GEOLOGICA CARPATHICA, JUNE 2007, 58, 3, 291—300
www.geologicacarpathica.sk
Reconstruction of Pliocene fluvial channels feeding Lake
Pannon (Gödöllő Hills, Hungary)
ANDRÁS UHRIN and ORSOLYA SZTANÓ
Eötvös Loránd University, Department of Physical and Historical Geology, Pázmány Péter sétány 1/C, H-1117 Budapest, Hungary;
uhrin-a@freemail.hu; sztano@ludens.elte.hu
(Manuscript received February 23, 2006; accepted in revised form December 7, 2006)
Abstract: During Pliocene to Pleistocene times, following the infill of Lake Pannon an alluvial plain was formed in the
Pannonian Basin. The main bulk of terrestrial sediments are floodplain silts and clays; sandy to gravelly channel fill deposits
comprise a smaller volume, but are excavated in several outcrops, as in Gödöllő Hills. In the studied outcrops five facies
units were described: medium-grained trough cross-bedded sand, medium-grained sand with compound cross-bedding,
small channel fills with bedding parallel to basal concavity, fine-grained cross-laminated sand and massive silt to mud.
Significant fining upward is rare, but decreasing thickness of cross-sets is often present in sands, defining cycles with heights
of about 5 m. In the same outcrops, lateral accretion surfaces were also observed; together with cyclicity, they indicate point
bar deposits of meandering rivers. Data from boreholes and a railroad cut has shown that channel fill sands are isolated
within the floodplain sediments, but amalgamation of 3—4 channel fill sandbodies is common. The bankfull depth of the
rivers depositing the studied sediments were estimated as 5—8 m. Direction of bend migration is indicated by lateral accretion
surfaces, whilst a compound cross-bedded sandbody proved to be a confluence bar. Compared to the size of recent rivers,
it can be concluded that one of the trunk rivers infilling the basin crossed the study area in the Pliocene. Paleocurrent data
show that the river flowed to the southeast, towards the eastern subbasin of Lake Pannon.
Key words: Pliocene, Gödöllő Hills, sedimentology, field observations, channel development, alluvial plain, bars.
Introduction
It has been known for decades (Mucsi & Révész 1975) that
the Late Miocene to Pliocene Lake Pannon was filled up by
huge amounts of siliciclastic sediments distributed via a flu-
viodeltaic system, and fluvial deposits in the upper part of
the basin infill were recognized much earlier (Lőrenthey
1905). However, the sedimentary features of the rivers –
namely their sinuosity, single- or multi-channel character,
direction and size of channels – remained unknown. Only
the occasionally preserved terrestrial fauna of the fluvial de-
posits, mostly mammals (Mottl 1939; Gaál 1946, 1953; Jas-
kó & Kordos 1990), got highlighted. There might be
several reasons for ignorance: poor and ever-changing out-
crop conditions, the lack of fossils as tools of stratigraphic
correlation and last, but not least the lack of hydrocarbon
reservoirs in the youngest – fluvial – part of the system.
The aim of this paper is twofold. We would like to demon-
strate how detailed reconstruction – including develop-
ment of individual channels or processes building different
types of bars – of a fluvial paleoenvironment can be made
even in an area where sediments are visible only in a few
small outcrops. Secondly, we would like to show how these
small details can contribute to understanding the features of
the fluvial system.
Geological setting
Pliocene sediments of Gödöllő Hills were deposited in
the latest phase of the infilling of the Late Miocene to
Pliocene (11.5—4 Ma) Lake Pannon. The lake reached its
greatest extension about 9 million years ago, when it cov-
ered the whole Pannonian Basin (Fig. 1) and had more than
1000 m of depth in its deepest parts (Magyar et al. 1992),
but huge deltaic systems carrying sediments from the Alps
and the Carpathians started to fill it rapidly mainly from the
northwest and the northeast. As the infill progressed, a
widespread alluvial plain was formed in the area, first in the
northern part of the basin, than it expanded southwards.
Thickness of alluvial succession reaches 1000 m in some
parts of the central areas of the basin (Juhász 1994).
