Global Geology，18( 3) ∶ 155-163( 2015)

doi: 10. 3969 / j. issn. 1673-9736. 2015. 03. 02Article ID: 1673-9736( 2015) 09-0155-09

Microbiostratigraphy and depositional environment ofEocene Shahbazan deposits at Chenareh section，

southwest of Iran

Azam Abdolnia1 and Iraj Maghfouri Moghadam2*

1． Department of Stratigraphy and Paleontology，Shiraz Branch of Islamic Azad University，Fars，Iran;

2． Department of Geology，Faculty of Science，Lorestan University，Khorram-Abad 68151-44316，Iran

Ｒeceived 2 April 2015，accepted 3 June 2015* Corresponding author ( Email: Irajmmms@ yahoo． co． uk)

Abstract: The Shahbazan Formation was deposited in the foreland basin in southwestern Iran ( Lorestan Ba-sin) ． In this research，microbiostratigraphy and depositional environmental implications related to the Shahba-zan Formation at the northern flank of Chenaerh anticline are discussed． Carbonate sequences of the ShahbazanFormation consist mainly of large benthic foraminifera along with other skeletal and non-skeletal components．Two biozones have been recognized by distribution of large foraminifera in the studied area that indicate middleEocene age ( Lutetian) ． Based on analysis of large benthic foraminiferal assemblages and microfacies features，7 different microfacies have been recognized，which can be grouped into three depositional environments: in-ner，middle and outer ramps．Key words: Shahbazan Formation; Eocene; Lorestan Basin; biostratigraphy; carbonate ramp environment

IntroductionThe Eocene Shahbazan Formation is a thick car-

bonate succession of the Tertiary which has been de-posited in flanking shelves of the Lorestan Basin． Itsname is after the village of Shahbazan in LorestanProvince． The type section of the Shahbazan Forma-tion was measured in the Thaleh Zang rail road stationin Khuzetan Province by James and Wynd ( 1965 ) ．The Shahbazan Formation at its type section ( 32°47'38″N，48° 42' 00″ E ) consists of 338． 3 m of lime-stones，dolomitic limestones and argillaceous lime-stones ( Motiei，1993) ． In the type section and in themost places of the Lorestan Basin，the Shahbazan For-mation conformably overlies the upper clastic units of

the Kashkan Formation ( Paleocene) ． In southern andsouthwestern parts of Lorestan Basin，the ShahbazanFormation interfingers with marls of the Pabdeh For-mation ( Fig． 1 ) ． The upper contact with the lime-stones of the Asmari Formation is disconformable( Stocking and Setudehnia，1991 ) ． In northern andnortheastern parts，the Shahbazan and Asmari forma-tions form a prominent topographic unit． It is differen-tiated from the Asmari Formation by an interveningconglomeratic leached zone and a change from lime-stone of the Asmari Formation to the dolomite of theShahbazan Formation． This boundary is often difficultto place，thus making it necessary to map the two for-mations as one unit． In this case，the two names areby hyphenated ( James and Wynd，1965) ． The main

objective of this research is microbiostratigraphy andseqencestratigraphy of the Shahbazan Formation at the

Chenareh anticline located at the southeastern part ofLorestan Basin．

Fig． 1 A schematic stratigraphic section of Cenozoic deposits in southwestern Iran ( after James ＆ Wynd，1965)

1 Geologic setting1． 1 Tectonics

Based on the sedimentary sequence，magmatism，

metamorphism，structural setting and intensity of de-formation，the Iranian Plateau has been subdividedinto eight continental fragments， including Zagros

( Divided to Simply folded Zagros and ImbricatedZagros) ，Sanandaj-Syrjan，Urumieh-Dokhtar，CentralIran，Alborz，Kopeh-Dagh，Lut，and Makran ( Fig．2a ) ． The Zagros Basin extended fromnortheasternSyria through northern and northeastern Iraq intosouthwestern Iran． It was a part of the stable super-continent of Gondwana in Paleozoic，and a passive

Fig． 2 Sketch maps ( a) General map of Iran showing nine geologic provinces ( Stocklin，1968) ; ( b) Structural-sedimen-tary zones ( after Sherkaty ＆ Letouzey，2004)

