Mohammad et al 2010

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ISSN 0891-2556, Volume 25, Number 2

ORIGINAL ARTICLE

The Asmari Formation, north of the Gachsaran (Dill anticline),southwest Iran: facies analysis, depositional environmentsand sequence stratigraphy

Mohammad Allahkarampour Dill • Ali Seyrafian •

Hossein Vaziri-Moghaddam

Accepted: 27 May 2010 / Published online: 11 June 2010

� Springer-Verlag 2010

Abstract The Asmari Formation is exposed at the Dill

anticline in the Dezful Embayment of the Zagros foreland

basin with 206 m thickness comprising thin, medium and

thick to massive bedded carbonates. The Asmari Formation

is Late Oligocene (Chattian)–Early Miocene (Burdigalian)

in age at the study area. Ten facies characterizing upward

gradual shallowing trend of an open marine (MF 1–3),

shoal (MF 4), semi-restricted and restricted lagoon (MF

5–9) and near-shore lagoon (MF 10) depositional envi-

ronments were identified. Based on environmental inter-

pretations, a homoclinal ramp consisting of inner and

middle parts prevails. MF 4–10 were characterized by the

occurrence of large and small porcelaneous benthic

foraminifera representing a shallow-water setting of an

inner ramp influenced by wave and tide processes. MF

1–MF 3 with large and small hyaline benthic foraminifera

represent a deeper fair water wave base of a middle ramp

setting. Three-third-order depositional sequences were

recognized. Sequence 1 mostly consists of an open marine

to the lower part and is followed by semi to restricted

lagoon facies. Sequences 2 and 3 are characterized by semi

to restricted lagoon facies. Moreover, the relative sea-level

change curves correlate with the global sea-level change

curves.

Keywords Asmari Formation � Oligocene–Miocene �Dill anticline � Benthic foraminifera �Sequence stratigraphy � Carbonate ramp

Introduction

The Asmari Formation (a giant hydrocarbon reservoir) is a

thick carbonate sequence of the Oligocene–Miocene in

Zagros foreland basin, southwest Iran. It is best developed

in the Dezful Embayment Zone. Lithologically, the Asmari

Formation consists of thin, medium to thick and massive

carbonate layers. Some sandstone layers (Ahvaz Member)

and anhydrite deposits (Kalhur Member) are also present.

The Kalhur evaporite deposits in the Lurestan province and

Ahvaz sandstone deposits in southwest Dezful Embayment

are two members of the Asmari Formation (Fig. 1).

The Asmari Formation at the representative section

consists of 314 m of limestones, dolomitic limestones and

argillaceous limestones (Motiei 1993).

The thickness of the Asmari Formation varies from area

to area. The Asmari Formation was deposited during the

Oligocene (Rupelian)–Miocene (Burdigalian). The base of

the Asmari Formation varies in age. For instance, toward

the coastal Fars area, it is mainly Rupelian in age; in the

Dezful Embayment, it ranges from Rupelian to Chattian in

age (Fig. 1), (Motiei 1993). The top of the Amari Forma-

tion, mostly Burdigalian in age, remains constant. But,

toward the coastal and interior Fars, it is Chattian in age.

The present study focuses on the facies analysis, depo-

sitional environments, relative sea-level changes together

with sequence stratigraphic framework of the Asmari

Formation in the Dill anticline outcrop.

Geological setting

The Zagros basin is defined by a 7–14 km thick succession

of cover sediments deposited over a region along the

north–northeast edge of the Arabian plate. This basin was

M. Allahkarampour Dill � A. Seyrafian (&) �H. Vaziri-Moghaddam

Department of Geology, Faculty of Sciences,

University of Isfahan, 81746-73441 Esfahan, Iran

e-mail: [email protected]

123

Carbonates Evaporites (2010) 25:145–160

DOI 10.1007/s13146-010-0021-6 Author's personal copy

part of the stable Gowndwana supercontinent in the

Paleozoic era and a passive margin in the Mesozoic era. It

became a site of convergent orogeny in the Cenozoic era

(Bahroudi and Koyi 2004).

On the basis of lateral facies variations, the Iranian

Zagros fold-thrust belt is divided into different tectono-

stratigraphic domains that are from SE to NW: the Fars

Province or eastern Zagros, the Izeh Zone and Dezful

Embayment or Central Zagros and finally the Lurestan

Province or Western Zagros (Motiei 1993) (Figs. 1, 3).

Also, from southwest to northeast of the Zagros basin are

the Zagros folded belt, folded and thrusted belt and High

Zagros and crush zone. The Dezful Embayment is part of

the Zagros folded belt (Falcon 1974; Colman-Sadd 1978;

Sepehr and Cosgrove 2004; Sherkati and Letouzey 2004;

Fakhari et al. 2008).

