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Oligocene ramp system (Asmari Formation) in the west of Fars province: Microfacies and

sedimentary environment

Samir Akhzari*1

, Ali Seyrafian2, Hossein Vaziri-Moghaddam

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*1University of Isfahan, Faculty of Sciences, Department of Geology; E-mail: samir.akhzari@gmail.com

2University of Isfahan, Faculty of Sciences, Department of Geology; E-mail: Seyrafian_ali@hotmail.com 3University of Isfahan, Faculty of Sciences, Department of Geology; E-mail: avaziri7304@gmail.com

Abstract

In this paper, microfacies and sedimentary environment

of the Asmari Formation are investigated at the

northwest of Deris village, located at 7 km northwest of

Kazerun city (west of Fars province). The Asmari

Formation in this section consists of 460 m massive,

thick, medium and thin bedded gray and cream to gray

limestone, slightly dolomitic with interbeds nodular and

marly limestone.

More than 270 thin sections from this Oligocene

carbonate interval have been prepared, studied and

analyzed. Facies analysis on the basis of depositional

texture, benthic foraminifera and non-foraminifera

caused to distinguish 14 microfacies and 4 subfacies:

O1- Bioclastic nummulitidae floatstone-rudstone, O1-1-

Bioclastic nummulitidae echinoids floatstone-rudstone,

O2- Bioclastic lepidocyclinidae nummulitidae

Neorotalia rudstone, O3- Bioclastic nummulitidae

Neorotalia packstone-rudstone, O3-1- Bioclastic

nummulitidae Neorotalia corallinacean packstone-

rudstone, O4- Bioclastic lepidocyclinidae Neorotalia

packstone, O4-1- Bioclastic lepidocyclinidae Neorotalia

corallinacean packstone, O5- Bioclastic Neorotalia

corallinacean packstone, O6- Bioclastic coral

corallinacean Neorotalia packstone-rudstone, Sh-

Neorotalia corallinacean echinoids packstone-

grainstone, L1- Coral benthic foraminifera floatstone-

rudstone, L2- Benthic foraminifera (perforate and

imperforate) peloidal packstone, L3- Bioclastic

imperforate foraminifera corallinacean packstone, L4-

High diversity imperforate foraminifera bioclastic

packstone-grainstone, L5- Bioclastic miliolids

wackestone-packstone, L5-1- Bioclastic miliolids

Valvulinid wackestone-packstone, L6- Small rotaliids

Discorbis bioclastic wackestone, and T- Sandy

mudstone. Based on these facies association lagoon and

open marine environments related to inner and middle

parts of a homoclinal ramp have been considered for

deposition of the Asmari Formation. Middle ramp fauna

are mostly characterized by hyaline benthic foraminifera

specially nummulitidae and lepidocyclinidae, and inner

ramp is dominated by imperforate foraminifera.

Keywords: Asmari Formation, Oligocene, Microfacies,

Sedimentary environment, Homoclinal ramp.

Introduction

The Asmari Formation, a thick carbonate sequence of

the Oligocene-Miocene, is present in the most of the

Zagros basin, consists of limestone, dolomite

limestone, dolomite and marly limestone [1]. The most

recent studies of the Asmari Formation are on

biostratigraphic criteria [2 and 3], microfacies and

depositional environment [4] and depositional

environment and sequence stratigraphy [5 and 6].

Regional setting

Iran territory stands in middle part of Alp-Hymalia

folded belt [7] "Figure 1". Based on sedimentary

sequences, magmatism and metamorphism and

structural setting, the Iranian plateau consists of eight

continental fragments, including sanandaj-Syrjan,

Urumieh-Dokhtar, Central Iran, Alborz, Kopeh-Dagh,

Lut, Makran and Zagros [8] "Figure 2". The study

section is located in the Zagros basin.

The Zagros basin is located to the southwest of Iran

and consists of a thick sediment deposits that covers the

Precamberian basement formed during the Pan-Africa

orogeny [9]. It is divided into six major

tectonostratigraphic regions: the interior and coastal

Fars province, Dezful embayment, the Izeh zone, the

Lurestan province and the high Zagros zone [10 and

11] "Figure 3".

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This study is based on a section of the Asmari

Formation in the interior Fars province of Zagros at

northwest of Deris village, located at 7 km northwest of

Kazerun city in west of Fars province. The coordinates

of such section are N: 29o 41' 59", E: 51

o 32' 26" [12]

"Figure 4".

Figure 1. Iran plateau position in Alp-Hymalia folded belt [7].

Figure 2. Iranian plateau subdivisions (adopted from [8]).

Figure 3. Zagros structural subdivisions [11].

