GeolCarp_Vol69_No1_89

GEOLOGICA CARPATHICA

, FEBRUARY 2018, 69, 1, 89–113

doi: 10.1515/geoca-2018-0006

www.geologicacarpathica.com

Lower Badenian coarse-grained Gilbert deltas in the

southern margin of the Western Carpathian Foredeep basin

SLAVOMÍR NEHYBA

Institute of Geological Sciences, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic; slavek@sci.muni.cz

(Manuscript received May 15, 2017; accepted in revised form December 12, 2017)

Abstract: Two coarse-grained Gilbert-type deltas in the Lower Badenian deposits along the southern margin of the

Western Carpathian Foredeep (peripheral foreland basin) were newly interpreted. Facies characterizing a range of

depositional processes are assigned to four facies associations — topset, foreset, bottomset and offshore marine pelagic

deposits. The evidence of Gilbert deltas within open marine deposits reflects the formation of a basin with relatively steep

margins connected with a relative sea level fall, erosion and incision. Formation, progradation and aggradation of the

thick coarse-grained Gilbert delta piles generally indicate a dramatic increase of sediment supply from the hinterland,

followed by both relatively continuous sediment delivery and an increase in accommodation space. Deltaic deposition is

terminated by relatively rapid and extended drowning and is explained as a transgressive event.

The lower Gilbert delta

was significantly larger, more areally extended and reveals a more complicated stratigraphic architecture than the upper

one. Its basal surface represents a sequence boundary and occurs around the Karpatian/Badenian stratigraphic limit. Two

coeval deltaic branches were recognized in the lower delta with partly different stratigraphic arrangements. This different

stratigraphic architecture is mostly explained by variations in the sediment delivery and /or predisposed paleotopography

and paleobathymetry of the basin floor. The upper delta was recognized only in a restricted area. Its basal surface

represents a sequence boundary probably reflecting a higher order cycle of a relative sea level rise and fall within the

Lower Badenian. Evidence of two laterally and stratigraphically separated coarse-grained Gilbert deltas indicates two

regional/basin wide transgressive/regressive cycles, but not necessarily of the same order. Provenance analysis reveals

similar sources of both deltas. Several partial source areas were identified (Mesozoic carbonates of the Northern

Calcareous Alps and the Western Carpathians, crystalline rocks of the eastern margin of the Bohemian Massif, older

sedimentary infill of the Carpathian Foredeep and/or the North Alpine Foreland Basin, sedimentary rocks of the Western

Carpathian/Alpine Flysch Zone).

Keywords: coarse-grained Gilbert deltas, facies analysis, key stratal surfaces, depositional settings, provenance

Introduction

All major architectonic elements of the foreland basins are

conventionally considered to accumulate due to flexural

subsidence of the foreland plate, with typical regional

orogen- ward thickening on a basinal scale (Beaumont 1981).

The classical wedge shape of the basin infill with distinct four

depozones, namely wedge-top, foredeep, forebulge, and back-

bulge was introduced by DeCelles & Giles (1996). However,

modern foreland basins contain a number of smaller-scale

depositional features and sedimentary trends, which might be

unrecognized in ancient successions owing to the fact that

regional data sets are required for their identification (e.g.,

DeCelles & Cavazza 1999; Shukla et al. 2001; Hartley et al.

2010; Weissmann et al. 2010). When reconstructing fluvial

and deltaic stratigraphy it is especially necessary to obtain

a regional, three-dimensional data set. Collecting of such

a data set is time consuming and complicated. On the other

hand, data about the depositional architecture of deltas provide

a useful tool in reconstructing the complicated synsedimentary

history (interplay between tectonics, eustasy, climate, basin

physiography and sediment supply) of the foreland basin.

The Neogene Carpathian Foredeep basin provides an oppor-

tunity to study the characteristics of a series of marine coarse-

grained deltaic systems.

The main aims of the presented paper are: a) to propose

a novel interpretation of the Lower Badenian “basal or mar-

ginal coarse clastics” in the southernmost part of the Western

Carpathian Foredeep as deposits of coarse-grained Gilbert

deltas; b) to reconstruct the stratigraphic architecture of these

deltas to demonstrate coarse grain delta deposits as an indi-

cator of the infill history of the basin, especially along the

basin margins where biostratigraphic evidence is poor; and c)

identification of the source area of the deltas.

Geological setting

The Western Carpathian Foredeep Basin represents a peri-

pheral foreland basin formed during the lithospheric flexure of

the Bohemian Massif in response to the thrust load of the

Western Carpathians and the Eastern Alps. The studied area is

part of the southernmost segment of the basin where the

Carpathian Foredeep continues into the North Alpine Foreland

Basin (Alpine Molasse Zone) in the southwest (see Fig. 1A).

The stratigraphic range of the sedimentary infill of the basin

LOWER BADENIAN GILBERT DELTAS IN THE WESTERN CARPATHIAN FOREDEEP BASIN

GEOLOGICA CARPATHICA

, 2018, 69, 1, 89–113

segment is Oligocene/Lower Miocene (Egerian) to Middle

Miocene (Lower Badenian) (Brzobohatý & Cicha 1993).

The study area is located above the Iváň canyon, which

represents a shelf-indenting canyon that formed due to a com-

bination of isostatic rebound along a terminating thrust front

and sea-level change during the terminal Early Miocene/

Karpatian (Dellmour & Harzhauser 2012).

The Lower Badenian deposits of the Western Carpathian

Foredeep reveal distinctive basin infill geometry because they

almost symmetrically thicken towards the basin centre

(Nehyba & Šikula 2007). Two lithofacies strongly dominate,

both areally and volumetrically. The first lithofacies are

monotonous basinal pelites (“tegel”) with a maximum thick-

ness of ~ 600 m in the central part of the basin. The pelites

reflect a marine depositional environment of the middle to

outer shelf and their abundant fossil content indicates the

Middle Miocene (Lower Badenian of Central Paratethys

regional stages) age with some evidence for Zone NN 5

(Tomanová-Petrová & Švábebnická 2007). A changeable

paleoenvironment, especially sea-level fluctuation and unstable

conditions were documented (Nehyba et al. 2008). Rare and

thin interlayers of acidic tuffs and tuffites are interpreted as

distal tephra fallout (Nehyba et al. 1999).

Pelites commonly overlie the other dominant lithofacies,

namely “basal or marginal coarse clastics”. These sandy gra-

vels and gravelly sands have been evaluated without detailed

sedimentological studies by Menčík (1973), Krystek (1974),

Novák (1985, 1986a,b) and Stráník et al. (1999) and are known

by several local names. They were recognized as a product of

Lower Badenian transgression and interpreted as gravel beach

deposits (Menčík 1973; Krystek 1974), or shoreline marine

bars and marine deltas formed during the end of the Karpatian

and start of the Badenian (Stráník et al. 1999). The Lower

Badenian age of the gravels was documented by Uvigerina

macrocarinata and Orbulina suturalis, which were recognized

in clayey and sandy intercalation in gravels (Čtyroký 1993).

These gravels represent an important aquifer of the area

(Kryštofová 2007).

Red-algal limestones widely known from the Lower

Badenian succession of the basin (Doláková et al. 2008) are

very exceptional in the studied area. Thin lignite beds were

rarely described within the Lower Badenian deposits in the

area under study.

Methods

The study area is located in south-eastern Moravia between

the border of the Czech Republic and Austria in the south and

the town of Pohořelice in the north. Individual exposures

are rare and not extensive here. The paper presented is based

on the study of 4 outcrops (Novosedly 48°50’58.2” N,

16°30’47.6” E; Troskotovice 48°54’41.7” N, 16°25’18.4” E;

Brod nad Dyjí 48°52’22.3” N, 16°33’24.1” E; Iváň 48°55’47.8” N,

16°34’18.9”E) and the results of 71 boreholes. These bore-

holes have been drilled during the last six decades and mostly

only general descriptions of lithology and stratigraphy are

available. Preserved cores are rare, discontinuous and small.

The exceptions are represented by two relatively modern bore-

holes (Iváň 1 and 22-41 D Pasohlávky). Locations of both out-

crops and boreholes are shown in Fig. 1B.

Conventional field methods of sedimentological analysis

were used, such as detailed logging, measurement of bedding

attitude and paleocurrent directions, and a line drawing of bed-

ding architecture on outcrop photomosaics (Tucker 1988;

Walker & James 1992; Collinson et al. 2006). Lithofacies

analysis in the outcrops is based on primary sedimentary

structures and textures. However, facies analysis of borehole

cores is based mainly on grain-size, because sedimentary

structures were obliterated by drilling in these loose deposits

and/or were not recognized in primary descriptons. Lithofacies

were grouped into facies associations, meaning assemblages

of spatially and genetically related facies, which are also the

expressions of different sedimentary environments.

Pebble and cobble petrography, shape and roundness were

determined both in outcrops (clasts larger than 1.6 cm) and in

borehole cores (data from 8 boreholes — clasts larger than

8 mm). Shape and roundness were estimated mostly visually

using the methods of Zingg (1935) and Powers (1953).