The Dezful Embayment is surrounded by three faults:

Montain Front Fault (MFF), Balarud Fault (BF) and

Kazerun Fault (KF). The Dezful Embayment is also one of

the most prolific oil reservoirs in the Middle East. The

study area (Dill anticline) is located in the northern part of

the Dezful Embayment close to the Izeh Zone (Sherkati

et al. 2006; Ahmadhadi et al. 2008) (Figs. 2, 3).

Study area and methods

The Dill anticline is 25 km north of Gachsaran City at

the Kohgiluyeh and Bouyer Ahmad Province. Several

outcrop sections of the Asmari Formation were examined

in the Dill anticline. Detailed field analysis and sampling

were located in the Tang-e-Zohr valley at 30�330N and

50�440E (Fig. 4). The lower boundary of the Asmari

Formation is not exposed at the Dill anticline. Samples

from 206 m thickness of the Asmari Formation were

taken. Collection of the 112 samples was based on field

evidences and lithofacies changes. However, the upper

boundary is exposed and overlain by the Gachsaran

Formation.

Previous work

The Asmari Formation was named after the Asmari anticline

located in the northern Dezful Embayment. It was referred to

a sequence of Cretaceous-Eocene in age (Busk and Mayo

1918). The Asmari Formation was measured and defined as

an Oligocene Nummulitic limestone by Richardson (1924)

and described by Thomas (1948) as an Oligocene–Miocene

carbonate interval. James and Wynd (1965) summarized

previous viewpoints and finally formally defined the Asmari

Formation.

Recent local researches on the biostratigraphy, deposi-

tional environment and sequence stratigraphy included

those by Seyrafian et al. (1996), Seyrafian (2000), Seyrafian

and Hamedani (2003), Seyrafian and Mojikhalifeh (2005),

Vaziri-Moghaddam et al. (2006), Amirshahkarami et al.

(2007) and Hakimzadeh and Seyrafian (2008).

Fig. 1 Cenozoic stratigraphy of the Zagros basin, after James and Wynd (1965)

146 Carbonates Evaporites (2010) 25:145–160

123

Author's personal copy

Ehrenberg et al. (2007) and Laursen et al. (2009)

examined the Asmari Formation based on Sr isotope stra-

tigraphy and revised age ranges mostly for the lower and

middle parts of the Asmari Formation. Moreover, salinity

changes during the Late Oligocene to Early Miocene for

deposition of the Asmari Formation have been described

by Mossadegh et al. (2009).

Biostratigraphy

The biostratigraphic criteria of the Asmari Formation

studied by Wynd (1965) and reviewed by Adams and

Bourgeois (1967) proposed assemblage zones.

Ehrenberg et al. (2007) improved the stratigraphy of the

Asmari Formation on the basis of strontium isotope

method. No assemblage was suggested. The systematic

application of the strontium isotope dating and funda-

mental revision of the biostratigraphic zonation from

Rupelian to Chattian across the Dezful Embayment were

proposed by Laursen et al. (2009) (Table 1).

On the basis of the Asmari Formation biozones descri-

bed by Laursen et al. (2009), the following foraminiferal

assemblages were identified for the study area:

Assemblage-1

From the base upward to 115 m, Miogypsinoides sp.,

Archaias kirkukensis, Miogypsinoides complanatus,

Archaias sp., Valvulinid sp.1, Valvulinid sp., Elphidium sp.,

Heterostegina sp., Spiroclypeus blankenhorni, Dendritina

rangi, Peneroplis sp., Borelis sp., Rotalia viennotti and

Amphistegina sp. are present. The faunal assemblage is

time equivalent to the Archaias asmaricus–Archaias hen-

soni–Miogypsinoides complanatus assemblage zone of the

Chattian age.

Assemblage-2

From 115 to 125 m, Elphidium sp.14, Miogypsina sp.,

Rotalia sp. Elphidium sp., Valvulinid sp.1 and Dendritina

rangi are mainly present and correspond to Miogypsina–

Elphidium sp. 14–Peneroplis farsenensis assemblage zone

of Aquitanian age.

Assemblage-3

From 125 to 190 m, Discorbis sp., Rotalia sp., Dendritina

rangi, Peneroplis sp. and fragments of miliolids, bryozoa

and echinoid debris are associated. Due to the appearance

of unidentified and long range fauna together with dolo-

mitic texture within the interval, an indeterminate zone of

the Aquitanian age is applied.

Assemblage-4

From 190 to 206 m, Meandropsina iranica, Borelis melo

curdica, Dendritina rangi, Peneroplis sp., Borelis sp.,

Discorbis sp., miliolids and echinoid debris are present.