Pliocene alluvial sediments – composed of mostly flood-
plain silts and clays with intercalating sandy channel fill
deposits – are known as Nagyalföld Variegated Clay For-
mation in the studied part of the basin (Gajdos & Papp
1997). This formation correlates with the lithologically sim-
ilar Hanság Formation in the Little Hungarian Plain, and
Kolárovo Formation in the Slovak part of the Danube Ba-
sin, of which the latter consists of a remarkable amount of
gravel beside sands and clays. On the northern edge of the
Pannonian Basin, Quaternary uplift has inverted the basin
fill (Horváth 1995). As a consequence in the study area –
at the junction area of the uplifting hills and the subsiding
plains – much of the succession has been eroded, so that
the thickness of the remaining part of these alluvial sedi-
ments is not more than 100—200 m, and they crop out at
many places. Borehole data demonstrate that the bulk of
them consist of variegated clay and silt formed on flood-
plains, while sandy or gravelly channel fill deposits com-
prise a smaller volume. In the Gödöllő Hills three major
outcrops (Fig. 2) – a sand pit and the railroad cut in
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UHRIN and SZTANÓ
Gödöllő and an another abandoned sand pit at Galgahévíz
– were studied, where mainly the coarse-grained litholo-
gies are exposed.
The age of the succession cropping out in the Gödöllő
area was determined first by Mottl (1939) using mammali-
an fossils found during the construction of the railroad
cut. Her results showed that the sand was deposited in the
Middle Pliocene ( ~ 4 Ma); later Gaál (1953), Jaskó & Kor-
dos (1990) also got Early—Middle Pliocene ages from the
same fossils. Unfortunately, fossils were not found in any
other place, therefore sediments of individual outcrops
cannot be correlated exactly within the interval given by
their stratigraphic position. Alluvial origin of the sand was
already mentioned by Szentes (1943). Palynological anal-
ysis of the Nagyalföld Formation (Nagy 2005) and the
abundance of variegated clay in its deposits in many parts
of the Pannonian Basin (Pécsi 1985) indicate a climate
warmer and drier than the present one (with annual aver-
age temperature about 13 ºC), with no major changes dur-
ing the Pliocene.
Description of the studied outcrops and facies units
Facies classification
In the last few decades many systems were proposed for
classifying sedimentary structures of fluvial deposits, of
which the classification of Miall (1988) is one of the most
commonly used systems. Three lithofacies of Miall’s clas-
sification (unit St, Sp, Sr) appear unambiguously in the
studied outcrops (Table 1); the lithology of a fourth one
Fig. 1. Contour of Lake Pannon at about 9.5 and 5 million years ago (after Magyar et al. 1992 and Magyar, pers. comm., 2006). The
main directions of infilling river systems are indicated by the arrows.
Fig. 2. Location map of the studied outcrops and boreholes.
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RECONSTRUCTION OF PLIOCENE FLUVIAL CHANNELS (GÖDÖLLŐ HILLS, HUNGARY)
(Fm) was described by Ferenczi (1936) in the documenta-
tion of the railroad cut and can also be recognized in the
successions of the studied boreholes. Although the origi-
nal classification of Miall (1988) does not contain it,
sand with bedding parallel to basal concavity (unit Sc)
was described as an individual lithofacies by García-Gil
(1993). Miall has used the term “scour fill” for this facies
characterized by ‘erosional scours’ and ‘crude’ cross-bed-
ding.
In the studied outcrops, where macrostructures at the
scale of channel size are also visible, identifying the facies
units already yields information about the processes
Fig. 3. Sketch, logs and facies units of the Gödöllő sand pit.
which have created the bedforms. Some of these lithofa-
cies and some distinctive spatial associations of them can
be interpreted as different architectural elements – such
as floodplain deposits or different types of bars – within
the river system.
Gödöllő sand pit
This outcrop consists of a shorter (15 m) west-east orient-
ed and a longer (50 m) north-south oriented wall (Fig. 3).
The calcareous cement of the sand forming horizons
mostly follows the bigger-scale bedforms, making them
Table 1: Classification of lithofacies identified in the study area (cf. Miall 1988).
294
UHRIN and SZTANÓ
more easily recognizable, elsewhere cementation has made
the observations difficult in some parts of the outcrop.
One of the most remarkable cemented horizons appears
at a height of 2 m at the joining of the two walls. From this
point, it apparently dips both to the west and to the south,
its real dip is about 15º to the southwest. Another, seem-
ingly horizontal cemented layer is visible at a height of
4 m on the north-south oriented wall. Both cemented beds
follow erosional surfaces, as they cut into units made up of
fine-grained cross-laminated sand described later. These
surfaces will be referred later on as ES1 and ES2 respec-
tively. The upper one (ES2) divides the outcrop into two
main parts. Four facies units were identified on the basis of
grain size and sedimentary structures (Figs. 4, 6).
