clastic deposits in main and marginal

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Sedimentation of Late Pannonian
clastic deposits in main and marginal
basins

B. Vrbanac, J. Velić & T. Malvić (Croatia)

EGU General Assembly 2008, Vienna, April 13
-
18, 2008

INTRODUCTION

The Sava depression and the Bjelovar subdepression are both located at the very
southwest margin of the Pannonian basin
.


The main difference is in
their

scale
: t
he Sava depression is independent regional
geotectonic unit,
while

the smaller Bjelovar subdepression
represents

only the
southern part of the Drava depression
.


Both
basins

were formed contemporaneously by Neogene extension.


Previously observed differences were interpreted as a consequence of different
source areas and variable palaeogeography
.



Here was proved that these area created and developed simultaneously, only with
different depths.



Geo
graphic
positions of

the Sava depression

and the Bjelovar subdepression

Geotectonical position of

the Sava
depression

and the Bjelovar
subdepression

SAVA depression

BJELOVAR subdepression

Geological map of
the study
area

(Upper Pannonian deposits are marked red)

SAVA depression

BJELOVAR subdepression

LITHOSTRATIGRAPHIC SECTIONS

Lithostratigraphic sections

are
different

for the Sava depression and for the
Bjelovar subdepression.


Upper Pannonian

d
eposits

are represented

by alternation of

marls
,
sandstones

and
siltites
.


There are
two main sandstone members
:

-
t
he
Okoli ss.

can be followed
in both
region
s
,

-
the

IVA s
s.

are
found

only in the Sava
depression
.


Pure

sandstones

are deposited only
in the central parts

of depressions
.


Marls

completely substituted sandstone members
in marginal areas

(
as a
result of the

basin plain sedimentation).


Correlation table of the chronostratigraphic

and lithostratigraphic units

and positions of EL
-
markers

MAPS AND FACIES








2
2
t
y
t
x
t
a


Isopach map of the Rs5
-
Z’ interval (Ivanić
-
Grad formation of the
Late

Pannonian age)

The maximum
thickness

of 800 m

600 m

500 m

The maximum
thickness

of 800 m








2
2
t
y
t
x
t
a


Sandstones


fine
-
grained, rarely medium
-
grained (diameter up to 0.5 mm)
;
porosity
variable from

10
-
33%, porosity values
decrease

toward SE (in direction of

palaeotransport).


Petrographic composition is very homogeneous: quartz (
>
60%)

and

rock
fragments
-

mostly carbonates, cherts, schists, gneisses, granites (18
-
35%).


Source area is determined after accessory minerals

indicating

metamorphic rocks
(from epi
-

and mesozones), limestones and cherts, all
probably
originat
ing

from
the
Alps.


Siltites


the mineral composition is identical like in sandstones, only the mica
content is increased.


Marls


the matrix is clay and carbonate, cryptocrystalline. The content of CaCO
3

is
ca.

60%.


Lithological description

Facies
found

within

core samples

(1)

F
4



massive marls

F
1



thick
-
layered

to massive sandstones

Facies of massive marls (F
4
)


homogeneous,
massive, mostly unstratified marls

with

clearly

visible

bioturbations. Sometimes stratification can
be recognized by
relics

of lamination, colour
changes and thin
siltite

film
s
.




M
arls
were formed by

continuous sedimentation of
pelitic detritus.


Facies

description

(1)

Facies
found

within

core samples

(1)

F
4



massive marls

F
1



thick
-
layered

to massive sandstones

Facies of thick
-
layered to massive sandstones
(F
1
)



this facies includes thick
-
layered to massive,
homogeneous sandstones
, r
arely
with thin

beds

or
lamin
a
e

of siltites and marls. Thickness of
sandstone

interval
s varies between 0.5
-
6 m
;
thick
er
beds are

probably result of amalgamation.



Gradation and cross
-
bedding

are hardly
visible
.


Convolution

is more often recognizable. Lower
bedding

surface
s

are characterized by erosional marks
.


Sandstones are deposited from denser turbidites.


Sedimentation is result of directed, fast and
massive deposition from suspension.

Facies

description

(2)

Facies of thin sandstone layers (F
2
)



represented by sandstone layers up to 0.5 m

thick
.
Horizontal and cross bedd
ed

lamination is very
eas
il
y observable in sandstones and siltites.







Very often erosional marks can be observed on the
upper bedding plane
s
. Also, convolution and marls
clasts can be found

frequently
.

These
deposits

we
re
also deposited from turbidites, but in areas
distally from

the main current
s
.


F
3



laminated
sandstones, silts and
marls

F
2



thin sandstones
layers

Facies

description

(3)

Facies
found

within

core samples

(2)

Facies of laminated sandstones, silts and
marls (F
3
)


monotonous alternation of thin
sandstone layers, which are in upper part
gradually
substituted

by siltites and marls.





Textures encompass horizontal and cross lamination,
convolution and sandstone veins and dykes.

These

rocks

we
re deposited from low
-
density turbidites, but
probably

some

of these strata are results of re
deposi
tion
caused by
normal

sub
marine

currents
along the

sea
bottom.



