In this article, Oligocene microfacies and sedimentary environment of the Asmari Formation in Zagros basin of Iran are interpreted.

1. 1
Oligocene ramp system (Asmari Formation) in the west of Fars province: Microfacies and
sedimentary environment
Samir Akhzari*1
, Ali Seyrafian2
, Hossein Vaziri-Moghaddam3
*1
University of Isfahan, Faculty of Sciences, Department of Geology; E-mail: samir.akhzari@gmail.com
2
University of Isfahan, Faculty of Sciences, Department of Geology; E-mail: Seyrafian_ali@hotmail.com
3
University 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".

2. 2
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: 51o
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

3. 3
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.,

4. 4
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.

5. 5
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).

6. 6
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.

7. 7
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