651 Azam Abdolnia and Iraj Maghfouri Moghadam

margin in Mesozoic，and became convergence orogenin Cenozoic time ( Bahroudi ＆ Koyi，2004 ) ． In Ju-rassic time，orogenic movement caused Zagros Basindivide into different basins，including the Fars Arc，

Dezful Embayment ( Khuzestan Province) and Lores-tan basins，from south to northeast respectively ( Fig．2b) ．2． 2 Stratigraphy

The studied section is located on the southeasternflank of the Chenareh Anticline in the southeastern

Lorestan Basin． In this section，the Shahbazan For-mation overlies limestone of the upper part of the Pab-deh Formation gradually and underlies the AsmariFormation． The Asmari Formation is late Oligocene-early Miocene ( Chattian-Burdigalian ) in age( Moghaddam et al．，2013 ) ． The thickness of theShahbazan Formation at Chenareh section is 294 m．The Chenareheh Anticline which has been named Ki-alu Anticline in some maps is located in southeastenLorestan． It is 65 km long by 8 km wide ( Fig． 3) ．

Fig． 3 Geological map of the studied area in Lorestan Basin ( after Liewellyn，1974)

2． 3 SampleSamples were collected from every 1． 5 to 2． 0

meters in ( lithified ) limestone beds． Sampling wasbased on facies variation． Approximately 100 sampleswere studied． Definition of microfacies is based ondepositional texture，grain size，grain composition andfossil content． The classification of carbonate rocksfollowed the nomenclature of Dunham ( 1962) ．

3 BiostratigraphyThe biozonation of the Shahbazan Formation in

the studied section is mainly based on the foraminifer-al assemblages of Wynd ( 1965) ．

Two assemblages have been recognized in thestudied section ( Fig． 4 ) ． They are discussed in as-cending stratigraphic order as following:

3． 1 Somalina sp． range zoneＲeference section: Jahrum Formation， Tange

Ab，near Jahrum City，Fars Basin and at the typesection of Tale Zang Formation in Lorestan Basin．

Definition: Ｒange of Somalina sp． in the refer-ence section． The lower limit of this zone is marked

751Microbiostratigraphy and depositional environment of Eocene Shahbazan deposits at Chenareh …

Fig． 4 Stratigraphic column，benthic foraminiferal distribution and vertical microfacies distribution of the ShahbazanFormation in Chenareh Anticline

by the first occurrence of zonal taxa and the upperlimit by the last occurrence of somalina sp．

Charaterization: This interval is also character-ization by the larger foraminiferal assemblages． Impor-tant taxa include ( Fig． 5) Alveolina sp．，A． rotumey-eri，Asselina sp．，Halyarkida sp．，Discocyclina sp．，Gypsina sp．，Nummulites sp．，Orbitides complanatus，Ｒobulus sp．，and Somalia stephani． The interval large-ly consists of limestone and marl．

Age: middle Eocene( a) Somalina stephani，sample no． 27，X 20;

( b) Praerhapydionina sp．，sample no． 84，X35; ( c)

Quincueloculina sp．，sample no． 93，X35; ( e) Disco-cyclina sp．，sample No． 46，X35; ( f ) Halkyardiasp．，sample no． 19，X35; ( g) Nummulites globulus，sample no． 73，X35; ( h ) Orbiotolites complanatus，sample no． 14，X25; ( i) Ｒobulus sp．，sample no． 15，

Z25 ( j) Alveolina rutmeyeri，Sample no． 14，X35．

851 Azam Abdolnia and Iraj Maghfouri Moghadam

Fig． 5 Ｒepresentative foraminiferal taxa of Sahbazan Formation in Chenareh section

3． 2 Nummulites-Alveolina assemblage zoneＲeference section: upper most part of Jahrum

Formation，Tange Ab，near Jahrum City，Fars Basinand at the type section of Thaleh Zang Formation in

951Microbiostratigraphy and depositional environment of Eocene Shahbazan deposits at Chenareh …

Lorestan Basin．Definition: The lower limit of this zone is marked

by the last occurrence of Somalina sp． and the upperlimit by the first occurrence of Pellatispira sp．

Charaterization: This interval is also character-ization by the larger foraminiferal assemblages． Impor-tant taxa include Alveolina sp．，A． rotumeyeri，Asseli-na sp．，Discocyclina sp．，Idalina sp．，Nummulites sp．，Nummulites globulus，Perloculina sp．，and Quiquelo-culina sp． The interval largely consists of limestoneand marl．

