An uninterrupted section across the Cretaceous/Tertiary boundary at the town of Bjala, Black Sea coast (Bulgaria)

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Comptes
rendtis de
i'Academic
bulgare des
Sciences
Tome
45, N° 7, 1992
i
. ; , , : , ]'
CEOLOGIE
Stratigraphie
%•
•
AN
UNINTERRUPTED
SECTION
ACROSS
THE
CRETACEOUS/
TERTIARY
BOUNDARY
AT THE
TOWN
OF
BJALA,
BLACK
SEA
COAST
(BULGARIA)
K.
H. Stoykova, M. I. Ivanov =
(Submitted by Corresponding Member T. Nitiolov on
Marcfi
24, 1992)
In
the last
decade
the published account of A 1 v a r e z et al. [^] increased consi-
derably the interest in the Cretaceous/Tertiary boundary and the
iridium
anomaly re-
lated
to it. That anomaly is considered to be a result of an impact event of global
sig-
nificance.
The Cretaceous/Tertiary (K/T) boundary in Bulgaria has not been studied
with
regard to that
respect
so far. The rocks in the
Bjala
surroundings, Varna
district,
have not been an object of special stratigraphical study
till
the beginning of the
six-
ties.
They were assigned to the Upper Senonian (Upper Cretaceous) [^^'
Trifonova
proved the
presence
of Upper Maastrichtian, Danian and
Paleocene
in
the borehole sections
near
Bjala
on the
basis
of foraminiferas
[^-].
J u r a n o v carried
out
purposeful stratigraphie studies in the region of the villages of
Bjala
and Gorica
based
on core materials. The author proved the existence of a complete stratigraphie
section representing the whole
interval
from
the Maastrichtian
toJhe
Lower Eocene.
The K/T boundary has been drawn between the Abattiomphalus mayaroensis and Sub-
botina
pseudobuUoides. Zones
without
being
fixed
precisely ["' -].
In
1990 one of the authors (K. Stoykova) began a deliberate sarnpling of section
for
calcareous nannoplankton aiming to establish the K/T boundary in the
East
Bal-
kan*.
Since 1991 the investigations have been carried out in the framework of the
East-
West project "Mesozoic boundaries in Bulgaria" by an Austrian-Bulgarian team. The
project
was conducted by Prof. Dr A. Preisinger (TU
Wien)
and was
financially
sup-
ported
completely by the Austrian side.
During
the
work,
a necessity for a new approach arised in order to
find
and fix
the K/T boundary. That approach included: 1)
finding
a complete section
with
conti-
nuous sedimentation
across
the K/T boundary; 2)
fixing
the K/T boundary in concrete
sections (the accuracy is equal to several cm); 3)
finding
the characteristic "boundary
clay
bed" whose
forming
is believed to be a result of the K/T event. It was possible to
reach
these
purposes
only
by
means
of biostratigraphical methods. In this particular
case,
the nannofossil
successions
have been examined in detail. Preliminary
researches
and samplings of 12 outcrops in the
East
Balkan-including
those
carried out together
with
the Austrian colleagues in
April
1991, did not provide any positive results.
In
early
June
1991, during a
joint
field
trip,
both authors
found
a complete
unin-
terrupted
section
across
the boundary
interval
(K/T) in the surroundings of
Bjala
(Fig.
1).
That section is being described here. It is located at the Black Sea
coast,
some
800 m
north
of the central entry of the municipal plage and 26 m to the north of a big
dislo-
cation
(Fig. 2). The
lithological
succession of the whole section is described (Fig.
IB)
whereas
the nannofossils are
listed.in
the boundary
interval
only
(the topmost 5 m
of
packet No 1, as
well
as packets Nos 2 and 3).
*
Acknowledgements
are
due to
D.
Synnyovsky
and
I.
Kanchev
for their help
during
the
initial
fieldwork.
61

