Globigerina ouachitaensis, Tenuitella munda), indicating a
mid- to Late Oligocene age (Schuster and Wielandt
1999
).
In addition, sequences 1 and 2 yield an Amussiopecten
subpleuronectes-A. labadyei assemblage of late Chattian to
early Aquitanian age.
Bed 56 (sequence 2) marks the base of the Aquitanian
due to the first occurrence of Miogypsinoides formosensis
and M. bantamensis. For the associated marls an Early
Miocene age is indicated by planktic foraminiferal
assemblages including Globigerina praebulloides, Gg. cf.
ottnangiensis, Globigerinoides primordius, Gs. immatu-
rus, Gs. quadrilobatus, and Cassigerinella boudecensis.
While the Chattian/Aquitanian boundary is correlated
with the lowstand of the Ch 4/Aq 1 cycle of Hardenbol
et al. (
1998
), in section Chalheghareh the faunal assem-
blages
at
the
boundary
contradictorily
indicate
a
relatively deep setting, which may suggest local tectonic
movements of the nearby volcanic arc (Fig.
1
b). Because
sequence 2 is conformably overlying sequence 1, the
former has to be equivalent with cycle Ch 3 of Hard-
enbol et al. (
1998
).
Finally, pectinid faunas in sequences 3 and 4 are char-
acterized by Costellamussiopecten pasini, which suggests
an Aquitanian to early Burdigalian age.
Integrating lithostratigraphic and biostratigraphic data,
we correlate the bases of the upper gypsum deposits in the
Qom and Chalheghareh sections (Fig.
8
: bed 37 and Fig.
9
:
bed 95), with the Aquitanian/Burdigalian boundary. The
Aq 3/Bur 1 cycle then corresponds to sequence 6 in Qom
section and sequence 4 in Chalheghareh section, and the
lower gypsum unit was formed at the beginning of the Aq 2
cycle (sequence 5 in Qom section, sequence 3 in Chal-
heghareh section).
Implications for the paleogeographic and
paleobiogeographic evolution of the Tethyan Seaway
For the Esfahan-Sirjan and Qom basins, the gradual shifts
from continental to offshore facies (and vice versa) indicate
that sedimentation took place on extensive homoclinal
ramps as already stated by Okhravi and Amini (
1998
) for
the f-Member of the Qom Basin. The widespread occur-
rence of intertidal to shallow subtidal deposits and the
absence of gravitative sediments (turbidites, slumps, etc.)
indicate a gentle inclination of the ramps.
At first, the fore-arc basin was inundated by the marine
transgression, of the Qom Sea during the late Early Oli-
gocene (Ru 3 cycle), while in the back-arc basin marine
environments were not established until the beginning of
the Late Oligocene (Ru 4/Ch 1 cycle). Subsequently, nor-
mal marine conditions prevailed in both basins throughout
the Oligocene (Fig.
12
).
During the Early Miocene the situation changed because
in the Qom back-arc basin the gates to the open ocean
gradually became restricted due to the compressive tec-
tonic regime, preventing water exchange between shallow
landlocked areas and the open sea (Fig.
12
). During the
Aquitanian this process is displayed in the Qom Basin by
the episodic precipitation of evaporites at sea-level low-
stands at the beginning of the cycles Aq 2 and Aq 3/Bur 1.
Uplift of the volcanic arc area caused by ongoing plate
collision is shown by the emersion of the proximally
positioned Chalheghareh and Zefreh sections already in the
early Burdigalian (Bur 2). Final emersion of the Qom Basin
is evidenced by deposition of the continental Upper Red
Fm. during the mid-Burdigalian (Bur 3 cycle).
Shallow marine Oligo-/Miocene deposits also occur on
the opposite side of the Tethyan Seaway along the African/
Arabian coast. Sequences comparable to those of the Es-
fahan-Sirjan
Basin
are
described
from
the
Asmari
Formation in the Iranian Zagros Basin (Seyrafian and
Hamedani
1998
; Vaziri-Moghaddam et al.
2006
), which is
part of the African/Arabian Plate. Sedimentation took place
on a ramp-type carbonate platform (Vaziri-Moghaddam
et al.
2006
) and depositional environments include tidal
flat, shelf lagoon, platform margin and open marine envi-
ronments. Detailed descriptions and interpretations of the
lithologies from the Asmari Fm. are given by Seyrafian and
Hamedani (
1998
) and Vaziri-Moghaddam et al. (
2006
). For
these Oligocene and Miocene sections the authors dem-
onstrate that in most parts of the basin normal marine
conditions prevailed and a restricted marine milieu was
limited to marginal marine lagoons. However, in the NW
Zagros Basin evaporitic deposits (Kalhur Member) locally
interfinger with limestones of the middle Asmari Fm. (see
Ala
1982
), which has an Aquitanian age (Vaziri-Moghad-
dam et al.
2006
) and may therefore be synchronous with
evaporite formation in the Qom Basin. These evaporites
precipitated in smaller, restricted areas of the Zagros Basin
that formed due to the upthrust of the Zagros Mountains,
which divided the Tethyan Seaway into several subbasins
(Alsharhan and Nairn
1995
) (Fig.
12
).
For the uppermost Burdigalian part of the section in the
north-central Zagros Basin, Seyrafian and Hamedani
(
1998
) reconstructed increasingly restricted conditions.
This development culminated in the deposition of the
evaporitic Gachsaran Fm., which covers the Asmari Fm.
basin-wide at the end of the Burdigalian (Ala
1982
). At this
time the continental milieu of the Upper Red Fm.
was already installed in the Qom Basin and at least in
marginal areas of the Esfahan-Sirjan Basin. The extensive
distribution of terrestrial and hypersaline facies at the end
of the Burdigalian in the area of the Tethyan Seaway points
to separation of the Mediterranean and Indo-Pacific Tethys
at this time.
Int J Earth Sci (Geol Rundsch)
123