The maximum pebble/cobble size represents an average of the

longest axis (A axis) of the 10 largest found extraclasts in

a locality.

Heavy minerals were studied in 18 samples from 4 outcrops

and 6 boreholes in the grain size fraction 0.063–0.125 mm.

The chemistry of garnet was analysed for 151 grains and the

chemistry of rutile is based on data from 31 grains. Electron

microprobe analysis was done on a CAMECA SX electron

microprobe analyser (Faculty of Science, Masaryk University,

Brno). Samples for the chemistry of garnet and rutile origi-

nated from the Novosedly and Troskotovice outcrops and the

N 1 Novosedly, HJ 401 Troskotovice and IK 1 Iváň boreholes

(see Fig. 1).

Ground penetrating radar (GPR) scanning using Pulse Ekko

Pro radar, manufactured by the Canadian company Sensor &

Software, at a frequency of 50 MHz with an antenna distance

of 3 m was employed. The measurement interval was 0.5 m.

The field measurement and processing of the data were pro-

vided by Kolejkonzult Brno co. A map of the thickness was

created in Surfer 7 software (gridding method).

Results

Facies analysis, sedimentology

Sedimentological study of the outcrops led to the distinction

of 9 lithofacies and 8 facies have been identified within the

borehole cores. Detailed descriptions (lithology, stratification

and sedimentary structures) and interpretation of each facies

are given in Table 1A,B. The examples of both lithofacies and

facies associations within the logged section can be followed

in Figs. 2 and 3.

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, 2018, 69, 1, 89–113

Table 1A:
Topset (subhorizontal)
Symbol Description

Interpretation

Clast- to matrix-supported pebble to cobble gravel, massive. Subrounded to well-rounded

clasts mostly up to 10 cm in diameter. Matrix formed by coarse grained sand to gravelite. Bed

thickness ranges from 25 to 60 cm. Erosive slightly undulated base. Openwork horizons of

coarsest clasts (rare cobbles A max. 20 cm) along the base locally with a ‘rolling’ a(t) b(i)

fabric. Rarely flat lying cobbles along the top of the beds. Tabular beds with flat or convex up

top.

Sheetflood deposits, bedload deposition of gravel bars —

sheet bars (Nemec & Postma 1993)

Openwork cobble to pebble gravel, Erosive base. Well rounded cobbles up to 25 cm. Rare

intraclasts up to 50 cm to the top of the beds. Pebble horizons along the base with ‘rolling’ a(t)

b(i) fabric. Broadly lensoidal beds, bed thickness ranges from 25 to 35 cm, width of the beds

over 2m.

Tractional deposition of bedload gravel as pavement and

sheet bars (Nemec & Postma 1993; Miall 1996).

Foreset (steeply inclined 20-25

)

Gms

Massive/structureless gravelite to pebble gravel, pebble to cobble gravel, mostly clast-

supported less commonly clast to matrix (very coarse sand–gravelite) supported, non-graded or

coarse-tail inversely graded, forming solitary or amalgamated beds 20 to 350 cm thick with

non-erosional bases. Cobbles (extraclasts) up to 15 cm, intraclast up to 60 cm. Mostly non

preferred orientation of pebbles, rarely elongated pebbles arranged parallel to bedding.

Flat slightly irregular non-erosional top and bases, occasional listric shearing bands. Rare

shell debris.

Cohesionless debris flows subject to a low to moderate-rate

strain (frictional shear regime (Gobo et al. 2015).

Discontinuous horizons or thin lenses of openwork gravel commonly one clast/cobble or

boulder thick, or isolated large subspherical clasts. Thickness varies between 15 to 100 cm,

clasts commonly oriented parallel to bedding thick, with cobbly downslope ‘heads’ and

upslope-fining pebbly ‘tails’. Boulders (intraclasts) up to 350 cm, extraclasts up to 90 cm.

Deposition by debris fall (Nemec 1990), or modified beds

by erosional stripping of an overpassing turbidity current

(Gobo et al. 2015)

Alternation of massive clast supported pebble gravel or gravelite beds (2 to 6 cm thick) and

thicker (5 to 10 cm thick) beds of very coarse sand to gravelite, faintly laminated. Some beds

contain scattered very coarse pebbles at the base. Tabular shape of the beds. Composed beds are

up to 250 cm thick.

Deposition of high density turbidity currents (sensu Lowe

1982).

Mostly fine to medium sand, poorly sorted due to admixture of very coarse sand and rare

granules, plane parallel stratification, inclined bedding, bed thickness 4–10 cm, commonly

fining upward trend in beds, flat slightly undulated top and bases

Tractional deposition by low density turbidity current

(hyperpycnal flow (sensu Lowe 1982).

Smg

Medium to coarse sand, massive, normal distributional grading, sometime passing upward into

faintly planar parallel-stratified sand.Bed thickness varies between 10 ad 20 cm.

Deposition by high density turbidity current (sensu Lowe

1982).

Coarse to very coarse sand, poorly sorted, scattered granules or small pebbles up to 1 cm in

diameter, outsized clast are commonly aligned parallel to bedding, massive to faint lamination,

alternation slightly finer and coarser grains. Bed thickness varies between 20 ad 40 cm.

Sandy debris flow accompanied or

followed by low density turbidity current

(Postma et al. 1988; Mulder & Alexander 2001).

Alternation of laminas or thin beds of very fine sand, planar parallel laminated, micaceous,

relatively well sorted, and laminas of dark brown green silt to silty sand, calcareous, faintly

laminated to massive.

Traction to suspension deposits of low density turbidity

currents

Table 1B:
Symbol Description

Interpretation

Massive/structureless pebble gravel, clast to matrix supported, cobbles up to 10 cm. Well

rounded pebbles, limestone dominate in the pebble spectra. Both subhorizontal and inclined

beds. Horizon thickness highly varies and can reach tens of meters.

Subhorizontal beds — tractional deposition of bedload

gravel (Nemec & Postma 1993; Miall, 1996) (Equivalent to

Gm, Gi in outcrops). Steeply inclined beds — mass flow

deposits, most probably products of cohesionless debris

flows, outsized cobbles might be connected with debris fall

(Equivalent to Gms, Go in outcrops).

Beds of very coarse sand to gravelite, Some beds contain scattered very coarse pebbles to small

cobbles at the base. Large scale cross bedding /foreset sometimes evident. Composed beds are

tens of meter thick.

Mass flow deposits, most probably products of cohesionless

debris flows, debris fall and turbidity currents. (Equivalent to

lithofacies Gs in outcrops)

Fine to medium grained sand, faint to well developed planar paralel stratification.

Subhorizontal beds.

Deposition from low density turbidity currents (sensu Lowe

1982). (Equivalent to lithofacies Sl in outcrops).

Medium grained sand, structureless, relatively well sorted. Subhorizontal beds, bed thickness

about 20 cm.

Deposition from mass flows - sandy debris flow to high

density turbidity currents (sensu Lowe 1982).

Fine to very fine sand, massive scattered pebbles to small cobbles (up to 10 cm in diameter).

Sand relatively well sorted. Limestone pebbles dominate in pebble spectra. Subhorizontal beds,

thickness of amalgamated beds up to 1.4m.

Deposition from low density turbidity currents (sensu Lowe

1982). Pebbles/cobbles can originate from debris fall.

Rhythmic alternation of laminas of very fine sand and silt, planar parallel laminated or massive

silty mud. Typical occurrence of scattered pebbles up to 3 cm diameter, rare cobbles up to

10 cm. Pebble strings or even thin beds (up to 6 cm thick) of gravel (pebbles up to 5 cm in

diameter). Pebbles are well rounded, limestone dominates in pebble spectra. Pebble gravels

are clast supported to openwork. Subhorizontal beds, individual bed thickness about

20 cm, amalgamated beds several m thick.

Deposition from low density turbidity currents (sensu Lowe

1982), attributed to river-generated hyperpycnal flows

descending subaqueous delta slope (Nemec 1995). Scattered

pebbles can originate from debris fall. Isolated thin interbeds

of facies S4 within monotonous gravel succession can also

represent large intraclasts.

Fine to medium grained sand, well sorted, calcareous, massive, shell debris. Subhorizontal

beds, bed thickness up to 1 m. Alternation with beds of facies M1.

Occasional input of the sandy material is connected with

storms or mass flows into open marine environment.

Grey, green-grey massive silty claystone, calcareous, well sorted, occurrence of shell debris.

Subhorizontal beds, bed thickness more than 10 m.

Open marine suspension deposits, hypopycnal suspension

plumes? (Nemec 1995)

Table 1: A — Descriptive summary list of lithofacies of the studied deposits distinguished in the studied outcrops. B — Descriptive summary

list of lithofacies of the studied deposits distinguished in the studied boreholes.