Unit St: medium- to fine-grained trough cross-bedded
sand
This facies unit mainly consists of moderately sorted,
medium-grained sand. As in all the sandy facies units of
the studied outcrops, most of the grains are quartz, mixed
with only a few percent of feldspars, biotite and musco-
vite. Between the two main cemented layers, unit St is vis-
ible on the biggest surface, and it is the only recognizable
unit in the upper part of the outcrop. The height of indi-
vidual cross-sets is between 10 and 65 cm, their width is
up to 5 m; they are mainly wedge-shaped in sections par-
allel to paleocurrent direction. Paleocurrent directions in-
dicated by the dips of cross-strata in the axes of the
bedforms are mostly towards the east-southeast (Fig. 5c).
The size of bedforms or thickness of cross-sets decreases
upwards up to the cemented erosional surface ES2 (Fig. 3).
Parallelly in the uppermost cross-sets, grain size also starts
to decrease and fine-grained beds of unit Sr can also be
found. Collaterally the deviation of paleocurrent directions
increases: cross-strata of some sets dip to the northeast or to
the south. Above the erosional surface, both the thickness
of the trough cross-bedded sets and the grainsize is increas-
ing again. In addition paleocurrent directions are somewhat
different, they are towards the south-southeast.
Unit Sp: medium- to fine-grained planar cross-strati-
fied sand
Unit Sp forms a 30 m wide, at least 2 m high sandbody
on the northern part of the north-south oriented wall, sur-
rounded by trough cross-bedded sand. Its upper boundary
is a horizontal erosional surface (Fig. 3); downwards the
unit continues below the lower, inclined cemented layer
of the outcrop, but its lower boundary is not exposed.
The cross-strata of the planar cross-bedded sand are 2—5 cm
thick with dips about 15—20º to the south (Fig. 5a). How-
ever, they have compound structure: every single cross-
stratum is built up by 0.5—1 cm thick laminae dipping to
the southeast (Fig. 5b). Although lithological features and
dip directions do not vary significantly within the sand-
body, this unit contains several, low angle internal ero-
sional surfaces also dipping to the south.
Unit Sc: medium-grained sand with bedding parallel to
basal concavity
The lower boundary of this unit (Fig. 4) is a sharp, con-
cave erosional surface cutting into unit Sp and – in its
uppermost part – into unit St. Moderately sorted medi-
um-grained sand fills the big trough-like form. The upper
boundary is also an erosional, but almost horizontal sur-
face. The unit is made up of about 1 cm thick beds parallel
to the basal concave surface. Their dip is steep towards the
east-southeast measured in the axis of the form. As the pa-
Fig. 5. Rose diagrams (equal area) show that paleocurrent directions are generally towards the east-southeast with quite low deviation.
In unit Sp it is shown by smaller-scale bedforms, while bigger-scale cross-strata indicates the direction of bedform accretion itself.
Fig. 4. Unit Sc and the surrounding bedforms in the Gödöllő sand pit.
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RECONSTRUCTION OF PLIOCENE FLUVIAL CHANNELS (GÖDÖLLŐ HILLS, HUNGARY)
Fig. 7. Sections of railroad cut, Gödöllő (after Ferenczi 1936).
leocurrent direction differs from that of unit St, the bed-
form is much bigger than “troughs” of unit St, and as it
deeply cuts into the underlying beds, it was formed by dif-
ferent processes.
Unit Sr: fine-grained cross-laminated sand
This unit appears at two horizons: only a 2—5 cm thin
layer underlies the erosional surface ES2, but below ES1 a
50 cm thick bed made up mostly of climbing cross-lami-
nation (Fig. 6) is present.