Fig. 2 General view of Dill

anticline located in Dezful

Embayment and next to the Izeh

Zone

Carbonates Evaporites (2010) 25:145–160 147

123


Fig. 4 Carbonate succession of the Asmari Formation in Tang-e-Zohr valley. Note that the lower Asmari strata (Chattian) are mostly thick to

massively bedded, whereas the middle and upper parts (Aquitanian and Burdigalian) are medium to thiny bedded limestone

Fig. 3 a Map shows the geology of Iran with its structural provinces.

The study area is located in the Zagros province (adopted from

Heydari 2008). b Location of the Dezful Embayment in Zagros basin

(modified after Ghabeishavi et al. 2009). c Location of the studied

area within the Dezful Embayment


123


This faunal assemblage is time equivalent to Borelis melo

curdica–Borelis melo of the Burdigalian age (Fig. 4).

As a result of this study, the Asmari Formation in Dill

anticline is Late Oligocene (Chattian)–Early Miocene

(Burdigalian) in age.

Microfacies and sedimentary environment

Ten carbonate sedimentary facies were recognized for the

Asmari Formation in the study area. These facies are

related to the five depositional settings (lower part of

intertidal, restricted and semi-restricted lagoon, high-

energy shoal and storm-influenced open marine) of inner

and middle portions of a carbonate platform (Figs. 8, 10).

MF 1: Bioclast perforate foraminifera

packstone–wackestone

This facies is characterized by an association of the larger

benthic foraminifera (LBF) (Heterostegina, Spiroclypeus,

Amphistegina) and Miogypsinoides. Subordinate fragments

showed typically echinoid, bryozoa, Sphaerogypsina and

bivalve debris. There is occasional high occurrence of

coralline red algae range facies and the bioclast perforate

foraminifera corallinacean packstone (Fig. 5).

Interpretation

High taxonomic diversity of LBF with perforate walls,

corallinacea, echinoid, bryozoa, mud micrite matrix and

stratigraphic position represents deposition on a shallower

slope environment (Amirshahkarami et al. 2007). The red

algae association and larger benthic foraminifera were

identified as living in the oligophotic zone of the middle

ramp environment (Pomar 2001; Brandano and Corda

2002; Corda and Brandano 2003). Moreover, these

foraminifera (mainly Heterostegina and Amphistegina) live

in a tropical–subtropical environment over a wide bathy-

metric range, but are particularly frequent between depths

of 40 and 70 m (Hottinger 1983, 1997; Hallock and Glenn

1986).

MF 2: Bioclast perforate foraminifera coral

packstone–rudstone

The framework of MF 2 comprises coral fragments and

perforate foraminifera such as: Miogypsinoides and Ne-

orotalia. Corallinacean fragments and Heterostegina debris

occur as a minor constituent. Occasionally more grain-

supported matrix with the presence of sparry calcite cement

renames the MF 2 to bioclast Neorotalia Miogypsinoides

coral grinstone–floatstone (Fig. 5).

Interpretation

The presence of Miogypsinoides, Neorotalia and coral and

the debris of corallinacea and Heterostegina, as well as the

stratigraphic position, indicate that this facies was formed

in an open marine middle shelf environment under normal

marine salinity conditions with open water circulation and

medium hydrodynamic energy (Amirshahkarami et al.

2007; Amirshahkarami 2008). The presence of abundant

open marine skeletal fauna and larger foraminifera suggest

that an oligotrophic condition occurred (Pedley 1996; Po-

mar 2001).

MF 3: Bioclast corallinacean Neorotalia

Miogypsinoides packstone/grainstone

The main components of MF 3 are corallinacean red algae

and small perforate benthic foraminifera (Miogypsinoides,

Neorotalia). Additional components are bivalves together

with the particles of Heterostegina and Amphistegina. This

facies is characterized by packstone–grainstone texture of

poorly rounded grain-supported type (Fig. 5).

Table 1 Recent biozones introduced by Laursen et al. (2009) on the

basis of strontium isotope dating


123


Interpretation

The appearance of foraminifera with perforate walls such

as Neorotalia and Miogypsinoides, as well as an associa-

tion of red algae fragments with sparry calcite cement,

indicates deposition in a moderate to high-energy envi-

ronment of an upper part of a carbonate shelf slope.

Neorotalia prefer to live in the shallow turbulent water

environments, 0–40 m in depth (Geel 2000). Miogypsinoides

prefer shallow, less than 50 m depth and normal salinity

waters (Geel 2000).

MF 4: Bioclast miliolids and Neorotalia grainstone

The main features of MF 4 are dominated by Neorotalia

and miliolids. This facies has a grain-supported texture,

and sorted and rounded grains together with sparry calcite

Fig. 5 a MF 1, Bioclast

Neorotalia corallinacean

Heterostegina (Spiroclypeus)

packstone, a, e. Spiroclypeus b.

