Sequence Stratigraphy of the Qom Formation in Natanz area (Isfahan-Iran)

Abdolhosein Kangazian

Department of Geology, Islamic Azad University, Isfahan (Khorasgan) branch, Isfahan, Iran,

E-mail address: kangazian@khuisf.ac.ir

Telephone No. 00989133083282

Fax No. 00983115354060

Vahid Safaei-Jazi

Department of Geology, Islamic Azad University, Isfahan (Khorasgan) branch, Isfahan, Iran,

Abstract:

The Oligocene-Miocene Qom Formation which consists of thin to thick bedded limestones,

marls and sandy-limestones (ca. 606 m thick) was examined in Natanz area (northeast of

Esfahan). This succession is disconformably laid on the (Eocene) volcanoclastic rocks and

covered by the (recent) alluvial sediments. According to microscopic and field studies 7

microfasiec, referred to the inner (open marine, restricted marine and bar), mid and outer

portions of a homoclinal carbonate ramp were distinguished. Based on the sequence stratigraphy

data, three (3rd order) sequences were recognized. The first sequence which overlaid on the

volcanoclastic succession begins with a type-1 sequence boundary (SBI). The boundaries

between the first and the second sequences and also between the second and the third ones are

the type-2 sequence boundary (SBII). All of these sequences are composed of TSTs and HSTs

but the HST of the third one was covered by the recent alluvial deposits.

KeyWords- Qom Formation; Microfacies; Sequence stratigraphy; Natanz area

1- Introduction:

After Loftu’s report (1854), the Qom Formation has been examined since mid-19th century (e.g.

Abich, 1858; Tietze, 1875; Stahl, 1911; Okhravi and Amini, 1998; Lasemi et al., 2005; Seyrafian

et al., 2005; Seyrafian and Torabi, 2005; Mohammadi et al., 2009; Safaei jazi et al., 2009;

Hadavi et al., 2010). Nowadays the Formation is very considerable for exploration of

hydrocarbor or storage of natural gas (Mostofi and Gansser, 1957; Abaie et al., 1963 Rosenberg,

1975). We intend to illustrate the sequence stratigraphy characteristics of the Qom Formation in

the Natanz area (Darheneh-Montaine), at 120 km northeastern Esfahan (Iran). In one hand, the

sequence stratigraphy data of this Formation can be used for its correlation and, on the other

hand, can reveal reservoir potential of it.

2- Geological setting and location of the studied section:

The study succession is located at the east of the Esfahan-Kashan road, near Natanz; where Qom

Formation outcropped as an anticline flank. The base of the section is positioned at 3330'N,

5158'E (figure 1). It is divided into three lithostratigraphic units (figure2) as follows:

Unit 1) Medium to thick-bedded shale and marl (ca. 106 m thick), with thin to thick-layer

limestone beds. The thickness and abundance of shale and marl is reduced upward the

succession.

Unit 2) Thick to massive bedded limestones (ca. 300 m thick) with a few marl and shale beds. It

is characterized by patch reefs.

Units 3) Medium to thick bedded limestone beds (ca. 300 m thick) alternated with reef

boundstones. Among them, layers of shale or marl cannot be seen.

3- Materials and methods:

More than 200 thin sections were prepared and examined under the microscope- based on Flügel

(1982) and Flügel (2004). In each thin section, the allochems and orthochems amounts were

calculated by the point counting method. Limestones were classified according to Wright (1992).

Based on Lasemi (1980) and Carozzi (1989), microfacies were identified; and then were

compared with the Flügel’s RMF and RMF belts (Flügel, 2004). Sequence stratigraphy data was

analyzed based on Martin-Chivelet (2003).

4- Results

According to the field and microscopic studies of more than 200 samples from the Qom

Formation, seven carbonate microfacies were recognized. These microfacies, that simply showed

in table 1, are attributed to outer, mid, and inner (include of open marine, restricted marine and

bar) parts of a homoclinal ramp (figure 3, plate 1).

In Natanz area, two complete and one partial (3rd order) sequences were recognized (figure 4)

for the Qom formation, as follow:

1) sequence number 1(Sq1): this sequence, which is overlaid on Eocene volcanoclastic rocks

commences with the type-1 sequence boundary (SBI). The limestone alternating with shale or

marl (attributed to open marin and mid-ramp) have produced an aggradational TST. The end of

the system tract is a MFS. Then, a proggradational parasequence set (belong to open marine and

bar) make up the HST.

2) sequence number 2 (Sq2): TST (the keep-up type) of this sequence, which consists of two

aggradational (attributed to open-marine), a progradational, and two retrogradational (attributed

to open-marine and mid-ramp) parasequence sets is located on a “type-2” sequence boundary

(SBII). The thin HST of this sequence, that is composed of tow progradational (boundstone to

packstone, wackstone to packstone) parasequence sets ends with a SBII.

3) sequence number3 (Sq3): the alluvial deposit covered this sequence so only its TST is

observable. The TST consists of some proggradational (boundstone to grainstone facies) and

several aggradational (boundstone to grainstone facies) parasecuence sets (keep-up type).