Gödöllő, railroad cut
The railroad cut located in 100 m distance from the Sza-
badság Road (Fig. 2) outcrop exposed Pliocene sediments
on a much bigger surface in the 30’s, when it was docu-
mented by Ferenczi (1936; Fig.7). Nowadays observations
can be made only in some places because of soil and vege-
tation cover. In these places trough cross-bedded sand
equivalent to unit St is exposed. However, the section
made by Ferenczi shows that the bulk of the sediments is
silt and clay, which can be described as an individual fa-
cies unit (unit Fm sensu Miall 1988). These are either
brown or blueish grey, without macrofauna, containing
some coalified vegetable detritus and some calcareous
nodules with 1—2 cm of diameter. Within the fine sedi-
ments there are some 1—5 m high, 30—130 m wide, lens-
shaped interbeddings of sand (numbered III to V). Sand in
greater thickness appears only near the southwestern and
the eastern end of the railroad cut (sandbodies I and II). In
the southwestern part, west of a sharp surface dipping at
60º to the east – interpreted as a normal fault – sand is
exposed in the whole height of the outcrop with only a
thin interbedding of mud. The studied sand pit is also lo-
cated west of this fault (Fig. 7), within the same thick
sandbody (I). The smallest sandbodies (III and V) cannot
be followed on opposite cut walls, unlike the largest one
(IV). Present paleocurrent data from sandbodies IV and VI
are towards the SSE and ESE; the larger sandbodies I and
II show southeastern paleoflow (Fig. 5d), but in sandbody
II there are some larger-scale surfaces indicated by cemen-
tation dipping at low angles to the SW.
Trough cross-stratification (unit St) is not the only struc-
ture appearing in these sandbodies. In sandbody VI a 10 m
wide and 1 m high trough-like form appears, which has
been assigned to facies unit Sc. The dip of its cross-stratifi-
cation towards the SSE (measured in the axis of the form)
corresponds to the dips of the surrounding smaller –
mainly 30—60 cm high – cross-sets. Nearby, above the
trough cross-stratified sets there is a 15—20 cm thick fine-
grained sand layer with climbing ripples (unit Sr). At the
top of this layer a cemented horizon developed overlain
by mudstones.
Sand pit at Galgahévíz
Galgahévíz is situated 15 km to the east of Gödöllő
(Fig. 2). According to the geological map of Szentes
(1943) the same Pliocene sand is cropping out around the
village. The 5—10 m high outcrop consists of two, 150 m
Fig. 6. Climbing cross-laminated sand (unit Sr) in the Gödöllő
sand pit (scale is 2 cm).
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UHRIN and SZTANÓ
and 80 m long roughly perpendicular walls (Fig. 8) Unfor-
tunately large portions are covered by vegetation. Only two
facies units – known from the Gödöllő sand pit – can be
distinguished.
St: medium- and fine-grained trough cross-bedded sand
This unit composes the bulk of the outcropping depos-
its. Height of cross-sets varies between 10 and 120 cm, but
adjacent bedforms can be of different scale. Grainsize is
significantly larger in the higher cross-sets. In spite of this
diversity, cross-sets with thickness of 80—120 cm occur
only on the lowest part of the section, while 4—5 m above
the smallest ones become more frequent (Fig. 8). This is
followed by a covered interval which suggests that softer
(silty to clayey) lithology is present. At the uppermost part
of the section cross-stratified sand appears again. On the
NW-facing wall many “troughs” are asymmetrically trun-
cated by other ones on their northeastern side, although
paleocurrent directions (towards the E—SE) are nearly per-
pendicular to this wall. These erosional surfaces gently
dip to the NE. Cross-sets on the SW-facing wall are wedge-
shaped with foresets dipping also to the E—SE (Fig. 5e).
Sr: fine-grained cross-laminated sand
Normal to climbing cross-laminated sand occur as inter-
beddings between the bedforms of unit St. Most of these
have an overall thickness about 5—20 cm, rarely up to
50 cm and an average length of 1—2 m.
Boreholes in Gödöllő
In Gödöllő area, many boreholes (Fig. 2) drilled for wa-
ter exploration have penetrated the Pliocene fluvial de-
posits underlying a few meters of Quaternary sediments.
Borehole data (Fig. 9) show that the upper 100 m of the
succession is made up by sand and finer-grained sediments
in nearly equal quantities. One or more thick (10—30 m)
layers of sand – sometimes interrupted by thin (1—2 m)
pelitic horizons – can be found in almost every borehole.
Between the thick layers of sand, successions consist of
mainly silt and mud with some 1—3 meters thick sandy
interbeddings. The latter cannot be correlated to each
other even between neighbouring boreholes; however,
thick sandbodies can be followed within a distance of
500—1000 m.