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Four facies associations (FA) were identified and for sim-

plicity labelled with interpretive genetic names of depositional

environments, but their descriptions are separated from inter-

pretations in the text. They partly (FA 1–3) correspond to

a complete tripartite (proximal to distal) Gilbert type delta

profile. FA 1(topset) and FA 2 (foreset) were clearly identified

in outcrops. However, in many boreholes, commonly “a joint”

FA 1+ 2 is declared due to problems with clear identification of

FA 1 and FA 2 (poor primary description). FA 3 (bottomset)

and FA 4 (offshore marine pelagic deposits) were identified

only in boreholes. Logs illustrating the distribution of facies

associations both from outcrops and boreholes are presented

in Figs. 4, 5 and 6.

FA 1 — topset deposits

Topset deposits were identified in only one outcrop. Here

the facies associations consist of poorly sorted gravels of

facies Gm and Gi forming subhorizontal tabular packages

about 4 m thick with internal subhorizontal erosional surfaces

with relief of several dm (Fig. 2 A, B). The presence of fossils

was not observed. FA 1 deposits are present in some boreholes

(facies G1 in the uppermost part of the succession covering

facies S1), where they reach a thickness of about 12 m. A fining

upward trend is evident. Deposits of FA 1 overlie deposits of

FA 2. Deposits of FA 1 or FA 1+2 are mostly overlain by depo-

sits of FA 4, rarely by deposits of FA 3.

Interpretation: Recognized facies are interpreted as bed-

load deposits (gravel pavement and sheet bars). The sheet-like

geometry, limited incision and the lack of cross-stratification

suggest deposition of poorly confined flows, in broad and

shallow braided channels or overflows during periods of high

discharge. Rare evidence of FA 1 (compared to FA 2) could

point to its basinward thinning and/or its formation domi-

nantly during the terminal stage of the delta building. Deposits

of FA 1 are interpreted as fluvial-dominated topset. Marine

influence (wave, tide) was not recognized.

FA 2 — foreset deposits

FA 2 represents the volumetrically dominant facies associa-

tion. It consists of steeply inclined (15–25

), tangential, late-

rally continuous, sandy to gravelly beds oriented at directions

of 262

–

059

. The logged thickness of FA 2 varies between

4 and 21 m; however, its base was not reached (Fig. 2 C, D).

The thickness of FA 2 (or FA 1 + FA 2) in boreholes can reach

up to 160 m. Deposits of FA 2 here cover deposits of FA 3 of

FA 4 and are covered by FA 1 or FA 4.

FA 2 includes ten lithofacies (i.e. Gms, Go, Gs, Sl, Smg, Sg,

G1, G2, S2 and S4); however, only six of them (Gms, Go,

Gs, Sl, G1 and G2) form the larger portion of the association

(Fig. 3A, B). Common inclined planar parallel stratification is

obvious due to minor vertical changes in clast sizes between

adjacent strata — commonly only one clast thick — and is

highlighted by a plane-parallel clast orientation. Facies G1 and

G2 strongly predominate in boreholes, forming 89.5 to 100 %

of FA 2 there (see Fig. 6). The rest of facies (S2 and S4) form

0 to 10.5 %. Lithofacies Gms and Go (Fig. 2 G, H and Fig. 3 A, B)

mostly predominate on outcrops, forming 38.9 to 100 % of

FA 2 there (see Figs. 4 and 5). Lithofacies Gs and Sl (Fig. 3 C

and D) form a significant portion of FA 2 in one outcrop and

represent 34 to 57 % of the facies succession there (see

Fig. 4 B, C). Cobbles and boulders of mudstone intraclasts

were recognized in various stages of disintegration (angular

boulders with well preserved internal stratification vs. highly

irregular, rounded cobbles with irregular intrusions of pebbles;

sharp margin of clasts vs. coated rim of small pebbles)

(Fig. 2 E, F).

Interpretation: Lithofacies Gms represents cohesionless

debris flows, lithofacies Go is interpreted as debris fall depo-

sits. Lithofacies Gs and Sl are deposits of high- and low

density turbidity currents. Facies G1 and G2 are interpreted as

gravity flow deposits — products of cohesionless debris flows,

debris fall and high density turbidity currents. Facies S2 and

S4 are products of sandy debris flows or low and high density

turbidity currents. The lithofacies assemblage suggests steep

delta foresets dominated by the deposition of gravity flows

(Nemec 1990b). Evidence of a large scale foreset clearly

points to a Gilbert type delta. Gilbert deltas (Gilbert 1885) are

defined by their tripartite geometry of sub-horizontal topsets,

steeply inclined foresets and sub-horizontal bottomsets.

Gilbert-type deltas form in settings where the depth ratio

(channel depth over basin depth) is small and where bedload

transport is high. Variations in the direction of the dip of fore-

set are explained by evidence of several shifting deltaic lobes.

Superposition of shifting lobes was evident also from the

facies architecture of the outcrop. The progradation generally

towards WNW-NE points to the position of the deeper parts of

the basin.

The situation on the outcrops points to either debrite-domi-

nated foreset deposits (more common) and/or turbidite-domi-

nated foreset deposits (less common) (Gobo et al. 2015).

The different delta-slope sedimentation processes in these

cases might reflect the delta-front morphodynamic responses

to base-level changes, namely either (relatively more common

in the studied case) increased accommodation (relative sea

level rise) or deficit of the delta-front accommodation (still-

stand or relative sea level fall) (Gobo et al. 2015).

The described Gilbert-type delta deposits consist mainly

of sandy gravels and gravelly sand. Paucity of mud in the

matrix and the common occurrence of mud intraclasts is typi-

cal. The separation of the muddy fraction from the coarse

(sand +gravel) fraction is explained by the density contrast

between the sea water and the inflowing river water. Whereas

low-density fresh water plumes carried suspended sediment

out to sea, bedload sediment was damped close to the river

mouth. Mud-poor gravel mixtures avalanched for relatively

short distances; the lack of mud results in strong frictional

forces between the clasts. This lowers the mobility of the sedi-

ments and thus increases the stability of the slope and allows

the development of steep coarse-grained foresets (Nemec

1990 b).

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The presence of floating intraclast indicates that the gravity

flows were erosive at some stage. These intraclasts were

mostly eroded from exposed older Neogene basin infill or

from a delta plain. The semi-consolidated mud clasts will

normally become subject of disintegration during transport.

However, if an initial damping of turbulence occurs at the

same stage as erosion (rapid transformation of the flow

behaviour), the suspended intraclasts may survive intact

(Postma et al. 1988). The mud clasts largely moved over rather

short distances (a few hundred metres) and came to rest upon

the steep foresets. The outsized clasts are randomly scattered

within the succession of FA 2, occasionally forming clusters.

Occurrence of outsized boulders suggests a steep slope over

which sediments can gain high downslope mobility over-

coming the frictional resistance of the substratum.

The steeply inclined bedding, parallel to the depositional

slope, and large height of the preserved delta slope deposits

suggest that the coarse-grained Gilbert delta was formed along

a steep margin. The 150 m thickness of FA 2 points to a rela-

tively deep basin (a reasonable minimum estimation of the

paleowater depth is the second tens of m) and intense sediment

supply. With sufficient bedload material transported to the

delta rim, the delta slope may have prograded as a result of

semicontinuous to continuous downslope movement of sedi-

ment (Nemec 1990b; Eilertsen et al. 2011). Heavily laden trac-

tion currents at the river mouth may have continued downslope

as gravity-driven underflows during major floods (Massari &

Parea 1990).

FA 3 — bottomset and prodelta deposits

The thickness of this facies association varies between 0 and

58 m. FA 3 shows flat laying beds and comprises five litho-

facies (lithofacies G1, S1, S2, S3 and S4). Whereas occurrence

of lithofacies G1 and S1 is rare (they are commonly missing,

rarely reach up to 8 %) and they form relative thin interbeds,

lithofacies S4 is the most common (Fig. 3 F). Two lithofacies

assembleges were identified: i) monotonous monofacies

assemblages (mostly lithofacies S4, rarely S2); ii) interbedded

lithofacies assembleges (lithofacies G1, S1, S2, S3 and S4; but

with a strong prevalence of lithofacies S3 and S4) with gene-

rally coarsening upward trends. Both the top and base of FA 3

are sharp and abrupt. Deposits of FA 3 mostly overlie deposits

of FA 4, less commonly deposits of FA 1–2. Deposits of FA 3

are mostly overlain by deposits of FA 1–2, less commonly by

deposits of FA 4.

Interpretation: The dominant lithofacies S4 and also litho-

facies S1, S2, and S3 are interpreted as deposits of high- or

low density turbidity currents. Unique occurrence facies G1 is

interpreted as an arrival of mass flow deposits (cohesionless

debris flow) on the shallowly dipping delta front or deposits of

hyperpycnal flows (Mutti et al. 2003). These observations

correspond to the deposition of a subaqueous delta base where

foresets pass into more gently dipping horizontal bottomsets

(Backert et al. 2010). The coarsening upward trend is explained

by transition from distal to proximal bottomset.

