GEOLOGIJA 31, 32, 209-217 (1988/89), Ljubljana
UDK 56.61.62.02.551.781(497.2)-20
Nummulites and calcareous nannofossils from Lower Eocene beds
in the area of Veliko Tâmovo (North Bulgaria)
Kina Aladžova-Hrisćeva
Geologičeski institut pri BAN, Ul. Akad. G. Bončev 2, Sofia 1113, Bulgaria
Jernej Pavšić
Katedra za geologijo in paleontologijo. Univerza Edvarda Kardelja v Ljubljani, Aškerčeva 12,
61000 Ljubljana
Abstract
From two sections in the Veliko Tâmovo area nummulites and calcareous
nannofossils were studied. In the Šemševo section by comparison with nummuli-
tes the nannoplankton biozone NP 13, and in the Balvan section biozones NP
9 and NP 11 were separated.
Introduction
Individual beds of the Lower Eocene beds in the area of Veliko Târnovo contain
numerous small nummulites. Most of them have been studied and described (Alad-
žova-Hrisčeva in press). Some of these beds have been sampled for nannofossils.
The identified nannoplankton assemblages enabled to subdivide the respective zones.
The nummulite associations in the area of Veliko Târnovo show common features
with those of the Mediterranean where a nummulite zonal scheme was developed
(Schaub 1981).
The aim of the present paper is to study the joint distribution of nummulites and
nannofossils in the Eocene sedimentary rocks in the Veliko Târnovo area (Fig. 1), and
to correlate the nummulites and the nannoplankton zones identified.
The stratigraphy of the Lower Eocene beds in the area and the study of the
nummulite fauna were performed by K. Aladžova-Hrisčeva. The nannoplankton was
studied by J. Pavšič. The samples for this investigation have been presented to him by
K. Aladžova-Hrisčeva in 1985.
Note on the stratigraphy
The Lower Eocene sedimentary rocks in the Veliko Târnovo area overlie trans-
gressively and unconformably different levels of the Lovec Urgonian Group ranging
from the Lower to Upper Cretaceous. Due to the wide distribution of talus deposits
210
Kina Aladžova-Hrisčeva & Jernej Pavšič
Fig. 1. Distribution of Eariy Eocene beds in the Veliko Tâmovo area
1 Cretaceous; 2 Šemševo Formation (Early Eocene); 3 Avren Formation (Early Eocene);
4 Quaternary; 5 Sections of Early Eocene beds
the boundary Cretaceous/Paleogene is masked and the basal Paleogene horizons are
usually not exposed on the surface. The sections of the Lower Eocene beds are
commonly incomplete due to denudation of their upper parts. Two formations are
distinguished in the Lower Eocene: the lower - Šemševo siltstone-sandstone Forma-
tion and the upper - Avren Marl Formation (Aladžova-Hrisčeva 1988). The
Stokiški limestone member is distinguished in the Šemševo Formation and occurs
only in the western part of the area.
The beds of the Šemševo Formation contain nummulites and nannoplankton.
These fossils are lacking in the marls of the overlying Avren Formation, but the
nannoplankton assemblage is rich and diverse (Fig. 2).
Biostratigraphy
Nummulite zones
The nummulite fauna in the Veliko Tâmovo area is poorly preserved and extend
of conservation is govered by different composition of the containing sediments (Figs.
2 and 3).
The lowermost levels of Šemševo Formation contain only single specimens of the
species: Nummulites exilis Douvillé (Pl. 1), N. spirectypus Doncieux (PI. 2), N.
spileccensis Munier-Chalmas and few rounded small nummulites of the group N.
globulus whose species determination is very difficult. According to the scheme of
Schaub (1981) this association is characteristic of the Nummulites exilis zone with
stratigraphie range Middle-Upper Ilerdian.
In the higher parts of the Šemševo Formation the nummulite association changes
abruptly and is richer both in species composition and amount of specimens. Particu-
larly numerous are the nummulites in the detrital limestones of the Stokiški member.
Several tens of specimens of the following species have been found there: Nummuli-
tes praemurchisoni Nemkov & Barhatova, N. semiinvolutus Nemkov (PI. 2), N.
pemotus Schaub. Less common are iV. laxus Schaub, N. crimensis Nemkov & Barha-
tova (PI. 2). Among the large number of badly preserved nummulites have been
identified also N. cf. soerenbergensis Schaub, N. cf. involutus Schaub. This associ-
ation defines the Nummulites involutus zone whose stratigraphie range is Upper
Ilerdian (Fig. 3).
Nummulites and calcareous nannofossils from Lower Eocene beds...
211
Fig. 2. Section of Lower Eocene beds east of Šemševo
212
Kina Aladžova-Hrisčeva & Jernej Pavšič
Fig. 3. Section of Lower Eocene beds south of Balvan
] Clay marl; 2 Fine grained sandstone; 3 Clayey siltstones; 4 Quartzitoid
sandstones; 5 Sandy limestones; 6 Aleurolitic limestones; 7 Nummulites
- N, Nannoplankton - NP
The uppermost levels of the Šemševo Formation, directly below the marls of the
Avren Formation, contain single specimens of Nummulites giganteus Mayer-Eymar,
N. aff. laxus Schaub, N. soerenbergensis Schaub, N. cf. involutus Schaub, N. crimen-
sis Nemkova & Barhatova, N. pernotus Schaub which also fall within Nummulites
involutus zone.
Nummulites and calcareous nannofossils from Lower Eocene beds.,
213!