Interpretation of depositional environment and
architectural elements
Depositional environment
As it was described above, successions of boreholes in
Gödöllő area include thick (10—30 m) and thinner (1—3 m)
layers of sand interbedding between silts and muds; the
railroad cut also consists of mainly silt and mud with in-
terbedded sandbodies. The former are typically floodplain
deposits; such a high ratio (a bit higher than 50 %) of
them suggests that the sediments were deposited by mean-
dering rivers (cf. Bridge & Diemer 1983; Collinson 1986).
The thicker sandbodies can be interpreted as point bar
deposits; their thickness can significantly exceed the val-
ue established for individual point bars, as amalgamation
of sandbodies above each other is an abundant phenome-
non in such successions built up by channel and flood-
plain deposits. Although channel belts are relocated by
avulsion, later they usually get back to their previous po-
sition, above former channel deposits which can be eroded
more easily (cf. Puigdefabregas & Van Vliet 1978; Bridge
& Leeder 1979; Bridge 2003). 2—3 m thick layers of sand
found in the successions can be deposits of smaller chan-
nels, which existed only temporarily after an avulsion, but
flow did not become permanent in them, therefore their
channel floors were not eroded as deeply as it was in the
parent channels. Sandy layers not thicker than 1 m are in-
terpreted as deposits of crevasse splays. The presence of
crevasse sands also supports the meandering character of
the ancient rivers.
Prevalence of trough cross-stratification (unit St) and
variations in set thickness may indicate that sediments of
Fig. 8. Sketches and logs of Galgahévíz outcrop.
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RECONSTRUCTION OF PLIOCENE FLUVIAL CHANNELS (GÖDÖLLŐ HILLS, HUNGARY)
the Galgahévíz outcrop were also deposited by a meander-
ing river. This is supported by the succession of a borehole
located about 800 m from the outcrop, with alternating
layers of sand and silt.
Architectural elements
Unit St and Sr: point bar
Within the trough cross-stratified sand (unit St) at the
Gödöllő sand pit, below ES2 the size of bedforms definite-
ly decreases upwards, while the dispersion of paleocurrent
data increases and grain size also becomes smaller with
the appearance of cross-lamination (unit Sr). These chang-
es indicate gradual decrease of the energy of paleoflow.
Trough cross-stratification of unit St was formed by sinu-
ous-crested dunes with heights of about a few decimeters
(cf. Collinson 1986), while Sr was formed as current rip-
ples or climbing ripples. The latter may point to decreased
velocity of flow by cut-off (cf. Walker 1984). In a fluvial
environment, the most common reason for that is the in-
filling of a migrating channel as the main current is shifted
away by the accretion of a bar. Such cycles may occur in
braided rivers (Bridge 2003), but they are characteristic of
point bars of meandering rivers (Allen 1970). As the main
current is placed further by meander development, flood-
plain deposits close the cycle (e.g. unit Fm in the railroad
cut) or the next channel is incised and amalgamation of
channel-belt deposits occur (Puigdefabregas & Van Vliet
1978; Bridge & Leeder 1979).
Above ES2 of the Gödöllő outcrop, a new cycle starts
with larger – 50—60 cm high – trough cross-sets. On the
northeast-southwest oriented wall of the Galgahévíz sand
pit, unit St and Sr also build up at least two sedimentary
cycles. Whilst the cross-sets with thicknesses of about 1 m
occur only on the lowest part of the outcrop, smaller ones
Fig. 9. Lithological columns of boreholes in Gödöllő with possible sandbodies.
298
UHRIN and SZTANÓ
become more frequent at heights of 4—5 m, but the upper
boundary of the cycle is not exposed. However, it might
be located at about 6 m of height, because on the upper
part of the wall, thicker cross-sets (with height reaching
50 cm) occur again, which can belong to another cycle.
Thickness of cross-sets and grain size decreases upwards
also within this cycle: fine-grained cross-laminated sets
start to appear in its upper part.
Unit Sp: confluence bar
The large cross-stratified sandbody of the Gödöllő sand
pit is surrounded by the trough cross-bedded sand, in
which set thickness does not exceed 65 cm. In contrast,
unit Sp can be evaluated as one continuous cross-set with
at least 3 m of height, in which individual cross-strata are
up to 10 m long. These structures can be caused by trans-
verse bars, which are much bigger and have lower height/
length ratio than dunes (cf. Cant & Walker 1978). Com-
pound structures are also typical in large-scale bars, as
smaller bedforms can migrate on their surface parallelly to
the flow direction (Jackson 1976; Khan 1987).
