Stratigraphy and correlation of the sections
Facies analyses revealed four Upper Oligocene and three
Lower Miocene sequences in the fore-arc sections, as well
as three Upper Oligocene and three Lower Miocene
sequences in the back-arc sections. Within the biostrati-
graphic framework, the recognized depositional sequences
generally correlate well with third order fluctuations of the
eustatic sea-level curve of Hardenbol et al. (
1998
) and
allow a basin-spanning correlation of the Qom Fm.
(Fig.
11
). However, while sequence correlation in the Qom
section is reatively well-defined due to its central position
in the Qom back-arc basin, the close proximity of the
sections Chalheghareh and Zefreh to the volcanic arc
makes it likely that vertical tectonic movements overprint
the eustatic signal.
Stratigraphic framework of the Esfahan-Sirjan
fore-arc basin
In section Abadeh (Fig.
4
) the Rupelian/Chattian boundary
is pinned down by the first occurrence of lepidocyclinids
[Lepidocyclina (Nephrolepidina) morgani-type, L. (Eu-
lepidina) formosoides, L. (E.) favosa] recorded in bed 14 in
the lower part of sequence 2 [BouDhager-Fadel and Banner
1999
; Cahuzac and Poignant
1997
(SB22b)]. Therefore,
sequence 1 is considered to be late Rupelian in age and is,
accordingly, correlated with cycle Ru 3 of Hardenbol et al.
(
1998
). The early Chattian age of sequence 2 is corrobo-
rated by planktic foraminiferal assemblages, comprising
Paragloborotalia opima opima, Globigerina ciperoensis,
Chiloguembelina, Guembelitria, and Tenuitella munda
from beds 16–20 (sequence 2), which are referred to zone
P21b of Berggren et al. (
1995
). For the upper part of sec-
tion Abadeh (above the fluvial conglomerate of sequence
3), pectinid and gastropod faunas also point to a Chattian
age (Mandic
2000
; Harzhauser
2004
). Consequently,
sequences 2 and 3 are correlated with cycles Ru 4/Ch 1 and
Ch 2, respectively.
In the Zefreh area, Chahida et al. (
1977
) placed sedi-
ments of the Qom Fm. into the Early Miocene (Aquitanian
to early Burdigalian) based on the occurrence of Miogyp-
sina (Miogypsinoides) dehaarti. The section analysed in
this study (Fig.
6
) also contains Miogypsina tests in beds
15–19 (sequence 2). Although their poor preservation does
not allow a specific identification, their association with the
planktic foraminiferal assemblage of Globigerina cf. cip-
eroensis,
Globoturborotalia
woodi,
Globigerinoides
trilobus, Gs. immaturus, Gs. quadrilobatus, Globigerinella
obesa, Globoquadrina cf. dehiscens, and Paragloborotalia
cf. semivera indicates an Early Miocene age (Aquitanian to
early Burdigalian) for sequence 2. Furthermore, sequences
1 and 2 contain an Amussiopecten subpleuronectes-A. la-
badyei assemblage, which suggests a late Chattian to early
Aquitanian age.
Fig. 11
Correlation chart of
sedimentological events in the
studied sections with the global
sea-level curve of Hardenbol
et al. (
1998
). Members refer to
the lithostratigraphic scheme of
Furrer and Soder (
1955
)
Int J Earth Sci (Geol Rundsch)
123
For sequence 4 an early Burdigalian age is indicated by
the Costellamussiopecten pasini assemblage, as well as by
the absence of the mid-Burdigalian Gigantopecten holgeri–
Amussiopecten expansus–Manupecten puymoriae assem-
blage (Mandic and Steininnger
2003
), which is, in turn,
present in sequence 7 in the Qom section. Consequently,
the four upper Rupelian to lower Burdigalian depositional
sequences of Zefreh section correspond to the Ch 3 to Aq
3/Bur 1 sequences of Hardenbol et al. (
1998
).
Stratigraphic framework of the Qom back-arc basin
Two episodes of evaporation produced well traceable
lithostratigraphic markers (gypsum beds) in the Qom and
Chalheghareh areas that can be correlated with the
d-Member of Furrer and Soder (
1955
). At both localities
a unit of green marls follow above, which are covered by a
unit of skeletal limestones that are equivalent with the
e-Member (green marls) and the f-Member (skeletal
limestones) of the lithostratigraphic scheme presented by
Furrer and Soder (
1955
). This allows us to correlate the
gypsum-bearing sequences 3 and 4 of the Chalheghareh
section with sequences 5 and 6 of the Qom section,
respectively (Fig.
11
). Sequences 2 and 1 of section
Chalheghareh are accordingly correlated with sequences 4
and 3 of section Qom, respectively.