Bottomsets were defined by Gilbert (1885) as gently

inclined (≤ 10

) fine grained sediments. Similarly Colella

(1988) or Nemec (1990a) point to their “low angle” dip and

Massari & Parea (1990) or Chough & Hwang (1997) point to

their ‘fine-grained’ nature. Bottomsets are here defined simi-

larly as by Ford et al. (2007) or Backert et al. (2010) as the

down-dip terminations of foresets, where the facies associa-

tion is transitional, deposited by both gravitational flow and

suspension fallout processes. The facies transition can be

abrupt or very gradual. Variations in thickness of the bottom-

set deposits probably reflect a lobate shape, common at the

base of steep-gradient delta slopes (Lee & Chough 1999).

The pebble- to cobble-sized openwork gravel lenses in thin to

medium-thick sandstone beds are typical of mass-flow-

dominated deposits at the base of gravelly steep-gradient delta

slopes and prodelta environments (Postma & Cruickshunk

1988; Lee & Chough 1999). The scattered pebbles and c obbles

were emplaced by coeval debris falls from the steep foreset

slope (Nemec et al. 1999) or, alternatively, may represent

outrunning clasts from cohensionless debris flows (Sohn

et al. 1997).

FA 4 — offshore marine deposits

The facies association comprises tens to hundreds of metre-

thick successions, in which mudstones (facies M1) absolutely

predominate (Fig. 3 H). The mudstones are generally massive

to faintly laminated and contain thin sandstone interbeds

(facies S5) or randomly scattered sandy grains. Mudstones are

calcareous and rich in marine fossil content. FA 4 occurs either

above or below FA 1, FA 2 and FA 3. FA 4 was rarely recog-

nized interfingering within deposits of FA 3 and FA 2.

Interpretation: Deposits of FA 4 are interpreted as suspen-

sion fallout deposits in an offshore marine pelagic environ-

ment. The transport of the mud into the basin might be (partly?)

connected with river-derived hypopycnal suspension plumes

(Nemec 1995). Contact of the basinal clays (FA 4) and gravels

(FA 1+2) is attested as the topset breakpoint path (Backert et al

2010). Such a topset breakpoint path is a key stratal surface,

which records a significant landward facies shift and indicates

a rapid increase in accommodation/sediment supply. Deposits

of FA 4 occur below and above the deposits of coarse grained

deltas, so they are either Karpatian or Lower Badenian in age

(Čtyroký 1993; Stráník et al. 1999; Tomanová-Petrová &

Švábenická 2007; Nehyba et al. 2008).

Areal distribution and stratigraphic architecture

Investigations into the stratigraphic architecture are based

mainly on borehole data. The total thickness of the “basal or

marginal coarse clastics“ (namely the Gilbert delta deposits)

ranges from zero to 158.5 m (Fig. 7) in the area under study

and the greatest one was found around borehole HJ 417.

Menčík (1973) estimated their maximum thickness at about

190 m. Coarse clastics are generally prolonged in the SW–NE

direction along the active margin of the basin with a general

LOWER BADENIAN GILBERT DELTAS IN THE WESTERN CARPATHIAN FOREDEEP BASIN

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(m)

Mud

VFS

VCS

GRAN

PEB

COB

Lithofacies

Sedimentary structures

Lithology

Facies

association

FA1

FA2

Lithofacies

(m)

Gms

Sedimentary structures.

Lithology

FA2

Facies

association

Mud

VFS

VCS

GRAN

PEB

COB

Lithofacies

(m)

Gms

Sedimentary structures.

Lithology

FA2

Facies

association

Mud

VFS

VCS

GRAN

PEB

COB

Lithofacies

(m)

Gms

FA2

Sedimentary structures

Lithology

Facies

association

Mud

VFS

VCS

GRAN

PEB

COB

Brod nad Dyjí

roskotovice

Ivá

Fig. 4

. Sedimentological core logs of outcrops:

— Brod nad Dyji;

—

Troskotovice

—

Troskotovice

— Iváň.

NEHYBA

GEOLOGICA CARPATHICA

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they are in contact with each other in a landward position.

The northern branch is prolonged by about 6 km in a SE–NW

direction and by about 7 km in a NE–SW direction, so it covers

a surface area of about 28 km

. The southern branch is pro-

longed by about 9 km in a SE–NW direction and of about

5.5 km in a NE–SW direction, so it covers a surface area of

about 30 km

. The spatial position of the two branches resem-

bles two divergent aprons (see Fig. 7). The partly different

stratigraphic arrangement and different occurrence of KSS can

be followed in these two deltaic branches. The northern delta

branch is characterized by the spread of FA 1+2 deposits over

a wide area, relatively uniform lithology, evidence of only two

KSS (KSS 1 on the base and KSS 7 on the top) and significant

thickness (up to 110 m). However, variations in the tilt of both

KSS 1 and KSS 7, together with variations of thickness of

FA 1+2 in individual cores in the basinward direction and the

vertical and lateral arrangement of the deposits (see Fig. 8A),

all point to more complex pattern of the northern branch of

D 1. The southern branch is characterized by multiple alterna-

tion of FA 1+2 and FA 3 deposits, their interfingering with

FA 4 (especially in the southern margins of the delta), rela-

tively common and thick FA 3 deposits spread over a relatively

wide area and significant total thickness of deltaic deposits (up

Fig. 5. A — Sedimentological core logs of the Novosedly outcrop and

B — an explanatory legend to symbols used in Figs. 4–6.

Log legend

Pebble to cobble gravel

Gravelite,
pebbly sand

Sand

Outsized cobbles/boulders

Intraclats of mudstone

Sandy mud, silt

Shells

Orientation of foreset dip

Imbrication of pebbles

RSFE

Flooding surface

Transgressive surface

Regressive surface of marine erosion

Sequence boundary

Lithofacies

(m)

Gms

Sedimentary structures
Lithology

VFS

VCS

GRA

COB

BOLD

Novosedly

Facies

association

trend of basinward (westward) thickening. The area of the

maximum total thickness of clastics generally coincides with

the area of the maximum thickness of total succession of the

Lower Badenian deposits (Nehyba & Šikula 2007). A signifi-

cant role of post Badenian tectonics was not documented from

the area under study.

Delta architecture is simplified and projected onto two pro-

files, one NW to SE (Fig. 8A) and the other NNE to SSW

(Fig. 8B). The borehole data shows that two deltas can be

identified in the area under study with different areal extent,

thickness and stratigraphic position. Several key stratal sur-

faces (KSS) separating individual FA packages are identified

and correlated across significant parts of the deltas.

The lower delta (D 1) represents the main deltaic body with

significantly higher thickness and areal extent than the upper

delta (D 2). The lower boundary/base of D 1 deposits, namely

KSS 1 corresponds to the laterally traceable surface, separa-

ting the underlying basinal mudstones of FA 4 and overlaying

coarse grained deposits of FA 1+2 or FA 3. The D 1 occurs in

two segments/deltaic branches. The thickest deposits repre-

sent the axial portion of the delta branch. Lateral (interbranche)

and distal areas are represented by thinner deposits. The two

deltaic branches are partly spaced in a basinward position and

LOWER BADENIAN GILBERT DELTAS IN THE WESTERN CARPATHIAN FOREDEEP BASIN

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Facies

FA1+2

FA3

FA4

Facies

associations

100,

120,

40,

20,

60,

80,

Mud

VFS

VCS

GRAN

PEB

COB

Sedimentary structures.

Lithology

(m)

HJ 501

Facies

FA1+2

FA3

FA4

Facies

associations

160,

120,0

180,

140,

100,

200,

220,

240,

260,

40,

20,

60,

280,

300,

80,

Mud

VFS

VCS

GRAN

PEB

COB

Sedimentary structures

Lithology

)

HJ 418

0,0

20,0

40,0

60,0

Mud

VFS

VCS

GRAN

PEB

Quaternary

cobbles

Facies

Sedimentary structures

Lithology

)

Facies

association

FA1+2

FA4

FA3

HJ 401

200,

160,

220,

180,

240,

260,

280,

300,

320,

340,

360,

Mud

VFS

VCS

GRAN

PEB

COB

FA1+2

FA3

FA4

Facies

Sedimentary structures.

Lithology

(m)

Facies

associatio

HJ 419

SB/

RSFE

160,0

120,0

180,0

140,0

200,0

220,0

240,0

260,0

280,0

300,0

320,0

Mud

VFS

VCS

GRAN

PEB

FA1+2

FA4

FA3

Facies

Sedimentary structures

Lithology

)

Facies

association

HJ 417

SB/

RSFE

SB/

RSFE

SB/

RSFE

SB/

RSFE

SB/

RSFE

)

86,0

82,0

88,0

84,0

80,0

90,0

92,0

94,0

96,0

66,0

56,0

46,0

42,0

62,0

52,0

58,0

48,0

44,0

68,0

64,0

54,0

60,0

50,0

70,0

72,0

74,0

76,0

98,0

100,0

78,0

Mud

VFS

VCS

GRAN

PEB

COB

Facies

Sedimentary structures

Litholog

22-41D

FA1

FA4

FA2

FA3

Facies

association

Fig. 6.