Foresets of unit Sp dipping towards the east-southeast
indicate that this was the flow direction in the ancient
channel. Large-scale cross-strata of unit Sp dip, however,
towards the south (Fig. 5a), which shows the direction of
accretion of the sandbody. The difference between these
values would be enough to evaluate unit St as a laterally
accreted bar, but dip directions of cross-laminae within its
compound cross-strata (137º on average) indicate that
above this bar water flowed to the southeast, which is clos-
er to the direction of accretion. Thus the difference be-
tween the directions of paleoflow and bar accretion is only
33º, which means an intermediate stage between lateral
and downstream accretion. This can be developed on bars
of braided rivers during floods, when they are covered by
water; the other possible explanation is a bar building up
of sediments carried by a confluencing channel (cf. Bridge
2003; Fig. 10a). In this case the confluence bar accretes
obliquely to the main channel, while stream direction
above it can be anywhere between the direction of the
main and the confluencing flow.
In meandering systems, confluence bars mostly form
where chute channels cutting point bars join the main
channel: these bars are referred to as “chute bars” (Collin-
son 1986). As channels cannot be followed further be-
cause of the lack of outcrops, we cannot decide whether
the planar cross-stratified sandbody in Gödöllő outcrop
was a chute bar or a bar formed at the confluence of two
distinct channels.
After the influent flow ceased, sediments of unit St con-
tinued to deposit on the top of the confluence bar. Later on,
probably during a successive flood, erosion has formed a
secondary bar-top channel (Fig. 10b) cutting into the sedi-
ments of unit Sp and St. Unit Sc was most probably created
by rapid infill of that channel. Paleoflow direction of the
bar-top channel fits the directions of the main flow.
Thus, in summary, two fining-upward sedimentary cy-
cles of a meandering river appear in the Gödöllő sand pit.
Fig. 10. a – Bar accretion and current directions at the conflu-
ence. b – Secondary channel cutting into sediments of the former
confluence bar (without scale).
The river flowed towards the east-southeast at this place.
Within the lower cycle, a larger-scale bar was formed,
probably at the confluence of a chute channel and the
main one.
Accretion of point bars
Point bars on the inner side of meanders accrete lateral-
ly, nearly perpendicular to flow direction. This is often – but
not always – shown by lateral accretion surfaces dipping
at low angles (from 1—2º – which is hardly visible in an
outcrop – to 15—20º) towards the direction of accretion
(Collinson 1986; García-Gil 1993; Willis 1993a,b). Later-
al accretion surfaces are outlined in sandbody II of the
railroad cut.
Paleoflow directions indicated by dip of cross-strata in
this sandbody were generally from the northwest towards
the southeast. Cemented horizons in the same sandbody,
dipping towards the southwest – perpendicularly to the pa-
leoflow – probably follow the surfaces of lateral accretion.
Based on this, at the given stratigraphic level the ancient river
which ran to the SE, had a meander loop migrating to the SW.
Compared to the Gödöllő outcrop, decrease of cross-set
size is less uniform in Galgahévíz: there are significant
differences between the sizes of adjacent bedforms. Larger
variability of the discharge of the ancient channel is a
probable reason for this: although the channel became
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RECONSTRUCTION OF PLIOCENE FLUVIAL CHANNELS (GÖDÖLLŐ HILLS, HUNGARY)
shallower gradually at a given point as the point bar ac-
creted, alternating periods of floods and low water could
result significant variability in water depth and size of the
developing bedforms. This variability could cause quite
long breaks in lateral accretion of the point bar; probably
erosion during these periods produced erosional surfaces a
few meters long gently dipping towards the north-north-
east within unit St (Fig. 8). The dip directions of these in-
ternal erosional surfaces indicate the direction of point bar
accretion towards the north-northeast, compared to the
perpendicular paleocurrent directions (Fig. 5). The direc-
tion of lateral accretion is supported by the fact that most
cross-sets are eroded on their northeastern side.
Size of the ancient channels
The depth of an ancient channel can be estimated on the
basis of the thickness of exposed cross-sets. Allen (1968)
proposed the following equation for this estimation:
h = 0.086 . H
1.19
, thus:
9
1
.
1
086
.
