However, in the Qom Basin a faunal turnover and
change in faunistic relationships of pectinids already took
place during the Bur 2 cycle (sequence 7 of Qom section)
when the central seaway was still open. Whereas pectinid
faunas from the a- to e-Members show a clear Indo-Pacific
affinity until the early Burdigalian, f-Member pectinid
faunas indicate a strong Mediterranean signal towards the
mid-Burdigalian (Mandic
2000
). The same development is
shown by corals and gastropod faunas from the central
Iranian Basins, in which Indo-Pacific elements diminish
during the Early Miocene (Schuster and Wielandt
1999
;
Harzhauser et al.
2002
; Harzhauser et al.
2007
). This
suggests that faunal exchange between the Qom Basin and
the Indo-Pacific ceased, while at the same time the con-
nectivity with the Mediterranean was enhanced. Even if
Eurasia and Africa were not connected by a fully formed
landbridge at this time, it is plausible that hypersaline
waters in the shallow remaining Tethyan Seaway possibly
formed a biogeographic barrier for Indo-Pacific marine
biota shortly before the TTE.
Summary and conclusions
The Qom Formation was deposited on extended, mixed
carbonate-siliciclastic, homoclinal ramps at the north-
eastern coast of the Tethyan Seaway in the Qom back-arc
and Esfahan-Sirjan fore-arc basins (central Iran). It
includes seven stratigraphic sequences that span the late
Rupelian to mid-Burdigalian time interval and can be
attributed to the Ru 3 to Bur 2 third order sea-level fluc-
tuations of Hardenbol et al. (
1998
). The recognition and
dating of these sequences in the Esfahan-Sirjan Basin and
in the Qom Basin allows a basin-spanning stratigraphic
correlation of the Qom Formation for the first time.
The elaborated stratigraphic scheme documents contin-
uous restriction of the Tethyan Seaway with advanced
collision of the African/Arabian and Eurasian/Iranian
plates. For the Oligocene a normal marine milieu is dis-
played throughout the sedimentary successions of the fore-
and back-arc basins. In the fore-arc basin these conditions
endured during the Aquitanian and Early Burdigalian,
while in the Qom Basin, as well as in other marginal basins
on both coasts of the Tethyan Seaway, intermittent epi-
sodes with restricted marine conditions and emersions are
documented. However, persisting gateways connecting the
Qom Basin with the Mediterranean and Indo-Pacific sides
of the Tethyan Seaway until the late Early Burdigalian is
documented by Tethyan faunal elements from both sides of
the Tethyan Seaway in the central Iranian basins. Yet after
the Bur 2 lowstand emersion of the Qom Basin its recol-
onization occurred from the Mediterranean region only.
Thus, the differentiation into an Atlantic-Mediterranean
and an Indo-Pacific bioprovince took place before the final
Fig. 12
Schematic block diagrams illustrating the development of
the Esfahan-Sirjan Basin and Qom Basin from Late Oligocene to
Early Miocene times (paleogeographic maps modified from Harzhau-
ser and Piller
2007
). The black arrows in paleogeographic maps tag
the positions of the Zagros Basin (ZB), Qom Basin (QB) and Esfahan-
Sirjan Basin (ESB); white arrows in block diagrams show the
permeability of gateways. Oligocene: broad Tethyan Seaway and
fully marine connections to the western Tethys via seaways to the
north and south of Qom Basin. Early Miocene: restriction of the
Tethyan Seaway, and closure of the gateways between Qom Basin
and the Proto-Indopacific due to ongoing collission of Africa/Arabia
with Eurasia, and upthrust of the Zagros mountain ranges. Transpar-
ent gray arrows indicate evaporation in the Qom Basin
Int J Earth Sci (Geol Rundsch)
123
paleogeographic disjunction of the Tethys (TTE), as shown
by the comparison with the Zagros Basin, where marine
sedimentation continued until the end of Burdigalian. It is
therefore suggested that the continuous shallowing and
restriction of the seaway generated an increasingly hostile
hypersaline environment and a biogeographic barrier for
marine biota.
Acknowledgments
Our special thanks go to the other members of
the research projects on Late Oligocene—Early Miocene circum-
Mediterranean palaeobiogeographical relations, the project leaders
F. F. Steininger (Eggenburg) and J. H. Nebelsick (Tu¨bingen), as well
as the researchers M.W. Rasser (Stuttgart), J. Reisinger (Korneuburg),
F. Schuster (Freiburg), and J. Schlaf (Aberdeen). We also appreciate
the comments from an anonymous reviewer. This work was supported
by the Austrian FWF (P11886-GEO) and the Deutsche Forschungs-
gemeinschaft (STE 857/1-1; NE 537/1-1, -2).
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