Figure 4. Geological map of the study area [12]. The study

section is shown by A-A'.

Methods and microfacies analysis

The 460 m thickness of the Asmari Formation outcrops

were measured and sampled bed by bed in order to

view and analyse any lithology, thickness, color,

sedimentary structures and fossil evidences as well as

facies variations. 450 samples from this interval were

taken, and more than 270 thin sections were

investigated. This study resulted in 6 open marine, 1

shoal, 6 lagoon and 1 tidal flat microfacies

environment.

The Asmari Formation deposits in the study section

are Oligocene (Rupelian-Chattian) in age. In the

following paragraphs, microfacies are illustrated and

interpreted in these order for deep to shallow depth

environment:

O1- Bioclastic nummulitidae floatstone-rudstone

The main components are relatively diverse assemblage

of nummulitids (Nummulites, Operculina and

Heterostegina) "Figure 5-A". Other bioclasts include

Elphidium sp.1, Amphistegina sp., Dendritina rangi,

Planorbulina sp., Neorotalia viennoti., bryozoan,

echinoids and coralline red algae. Because of

abounding echinoids in some thin sections, the O1-1-

Bioclastic nummulitidae echinoids floatstone-

rudstone, subfacies is introduced.

There are abundant B-form hyaline foraminifera

species in a carbonate matrix with floatstone-rudstone

texture. The size of most of them is more than 2 mm.

The presences of large flat nummulitids indicate that

sedimentation took place in relatively deep water.

Flatter test and thinner walls with increasing water

depth reflect the decrease light levels at greater depth.

The presence of high diverse stenohaline fauna such as

large foraminifera, red algae, bryozoan and echinoids

indicate that the sedimentary environment was situated

in the oligophotic zone environment, with low-medium

hydrostatic energy and under the fair weather wave

base in distal mid-ramp [13, 14 and 15]. Due to the

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presence Nummulites vascus and Nummulites fichteli

species, such microfacies is restricted to the Rupelian

stage [2, 16 and 17].

O2- Bioclastic lepidocyclinidae nummulitidae

Neorotalia rudstone

This microfacies possess Neorotalia viennoti,

nummulitidae and lepidocyclinidae as major

components. Lepidocyclinidae includes B-form

Nephrolepidina sp. and Eulepidina sp., and also

accessory allochems include Elphidium sp.1,

Planorbulina sp., Amphistegina sp. and coralline red

algae. Such microfacies is grain-supported in a

carbonate micrite with rudstone texture "Figure 5-B"

[18, 19 and 20].

Like the previous microfacies, this one also shows

the Asmari Formation sedimentation took place in low

to moderate hydrostatic energy environment under the

fair weather wave base in lower parts of middle ramp

[13].

O3- Bioclastic nummulitidae Neorotalia packstone-

rudstone

In this microfacies nummulitids and Neorotalia

viennoti are the dominant biotic components, but fine

fragments of coralline red algae (Subterranophyllum

thomasi and Lithophyllum sp.) are also present. Then,

the O3-1- Bioclastic nummulitidae Neorotalia

corallinacean packstone-rudstone microfacies has

been determined. Other allochems include Elphidium

sp.1, Planorbulina sp., Dendritina rangi, textularids,

echinoids and bryozoan. This microfacies is composed

of coarse-grained packstone and rudstone with sand-

sized nummulitic fragments "Figure 5-C".

A common feature of this facies (like microfacies

O1), is dominanly occurs in the Rupelian, and also

composed of larger, robust to flat forms of Nummulites

tests. The abundance of abraded and randomly

orientated A- and B- form Nummulites, may represents

sedimentation within the middle ramp [13].

O4- Bioclastic lepidocyclinidae Neorotalia packstone

Robust lepidocyclinids and Neorotalia viennoti are the

major bioclasts "Figure 5-D". Subordinate biotic grains

consist of Elphidium sp.1, Amphistegina sp.,

Heterostegina sp., Operculina sp., Planorbulina sp.,

textularids, echinoids and corallinacean algae. In a few

samples with increasing coralline red algae fragments,

the name of this microfacies changes to O4-1-

Bioclastic lepidocyclinidae Neorotalia corallinacean

packstone.

The presence of hyaline large benthic foraminifera

with robust and thick tests in packstone texture

determined that the sedimentation of this facies took

place in high energy condition in proximal mid-ramp

[21].

O5- Bioclastic Neorotalia corallinacean packstone

It is characterized by co-occurance corallinacean algae

and Neorotalia viennoti. Subordinate bioclasts include

Heterostegina sp., Planorbulina sp., Amphistegina sp.,

Elphidium sp.1, miliolids, bryozoan and echinoids

"Figure 5-E".