Age: middle Eocene

4 Depositional environment and mi-crofaciesSedimentological and paleontological studies

show that a ramp type carbonate platform sedimentarymodel can be fully applied to these ancient carbonatedeposits ( Tucker ＆ Wright，1990 ) ( Fig． 5 ) ． Ac-cording to Burchette and Wright ( 1992 ) ，carbonateramp environments are separated into inner ramp，

middle ramp and outer ramp． The inner ramp facieshave been deposited above fair weather wave base( FWWB) and divided into medium to low energy andhigh energy conditions． The middle ramp facies havebeen deposited between FWWB and storm wave baseand outer ramp facies have been deposited belowstorm wave base．

The microfacies analysis of samples of the Shah-bazan Formation in Chenareh Anticline is led to recog-nition seven microfacies ( Fig． 6 ) of four facies belts( depositional environments ) based on Flügel 2010，

including open marine ( A) ，bar ( B) ，lagoon ( C)

and tidal flat ( D) environments． The vertical changessurvey and the comparision with modern and old depo-sitional environments indicate that the facies of theShahbazan Formation in the studied section have beendeposited in a carbonate ramp ( Fig． 7) ．

Open marin facies zone ( A) consists of biotur-bated mudstone ( A1 ) ，Nummulite bioclast wackstoneto Packstone ( A2 ) ． Bar facies zone ( B) consists offoraminifera grainstone． Lagoon facies zone ( C) con-

sists of ( C1 ) Benthic foraminifera ( perforate and im-perforate) bioclast wackestone - packstone and grain-stone，( C2 ) Imperforate foraminifera bioclast wacke-stone-packstone-grainstoneand ，( C3) Bioclast peloi-dal wackstone-packstone． Tidal flat facies zone ( D)

consists of Mudstone．4． 1 Open marine

Microfacies A1 : Bioturbated mudstone ( Fig． 6a)

The most important feature of this facies biologicalprocess is chaos，dark brown spots． It was clear thatmottled fabric ( Mottled fabric) is created． Microfacies6 consists of dark micrite and planktonic foraminifera，

and alternating with grey marls and argillaceous． It isinterpreted to have been below Fair Weather WaveBase ( FWWB) in a deep outer- ramp setting close tothe basin edge ( Wilson and Evans，2002) ．

Microfacies A2 : Nummulitidae bioclast wack-stone Packstone ( Fig． 6b)

Nummulitidae with small size test ( A form) areabundant biogenetic components in microfacies A2 ．Other bioclast are small debris of echinoids and disco-cyclinidae． Fragmentation of larger foraminifera israre． Megascopically，it is medium-bedded limestonecontaining echinoid fragments． Grains are coarse sandto granule in size and are in a fine-grained carbonatematrix．

A form dominated fossil communities are likely tohave formed in the shallowest or deepest part of depthrange． These two environments can be distinguishedon the basis of the matrix and stratigraphic position( Beavington-Penny and Ｒacey，2004) ． The relativelyhigh degree of fragmentation of the nummulitidae indi-cates that these deposits formed in the peoximal partof the middle ramp．4． 2 Bar

Microfacies B: Bioclast grainstone ( Fig． 6c)

This facies zone is characterized with abundantforaminifera． Frequency of intraclasts such as foramin-ifera indicates a very high energy condition in barriersetting． Depositional texture is represented by grain-stone． It consists of medium-bedded to thick-beddedgrey to brownish limestone beds．

061 Azam Abdolnia and Iraj Maghfouri Moghadam

The sorting and grainy texture suggests a high en-ergy environment for this microfacies． The sedimentswould have been deposited in a shoal environmentwhich separating the open marine from more restrictedmarine environment ( Flügel，2010) ．4． 3 Lagoon

Microfacies C1 : Benthic foraminifera ( perforate

and imperforate) bioclast wackestone-packstone-grain-

stone ( Fig． 6d)

This microfacies is composed of variable propor-tion of benthic foraminifera． Porcelaneous foraminiferasuch as Somalina sp． and hyaline foraminifera ( Ｒobu-lus sp． ) are the most important foraminifera in thismicrofacies． C1 include different texture ranging fromwackestone，packstone to grainstone． It is dominatedby olive green to grey thick-bedded to medium-bedded