Sphaerogypsina c. Neorotalia d.

Corallinacea red algae, b MF 1,

bioclast perforate foraminifera

(Heterostegina) packstone–

wackstone, c MF 2, bioclast

perforate foraminifera

(Neorotalia, Miogypsinoides)

coral packstone–rudstone, d MF

2, bioclast Neorotalia

Miogypsinoides coral

grainstone–floatstone, e MF 3,

bioclast corallinacean

Neorotalia Miogypsinoides

packstone to grainstone


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cement. The particles of Peneroplis and corallinacea red

algae are the minor elements (Fig. 6).

Interpretation

Grain-supported matrix with an abundance of benthic

foraminifera (Neorotalia and miliolids), particles of coral-

linacea red algae and Amphisteginid suggest that a mod-

erate to high-energy bottom current environment occurred

(Fournier et al. 2004). These biota and grain-supported

matrix associated with MF 4 are interpreted as having been

formed in high-energy sand shoals, located at the platform

margin, separating the open marine from the more

restricted marine environments (Amirshahkarami et al.

2007). This interpretation is corroborated by the lack of

foraminiferal deeper water markers (Fournier et al. 2004).

MF 5: Bioclast perforate and imperforate foraminifera

packstone

Porcelaneous foraminifera such as miliolids (Austrotrillina,

Pyrgo, Quinqueloculina and Triloculina), Archaias, Den-

dritina and hyaline foraminifera (Heterostegina, Neorotalia

and Miogypsina) are abundant within the MF 5. Fragments

of corallinacea red algae, echinoid and bryozoa are present.

MF 5 is mostly grain-supported matrix with micritic

groundmass (Fig. 6).

Interpretation

The occurrence of perforate and imperforate benthic

foraminifera reflects that sedimentation took place in a

shelf lagoon (Romero et al. 2002). This association

Fig. 6 a MF 4, bioclast

miliolids Neorotalia grainstone,

b MF 5, bioclast perforate

imperforate foraminifera

packstone, a. Neorotalia, b.

Heterostegina, c.

Miogypsinoides, d. miliolid, cMF 5, bioclast Neorotalia

miliolids packstone, a.

Neorotalia b. miliolid, d MF 5,

bioclast perforate imperforate

foraminifera packstone, a.

Miogypsina, b. Elphidium, c.

Rotalia, d. miliolid, e.

Dendritina, e MF 6, bioclast

miliolids corallinacea

wackestone–packstone to

grainstone, f MF 6, bioclast

miliolids coral wackestone/

floatstone


123


together with red algae debris characterizes an inner-shelf

depositional setting (Corda and Brandano 2003).

MF 6: Bioclast miliolid corallinacea

wackestone–packstone to grainstone

Miliolids and corallinacea red algae are dominant compo-

nents of the MF 6. The Archaias, Valvulinid, Textularia,

rotaliids and Amphisteginids with bivalve debris are the

minor bioclasts.

Occasionally, coral fragments replace corallinacea red

algae debris. The Valvulinid, Austrotrillina and bivalve

debris also occur as minor constituents. As a result, the

wackestone–packestone to grainstone facies changes to

bioclast miliolid coral wackestone/floatstone (Fig. 6).

Interpretation

Well-preserved coralline algal growth framework in the

coralline algae facies indicates a relatively quiet-water

environment with stable substrate and low sedimentation

rates (Nebelsick and Bassi 2000). The associations of

miliolids within MF 6 support the additional interpretation

of a relatively protected environment, probably an inner

part of a platform (Fournier et al. 2004). Rotaliids and

Amphisteginids can tolerate a variety of water salinity

concentrations. They are common in modern sea grass

environments of relatively high salinities (Sen Gupta

1999). Agglutinated foraminifera also represent a shallow-

water lagoon and open marine environments (Geel 2000).

Rotalids and Amphisteginids together with agglutinated

foraminifera also could be evidence of the semi-protected

waters.

The porcelaneous foraminifera associated with bioclast

miliolid coral wackestone/floatstone facies is typical of

shallow and illuminated habitats where sea grass flats

intersect adjacent non-vegetated areas (Brandano et al.

2008). Moreover, scattered branching coral are character-

istics of reduced water energy in the lowest part of the

euphotic zone (Schuster and Wielandt 1999). However, the

common coral debris may have derived from adjacent

patch reefs or could also have been produced in situ from

isolated colonies that are known to grow in sea grass

environments (Brasier 1975).

MF 7: Bioclast porcelaneous foraminifera

wackestone–packstone

MF 7 consists of wackestone to packstone textures as well

as porcelaneous foraminifera rich in miliolid, Archaias,

Dendritina and Borelis. Debris of corallinacea red algae,

Textularia, small rotaliids and rare ostracod are minor

elements (Fig. 7).