F
3



laminated
sandstones, silts and
marls

F
2



thin sandstones
layers

Facies

description

(4)

Facies
found

within

core samples

(2)








2
2
t
y
t
x
t
a


Facies a
ssociations
indicated by


spontaneous potential

and resistivity curves

in the Sava
depression

FACIES ASSOCIATION

A
nalysis can be followed through four (4)
facies associations

(F
A
, F
B
, F
C
, F
D
).

F
A

or

Channel filling facies association



This
facies association consists of thick
-
bedded
sandstone facies (F
2
) and thin
-
bedded
sandstone facies (F
3
).




F
B

or

Depositional lobe facies association



This facies association is composed of F
3

facies
in lower part and facies F
2

in its upper part.








2
2
t
y
t
x
t
a


Facies a
ssociations
indicated by

spontaneous
potential and resistivity curves

in the Sava
depression

F
C

or

Lateral or distal turbidite facies
association


This facies association
consists of a monotonous alternation of very
thin, thin and medium thick sandstone beds
passing into siltites and marls. They are
represented by facies F
3

and F
2
.



F
D

or Massive marls facies association



this association is mostly represented by
massive marl facies F
1

with rare intercalations
of thin siltite or sandstone laminae or beds.








2
2
t
y
t
x
t
a


Facies a
ssociations
indicated
by

spontaneous potential

and resistivity curves

in the
Bjelovar sub
depression








2
2
t
y
t
x
t
a


Vertical
and lateral
facies correlation section over SW part of the Žutica field

in the Sava depression








2
2
t
y
t
x
t
a


Lateral
facies correlation section over Letičani, Galovac
-
Pavljani and Velika Ciglena fields

in the
central part of the B
jelovar

subdepression








2
2
t
y
t
x
t
a


Lateral

facies correlation
within

the Žutica
field

in the Sava depression








2
2
t
y
t
x
t
a


Lateral

facies

correlation
within

the

Šandrovac

field

in the
Bjelovar subdepression;

(modified after Bokor et al.,
2000)








2
2
t
y
t
x
t
a


Schematic lateral

facies correlation
between

the
Sava and Bjelovar depressions

CONCLUSIONS (1)

1.
Two

mechanism
s

of transport and deposition of the Ivanić Grad formation were
proposed:
m
assive marls are product of “normal” basin hemipelagic deposition

(F
1

facies)
, while p
eriodic
turbidity

currents

transported

coarser sand
-
size
d

detritus
, as
well as finer grained detritus of siltite and mud size

as a

turbidite facies (F
2
-
F
4
).


2.
It can be concluded that the
depositional environment was calm and stable, owing
sufficient water depth (more than 200 meters
) that compensated for all water level
changes caused by either tectonic movements or cyclic climate changes.


3.
This
low
-
energy
environment was disturbed only by temporary turbidity

currents which
deposited most detritus in the depression.

CONCLUSIONS (2)

4.

Deposition of the I
va

sandstones (lower part of
Upper

Pannonian) occur only in the
Sava depression
.
Contemporaneous deposits of

the Bjelovar subdepression
are
represented

only by basin marls.


5.

Younger

Okoli sandstones are found in both depressions
, as a result of the
deeping
and

opening of the Bjelovar subdepression

to source
areas
and
transport by

turbidit
y

currents.


REFERENCES

1.
Bokor, N., Hernitz, Z., Sečen, J. and Steiner, I. (2000):
Bypassed oil: Šandrovac oilfield (Northern
Croatia)
. 62
nd

EAGE Conference & Techbical Exhibition, Glasgow, 29.5.
-
2.6. 2000. Extended Abstracts
Proceedings.


2.
Malvić, T. (2003): Vjerojatnost pronalaska novih zaliha ugljikovodika u bjelovarskoj uleknini (
Oil
-
Geological
Relations and Probability of Discovering New Hydrocarbon Reserves in the Bjelovar Sag
).
Unpublished PhD Thesis, Faculty of Mining, Petroleum and Geology, University of Zagreb, 123 p.


3.
Velić, J. (1980): Geološka građa zapadnog dijela Savske depresije
(
Geological framework of the western
part of the Sava depression
).

Unpublished PhD Thesis, Faculty of Mining, Petroleum and Geology,
University of Zagreb, 1
37

p.


4.
Vrbanac, B. (1996):
Paleostrukturne i sedimentološke analize gornjopanonskih naslaga formacije Ivanić
grad u savskoj depresiji (
Palaeostructural and sedimentological analyses of Late Pannonian
sediments of Ivanić
G
rad formation in the Sava depression
)
. Unpublished PhD Thesis, Faculty of
Natural Sciences, Geological department, University of Zagreb, 303 p., Zagreb.


5.
Vrbanac, B. (2002):
Chronohorizons Based on Resistivity Curve Variations


Upper Miocene
Sediments of the Ivanić Grad Formation in the Sava Depression (NW Croatia)
. Geologia Croatica, 55,
1, 11
-
24, Zagreb.


6.
Vrbanac (2002):
Facies and Facies Architecture of the Ivanić Grad Formation (Upper Pannonian)


Sava Depression, NW Croatia
. Geologia Croatica, 55, 1, 57
-
78, Zagreb.