Sedimentological

core

logs

the

selected

boreholes:

—

22-41

Pasohlávky;

—

417;

—

419;

—

401;

—

418;

—

501.

The

logs

show

the

stratigraphic

distribution of sedimentary facies (letter codes as in

Table

1) and distinction of facies associations (F

1–4).

100

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GEOLOGICA CARPATHICA

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to 150 m). Such an arrangement points to alternation of delta

progradation, retrogradation and aggradation. Several stratal

surfaces are identified in the southern part of the southern

branch. KSS 2 separates underlying FA 3 deposits and over-

lying FA 1+2, reflecting a D 1 progradation stacking pattern

(followed by aggradation). KSS 3 separating FA 1+2 and over-

lying (onlapping) FA 4 continues towards the N into KSS 4

separating FA 1+2 and overlying FA 3. Both surfaces reflect

a retrograding stacking pattern. KSS 5 separating FA 4 and

overlying FA 3 reflects a renewed progradation of the delta

branch. Further progradation is also reflected by successive

KSS 6 separating FA 3 and overlying FA 1+2. All KSS 2–6 are

not laterally continuous through D 1, but disappear towards the

north within a uniform FA 1+2 package. The final KSS 7 repre-

sents the top of the D 1 and separates underlying FA 1+2 or

FA 3 deposits and overlying FA

4. KSS

7 records a significant

retrogradation/landward shift in the topset breakpoint path and

finally termination of the deposition of D 1.

Upper delta D 2 was recognized only in the southernmost

part of the area under study (see Fig. 7). D 2 is significantly

smaller in both thickness and areal extent than the lower delta

D 1. The maximum total thickness of D 2 reaches 33 m. D 2 is

markedly prolonged in the NE–SW direction, where its radius

reaches about 5 km. However, prolongation in the SE–NW

direction is only 1.5 km. D 2 covers a surface area of about

7 km

. Lateral and vertical/stratigraphical separation of D 1

and D 2 suggests migration of the delta depocentre and evolu-

tion of the basin margin. Several stratal surfaces are identifed

in D 2. KSS

8 separates underlying pelagic mudstones of FA

from over lying FA

1+2 or FA

3 reflects relative sea level fall

followed by progradation and aggradation of D 2. KSS

9 sepa-

rating underlaying FA

3 deposits and overlying FA

1+2 reflects

D 2 progradation (followed by aggradation). KSS10 represents

the top of D 2 and separates underlying FA

1+2 and overlying

4. KSS

10 records a significant retrogradation/landward

shift in the topset breakpoint path and finally termination of

the deposition of D 2.

Interpretation: The stratigraphic arrangement of Gilbert

deltas is directed by the interplay between the available

accommodation space/A and the sediment supply/S, expressed

as the “A/S ratio” (Jervey 1988; Muto & Steel 1992, 1997;

Dart et al. 1994; Martinsen et al., 1999; López-Blanco et al.

2000; Backert et al. 2010; Martini et al. 2017). Accommodation

can be created by several factors, most notably tectonic-driven

subsidence and rises in base level and sea level (Gawthorpe &

Collela 1990;

Blum

Tornqvist 2000

). When 0 < A/S < 1

progradational stacking patterns are developed, when A/S > 1

retrogradational stacking patterns are developed and when

A/S = 1, aggradational patterns are observed (Shanley &

McCabe 1994). Each recognized KSS represents a change in

A/S ratio (Backert et al. 2010).

The basal surface KSS

1 of D 1 reflects incision, significant

migration of the basin depocentre and the start of development

of the Gilbert-delta, which is interpreted as reflecting a new

basin physiography with relatively steep margins connected

with a relative sea level fall (Sohn et al. 2001). The evident

convex down shape of KSS

1 (see Fig. 8 A) points to a major

erosion surface incising downward several tens of m into the

Karpatian Laa Fm. and also several km basinward. KSS

1 is

regarded as a sequence boundary (similarly Nehyba & Šikula

2007). Progradation (followed by aggradation) of the FA

deposits and significant km-long progradation of stacked

packages of FA

1+2 gravels observed above KSS1 indicate

a dramatic increase of sediment supply from the hinterland.

Arrangement of the northern branch of D 1 reveals a strong

progradation and aggradation stacking pattern of the deposi-

tional system, a relatively “continuous” sediment supply and

“continuous” low available accommodation space over the

time available. Spread of the thick monotonous coarse grained

1+2 deposits might suggest a general progradation and

aggradation motif for the northern branch — namely A/S > 1 or

A/S = 1. However, the northern branch of the D 1 succession is

composed of multiple stacked retrograding deltaic clinoforms

(instead of one thick delta pile). Although the generally retro-

grading stacking patterns are evident from the Fig. 8 A, the

clear identification of individual stages of D 1 evolution or

identification of individual deltaic clinoforms is not possible.

On the other hand, the succession in the southern branch of

D 1 reveals a more complicated arrangement with alternation

of phases of progradation (A/S > 1) and retrogradation (A/S < 1).

KSS

2 suggests a relative increase in A/S ratio, continuing

Fig. 7. Areal extent and map of thickness of deposits of both Lower

and Upper Gilbert delta (northern branch of lower delta D 1, southern

branch of lower delta D 1).

101

LOWER BADENIAN GILBERT DELTAS IN THE WESTERN CARPATHIAN FOREDEEP BASIN

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Karpatia

Karpatian

Lower Badenian

HJ404

cfMuš

HJ417

HJ419

cfMuš

HJ418

HJ501

HJ502

HV103

HV102

HV510

18 km

SSW

NNE

200

150

100

-50

-100

-150

-200

(m a.s.l.)

Explanations

Deposits of F

A1 and

Deposits of

Key stratal surface / KSS

(Retrogradation

A/S

(Progradation

A/S

< 1)

(A/S

KSS1 and

8 km

HJ404

HV105

cfM41

HJ415

Pas1

Karpatian

Lower Badenian

Karpatian

m a.s.l

200

150

100

-50

-100

-150

-200

. Carpathian

Flysch Zone

not in scale

KSS 2,5,6,9

KSS 3,4,7,10

Sequence boundary

Fig.

Representative

cross-sections

across

the

studied

part

basin

with

occurrence

Gilbert

deltas

with

position

key

stratal

surfaces:

—

cross-section

oriented

NW–SE

direction;

— cross-section oriented in NNE–SSW

direction.

102

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progradation and aggradation of D 2 after the start of its depo-

sition. Local evidence of KSS

2 points to a cuspate shape of

the surfaces cut down into FA

3 beds by overlying large foreset

packages (FA

1+2) and may be related to local erosional pro-

cesses at the foot of prograding foresets.

The thick pile of FA

1+2 above KSS

2 reveals a continuing

progradation and aggradation stacking pattern (A/S close to 1).

Especially KSS

3 and KSS

4 are obvious expressions of

an increase in A/S ratio and retrograding stacking pattern.

KSS

5 and KSS

6 points to a decrease in A/S ratio and renewed

progradation of the delta branch. The KSS

5 and 6 reveal

the same stacking pattern in different (i.e. proximal vs. distal)

settings. The deposition of both northern and southern

branches is terminated by KSS

7 and connected with the

drowning of the D 1 delta plain/topset. KSS

7 therefore reveals

a significant rapid increase in A/S ratio, rapid retrograding

stacking pattern, landward shift of the topset breakpoint and

termination of D 1 deposition and therefore is connected with

a transgressive event.

The stratigraphic evidence suggests coeval deposition of

both the delta branches, so the climatic and eustatic sea level

factors influenced the whole of D 1 in the same way. Although

the total thickness of coarse-grained delta deposits is generally

comparable in both branches, a greater thickness was recog-

nized in the southern branch. Similar evidence of progradation

above the basal surface KSS

1 reflects that erosional period

which occurred during a relative sea-level fall was followed

by an increase in A/S ratio. Such a situation indicates that the

accommodation space was initially formed almost uniformly

in the whole area under study. Thus, the differences in the

stratigraphic architecture of the northern and southern branch

of D

1 might be connected with variations in sediment delivery

(Martini et al. 2017) or might result from predisposed paleo-

topography (by incision) and paleobathymetry of the basin

floor. Lateral shifts of position of D 2 compared to D 1 gene-

rally towards the SSE, both the more complex stratigraphic

architecture and the higher thickness of southern branch of D 1

towards the southern margin of D 1 might reflect varied posi-

tion of the deltaic entry to the basin and/or indicate a relatively

rapid formation of accommodation space towards the southern

part of the basin during the studied stratigraphic interval.

This situation might be connected with the position of the

drainage system, or with possible connection between the

Vienna Basin and the Carpathian Foredeep (Brzobohatý &

Stráník 2011).

The evolution of D 2 is less complicated as it predominantly

records progradation. KSS

8 as the base of D 2 reflects a signi-

ficant decrease in A/S ratio (interpreted as a relative sea

level fall) and “localized” formation of a steep basin margin.