Interpretation

The occurrence of large number of porcelaneous imperfo-

rate foraminiferal tests may point to the depositional

environment being slightly hypersaline. Such an assem-

blage is described as being associated with a shelf lagoon

environment (Wilson 1975; Flugel 1982, 2004; Vaziri-

Moghaddam et al. 2006; Brandano et al. 2008). Some

porcelaneous perforate foraminifera (Peneroplis and

Archaias) live in recent tropical and subtropical shallow-

water environments, hosting dinoflagellate, rhodophycean

and chlorophycean endosymbionts (Lee 1990; Holzmann

et al. 2001). Furthermore, MF 7 could have originated in

sea grass-dominated environments due to the presence of

epiphytic foraminifera such as Borelis, Archaias and Pen-

eroplis (Brandano et al. 2008).

MF 7 is occasionally recognized as bioclastic peloidal

miliolids wackestone–packstone (Fig. 7). The predomi-

nance of mud-rich facies with oligotypic fauna (such as

miliolids) and the presence of a low-diversity foraminif-

eral association indicate a very shallow subtidal envi-

ronment with low to moderate energy (Ghabeishavi et al.

2009) and low water turbulence (Geel 2000), as well as

high salinity. Moreover, the scarce appearance of peloids

in lime–mud matrix with low diversity of fossils suggests

deposition in a restricted shallow subtidal water and slow

sedimentation rate (Wilson 1975; Flugel 1982; Wanas

2008).

To summarize, the prevalence of porcelaneous forami-

nifera strongly suggests deposition in a protected inner-

shelf environment.

MF 8: Bioclast peloidal Archaias, miliolids,

Dendritina packstone–grainstone

The main elements of MF 8 are skeletal and non-skeletal

components. Skeletal components comprise porcelaneous

benthic foraminifera (Archaias, miliolids and Dendritina),

and peloids and intraclasts are the predominant non-skel-

etal associations. The minor biota are represented by par-

ticles of coral and mollusca (gastropod and bivalve)

(Fig. 7).

Interpretation

These are often interpreted as having been formed in very

shallow marine environments as a product of tide or

combined tide and wave processes (Beavington-Penney

and Racey 2004). Moreover, these skeletal components and


123


grainstone textural rock type are interpreted as a high-

energy environment with high salinities ([40 psu salinity

range) (Mossadegh et al. 2009). Nonetheless, the presence

of sparry calcite cement is indicative of a high-energy

environment. Textural characteristics and prolific porcela-

neous foraminifera, as well as peloids and to some extent

intraclasts, suggest that a high-energy portion of a restric-

ted lagoon with a nearby tidal flat sedimentary environment

prevailed (Vaziri-Moghaddam et al. 2006). These evi-

dences indicate that the sediments associated with MF 8 are

commonly reworked and transported.

A similar facies was studied by Brandano et al. (2008)

from a grass-dominated inner ramp setting as suggested by

the presence of epiphytic foraminifera such as Archaias

and Dendritina.

MF 9: Bioclast laminated mudstone

MF 9 is represented by millimeter-thick laminae fabric. In

this facies type, the fossils are rare, although, mainly sparse

and parallel fine-sized particles of ostracod, mollusca and

echinid are present. Occasionally, very fine-sized quartz

grins are scattered within the matrix (Fig. 7).

Interpretation

The appearance of sparse fine-sized quartz grains together

with particles of bivalve, echinid and ostracods within a

micritic groundmass is typical of restricted inner lagoon

environments (Rasser et al. 2005). This facies is also

similar to SMF19 of Wilson (1975) and is characteristic of

Fig. 7 a MF 7, bioclast

porcelaneous foraminifera

wackstone–packstone, a.

miliolid, b. Borelis, c.

Dendritina, b MF 7, bioclast

porcelaneous foraminifera

wackstone–packstone

(dominant of Dendritina), c MF

7, bioclast peloidal miliolids

wackstone to packstone, d MF

8, bioclast peloidal Archaias

miliolids Dendritina packstone–

grainstone, e MF 9, bioclast

laminated mudstone, f MF 10,

sandy mudstone


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8 and 9 facies belts of a near-coast platform interior setting

(Flugel 2004).

MF 10: sandy mudstone

MF 10 is characterized by mudstone texture appearances

together with sparse fine-sized grains of quartz (Fig. 7).

Interpretation

MF10 indicates a lagoon to near-coast environment of an

inner-shelf setting. Furthermore, vertical relationship with

lagoon facies and low diversity of fauna, mud-rich texture

and the lack of subaerial exposure features in this facies are

indicators of a sedimentary environment of the lower part

of the intertidal setting. According to Flugel (2004), sedi-

ments composed of a mixture of carbonate and siliciclastic

material are common in near-coast and inner-shelf settings

as well as at high latitudes. However, the Asmari Formatin

was deposited in a tropical to subtropical low latitudes

(Heydari 2008).