Angled and crunched coralline red algae fragments

and Neorotalia viennoti species with abraded tests,

indicate deposition in a moderate to high energy

environment in shallow waters within middle ramp and

near normal wave base [21].

O6- Bioclastic coral corallinacean Neorotalia

packstone-rudstone

The major grains are Neorotalia viennoti, corallinacean

and coral fragments, and subordinate allochems consist

of Reussella sp., Valvulinid sp., miliolids, chinoids and

bryozoan "Figure 5-F".

Grain-supported texture shows high energy

environment. Abraded coralline red algae fragments,

crunched bioclasts and also absence autochthonous

coral boundstone structure, segregated this microfacies

from coral reef facies. Then, this facies is interpreted as

an open marine facies that formed seaward of the shoal,

within the normal wave base [22]. Also, open marine

and well-oxygenate conditions are indicated by the

diverse fauna [23].

Sh- Neorotalia corallinacean echinoids packstone-

grainstone

An invariant assemblage of well-sorted biotic

fragments include Neorotalia viennoti, corallinacean

algae and echinoids in lime mud is characteristic of this

microfacies. Less common bioclasts include

nummulitids, lepidocyclinids, Planorbulina sp.,

Dendritina rangi and miliolids "Figure 5-G".

The grains with low variation and well sorting in the

grain-supported texture, shows this microfacies has

been formed in a shoal environment with high

hydrostatic energy level [23].

L1- Coral benthic foraminifera floatstone-rudstone

This facies is predominantly composed of benthic

foraminifera such as Peneroplis sp., Valvulinid sp.,

Reussella sp., Discorbis sp., Haplophragmium slingeri,

miliolids, small rotaliids and fragments of coral

colonies. These grains are poorly sorted. Micrite and

coralline red algae fragments formed the matrix "Figure

5-H".

Like microfacies O6, because of absence

autochthonous coral boundstone structure, coral reef

facies has not been introduced. The presence hyaline

and imperforate foraminifera with corallinacean algae

indicate the revolving shallow waters in euphotic

environment. Then, this microfacies has been

sedimented in an open lagoon within inner ramp [13].

L2- Benthic foraminifera (perforate and imperforate)

peloidal packestone

The skeletal components include benthic foraminifera

such as lepidocyclinids, Heterostegina sp., Neorotalia

viennoti, Elphidium sp.1, Elphidium sp.14,

Meandropsina sp., Dendritina rangi, Borelis sp.,

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Valvulinid sp., Ditrupa sp., miliolids and coralline red

algae. In some samples, the predominant non-skeletal

carbonate grains are peloids "Figure 5-I".

Co-occurance of normal marine components

(perforate foraminifera and coralline red algae) and

imperforate foraminifera, indicates that sedimentation

took place in an open lagoon in inner ramp, and also

suggests the absence of an effective barrier in that time

[24].

L3- Bioclastic imperforate foraminifera

corallinacean packstone

The main characteristics of this microfacies is abundant

fragments of corallinacean. Imperforate foraminifera

that include miliolids, Meandropsina sp., Archaias sp.,

Austrotrillina sp., Valvulinid sp., Elphidium sp.1and

echinoids, bryozoan and bivalve fragments are also

present "Figure 5-J".

Crunched coralline red algae fragments and

packstone texture indicate moderate energy shallow

waters within a semi-restricted lagoon in inner ramp

[24].

L4- High diversity imperforate foraminifera

bioclastic packstone-grainstone

Skeletal grains consist of diverse fauna, including

imperforate benthic foraminifera such as Peneroplis sp.,

Archaias sp., Austrotrillina sp., Spirolina sp.,

Meandropsina sp., Elphidium sp.1, Valvulinid sp.,

Pyrgo sp., miliolids, textularids and bryozoan, coralline

red algae, dasycladacean algae and gastropod fragments.

Texture varies from packstone to (in few samples)

grainstone "Figure 5-K".

Grain supported fabric and diverse imperforate

foraminifera indicate that sedimentation took place in a

shallow lagoon, with bright and rather restricted

condition [13 and 21].

L5- Bioclastic miliolids wackestone-packstone

The most abundant bioclast is miliolids. Subordinate

biota grains consist of Archaias sp., Austrotrillina sp.,

Elphidium sp., Dendritina rangi, Valvulinid sp. and

bivalve fragments "Figure 5-L". Due to changes in type

and abundance of fauna in some samples, the name of

this facies changes to L5-1- Bioclastic miliolids

Valvulinid wackestone-packstone.

