GEOLOGICA CARPATHICA, JUNE 2010, 61, 3, 227—234 doi: 10.2478/v10096-010-0013-4
An Early Miocene biserial foraminiferal event in the
Transylvanian Basin (Romania)
, SORIN FILIPESCU
and RAMONA BĂLC
Babe -Bolyai University, Department of Geology, Kogălniceanu 1, 400084 Cluj Napoca, Romania;
Babe -Bolyai University, Faculty of Environmental Sciences, Fântânele 30, 400294 Cluj Napoca, Romania; firstname.lastname@example.org
(Manuscript received November 2, 2009; accepted in revised form March 11, 2010)
Abstract: Investigations of the Lower Miocene of the Transylvanian Basin reveal particularly high abundances ( > 90 % of
total foraminifera) of small sized biserial foraminifera (Bolivina/Streptochilus). This biotic event has not been noticed in
the Transylvanian Basin so far probably owing to the facies misinterpretation and the small size of the specimens. SEM
investigations allow more precise identification of biserial planktonic taxa and more accurate taxonomic interpretations.
The high abundance of Bolivina/Streptochilus assemblages provide evidence for paleogeographic connections to the Indo-
Pacific area and support new paleoenvironmental interpretations at the transition from the Early to Middle Miocene in relation
to the paleoceanographic events. Both planktonic foraminifera and calcareous nannoplankton suggest a late Burdigalian
age. A new Bolivina/Streptochilus Abundance Biozone is proposed just below the Early/Middle Miocene boundary.
Key words: Lower Miocene, Paratethys, Transylvanian Basin, biostratigraphy, Bolivina, Streptochilus.
High abundances of small biserial foraminifera assigned ini-
tially to the benthic genus Bolivina have been recorded in
Lower Miocene sediments in the eastern Atlantic and north-
western Indian Oceans (Thomas 1987; Smart & Ramsay
1995). The Early Miocene ‘High Abundance of Bolivinid
(HAB) event’, as it was called, was considered as a synchro-
nous biotic event having paleoceanographic significance
(Smart & Murray 1994). Based on morphologic and isotopic
evidence, Smart & Thomas (2006, 2007) reassigned these Early
Miocene Bolivina to the planktonic genus Streptochilus.
High abundance (> 90 % of total foraminifera) of small
biserial foraminifera have recently been observed in the
uppermost Lower Miocene sediments of the Transylvanian
Basin (top of the Hida Formation). In this study, we present
new data on the biostratigraphic, biogeographic and paleo-
environmental significance of these peculiar assemblages.
Material and methods
Small biserial foraminifera were recovered from several
sections at the top of the Lower Miocene Hida Formation. The
representative sections are located in the northwestern part of
the Transylvanian Basin (Romania), at Ciceu-Giurge ti
(N47.24549; E24.03438), Zagra (N47.26599; E24.28779),
and oimeni (N46.95658; E23.53370) (Fig. 1).
Fourteen samples were analysed for foraminifera and
nannofossils, collected from several small outcrops in distal
epiclastic turbidites and hemipelagites (targeted fine-grained
intervals), with a sampling resolution of 0.5 meters. Sedi-
ment samples were processed using standard micropaleonto-
logical methods and the foraminifera were recovered from
the > 63 µm fraction. Due to their small size, after a prelimi-
nary observation under the stereomicroscope, the species
were examined and their taxonomic identification confirmed
by SEM examination (JSM-JEOL 5510 LV scanning electron
microscope). Calcareous nannofossils were processed by the
standard smear slide technique and examined under the light
microscope (Axiolab Zeiss).
Biserial foraminiferal assemblages
The samples collected from Ciceu-Giurge ti and Zagra con-
tain almost exclusively small biserial planktonic foraminifera
belonging to Streptochilus pristinum (Fig. 2), a species de-
scribed from the Indo-Pacific region by Brönnimann & Resig
(1971). This kind of assemblage is documented for the first
time in the Lower Miocene of the Transylvanian Basin and, to
our knowledge, in the Paratethyan area.