Facies association and depositional model

Based on discussed microfacies, stratigraphy and sedi-

mentary analysis together with lithofacies distribution and

gradual shallowing trend from the basin into the shallow

platform, the homoclinal ramp depositional profile is sug-

gested for the deposition of the Asmari Formation at the

study area (Fig. 8).

During the Oligo–Miocene, distally steepened and ho-

moclinal ramps were widespread in the Mediterranean

areas (Pedley 1996).

Distally steepened ramp resulted from an increased

accumulation of both in situ gravel-sized skeletal compo-

nents and finer-grained sediments transported from the

shallower euphotic zone (Brandano et al. 2008). In-place

boundstone fabrics as widespread massive reefs constituted

the rimmed shelf.

Since features associated with distally steepened ramp

and rimmed shelf are not present in the studied section, a

homoclinal ramp depositional setting is suggested.

Carbonate ramp environments are characterized by: (1)

the inner ramp, between the upper shore face and fair-

weather wave base, (2) the middle ramp, between fair-

weather wave base and storm-wave base, and (3) the outer

ramp, below normal storm-wave base down to the basin

plain (Burchette and Wright 1992).

Based on studied sedimentary facies, two depositional

environments (inner and middle portions of a carbonate

ramp) are proposed for deposition of the Asmari

Formation.

Middle ramp

Three main microfacies (MF 1–MF 3) are subjected to an

open marine environment of a middle ramp. More common

components of these facies are biota association, such as

LBF (Heterostegina, Spiropclypeous and Amphistegina),

SBF (Neorotalia and Miogypsinoides), corallinacea red

algae and scattered coral fragments.

Fig. 8 Sketch of facies model of carbonate ramp showing the distribution of facies associations and the related depositional environments of the

Asmari Formation in Dill anticline, southwest Iran


123


LBF of the genera Heterostegina and Amphistegina (MF

1 and 3) are of particular ecological importance (Brandano

and Corda 2002). These live in tropical to subtropical

environments over a wide bathymetric range, but are par-

ticularly frequent between depths of 40 and 70 m (Hottinger

1983, 1997). Moreover, the red algae association with these

larger foraminifera places the middle ramp in an oligophotic

(Brandano et al. 2008, 2009; Corda and Brandano 2003;

Brandano and Corda 2002) to mesophotic zone (Hottinger

1997; Pomar 2001).

The Miogypsinoides and Neorotalia with packstone–

grainstone–floatstone textural rock types (MF 2 and MF 3)

are formed in an open marine environment under normal

marine salinity conditions, normal water circulation and

medium hydrodynamic energy (Geel 2000). As a result,

MF 1–MF 3 of the Asmari Formation in the Dill anticline

represents the tropical to subtropical environments in an

oligophotic to mesophotic zone of the middle ramp setting.

Furthermore, the presence of epiphytic porcelaneous

foraminifera (Borelis, Archaias, Peneroplis and Sorites)

and encrusting forms (Planorbulina) correspond to a

deposition within the photic zone, in a sea grass-dominated

environment (Brandano et al. 2008, 2009). Due to the

absence of these foraminifera, a middle ramp setting with

no vegetation is proposed.

Inner ramp

Seven main microfacies (MF 4–MF 10) are subjected to the

shoal, lagoon and near-shore lagoonal environments of an

inner ramp setting. Bioclastic shoal facies (MF 4) isolated

the inner from the middle ramp.

Large and small benthic foraminifera that include

agglutinate, perforate and imperforate forms are present.

Moreover, other components of this setting are scattered

coral and red algae together with sea grass meadows.

The large porcelaneous foraminifera types such as

Archaias, Peneroplis, Dendritina and Borelis are present in

MF 6, 7 and 8. The occurrence of Archaias and Peneroplis is

typical of recent tropical and subtropical shallow-water

environments (Lee 1990; Holzmann et al. 2001) and are

characteristics of the upper part of the upper photic zone

(Brandano et al. 2009). Furthermore, these large porcela-

neous foraminiferas are also common fossils in the Meso-

zoic and Cenozoic neritic sediments (Brandano et al. 2009).

The small porcelaneous foraminifera types mainly

dominated as the abundant miliolids (MF 4–8). In addition,

some larger hyaline foraminiferas such as rare Heteroste-

gina and Amphistegina (MF 5 and 6) were observed. The

Heterostegina and Amphistegina genera are typically

representative of warm water environments (Reiss and

Hottinger 1984; Betzler et al. 1997) and show relatively

high ecological tolerances compared to other LBF (Langer

and Hottinger 2000).