The restricted condition within the inner ramp is

suggested by the absence normal marine biota and

abundant skeletal components of restricted biota

(imperforate foraminifera) such as miliolids, Valvulinid

sp., Archaias sp. and Dendritina rangi.

Mud supported fabric and presence miliolids with

thick tests (due to increase light intensity) and invariant

genera, indicate a restricted lagoon with low energy

shallow water [13 and 21].

L6- Small rotaliids Discorbis bioclastic wackestone

Identifiable grains of this facies include benthic

foraminifera (small rotaliids and Discorbis sp.).

Reussella sp., Elphidium sp.1, Valvulinid sp., miliolids

and coralline red algae are less common. Texture is

dominantly wackestone and foraminifera assemblages

have very low diversity "Figure 5-M".

This facies has been deposited in hypersaline wares in

restricted lagoon with low hydrostatic energy level [21].

T- Sandy mudstone

This microfacies is composed of dense lime mudstone

with detrital small quartz grains, wheras bioclasts are

lacking "Figure 5-N". These quarts grains in the micrite

matrix have no any lamination. Detrital substances

maybe have been slidded to the deeper parts of

sedimentary basin and made this microfacies. This

facies type is common in tidal flat sediments [21], and it

is occurs in upper parts of the Asmari Formation in the

study section.

Lime mudstone with small quarts grains and no

evidence of biotic fauna, has been deposited under the

hypersaline condition in tidal flat environment [21].

As can be seen in the table 1, the study section of a

thickness of 159 to 221 meters, consists of massive and

thick to medium cream to gray dolomite. These are

secondary dolomites that have been resulted by

dolomitization of calcite, such that primary fabrics

completely destroyed "Figure 5-O". So, for this

thickness no microfacies has been introduced "Table 1".

Sedimentary environment and conclusion

Based on the study of taxons, sedimentary textures and

microfacies vertical variations, the sedimentary model

of the Asmari Formation was introduced.

Due to presence of shoal facies and gradual

microfacies variation, and absence of turbulent flows

effects and autochthonous reef structure, a homoclinal

ramp platform is recommended for the Asmari

Formation in Deris village section "Figure 6". This

sedimentary environment consists of middle ramp and

inner ramp. Because of absence marine deep facies and

planktonic foraminifera, the outer ramp environment is

not introduced.

In distal mid-ramp there are B-form hyaline large

benthic foraminifera with coralline red algae and

echinoids, in floatstone-rudstone texture. The proximal

mid-ramp is characterized by A-form and robust hyaline

foraminifera with thick tests and abounding Neorotalia

viennoti. The shoal facies consists of Neorotalia

viennoti, corallinacean algae and echinoids in grain

supported texture.

Inner ramp consists of euphotic zone with shallow

saline waters and characterized by imperforate

foraminifera.

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Figure 5. A: O1- Bioclastic nummulitidae floatstone-rudstone, sample no. B0; B: O2- Bioclastic lepidocyclinidae nummulitidae

Neorotalia rudstone, sample no.B63; C: O3- Bioclastic nummulitidae Neorotalia packstone-rudstone, sample no. B17; D: O4-

Bioclastic lepidocyclinidae Neorotalia packstone, sample no. B72; E: O5- Bioclastic Neorotalia corallinacean packstone, sample no.

B69; F: O6- Bioclastic coral corallinacean Neorotalia packstone-rudstone, sample no. B135; G: Sh- Neorotalia corallinacean echinoids

packstone-grainstone, sample no. B100; H: L1- Coral benthic foraminifera floatstone-rudstone, sample no. B435; I: L2- Benthic

foraminifera (perforate and imperforate) peloidal packestone, sample no. B225; J: L3- Bioclastic imperforate foraminifera

corallinacean packstone, sample no. B313; K: L4- High diversity imperforate foraminifera bioclastic packstone-grainstone, sample no.

B449; L: L5- Bioclastic miliolids wackestone-packstone, sample no. B384; M: L6- Small rotaliids Discorbis bioclastic wackestone,

sample no. B252; N: T- Sandy mudstone, sample no. B396; O: Dolomite in xpl light, sample no. B175.

Nummulites (N), Heterostegina (H), Operculina (Op), lepidocyclinids (Lep), Neorotalia (R), coralline red algae (Ral), echinoids (E),

textularids (T), coral fragment (C), bioclast fragment (Bio), Discorbis (D), Austrotrillina (As), Archaias (A), Reussella (Re), miliolids

(M), Meandropsina (Me), Elphidium (El), Peneroplis (P), peloid (Pel).

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Figure 6.Oligocene sedimentary schematic model of the Asmari Formation in Deris village section.

Table 1. Microfacies, sedimentary environment and sea level changes column of the Asmari Formation in Deris village section.

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