At oimeni, the biserial foraminiferal assemblage is domi-
nated by specimens of Bolivina (B. dilatata dilatata Reuss, B.
dilatata brevis Cicha & Zapletalova, B. molassica Hofmann)
(Fig. 3), which have been traditionally considered to be a part
of benthic communities. There are no typical Streptochilus
specimens in the studied assemblages in this section.
According to Brönnimann & Resig (1971) and Smart &
Thomas (2007), the genus Streptochilus has particular mor-
phological features compared to Bolivina: a loop-shaped
aperture bordered by a high, collar-like projection, except for
an inturned portion at the inner margin. The internal plate
that connects succeeding apertural borders does not project
freely into the aperture as a tooth plate (as in the case of
Bolivinitidae). Recent genetic work has shown that the
BELDEAN, FILIPESCU and BĂLC
planktonic S. globigerus and the benthic B. variabilis are the
same biological species (Darling et al. 2009). Regardless of the
taxonomic problem (Streptochilus or Bolivina), the presence
of abundant biserial taxa gives a new approach to the paleo-
geographic and biostratigraphic interpretations.
In the Hida Formation, the planktonic foraminiferal assem-
blages contain common globigerinids including: Globigerina
ottnangiensis Rögl, Globigerina dubia Egger, Globigerina
pseudociperoensis Blow, Globigerina tarchanensis Subbotina
& Chutzieva, Tenuitellinata selleyi Li, Radford & Banner.
Rögl (1994) and Rögl et al. (2002) have mentioned these as
common in the Lower Miocene (Karpatian) of the Central
Paratethys. The high abundance biserial foraminifera event
is biostratigraphically “sandwiched” between these assem-
blages and the first Middle Miocene (Badenian) Praeorbulina
assemblages (M5a Zone in the classification of Berggren et
al. 1995 – see Fig. 4).
The biserials seem to be associated with a newly observed
transgressive event, considered here as belonging to the
TB2.2 cycle of Haq et al. (1988) or Bur4 sequence of Harden-
bol et al. (1998). This event produced a change in the sedi-
mentation pattern, from lowstand coarse turbidites and
channel fills of the upper Hida Formation to hemipelagic
sediments marking the transition to the Middle Miocene Dej
Calcareous nannofossils (Fig. 5) have low abundance and
diversity, moderate to poor preservation, and there are fre-
quent reworked Cretaceous and Paleogene taxa in all sec-
tions. The common species indicate the NN4 Zone with
Helicosphaera ampliaperta (defined from the LO of Spheno-
lithus belemnos and LO of H. ampliaperta) positioned in the
latest Burdigalian (Martini 1971). At oimeni, H. ampli-
aperta (Bramlette & Wilcoxon) was identified only in one
sample, but in Ciceu-Giurge ti and Zagra, this taxon is com-
mon and well preserved. The assemblages consist of several
other common species: Reticulofenestra minuta (Roth), R.
pseudoumbilicus (Gartner), Coccolithus pelagicus (Wallich),
and Cyclicargolithus floridanus (Roth & Hay). Calcidiscus
spp. and Sphenolithus heteromorphus Deflandre have not
been identified in the assemblage, although they are com-
mon in the Central Paratethys (Austria, Slovenia, Czech Re-
Fig. 1. Location of investigated sections on the simplified geological map (1:200,000). 1 – Paleogene, 2 – Lower Miocene shallow marine
formations, 3 – Hida Formation, 4 – Badenian, 5 – Sarmatian, 6 – metamorphics, 7 – Quaternary, 8 – investigated sections.
BISERIAL FORAMINIFERAL EVENT IN THE LOWER MIOCENE (ROMANIA)
Streptochilus pristinum Brönnimann & Resig from Ciceu-Giurge ti and Zagra. 1, 3, 6, 7, 9 – from Ciceu-Giurge ti; 2, 4, 5, 8 – from
BELDEAN, FILIPESCU and BĂLC
Fig. 3. Bolivinid assemblages from oimeni. 1, 2, 4 –Bolivina dilatata dilatata Reuss; 3 – Brizalina alata Seguenza; 5, 6 – Bolivina dilatata
brevis Cicha & Zapletalova; 7, 8 – Bolivina molassica Hofmann.
public – as mentioned by Martini & Muller 1975; Spezza-
ferri & Ćoric 2001; Švábenická et al. 2003).