The perforate genus of encrusting foraminifera (Plano-

rbulina) and small benthic agglutinated foraminifera

involve Textularid, Bigenerina and Valvulinid and are

associated with MF 5 and 6, respectively. The scattered,

thinly branching coral fragments occurring in MF 6 (bio-

clastic miliolids coral floatstone) represent reduced water

energy in the deepest part of the euphotic zone of the

lagoonal environment (Schuster and Wielandt 1999). The

foraminifera (Peneroplis and miliolids) assemblage of

this facies is typical of shallow-water and illuminated

habitats, where sea grass meadows interfinger with adja-

cent unvegetated areas (Brandano et al. 2008).

Consequently, the biotic association of the inner ramp in

this area could have originated from tropical and subtropical

shallow waters (Lee 1990; Betzler et al. 1997; Holzmann

et al. 2001; Heydari 2008; Brandano et al. 2009) in sea

grass-dominated environments, as suggested by the pres-

ence of epiphytic foraminifera such as Archaias, Peneroplis

and Borelis (Brandano et al. 2008).

Based on these microfacies, a shallow-water setting of

an inner ramp influenced by wave and tide processes is

suggested for the deposition in MF 4–10.

Relative sea-level changes

The sea-level changes are of two types, local and eustatic.

Locally, these changes take place in a relatively smaller

setting, whereas the eustatic changes are connected with

global sea-level changes used long term, at a geological

timescale (Gradstein et al. 2004).

The relative sea-level change curve in the Dill anticline

for the Oligocene–Miocene reflects the index peaks of

deepening and shallowing within the carbonate of the

Asmari Formation. Relative consistency is observable

between the global sea-level change curve (Haq et al.

1987) and shallowing and deepening peaks drawn for the

study area (Fig. 9). This could lead to eustatic type of sea-

level changes that principally effected deposition at the

Asmari Formation.

Sequence stratigraphy

Sequence stratigraphy is one of the most important meth-

ods for regional correlation and sedimentary environment

analyses. This method has been applied by geologists and

oil companies for oil fields exploration. Sequence strati-

graphic interpretation, based on standard criteria as bed-

ding patterns, is made difficult by the poor quality of

exposure of the large-scale geometries. Nevertheless, it


123


was carried out by considering the relative position of

facies belts and the sequence boundaries (Corda and

Brandano 2003). Each sequence consists of a package of

transgressive and regressive sedimentary facies (systems

tracts or facies tracts) and is bracketed by two sequence

boundaries (Wanas 2008). In marine shelf homogeneous

carbonate environments, it is sometimes difficult to dis-

tinguish the different system tracts of a depositional

sequence (Vail et al. 1984; Posamentier and Vail 1988;

Sarg 1988). Hence, the various markers of high and low

sea-level phases, such as benthic foraminifera, seem to

provide particularly reliable data as they are very sensitive

to any change in the environment (Vaziri-Moghaddam

et al. 2006). Therefore, the facies distribution, stratal pat-

terns and sequence boundaries permit the identification of

three separate third-order depositional sequences and the

sequences that occurred at a particular age of the Oligo-

cene–Miocene time (Fig. 10). Each third-order deposi-

tional sequence is composed of a TST (transgressive

systems tracts), HST (highstand systems tracts) and MFS

(maximum flooding surface).

Sequence 1 (Chattian)

Sequence 1 is a massive to thick bedded limestone suc-

cession. It is coincident mostly with the lower part of the

Asmari Formation and is well exposed in outcrops

(Fig. 10).

The TST of sequence 1 is characterized by an open shelf

lagoon facies, rich in small benthic hyaline and porcela-

neous foraminifera (MF 5). MFS is marked by the occur-

rence of an open marine facies, rich in large benthic

hyaline foraminifera (MF 1).

Sequence 1 is followed by a relative sea-level fall,

passing of an open marine facies (MF 1) to bioclast high-

energy shoal facies (MF 4) and to lagoonal facies (MF 5,

6 and 7). This upward shallowing trend reflects on HST.

However, the HST facies indicate an aggradationally

lagoonal environment that was prominent. HST is termi-

nated by a near-shore lagoonal facies (MF 8). The

boundary between sequence 1 and sequence 2 is marked

by SB 2 (evidence of subaerial exposure was not

observed). The upper limit of sequence 1 (SB 2) is almost

coincident with very late Chattian and Chattian–Aquita-

nian boundary. This boundary is more or less correlatable

to the eustatic fall of sea level as indicated by Haq et al.

(1987) (Fig. 9).

Sequence 2 (Aquitanian)

Sequence 2 is a thin, medium and thick bedded limestone

succession. It is coincident mostly with the middle part of

the Asmari Formation and is well exposed as outcrops

(Fig. 10).

