Islamic Azad University, Khorasgan (Esfahan) Branch for providing the laboratory facility. Thanks also to Mr. A. Sarmad for helping in the field, Dr. S. Davies for helping in language editing and Professor W. D. Huff for his comments on the style of the manuscript. We also express our gratitude to Dr. Pat Eriksson (Editor of the Journal of African Earth Sciences) and anonymous referees who made precise reviews, which helped us in enhancement of final version. References Adabi, M., 2009. Multistage dolomitization of upper jurassic mozduran formation, Kopet-Dagh Basin, n.e. Iran. Carbonates Evaporites 24, 16–32. Adabi, M.H., Salehi, M.A., Ghabeishavi, A., 2010. Depositional environment, sequence stratigraphy and geochemistry of Lower Cretaceous carbonates (Fahliyan Formation), south-west Iran. Journal of Asian Earth Sciences 39, 148–160. Al-Fares, A.A., Bouman, M., Jeans, P., 1998. A new look at the Middle to Lower Cretaceous stratigraphy, offshore Kuwait. GeoArabia 3, 543–560. Alavi, M., 1980. Tectonostratigraphic evolution of the Zagrosides of Iran. Geology 8, 144–149. Alavi, M., 1994. Tectonics of the Zagros orogenic belt of Iran: new data and interpretations. Tectonophysics 229, 211–238. Alavi, M., 2004. Regional stratigraphy of the Zagros Fold-Thrust Belt of Iran and its proforeland evolution. American Journal of Science 304, 1–20. Alavi, M., 2007. Structures of the Zagros Fold-Thrust Belt in Iran. American Journal of Science 307, 1064–1095. Archie, G.E., 1952. Classification of carbonate reservoir rocks and petrophysical considerations. AAPG Bulletin 36, 278–298. Bachmann, M., Hirsch, F., 2006. Lower Cretaceous carbonate platform of the eastern Levant (Galilee and the Golan Heights): stratigraphy and second-order sea-level change. Cretaceous Research 27, 487–512. Bodzioch, A., 2003. Calcite pseudomorphs after evaporates from the Muschelkalk (Middle Triassic) of the Holy Cross Mountains (Poland). Geologos 7, 169–180. Burchette, T.P., Wright, V.P., 1992. Carbonate ramp depositional systems. Sedimentary Geology 79, 3–57.

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