The pro gress of progradation and aggradation of coarse-

grained Gilbert delta deposits is connected with increase of

accommodation space. KSS

9 suggests a relative increase A/S

ratio, continuing progradation and aggradation of D 2. Local

evidence of KSS

9 points to a cuspate shape of the surfaces cut

down into FA

3 beds by overlying large foreset packages. It is

proposed that KSS

6 and 9 record local erosion due to

emplacement processes at the base of prograding foresets

(FA

1+2). These surfaces are therefore autocyclic erosional

surfaces that post-date the increase in A/S (Backert et al.

2010). The thick pile of FA

1+2 above KSS

9 reveals a con-

tinuing progradation and aggradation stacking pattern, so

an A/S ratio close to 1. KSS

10 representing the top of D 2

records a significant retrogradation stacking pattern, flooding,

landward shift in the topset breakpoint and finally termination

of the deposition of D 2, all indicating a rapid further increase

in A/S ratio due to transgression. The KKS

10 is relatively flat,

pointing to a flat delta plain and rapid flooding. Debrite-

dominated foreset deposits are typical for the upper delta D 2.

A marked bathymetric gradient towards the west or north-

west and south-west in the area under study indicate that the

basin axis was probably influenced by the active (eastern)

basin margin. Although the position of studied coarse grained

deltas generally coincides with eastern wall of the Karpatian

Iváň canyon (Dellmour & Harzhauser 2012), the studied

deltas are not parts of the canyon infill. The top part of the

canyon was eroded around the Early/Middle Miocene boun-

dary, capped by marine marls during the subsequent early

Middle Miocene transgression, and also the seismic data does

not show the presence of deltaic foresets (see Dellmour &

Harzhauser 2012). However, presence of this structure might

have affected the predominant Neogene drainage system of

the area (differential subsidence and more rapid formation of

the accommodation space) and the actual position of the

deltas. The marked prolongation of D 2 in the N–S/SW direc-

tion together with the orientation of the foreset dip on outcrops

(NNE-NE ward) point to the existence of several delta lobes

and complex progradation generally basinwards.

Ground penetrating radar

Two georadar cross sections were measured in the locality

of Novosedly. The longer cross section L 0 shows the internal

organization of the studied deposits of upper delta D 2 in the

NW–SE direction and the shorter cross section L1 reflects the

internal organization of D 2 in the NNE–SSW direction

(Fig. 9 A, B, D). Georadar profiles were oriented along the out-

cropped walls of the sand pit, so the image can be to some

extent calibrated by the visually observed depositional set-

tings. Four main georadar units (GRU) were defined based on

the characteristic reflection configuration (parallel to sub-

parallel reflections in each unit) (Fig. 9 C, E).

GRU 1 is characterized by continuous, horizontal, planar to

slightly undulated parallel reflections and was developed in

the uppermost part of the profile. GRU 1 is of tabular shape, its

thickness is relatively stable between 1 and 1.5 m and its base

is almost flat. GRU 2 is characterized by low amplitude, highly

variable (slightly undulated to almost planar, short, horizontal

to subhorizontal, sub-parallel) reflection. GRU 2 is of slightly

irregular shape with both base and top undulated; however, the

base is significantly more uneven than the top. The thickness

of the unit varies between 0.5 and 1.5m (due to the uneven

base), but is mostly about 1 m. The base of the unit truncates

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100

200

250

300

100

150

100

300

1:2 000

outcrop

Novosedly

GRU

GRU2

GRU

Foreset

Quaternary Topse

Bottomest

fshore mud

GRU

GRU2

GRU3

GRU4

Foreset

Quaternary

opset

Bottomest

fshore mud

Fig. 9.

Ground

penetrating

radar

profiles

across

the

Novosedly

outcrop.

—

Location

profiles;

—

Profile

- raw

data;

—

Profile

- interpretation;

—

Profile

- raw

data;

—

Profile

L0 - interpretation.

White arrows mark scoop-shaped scours.

104

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the underlying reflections of GRU

3. GRU

3 is generally

charac terized by the dominance of steeply inclined reflections.

These reflections are generally continuous and parallel.

Occasionally, short, horizontal, concave up and concave down

reflections occur between more continuous dipping reflec-

tions. The top of GRU

3 is slightly irregular (undulated).

The base of GRU

3 is very uneven, with numerous significant

undulations (see white arrows in Fig. 9 D). The thickness of

GRU

3 ranges from 6 to 12 m and the unit is of generally

tabular to wedge shaped. GRU

4 is generally characterized by

mostly discontinuous, short, horizontal, planar to concave up

or concave down reflections. GRU

4 forms the lower parts of

the profile. The contact between GRU

3 and GRU

4 is mostly

sharp and very uneven. Transition from GRU

3 into GRU

was observed only rarely and is connected with the gradual

passage of inclined reflections to flat laying horizontal ones

(gradual decrease of the dip).

Interpretation: The comparison of the GPR profiles with

depositional setting in the outcrop walls shows that GRU

represents the Quaternary sedimentary cover. GRU

2 is inter-

preted as deposits of FA

1. Continuous, horizontal to subpa-

rallel, and in places hummocky reflections sharply truncating

underlying inclined reflections of foreset are typical of fluvial

topsets of Gilbert deltas in GPR cross sections (Jol & Smith

1991), and are generally also characteristic for alluvial hori-

zontally bedded sands and gravels (Ékes & Friele 2003).

GRU

3 is compared with FA

2. This interpretation is confirmed

by inclined continuous (large scale) parallel reflections with

some differences in the dip, which are typical of the GPR

record of the foreset (Roberts et al. 2003; Eilertsen et al. 2011).

The upper boundary of GRU

3 with overlying GRU

2 is inter-

preted as toplap, and contact with underlying GRU

4 as down-

lap. Toplap of inclined reflections (GRU

3) in contact with

subhorizontal reflections (GRU

2) reveals the erosional rela-

tion of the topset and foreset. Variations in continuity, fre-

quency and also in the orientation of the dip of individual

series of inclined reflections reveal variations in the type of

mass flows (turbidity currents vs. debris flows). Short subhori-

zontal reflections recognized within a series of continuous

inclined ones are interpreted as backset bedding, cut and fill

(chutes) or slope failure structures (Roberts et al. 2003;

Eilersten et al. 2011). Progradation of foresets towards the

S-SSE is evident from the position of profiles (Fig. 9 A).

Comparison with the situation in the outcrop (Fig. 5 A) where

the transport direction was generally towards the NNE-NE

points to the existence of several deltaic lobes. Deposits of

GRU

4 are not outcropped. They have been interpreted either

as FA

3 or as FA

4 and so also with respect to the results of the

drillings in the close surrounding. Parallel reflections with

a low, subhorizontal angle of dip which underlie the inclined

reflections of the foreset are commonly interpreted as bottom-

set deposits of Gilbert deltas (Jol & Smith 1991; Eilersten et

al. 2011). Locally observed lateral transition of steeply

inclined reflections of GRU

3 into low-inclined reflections of

GRU

4 (white arrows in Fig. 9 D) can be interpreted as repre-

senting basinward transition of the foreset to the bottomset

(Eilersten et al. 2011). They overlie the basal unconformity

and show scoop-shaped scours (about 1 m deep and about

10 m wide). These structures resemble “spoon-shaped depres-

sions” (Breda et al. 2007, 2009; Leszczyński & Nemec 2015)

formed by turbidity currents descending a steep subaqueous

slope and undergoing a hydraulic jump at its toe. However,

sharp contact of gravels and underlying offshore mudstones of

4 is very common in the surrounding drill holes. Downlap

of the inclined reflection of GRU

3 on the highly irregular top

of GRU

4 with little preservation of their transition reveals

prograding of the foreset on the eroded top of underlying beds.

Variations in occurrence of the bottomset might also be partly

attributed to the bedrock morphology.

Provenance analysis

Provenance analysis is based on the pebble petrography and

analyses of heavy minerals.

Petrography and size of pebbles and cobbles, shape and

roundness of pebbles

The gravels can be classified as polymict. Dominance of

light beige, brown, grey, dark in colour, bituminous, micritic

or bioclastic limestone and dolomite pebbles and cobbles is

a typical feature. The content of carbonates usually varies

between 30 % and 82.0 % (average/AVG 40.9 %). The cobbles

or boulders of carbonates typically form the largest found

extraclast (max. 65 cm) (Fig. 2 D). Carbonate pebbles are

mostly discs (38–49 %), less common are spheres (27–29 %),

blade pebbles (16–22 %) or rods (8–15 %). Their average

value of form ratio (Sneed & Folk 1958) varies between 0.27

and 0.43. The average value of sphericity (Sneed & Folk 1958)

varies between 0.65 and 0.68 and average value of sphericity

(Krumbein 1941 in Carver 1971) between 0.69 and 0.7.

The average value of flatness ratio (Cailleux 1945 in Carver

1971) varies between 1.94 and 2.04. The average value of

isometry index (Sarkisjan & Klimova 1955) varies between

1.02 and 1.04. The average elongation index (Folk 1965) is

between 0.73 and 0.75 (equant). The average value of round-

ness index (Wentworth 1933 in Carver 1971) varies between

0.73 and 0.75 (well rounded).