The TST of sequence 2 is characterized by a lagoonal

facies rich in porcelaneous foraminifera together with coral

and corallinacean debris (MF 6). MFS is marked by the

occurrence of a semi-restricted lagoonal facies rich in

hyaline and porcelaneous benthic foraminifera (MF 5).

HST sequence 2 reveals a relative sea-level fall and a

prevailing restricted lagoonal environment. HST is char-

acterized by an occurrence of porcelaneous foraminifera

together with coral and corallinacean debris (MF 6) and

terminated by rich porcelaneous benthic foraminifera

(MF 7).

The boundary between sequence 2 and sequence 3 is

marked by SB 2, due to the occurrence of bioclast lami-

nated mudstone of near-shore lagoon environment (MF 9).

The upper limit of sequence 2 (SB 2) is coincident with the

Aquitanian–Burdigalian boundary. This boundary is cor-

relatable to the eustatic fall of sea level as estimated by

Haq et al. (1987) (Fig. 9).

Sequence 3 (Burdigalian)

Sequence 3 is a thin to medium bedded limestone succes-

sion. It is coincident mostly with the upper part of the

Asmari Formation and is well exposed as outcrops

(Fig. 10).

The TST package of sequence 3 consists of sediments

deposited in restricted and semi-restricted lagoonal envi-

ronments. Facies rich in porcelaneous benthic foraminifera

(MF 7) is followed by hyaline and porcelaneous benthic

foraminifera together with echinoid, bryozoa and bivalve

debris facies (MF 5). MF 5 was set as a maximum flooding

surface.

Fig. 9 Relative correlation exists between sea-level change curves of

the study area and global model for Oligocene–Miocene time interval

(Haq et al. 1987)


123


Fig. 10 Vertical facies distribution showing paleoenvironmental, relative sea-level changes and sequence stratigraphic characteristics of the

Asmari Formation at Dill anticline in Zagros basin


123


HST of sequence 3 shows a shallowing upward trend

with passing of a restricted to near-shore lagoonal envi-

ronment. An imperforate benthic foraminifera facies (MF

7) is replaced by the sandy mudstone facies (MF 10). The

sequence boundary (SB 1) reflects closing of the Asmari

Formation carbonate deposition and the appearance of

Gachsaran Formation evaporitic cycles.

Regional sequence boundaries

As discussed, three sequence boundaries were identified

for the exposed Asmari Formation at the study area.

Sequence boundary presents at the base of the sequence 2,

near the late Chattian to Aquitanian border. This bound-

ary is close to Aq. 10 of global regressive sea level

(Gradstein et al. 2004) and is coincident with basal Kal-

hur anhydrite in the Dezful Embayment (Ehrenberg et al.

2007). It can be referred to as an index sequence

boundary between carbonate successions of the Asmari

Formation in the Zagros basin of the Chattian–Aquitanian

age (Fig. 11).

Sequence boundary presents at the base of sequence 2 at

the Aquitanian–Burdigalian border. This boundary is close

to Bu. 20 of global regressive sea level (Gradstein et al.

2004) and is coincident with the top of middle Kalhur

anhydrite in the Dezful Embayment (Ehrenberg et al.

2007). This boundary occurs between carbonate succes-

sions of the Asmari Formation in Zagros basin at the

Aquitanian–Burdigalian boundary (Fig. 11).

The third sequence boundary occurs in the Burdigalian

time and reflects closing of the Asmari Formation car-

bonate deposition and appearance of the Gachsaran For-

mation evaporitic cycles (Fig. 11).

Conclusions

The Asmari Formation, exposed at the Dill anticline part of

the Dezful Embayment in the Zagros foreland basin, was

examined. The Asmari Formation of the Late Oligocene

(Chattian)–Early Miocene (Burdigalian) time is interpreted

based on field observations, facies analysis and distribution

of the foraminiferal contents to reconstruct depositional

environments and sequence stratigraphy. Ten facies, char-

acterizing a gradual shallowing upward trend, of an open

marine, shoal, semi- restricted and restricted lagoon

depositional environments were identified. Environmental

interpretations show that an inner and middle parts of a

homoclinal ramp prevailed during the deposition of the

Asmari Formation. Moreover, relative sea-level change

curves were drawn and correlated with global sea-level

change curves (Haq et al. 1987) during the Oligocene–

Miocene age. Three-third-order depositional sequences

were recognized and sequence boundaries were correlated

with the Dezful Embayment of the Zagros basin.

Acknowledgments The authors wish to thank the University of

Isfahan for the financial support. Also, thanks are due to the National

Iranian Oil Research and Development for supporting this research.

Grateful thanks are extended to Professor James W. Lamoreaux and

to the Carbonate and Evaporate reviewers for their constructive

comments on this manuscript.

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