This lithostratigraphic correlation is supported by bio-
stratigraphy. The sections are pinpointed using microfossils
at the Chattian/Aquitanian (sequences 3/4) and Aquitanian/
Burdigalian (sequences 5/6) boundaries in the Qom sec-
tion, as well as at the Chattian/Aquitanian boundary
(sequences 1/2) in the Chalheghareh section. Age indica-
tions of a variety of faunal assemblages throughout the
sections corroborate the correlation of the remaining
sequences, which will be discussed in detail below.
In Qom section, the lower part of the b-Member
(sequences 1–3) contains nannoplankton assemblages
(beds 7 and 11) indicating a late Rupelian to Chattian age
(NP24-NP25 of Martini
1971
) due to the co-occurrence of
Reticulofenestra abisecta, R. bisecta, and Zygrhablithus
bijugatus. This age estimate is supported by a planktic
foraminiferal fauna from bed 9, which includes Globige-
rina anguliofficinalis, Gg. ciperoensis, Gg. labiacrassata,
Gg. ouachitaensis, and Globigerinoides primordius, also
denoting a late Rupelian to early Chattian age. Lepido-
cyclinids from the middle subunit of the c-Member
(lowstand of sequence 4) are represented by Lepidocyclina
(Nephrolepidina) morgani-type and L. (Eulepidina) dila-
tata and point to a Chattian age. Therefore, the lower
sequences 1–3 that span the Chattian are correlated with
the Ru 4/Ch 1 (sequence 1), Ch 2 (sequence 2) and Ch 3
(sequence 3) cycles of Hardenbol et al. (
1998
).
The Oligo-/Miocene boundary is recognized between
beds 29 and 30 in the lower part of sequence 4 [=Ch 4/Aq 1
cycle of Hardenbol et al. (
1998
)] because the succeeding
marls contain the planktic foraminifers Globigerina cf.
ciperoensis, Globorotalia woodi, Globigerinoides primor-
dius, and Gs. immaturus, which characterize the Early
Miocene.
The Aquitanian/Burdigalian boundary coincides with
that of the d-/e-Member, as marls above the thick upper
gypsum package (38) contain a diversified Globigerino-
ides
assemblage
[Globigerinoides
altiaperturus,
Gs.
immaturus, Gs. quadrilobatus, Gs. subquadratus, Gs.
trilobus, Gs. cf. trilobus (‘‘bisectus’’ type), Globoquadrina
langhiana, Catapsydrax unicavus, Cassigerinella bou-
decensis]. This assemblage is characteristic for the base of
N5 according to Bolli and Saunders (
1985
), which is
equivalent to the base of the Globigerinoides altiapertu-
rus–Catapsydrax dissimilis Zone of Bizon and Bizon
(
1972
) and Iaccarino (
1985
), and which defines the base
of the Burdigalian in the Mediterranean. The Aquitanian/
Burdigalian boundary corresponds with a sea-level low-
stand (Aq 3/Bur 1) in the Hardenbol et al. (
1998
) sea-
level curve (Fig.
11
).
Nannoplankton assemblages from the lower (41) and
upper part (52) of the e-Member (sequence 6) additionally
indicate an Early Miocene age owing to the presence of
Sphenolithus cf. umbrellus, Helicosphaera cf. carteri, and
Coccolithus cf. micropelagicus, and the coeval absence of
the typical Chattian forms Reticulofenestra bisecta and
Zygrhablithus bijugatus.
For the f-Member in section Qom, a rich association
of larger benthic foraminifers with Borelis melo curdica,
B. haueri, Peneroplis thomasi, P. armorica, Archaias cf.
asmaricus, A. hensoni, A. sp., and Dendritina rangi shows
strong affinities to other Burdigalian faunas from the
Middle East (Adams
1968
; Ctyroky et al.
1975
; Schuster
and Wielandt
1999
). Moreover, planktic assemblages from
beds 102 to 108 (sequence 7) with Globigerina dubia, Gg.
lentiana, Gg. ottnangiensis, Gg. praebulloides, Cassigeri-
nella boudecensis, and C. chipolensis are similar to those
of the mid-Burdigalian in the Mediterranean. The pectinid
assemblage with Gigantopecten holgeri, Amussiopecten
expansus and Manupecten puymoriae, which is character-
istic for the upper part of the f-Member indicates a mid-
Burdigalian age (Mandic and Steininger
2003
). This
assemblage neither occurs in the Zefreh sections nor in
section Chalheghareh, which indicates a shorter strati-
graphic range of these sections.
In section Chalheghareh, sequence 1 (beds 23–29)
contains eu- and nephrolepidinids [L. (E.) dilatata, L. (E.)
cf. undulosa, L. (E.) sp., L. (Nephrolepidina) spp.], which
are regarded as Chattian in age. This assumption is sup-
ported by the associated planktic foraminifers (e.g.
Int J Earth Sci (Geol Rundsch)
123