Quartz pebbles are also quite common forming 4.4–49.9 %

(AVG 18.4 %) of the pebble spectra. Various varieties of

quartz are present. Whitish, milky quartz is the main one, with

dark or light grey and pinkish types subordinating. Quartz

pebbles are mostly discs (49 %), less common are spheres

(19 %), blades (16 %) or rods (16 %) with maximum diameter

dominantly between 1 and 5 cm. The average value of form

ratio (Sneed & Folk 1958) of quartz pebbles is 0.39, and

the average value of sphericity pebbles is 0.65 as defined

by Sneed & Folk (1958) and 0.69 as defined by Krumbein

(1941 in Carver 1971). Their average value of flatness ratio

(as defined by Cailleux 1945 in Carver 1971) is 2.04.

The average value of isometry index (Sarkisjan & Klimova

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LOWER BADENIAN GILBERT DELTAS IN THE WESTERN CARPATHIAN FOREDEEP BASIN

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, 2018, 69, 1, 89–113

1955) is 1.01. The average elongation index (Folk 1965) is

0.74 (equant). The average value of roundness index

(Wentworth 1933 in Carver 1971) is 0.74 (well rounded).

Sandstone pebbles (fine, middle or coarse-grained arkoses,

greywackes, quartzose sandstones, calcareous sandstones)

were identified in all samples, exceptionally forming up to

32.2 % of the pebble spectra (AVG 9.7 %). Presence of cherts

(dark grey, brown or red brown) is typical, which can reach up

to 11.9 % (AVG 10.4 %). A radiolarite pebble was described

by Přichystal (2009). Typical occurrence of mudstone (silty

clays) intraclasts can exceed 10 % of the pebble spectra, how-

ever their content is often difficult to quantify in relatively

small drill cores. The intraclasts are typically significantly

larger than the associated extraclasts and their size can some-

times reach over one metre in diameter (max. 3.5 m).

Micropaleontological study of intraclasts (Švábenická &

Čtyroká 1999; Petrová 2002) confirms the source mostly from

the deposits of the Laa Fm. (Karpatian), less commonly from

the Grund Fm. (Early Badenian) and canibalization of the

older basin infill. Čtyroký (1993) connected the source of

intraclasts with processes in the thrust front. Coal cobbles

were recognized exceptionally (Nehyba et al. 2008).

Crystalline rocks in general form only the minor portion of

the pebble suite; however, exceptionally, they can represent

more than 30 % (AVG 8.2 %). Metamorphic rocks are a stable

pebble component and are mostly represented by gneisses (up

to 22 %), quartzites (up to 7.4 %) or mica schists (up to

11.9 %). Pebbles of magmatic rocks were described in the

majority of samples forming up to 8.1 % (AVG 2.4 %). Two

types of magmatites were recognized, namely granitic rocks

(granites, granodiorites, aplites) and volcanic (melaphyre,

diabase, rhyolites) rocks (similarly Přichystal 2009). Some

differences in the content of individual rocks in the pebble

spectra are influenced by varied grain size of the samples (out-

crops vs. borehole cores).

Heavy minerals

Heavy minerals are sensitive indicators of the provenance,

weathering, transport, deposition and diagenesis (Morton &

Hallsworth 1994) especially if combined with the chemistry of

selected heavy minerals (Morton 1984). The ZTR ( zircon +

tourmaline + rutile) index is widely accepted as a criterion for

the mineralogical “maturity” of heavy mineral assemblages

(Hubert 1962; Morton & Hallsworth 1994) in the case of deri-

vation from a similar source. Garnet and rutile represent com-

mon heavy minerals in the studied deposits, being relatively

stable in diagenesis and having a wide compositional range,

thus enabling further evaluation in detail.

Heavy mineral assemblages

Garnet always dominates in the heavy mineral spectra and

its content varies between 69.1 and 93.4 % (AVG 77.6 %).

Zircon (0.5–11.9 %, AVG 5.4 %), represents the second most

common mineral. Staurolite (0.7–5.2 %, AVG 6.0 %), rutile

(0.3–8.2 %, AVG 4.3 %), disthene (0 – 6.4 %, AVG 3 %),

apatite (0 –7.0 %, AVG 2.6 %), tourmaline (0.2– 6.0 %, AVG

1.5 %) and amphibole (0 –8.5 %, AVG 0.1 %) represent acces-

sory but relatively common heavy minerals. The presence of

titanite, anatase, epidote, monazite, andalusite, pyroxene and

sillimanite was rather exceptional. The heavy mineral assem-

blage can be mostly (89.5 %) described as garnet rarely

(10.5 %) as garnet-zircon. The value of ZTR ranges between

1.9 and 11.8 (AVG 7.8).

Garnet

The chemistry of detrital garnet is widely used for the deter-

mination of provenance (Morton 1991).

The garnet composition was typical with its predominance

of an almandine component. Ten garnet types (T 1–T 10)

were determined in detail. The most common T 1 forms

35.5 % of the garnet spectra and is represented by gros sular–

almandine garnets with a composition in the range

Alm

58–78

Grs

10–32

Prp

3–9

Sps

0–9

. T 2 forms 21.5 % and is com-

posed by pyrope–almandine garnets with their typical compo-

sition in the range Alm

53–85

Prp

11–45

Grs

0–9

Sps

0–8

. T 3 forms

20 % and is represented by grossular-almandine garnets

with increased contents of pyrope and compositions of

Alm

47–75

Grs

13–30

Prp

10–24

Sps

0–5

. T 4 forms 6.5 % and is com-

posed of almandine garnets with low contents of pyrope, gros-

sular and spessartine components and the usual composition is

in the range Alm

80–86

Prp

5–9

Grs

0–9

Sps

1–9

. T 5 and T 6 both equally

form 5 % of the garnet spectra. T 5 consists of pyrope–

almandine garnets with increased grossular contents and the

composition Alm

52–71

Prp

12–20

Grs

11–17

Sps

0–2

. T 6 is represented by

gros sular–almandine garnets with an increased content of

spessartine and a composition in the range Alm

65–66

Grs

18–19

Sps

10–11

Prp

4–5

. T 7 is represented by spessartine–almandine

garnets with a composition of Alm

48–76

Sps

14–38

Prp

3–9

Grs

3–8

and

forms 4.8 %. T 8 forms 2.5 % and is composed of spessartine–

almandine garnets with an enriched grossular component and

composition in the range Alm

58–72

Sps

12–19

Grs

10–17

Prp

4–6

. Both

T 9 and T 10 are very rare and make up 0.5 % of the garnet

spectra equally. T 9 is represented by spessartine–almandine

garnets with an increased content of pyrope and composition

Alm

Sps

Prp

Grs

. T 10 is composed of grossular-almandine

garnets with an increased content of both pyrope and spessar-

tine and composition Alm

Grs

Sps

Prp

Classification diagrams (Mange & Morton 2007; Aubrecht

et al. 2009; Krippner et al. 2014) were used for evaluation of

the potential primary sources. The PRP−ALM+SPS−GRS

diagram (Mange & Morton 2007) in Figure 10A reflects the

most important role (59.6 %) of garnets from amphibolite–

facies metasedimentary rocks; significantly less common are

garnets from intermediate to felsic igneous rocks (24.0 %) or

garnets from high-grade granulite facies metasediments and

intermediate felsic igneous rocks (12 %). Only garnets from

high-grade mafic rocks are exceptional (4.4 %). The PRP–

ALM–GRS diagram (Aubrecht et al. 2009) in Figure 10B

indicates the most dominant (84.5 %) primary source of

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, 2018, 69, 1, 89–113

garnets derived from gneisses and amphibolites metamor-

phosed under amphibolite-facies conditions. Garnets reflec-

ting the source from gneisses or amphibolites metamorphosed

under pressure and temperature conditions transitional to

granulite- and amphibolite-facies metamorphism are not

common (8.5 %), similarly to garnets derived from granulites

(7.0 %).

The GRS–SPS–PRP diagram (Fig. 10 C) allows comparison

with possible source rocks along the eastern margin of the

Bohemian Massif (Otava et al. 2000; Čopjaková et al. 2002,

2005; Čopjaková 2007; Buriánek et al. 2012). Some garnets

can be compared to the Moravian Zone, the Moldanubicum, or

the Brno Massif; however, they are commonly outside the

diagnostic fields. The results are aligned with a noticeable

lateral elongation in the PRP–GRS line. This distribution sig-

nificantly differs from the distribution recognized for the

Myslejovice Fm. of Moravian–Silesian Paleozoic (Culmian)

deposits (Otava et al. 2000), where the source of Neogene

deposits of the Carpathian Foredeep is commonly traced

(Hladilová et al. 2014; Holcová et. al. 2015). The composed

diagram (Fig. 11) allows comparison with garnets from

Krosno and Ždánice–Hustopeče Fm. of the Western Carpathian

Flysch Zone, which represents an active margin of the basin.