Our abundant biserial assemblages seem to have a clear
biogeographic and biostratigraphic importance, owing to the
fact that Thomas (1987) correlated the HAB event in the
Atlantic Ocean with the calcareous nannofossil Zone NN4 and
paleomagnetic Chrons C5C—C5E (Fig. 4). Smart & Murray
(1994) consider that the equivalent event in the Atlantic hap-
pened 20—17 Ma. Smart & Ramsay (1995) dated the HAB
event in the Indian and Atlantic Oceans at 19.5—16.5 Ma,
while Smart & Thomas (2006, 2007) gave for the same areas
ages of 19—17 Ma.
BISERIAL FORAMINIFERAL EVENT IN THE LOWER MIOCENE (ROMANIA)
Fig. 4. Position of the Streptochilus-Bolivina Abundance Biozone within the stratigraphic zonations in use (zones based on Popescu 1975;
Haq et al. 1988; Berggren 1995; Hardenbol et al. 1998; Rögl et al. 2008).
livinids occur only where an oxygen minimum zone impinges
on the sea floor, under upwelling regions along continental
margins and in silled basins (Bernhard 1986; Bernhard & Sen
Gupta 1999; Nigam et al. 2007). It can be easily observed that
the high abundance biserial foraminifera event in the north-
western Transylvanian Basin was not associated with other
typical benthic foraminifera. Smart & Thomas (2006) suggest-
ed that benthic foraminiferal accumulation rates are low when
biserials are abundant. Low export productivity occurs where
the thermocline is deep and regeneration rates of organic mat-
ter are high (Fischer et al. 2003). This could also be correlated
to low oxygenation on the deep-sea floor during the transgres-
sive events due to the poor vertical circulation.
Bolivinids are common for bathyal and shallower water
low-oxygen environments (Sen Gupta 2002). Assemblages
are typically of low species diversity (2—3 species making up
around 80 % of the assemblage) and are associated with a
combination of low oxygenation and high organic-matter sup-
ply along the continental margin. The dominant species are
small, with thin tests, usually belonging to Bolivina (Sen Gupta
& Machian-Castilio 1993).
Smart & Thomas (2006, 2007) used the information pro-
vided by the apertural morphology, accumulation rates and
isotopic composition of the tests to show that the abundant
Early Miocene biserial foraminifera are in fact planktonic
foraminifera and should be assigned to the genus Strepto-
chilus. Assigning a planktonic mode of life to these foramin-
ifera could explain their large distribution, while their
abundance can be correlated to a high productivity event in
Elongated bi- and triserial planktonic foraminifera were
common during the Late Cretaceous, some of them as rare
survivors of the end Cretaceous mass extinction (Kroon &
Nederbragt 1990; Olsson et al. 1999). Biserial forms, usually
The distinctive occurrence of high abundance biserial fora-
minifera in relation to a transgressive event gives the assem-
blage good correlation potential. We therefore define a new
Streptochilus-Bolivina Abundance Biozone
Definition: The body of strata belonging to the transgres-
sive and highstand interval from the top of the Burdigalian
(Fig. 4), with a high abundance of small biserial foraminifera
assigned to the genera Streptochilus (S. pristinum Brönni-
mann & Resig) and Bolivina (B. dilatata dilatata Reuss, B. di-
latata brevis Cicha & Zapletalova, B. molassica Hofmann).
Age: Latest Burdigalian (?Ottnangian—Karpatian), above
the Early Miocene assemblage with small trochospiral globi-
gerinids (Globigerina spp., Tenuitellinata spp.) and below the
first occurrence of Middle Miocene Praeorbulina. Probably the
optimum abundance interval was between 16.8 and 16.4 Ma in
the classification of Rögl et al. (2008) – see Fig. 4.
Representative sections: Ciceu-Giurge ti, Zagra, oimeni.