Nannoplankton
The calcareous nannoplankton we studied from the Balvan and Šemševo sections.
In Balvan we found nannofossils in five samples. It is rather good preserved and
abundant only in the sample Balvan 5 (Fig. 3). In the first two samples there are
autochthonous floras cowered with the reworked species mainly from Cretaceous.
We determined following reworked species (PI. 2): Arkhangelskiella cymbiformis
Vekshina, Eiffelithus turriseiffeli (Deflandre), Watznaueria barnesae (Black), Strad-
neria crenulata (Bramlette & Martini), Eiffelithus eximius (Stover), Zeugrhabdotus
acanthus Reinhardt, Chiastozygus litterarius (Gorka), Microrhabdulus decoratus
Deflandre, Cribrosphaerella ehrenbergii (Arkhangelsky), Biscutum constans (Gorka).
Theses samples contain just a few Tertiary species, which correspond to the Lower
Eocene (Upper Paleocene) biozone Discoaster multiradiatus (NP 9) after Martini
(1971).
In biozone Discoaster binodosus (NP 11) we put the sample five, where appear
with tipical nannoflora also characteristical discoasters: Discoaster binodosus Mar-
tini and Discoaster barbadiensis Tan. In Šemševo section (Fig. 2) there are two
positive samples. Nannofossils are good preserved. According to tipical discoasters;
Discoaster salisburgensis Stradner, D. binodosus Martini and D. lodoensis Bramlette
& Riedel, we put it in the Lower Eocene biozone Discoaster lodoensis (NP 13).
Correlation of nummulites and nannofossils zones
The relationship between the zonal scheme based on nummulites and nanno-
plankton is shown in Fig. 4. There are some coincidences or incoincidences between
the boundaries of some zones, but they are to a certain extent conventional due to the
small number of sampled sections.
In the transgressive glauconite sandstones from the base of the paleogene section
south of Balvan the nannoplankton species Discoaster multiradiatus Bramlette
& Riedel has been found. Nummulites are lacking there. In the same glauconite
Fig. 4. Correlation between the Nannoplankton and Nummulite zones in
the Veliko Tàrnovo area
214 Kina Aladžova-Hrisčeva & Jernej Pavšič
References
Aladžova-Hrisčeva, K. 1988, Stratigrafii na dolnjoeocenskite sedimenti ot Balvan-
skata sinklinala (Velikotmovsko). Sp. Bulg. Geol. d-vo, 49/3, Sofia.
Aladžova-Hrisčeva, K., Ranoeocenski numuliti ot Velikotmovsko. God. Sof. univ. (in
press).
Martini, E. 1971, Standard Tertiary and Quaternary calcareous nannoplankton zonation.
Proc. II. Plankt. Conf., 845-856, Roma.
Schaub, H. 1981, Nummulites et Assilines de la Thétys paléogène. Taxinomie, Phyloge-
nese et biostratigraphie. Mém. Suisses Paléont., 104-106, 236 p., 97 pl., Basel.
Plate 1
1 Nummulites crimensis Nemkov et Barhatova (B), x 13
2 Nummulites crimensis Nemkov et Barhatova, x 15
3 Nummulites exilis Douvillé (B), x 8
4 Nummulites exilis Douvillé (B), x 8,5
5 Nummulites exilis Douvillé (A), x 10
6 Nummulites semiinvolutus Nemkov et Barhatova (B), x 8
7 Nummulites semiinvolutus Nemkov et Barhatova (B), x 8
8 Nummulites spirectypus Doncieux (A?), x 6,5
sandstones in the easternmost part of the area (near the Malak Ciflik Village), few
fossils of the Nummulites exilis zone have been found.
In beds rich in nummulite fauna of the Nummulites involutus zone (packet no. 5 in
the section south Balvan - the Stokiški Member) occur the diverse nannoflora of the
Discoaster binodosus biozone.
In the silty clayey limestones from the area of Sveta Gora, Veliko Târnovo, which
belong to the uppermost parts of the Šemševo Formation, the Tribrachiatus orthosty-
lus zone and Nummulites involutus zone have been distinguished.
The uppermost parts of the Eocene sections in the area comprise marls which lack
nummulites. They contain a rich and diverse nannofossil asemblage defining the
Discoaster lodoensis zone.
Nummulites and calcareous nannofossils from Lower Eocene beds.,
215^
216 Kina Aladžova-Hrisčeva & Jernej Pavšič
Plate 2
], 2 Toweius gammation (Bramlette et Sullivan), x 1800
3 Helicosphaera lophota (Bramlette et Sullivan), x 1800
4 Toweius pertusus Sullivan, x 1800
5 Discoaster multiradiatus Bramlette et Riedel, x 1500
6 Discoaster miras Deflandre, x 1500
7 Discoaster diastypus Bramlette et Sullivan, x 1500
8 Discoaster lodoensis Bramlette et Riedel, x 1500
9, 10 Sphenolithus radians Deflandre, x 3000
11 Sphenolithus pseudoradians Yramieite et VJilcoxon, x 3000
12,16 Micrantholithus vesper Deilaiidre, x 1500
13 Zygrhablithus bijugatus (Deflandre), x 3000
14 Eiffelithus eximius (Stover), x 1800
15 Rhabdosphaera perlonga (Deflandre), x 3000
1, 5-8, 12 under ordinary light, all others between crossed niçois
Nummulites and calcareous nannofossils from Lower Eocene beds.
217