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III
Bjala
limestone
— marl Formation** (N 5-1, Upper Maastrichtian —
Paleocene)
5. (more than 25 m)
Alternation
of
limestones
and marls dominated by the
limestones
~ yellowish to
beige,
medium-bedded
(0.6-0.8
m). Fossil
remains
of
echinides
are found
4. (16 m)
Regular
alternation of marls and clayey lime-
stones.
Both lithological
types
are in approxi-
mately
equal
proportions and form
beds
30 to
50 cm thick. The marls are grey and the lime-
stones
— light-yellowish to
beige,
compact.
3. (8 m)
Maris
interbedded
by thin
layers
of clayey lime-
stones.
Marls are grey to light-beige, indistinctly
bedded.
Marly
limestones
form
beds
of varying
thickness
(0.2-0.4 m) and
occur
almost
regular-
ly
at
each
1 m
across
the section. That
packet
is rich in
calcareous
nannoplankton which has
been
studied in detail (sampling at
each
se-
I
Z |Z
I'l'
veral
centimeters)
from the
base
of the packet).
4 •' f—T ' The
following
taxa
are
recovered
from 4 to 8m:
CruciplacoUthus
tenuis
(Stradner)
Hay & Mohler
(at 4 m), Coccolithus cavus Hay&Mohler, Pla-
cozygus sigmoides (Bramlette d Sullivan) Ro-
mein, Micracantholithus crenulatus Bramlette
&
Sullivan; from 1.8 to 4 m: CruciplacoUthus
yonmus
Perch-Nielsen
(at 1.80 m), C. interme-
dins van Heck&Prins (at 3 m), Prinsius petalo-
sus (Ellis&Lohmann) Romein, Markalius in-
versus
(Defl.&Fert) Bramlette&Martini; from
0.7 to 1.8 m: Biantholithus sparsus Bramlette
&Martini,
Micula decussata Vekshina,
Predis-
cosphaera cretacea (Arkhangelskii)
Gartner,
Cyctagelosphaera reinhardtii (Perch-Nielsen) Ro-
mein, C. alta
Perch-Nielsen,
Thoracosphaera
operculata Bramlette
&Martini,
T. saxea Strad-
ner; from O.l to 0.7 m: Markalius inversus
r
(Defl.:&Fert) Bramlette &
Martini,
Placozygus
iviV
sigmoides (Bramlette & Sullivan ) Romein,
•r-fi:4;4
Thoracosphaera operculata Bramlette &
Martini,
Braarudosphaera
turbinea
Stradner,
B.discula
PxH E^jj
Eiam\&ii&Si'Ri&d&\,
Cyctagelosphaera reinhardtii
1=14
(Perch
Nielsen) Romein, C. alta
Perch-Nielsen,
Biantholithus sparsus Bramlette
&Martini;
from
0 to 0.1 m: Arkhangelskiella cymbiformis Vek-
shina,
Micula
muras
(Martini)
hukxy,
Thoracos-
phaera operculata Bramlette &
Martini,
Cyctage-
losphaera alta
Perch-Nielsen,
C. reinhardtii
(Perch-Nielsen) Romein, Biantholithus sparsus
Bramlette
&
Martini
(at +1 cm).
2.Z.
M
E
Fig.
J. Section across the K/T boun-
dary
interval north of
Bjala:
A
— locality map; B — lithological
successions and stratigraphie subdi-
vision;
1 — limestones; 2 — marly limesto-
nes; 3 — marls; 4 — K/T boundary
clay
bed
**
That
Formation was introduced as to
"Bjala
clayey marls of Senonian age" (B o n c h e v
,
p. 17). For a long time it was considered as an informal lithostratigraphic unit — the so-called
mestone-marl series"!'.*]. Only recently its correct name,
rank
and chronostratigraphical assignement
were determined ['].
62

2.
(0.02-0.04
m)
Clay bed — dark grey, dark brown to black clays.
Rare
fragments
of poorly
preserved
nannofossils
are
recorded:
Thoracosphaera operculata Bramlette &
Martini,
T. saxea
Stradner,
Micula murus (Martini) Bukry, Cyclagelosphaera reinhardtii (Perch-Niel-
sen) Romein.
Marls —light to dark grey, indistinctly
bedded,
highly
cleaved,
of varying
car-
bonate
content.
In the interval
20-25
m below the K-T
boundary
well
preserved
ammonites,
echinides
and
inoceramids
are common. A rich and
diverse
Maastrich-
tian
nannoflora is
recorded;
the following
species
are
recorded
in the
highest
5
metres
of
the
packet:
29.90-30.00
m — Micula prinsii
Perch-Nielsen,
M. murus (Martini)
Bukry,
Mf decussata Vekshina, Braarudosphaera bigelowii (Gran & Braarud) Deflan-
dre, Arkhangelskiella cymbiformis Vekshina, Watznaueria barnesae (Black)
Perch-
Nielsen, Cribrosphaerella ehrenbergii (Arkhangelskii) Deflandre, Eiffellithus turri-
seiffeli
(Deflandre) Reinhardt. Markalius inversus (Deflandre) Bramlette&Martini,
Thoracosphaera operculata Bramlette &
Martini;
29.90-25.00 m — Arkhangelskiella
cymbiformis Vekshina, Micula prinsii
Perch-Nielsen,
M. murus (Martini) Bukry,
Ahmuellerella octoradiata (Gorka) Bukry, Prediscosphaera cretacea (Arkhangelskii)
Gartner,
P.
spmosa
(Bramlette
&
Martini)
Gartner,
Cyclagelosphaera reinhardtii
(Perch-Nielsen)
Romein.
Analysis of the
nannofossils
successions
in the
boundary
interval proved the
pre-
sence
of the Micula prinsii (Uppermost Maastrichtian), Biantholithus sparsus (Lower-
most
Danian) and CruciplacoUthus tenuis (Lower Danian)
Zones
(Fig. IB). The lower
boundaries
of
these
interval-biozones are drawn to coincide
with
the first
occurrence
of
its
index-species.
The first
occurrence
(FO) of Biantholithus sparsus has
been
esta-
blished immediately
above
the clay bed. The FO of CruciplacoUthus primus is at +1.80 m
and of CruciplacoUthus tenuis — at +4 m
above
the clay bed which
marks
the K/T
boundary.
Cretaceous/Tertiary
boundary
1.
(30 m)
Fig.
2. Geological
sketch
map of the K/T boundary interval
exposed
at the
section
near
Bjala
(see the key to Fig. 1)
63

Getting
knowledge of that continuous section made it possible to complex exa-
minations.
Later, in the end of
June
1991, the Bulgarian-Austrian team sampled the
section to study mineralogical, geochemical and paleomagnetical characteristics of
the boundary
interval.
Ir-content analysis of the boundary clay bed made in the
Atom
Institute
at the Universities of Vienna by Dr
Grass
has proved Ir-content values about
7ppb n
In
conclusion, the section studied
near
the
town
of
Bjala
comprises marly-carbonate
sediments
without
breaks and condensation. It yielded a variety of
micro-
and macro-
fossils
which
provides an exellent
possibility
to examine its mode of
extinction
and
degree
of
change
in relation to the K/T boundary. That section, having particular geo-
logical
and stratigraphical advantages, is one of the most representative in the
world
amongst the sections
known
so far
across
the Cretaceous/Tertiary boundary.
•
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1
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'
^ Bulgarian
Academy
of Sciences
•
1113 Sofia, Bulgaria
64