Discussion and interpretation
According to Brönnimann & Resig (1971), the foramin-
iferal assemblages in the eastern Atlantic, northwestern Indian
and western Pacific Oceans record a change just before the
Middle Miocene. Small, smooth-walled species of the genus
“Bolivina” (called Streptochilus by Smart & Thomas 2006,
2007) reached extremely high relative abundances in the
open-ocean (Thomas 1987; Smart & Ramsay 1995; Smart &
The high abundance of biserial foraminifera is difficult to
explain. In the modern oceans, such high abundances of bo-
BELDEAN, FILIPESCU and BĂLC
Fig. 5. Calcareous nannoplankton assemblages from oimeni (assemblage A) and Ciceu-Giurge ti (assemblage B).
BISERIAL FORAMINIFERAL EVENT IN THE LOWER MIOCENE (ROMANIA)
assigned to the genus Chiloguembelina, were common to
abundant in the Paleogene (Olsson et al. 1999; Huber et al.
2006), their abundance reflecting relatively high productivity
(Hallock et al. 1991). The genus Chiloguembelina was gener-
ally considered to have become extinct during the Oligocene,
while the genus Streptochilus was considered as its descen-
dant (Kennett & Srinivasan 1983).
Miocene Streptochilus spp. have been described as tropical
to warm-subtropical forms (Brönnimann & Resig 1971;
Kennett & Srinivasan 1983; Resig 1989), but they have been
reported from the northernmost Atlantic Ocean (Flower 1999)
through the Bahama Bank (Kroon et al. 2000), the equatorial
western Pacific (Premoli-Silva & Violanti 1981; Resig 1989)
and the eastern Indian Ocean (Resig 1989). High abundances
of biserial planktonic foraminifera are indicative of eutrophic
waters in the Paleogene (Hallock et al. 1991), while high
abundances of Miocene—Pliocene Streptochilus species were
correlated with high accumulation rates of plankton (Resig
1989). In our case, the hypothesis of eutrophic waters seems
to fit better, because an important planktonic bloom was re-
corded only subsequently, related to the advancing Middle
Darling et al. (2009) used the term “tychopelagic” to de-
scribe organisms that usually live as benthos but can survive
and grow in fairly large numbers as plankton and may be ad-
vected well offshore into open ocean assemblages. They re-
vealed that tychopelagic Bolivina may well evolve into true
pelagic forms and consequently biserials can become abundant
in the planktonic assemblage at some time intervals (isotopic
and distributional evidence suggest their truly pelagic life).
In the Transylvanian Basin, the Early Miocene Streptochi-
lus/Bolivina may have bloomed opportunistically in response
to highly fluctuating nutrient conditions caused by rapid trans-
gression or could have been a part of the planktonic invasions
from the Indian Ocean stimulated by the pattern of the surface
circulation. Assemblages with biserial planktonics have been
also reported from the upper Middle Miocene of the Transyl-
vanian Basin (Filipescu & Silye 2008). The Transylvanian
Early Miocene abundant biserial foraminifera assemblage, to-
gether with other regionally distributed planktonics, and the
obvious differences with the Mediterranean assemblages pro-
vide clear evidence for paleogeographic connections to the
The high abundance biserial foraminifera event identified in
the northwestern Transylvanian Basin seems to be related to
the last Early Miocene transgressive event (TB2.2 cycle of
Haq et al. 1988 or Bur4 sequence of Hardenbol et al. 1998).
These particular assemblages were produced by high organic
supply, which induced low sea-floor oxygenation and sup-
pressed benthic life. The occurrence of biserial planktonics in
relation to a transgressive event suggests open-sea connec-
tions of the Paratethys to the east.
The high abundance of biserial foraminifera are biostrati-
graphically and biogeographically important, making possible
the separation of a distinct biozone just below the Lower/Mid-
dle Miocene boundary. Although the assemblage is only re-
ported from a small number of localities so far, we suspect
that the assemblage has a wider distribution in the Paratethys.
Acknowledgments: This is a contribution to Project 473/2007
funded by C.N.C.S.I.S. Romania. The fieldwork was support-
ed from the Project 32/2008 funded by S.N.G.N. Romgaz S.A.
We thank Dr. M.A. Kaminski, Dr. Christopher Smart, and
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