Fig． 6 Photomicrographs of the identified microfacies in Shahbazan Formation

161Microbiostratigraphy and depositional environment of Eocene Shahbazan deposits at Chenareh …

limestone beds．Co-occurrence of normal marine fauna and pro-

tected fauna indicate that deposition took place in theinner ramp environment ( Brandano ＆ Corda，2002) ．

Microfacies C2 : Imperforate foraminifera bioclastwackestone-packstone-grainstoneand ( Fig． 6e)

Imperforate foraminifera bioclast wackestone-packstone-grainstone． Megascopically，it consists ofalternating grey thin-bedded limestone and nodularlimestone． No macrofossils have been observed． Thismicrofacies is dominated by occurrence of imperforateforaminifera ( Alveolinidae ) and bivalve debris． Thetexture ranges from common wackestone and pack-stone to less common grainstone． The occurrence oflarge number of porcelaneous imperforate foraminiferaltests may point to the depositional environment beingslightly hyper-saline． Such an assemblage described

to be associated with an inner ramp environment( Wilson，1975; Brandano ＆ Cord，2002) ． This mi-crofacies is dominated by miliolids and bioclasts suchas ostracods and bivalves． The matrix is fine grainedmicrite． Megascopically，it is thin bedded to nodularbedding containing bivalve fragments．

Microfacies C3 : Bioclast peloidal wackstone-packstone ( Fig． 6f)

This facies is characterized by the presence of a-bundant peloids． Sparse bivalve and milolids are alsopresent． The high abundance of peloid contribution in-dicates a subtidal zone with shallow，lagoonal environ-ment ( Ｒeuter et al．，2008) ． The scarcity or absence ofbenthic foraminifers，bioclasts，and the dominance ofpeloids indicate deposition in low-energy，restrictedshallow lagoonal setting with weak connection with anopen marine environment ( Tomasovych，2004) ．

Fig． 7 Inferred depositional model of middle Eocene succession ( Shahbazan Formation) at the Chenareh anticline

4． 4 Tidal flatMicrofacies D: Mudstone ( Fig． 6g)

This facies consists of debris of ostracods and mol-lusks scattered in a micritic groundmass． This facieszone is generally formed from mudstone，which are as-sociated with early fine dolomite．

Sparse quartz grains are also presented． The mud-

dy groundmass of the facies is representative of low en-ergy conditions． The presence of sparse fine-sizedquartz grains together with particles of bivalves，echi-noids and ostracods within a micritic groundmass istypical of restricted inner lagoon environments ( Ｒasseret al．，2005) ．

This facies consists of grained microcrystalline

261 Azam Abdolnia and Iraj Maghfouri Moghadam

dolomite． Bioclasts are lacking and mostly occurs withquartz mudstone． Presence of dolomites indicates inter-nal part of a tidal flat seating ( Shinn，1983) ．

5 ConclusionsAccording to the distribution of benthic foraminif-

eral assemblage，two biozones are recognized in thestudied section，the Somlina sp． range zone and theNummulites-Alveolina assemblage zone． The age of theKashkan Formation in the Chenareh Anticline is de-fined as middle Eocene ( Lutetian ) ． The ShahbaznFormation overlies the Pabdeh Formation and underliesAsmari Formation in the studied stratigraphic section．In this section，The Asmari Formation is Chattian inage． In this study，we introduced a paraconformity be-tween the Shabazan and Asmari formations which is at-tributed to the Priabonian-Chattian in age．

Microfacies analysis led to the recognition of sev-en microfacies which are varying in lateral and verticaldistribution，and showing that carbonates were deposi-ted on a ramp． The inner ramp consists of tidal flat，inner-outer lagoon and shoal environments． The tidalflat contains of mudstone． The wackestone-packstonefacies with imperfoated foraminifers is the most domi-nant facies of the inner lagoon． In the outer lagoon，

perforate and imperforate foraminifera together areabundant． The shoal facies is dominated by bioclastgrainstone． The middle ramp and outer ramp have beenidentified by Nummulitidae bioclast wackstone Pack-stone and bioturbated mudstone facies．

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361Microbiostratigraphy and depositional environment of Eocene Shahbazan deposits at Chenareh …