5- Discussion and conclusion:

Type and frequency of TST microfacies in the first sequence of the Qom formation shows that in

this time the restricted condition in the sedimentary basin has been in place first and then open

marine condition have prevailed. Type and frequency of HST microfacies in the sequence

indicates that the basin depth is gradually decreased and restricted and bar conditions has

occurred. The thick TST in this sequence illustrates that the sediment input and the

accommodation space rate have been equal. Due to decreasing in the fine clastic sediment supply

and relative increasing in energy, the thin HST has occurred. In the second sequence the patch

reef- boundstones extended in the basin and due to the fast growth of them a thick TST is

created. These events had been responsible for the extension of related grainstones facies as well.

The reduction in the accommodation space affected the boundston growth. Therefore its vertical

growth was changed to lateral growth, and, finally, barrier-reef was formed in this Sequence

HST. Increasing of the restricted marine microfacies is the result of this event. Also, this

condition had been continued, in the TST of the third sequence.

Reuter et al. (2007) believed that Qom Formation was deposited in two basin: 1) Qom basin (a

back-arc basin), and 2) Esfahan-Sirjan basin (a fore-arc basin). The studied succession is not as

the same as their sections, lithologicaly. It has some characteristics of their Qom and some

characteristics of their Zefreh sections. In addition, the studied succession located on the

volcanic and volcanoclastic rocks, directly. So it can be said that Qom Formation depositions in

this area occurred in an inner-arc basin, in the Tethyan Seaway.

6- Acknowledge

We would like to thank the logistical and financial support given to this study by the Islamic Azad

University, Khorasgan (Isfahan) branch.

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Table1: Description and interpretation of the Microfacies of The Qom Formation (in the study area

figure Mf. interpretation Mf. discription

Mf.

symbol Microfacies name Microfacies belt

Pl1/1 Bar

Main allochems: Large foraminifer, Red

algae, Echinoderm segments.

Minor allochems: Bioclastic fragments,

Brachiopod, Bryozoan and mollusk

fragments

Fabric: Grain supported.

6G Red algal, Foraminifer,

Echinoderm Grainstone. Inner ramp

Pl1/2

Restricted marine

Main allochems: Large foraminifer, Red

algae, Echinoderm segments.

Minor allochems: Ostracod, Brachiopod,

Bryozoa and mollusc fragments

Orthochem: Micrite, and Microsparite

Fabric: Grain supported.

5L Echinoderm, Red algal, Bentic

Foraminifer, Pakcstone

Inner ramp

Pl1/3

Main allochems: Miliolida, Red algae,

Echinoderm segments.

Minor allochems: Serpolid, Brachiopod,

Bryozoa and mollusc fragments

Orthochem: Micrite, and Microsparite

Fabric: Grain supported.

5M Echinoderm, Red algal,

Miliolida, Packstone

Pl1/4

Main allochems: Large foraminifer, Red

algae, Echinoderm segments.

Minor allochems: Ostracod, Ostra, Coral and

serpolit fragments

Orthochem: Micrite, and Microsparite

Fabric: matrix supported.

4L Echinoderm, Red algal, Bentic

Foraminifer, Wakcstone

Pl1/5

Main allochems: Miliolida, Red algae,

Echinoderm segments.

Minor allochems: mollusc, Coral fragments

Orthochem: Micrite, and Microsparite

Fabric: Matrix supported, Bioturbation.

4M Echinoderm, Red algal,

Miliolida, Wackstone

Pl1/6 Open marine

Main allochems: Coral

Orthochem: micrite 3 Boundstone (barrier like)

Pl1/7 Lens like build-ups between rudstone and

floatstone 3a Boundstone (Patch reef)

Pl1/8

Mid ramp

Main allochems: Boundstone fragments

Orthochem:micrite 2FR Rudstone and Floatstone Mid ramp

Fig1: Geographical setting and location of the studied section

Fig2: Lithostrarigraphy column of Qom Formation in the Natanz Area

Fig3: sedimentary model of the Qom Formation and the distribution pattern of allochemes and

orthochemes in it (Natanz area)

Plate1: Microscopic photographs of the microfacies

RedAlgae in microfacies of Echinoderm, RedAlgal, Foraminifera Grainstone (6G),Large

foraminifera in microfacies of Echinoderm, RedAlgal, Benthic Foraminifera Packstone (5L),

Miliolid Foraminifera in microfacies of echinoderm, redAlgal, miliolid Froaminifera

Packstone(5M),Large Foraminifera in microfacies of Echioderm, RedAlgal, and Benthic

Foraminifera wackestone(4L). Miliolid Foraminifera in microfacies of echinoderm,RedAlgal,

miliolid Foraminifera wackestone(4M). Meandering coral in microfacies of

Boundstone(3B),Bindstone in microfacies of Boundstone (patch reef-3a), Rock Fragment of

Boundstone in microfacies of Rudstone and floatstone(2FR)

Fig4: 3rd

order Sequences and their system tracts in the study area (see the text for scale)

Table1: Description and interpretation of the Microfacies of The Qom Formation (in the study

area)

Fig2: Lithostrarigraphy column of Qom Formation in the Natanz Area