0
h
H
where h is the height of dunes, H is the formative flow
depth over the sedimentary structure, in meters. Others
(Jackson 1976; Allen 1980; Flemming 2000; Bridge
2003) noticed that water depth/dune height ratios falls in
the range of 3—20, therefore Allen’s equation should be
considered as a rough estimation only. Counting with the
height of the thickest cross-sets – which are surely lower
than the dunes that formed them – the result show that
flow depth has reached at least 5 meters at the Gödöllő
outcrops. As larger-scale bars cannot rise above the surface
of the water, their height – in our case the thickness of
unit Sp – gives the minimal possible value for water
depth. In the outcrop, this unit is 3 m thick, but its real
thickness probably exceeds this, because the lower part of
the ancient bar is partially covered. In Galgahévíz, point
bar deposits are at least 6 m thick in the lower sedimentary
cycle. On the basis of the height of the largest cross-sets –
using the equation of Allen (1968) – channel depth may
have attained about 8—9 meters.
Conclusions
In the studied Pliocene sequence, based on the ratio of
channel and floodplain deposits and successions of sedi-
mentary structures, meandering rivers were reconstructed,
suggesting that the studied part of the Pannonian Basin
was probably dominated by meandering rivers in the
Pliocene. That indicates a paleotopography with quite low
relief and probably lower flux of sediment compared to
the northern Danube Basin, where chiefly braided river de-
posits of the same age were identified in the Kolárovo For-
mation (Kováč et al. 2006). This difference probably
indicates that the uplifting of the Gödöllő Hills had not
started by the time of deposition of the studied sediments.
Lack of gravels and relative abundance of floodplain fines
in the Pliocene of Gödöllő Hills is probably due to its
longer distance from the source of the sediments. Pale-
ocurrent directions show that the studied rivers were flow-
ing from the northwest towards the southeast, in harmony
with existing paleotopographical models (e.g. Magyar et
al. 1992) of Lake Pannon. Based on this direction, the
probable source area is thought to be in the Western Car-
pathians, which were already elevated and eroding in the
Pliocene.
The depth of the ancient rivers could be estimated from
the size of cross-sets, and the height of point bar deposits.
River depth at bankfull discharge may have been around
5 m in Gödöllő area, while it has reached about 8—9 m at
Galgahévíz. This larger river might have had a less uni-
form water discharge. Compared to recent rivers of the
Pannonian Basin, the latter means the scale of Tisza, the
second greatest river of the basin, while 5 m of bankfull
depth is typical of its major tributaries (like the Körös and
Maros). Under the quite arid paleoclimate the Pliocene
rivers should have had at least as large drainage areas as
the recent rivers with the same size. This supports the view
that their source area was not located closer than the West-
ern Carpathians, although parts of the Transdanubian and
North Hungarian Range could be already uplifting in the
Pliocene (Csillag et al. 2004).
However, it should be considered that the studied out-
crops and boreholes do not map the whole succession,
therefore they do not exclude the possibility of a larger
temporal and spatial variance of river styles (e.g. meander-
ing or braided) and paleoflow directions. Analysis of seis-
mic profiles show that some northwest—southeast trending
faults of the Gödöllő Hills area could have been active in
the Pliocene (Fodor et al. 2005), which could have locally
changed the topography and river style, but the quantity
of data accessible from the sedimentary succession does
not allow a more detailed reconstruction.
Within the point bar deposits of the Gödöllő outcrop, a
large compound cross-stratified sandbody was found,
which is thought to be a confluence bar, as its paleocur-
rent directions significantly differs from that of the sur-
rounding trough cross-bedded sand. Cemented beds and
low angle erosional features were interpreted as lateral ac-
cretion surfaces, dipping perpendicularly to flow direc-
tions. Thus migration of individual meander loops was
reconstructed to SW at Gödöllő and to NE at Galgahévíz.
On the basis of the reconstructed channel sizes and direc-
tions, we can conclude that one of the trunk rivers infill-
ing the basin flowed across the present Gödöllő area in the
Pliocene.
Acknowledgments: We are grateful to Áron Jámbor, Lász-
ló Fodor, Árpád Magyari (Geological Institute of Hunga-
ry), ubomír Sliva, Juraj Janočko and three anonymous
journal reviewers for their useful suggestions and com-
ments. Áron Jámbor is also thanked for providing unpub-
lished borehole data. The study was supported by the
Grant T 037724 of the Hungarian Scientific Research
Fund (OTKA).
300
UHRIN and SZTANÓ
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