Only part of the studied data fits with the diagnostic fields,

however; commonly bi-lateral distribution of the results can

be followed outside the diagnostic field.

Rutile

Rutile, which represents one of the most stable heavy

minerals, is commonly used for provenance analyses (Force

1980; Zack et al. 2004 a, b; Triebold et al. 2007).

The concentration of the main diagnostic elements (Fe,

Nb, Cr and Zr) varies significantly in the studied samples.

The concentration of Nb ranges between 388 and 5800 ppm

(average 1854.7 ppm), the concentrations of Cr vary between

418 and 1998 ppm (average 418 ppm), of Zr between 50 and

5389 ppm (average 429 ppm), and the value of log Cr/Nb is

mostly negative (76.5 %). The discriminate plot Cr vs. Nb

(Fig. 12) shows two different trends in the rutile provenance

GRS

SPS

PRP

ALM+SPS

GRS

PRP

ALM

GRS

1
2
3
4
5
6
7

Fig. 10. Ternary diagrams of the chemistry of detrital garnets

(ALM — almandine, GRS — grossular, PRP — pyrope, SPS — spes-

sartine). A — Discrimination diagram according to Mange and

Morton (2007) (1– pyroxenes and peridotites, 2 – high-grade granulite

facies metasediments and intermediate felsic igneous rocks, 3 – inter-

mediate to felsic igneous rocks, 4 – amphibolite facies metasedimen-

tary rocks); B — Discrimination diagram according to Aubrecht et al.

(2009) (1 – pyroxenes and peridotites, 2 – felsic and intermediate

granulites, 3 – gneisses and amphibolites metamorphosed under

pressure and temperature conditions transitional to granulite and

amphibolite facies metamorphism, 4 – gneisses metamorphosed

under amphibolite facies conditions); C — Ternary diagram of the

chemistry of detrital garnets in comparison with possible source areas

(1 – the Moravian Zone, 2 – the Moldanubicum, 3 – the Svratka

Crystalline Complex, 4 – granites of the Brno Massif, 5 – migmatites

of the Brno Massif, 6 – younger part of the Moravian–Silesian

Palaeozoic/Culmian, 7 – samples from studied coarse-grained deltas).

Data from source rocks according to Otava et al. (2000); Čopjaková

et al. (2002, 2005); Čopjaková (2007) and Buriánek et al. (2012).

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LOWER BADENIAN GILBERT DELTAS IN THE WESTERN CARPATHIAN FOREDEEP BASIN

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well. The majority of rutiles originated from metapelitic rocks

(64.7 %), whereas origins from metamafic rocks (23.5 %)

or pegmatites (11.8 %) are less common. The Zr-in-rutile

thermo metry of metapelitic zircons only (see Zack et al.

2004 a, b; Meinhold et al. 2008) indicates that 58.8 % of these

metapelitic rutiles belong to green schist metamorphic facies

and 41.2 % to the amphibolite/eclogite facies. The calculated

temperatures range between 246 –753 °C (equation Zack et al.

2004 b) showing the significant role of low-medium tempera-

ture metamorphic rocks in the source area. The diagnostic

criteria of Triebold et al. (2012) confirm prevailing prove-

nance from metapelites (61.8 %) over metamafic sources

(26.5 %).

These results differ from the data known for the Karpatian

or the Lower Badenian deposits of the Carpathian Foredeep

(Francírek & Nehyba 2016; Nehyba et al. 2016) and indicate

a different provenance of rutile in this case.

Interpretation of the provenance data

Polymict gravels with a dominance of Mesozoic carbonates,

broad spectra of further sedimentary rocks, and a content of

magmatic and metamorphic rocks all point to sources in the

Alpine–Carpathian orogene (cf. Nehyba & Roetzel 2004; 2011).

The high similarity in shape characteristics and well round-

ness of both carbonate and quartz pebbles reveal multiple

sources and the role of redeposition. Abundant intraforma-

tional clasts suggest enhanced erosion of subaerially exposed

distal offshore deposits (both Karpatian schlier and Lower

Badenian tegel), significant relative sea-level fall, large-scale

slope failures during deposition and attendant sediment gra-

vity flow processes.

The heavy mineral assemblage is very typical for the Lower

Badenian deposits of the Carpathian Foredeep. The dominant

presence of garnet confirms the important role of metamor-

phic complexes (crystalline schists) in the source area. Zircon,

tourmaline and rutile are common in acidic to intermediate

magmatic rocks, as in selected metamorphic rocks (von

Eynatten & Gaupp 1999) or can be connected with redepo-

sition from older deposits. The relatively stable heavy mineral

assemblage together with low and varied content of low- stabilty

heavy minerals (apatite, pyroxene, amfibole etc.) point to

relatively weathered rocks in the primary source area, formed

by both crystalline schists and magmatic rocks (a mature

continental crust). Low values of ZTR index are typical for

immature clastic deposits with a relatively low role of recy-

cling (or significant role of carbonates in the source area).

Dominance of almandine garnets is a very common feature

of the garnet spectra in the deposits along the eastern margin

of the Bohemian Massif. These are recognized in the

Moravian–Silesian Paleozoic (Culmian) rocks (Otava et al.

2000; Čopjaková et al. 2002, 2005; Čopjaková 2007), Permo–

Carboniferous deposits (Nehyba et al. 2012; Nehyba &

Roetzel 2015), Jurassic deposits of both the Gresten and

Nikolčice Formations (Nehyba & Opletal 2016; 2017),

Paleogene deposits of the Western Carpathian Flysh Zone

(Otava 1998; Otava et al. 1997; Stráník et al. 2007), and also

in Eggenburgian and Ottnangian, Karpatian (Francírek &

Nehyba 2016) and Lower Badenian (Nehyba et. al. 2016)

deposits of the Carpathian Foredeep itself. However, the dis-

tribution of recognized garnet types varies within these depo-

sits. Obtained garnet data can to some extent be compared

either with the results from the depositional unit III of

Karpatian deposits (Francírek & Nehyba 2016), or with the

Lower Badenian deposits along the marginal flank of the crys-

talline basement (Nehyba et al. 2016). However, here also the

studied garnet spectra differ in detail. The occurrence of spes-

sartine–almandine garnets (T 7–9) is remarkable and signifies

a source from the crystalline rocks of the basement, because

such garnets are generally very common in the gneisses,

migmatites and mica schists of the Bohemian Massif

(Čopjaková 2007; Francírek & Nehyba 2016). A direct source

from the passive margin of the basin formed by crystalline

rocks of the Bohemian Massif is highly improbable due to the

Metamafic
Rocks

Metapelites

1000

2000

3000

4000

5000

1000

2000

3000

4000

5000

6000

Nb (ppm)

Cr (ppm)

ALM

GRS

PRP

ALM

SPS

ALM

Ždánice-Hustopece Fm.

Krosno Fm.

Lower Badenian
coarse-grained delta

Fig. 12. Discrimination plot of Cr vs. Nb of investigated rutiles.

Fig. 11. Ternary diagram of the chemistry of the detrital garnets and

comparison with garnets from Krosno and Ždánice–Hustopeče Fm. of

the Western Carpathian Flysch Zone (ALM – almandine, GRS – gros-

sular, PRP – pyrope, SPS – spessartine).

108

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basin configuration, position of the studied deposits in the

basin and the depositional environment of the studied depo-

sits. Both diagrams (Fig. 10A, B) reveal the dominance of

the garnets from amphibolites and metamorphosed rocks of

amphibolite facies and a relatively uniform primary source.

The obtained provenance data can be summarized: 1) The

prove nance of the gravels is specific and partly differs from

the known data from the sedimentary infill of the Carpathian

Foredeep. 2) Several partial sources can be recognized: a)

provenance from Mesozoic carbonates of the Alpine–

Carpathian orogeny, b) crystalline rocks of the eastern margin

of the Bohemian Massif, c) older sedimentary infill of the

Carpathian Foredeep and/or Alpine Molasse Zone, and

d) sedi mentary rocks of the Carpathian Flysch Zone. The role

of these sources varies in individual samples (see the common

bilateral distribution of garnet data in diagrams). A source

from an area now below the surface is highly probable.

The composition of garnet, rutile and also pebble petro-

graphy of the deposits studied significantly differs from simi-

lar data obtained for the autochthonous Jurassic beds along the

eastern margins of the Bohemian Massif (Nehyba & Opletal

2016, 2017). This result indirectly challenges the provenance

of carbonate pebbles only from Jurassic beds (see Eliáš 1981;

Řehánek 2001). Part of the carbonates could originate from

older Triassic carbonate units of the Northern Calcareous Alps

(similarly Přichystal 2009). Several authors (Menčík 1973;

Eliáš 1981; Stráník et al. 1999; Řehánek 2001) supposed the

source to be from autochthonous (subsurface) Jurassic depo-

sits of the SE margin of the Bohemian Massif. However,

a detailed microfacies and provenance study of these carbo-

nates is missing.

Discussion

The creation of an adequate initial bathymetry and topo-

graphy necessary for the initiation of deposition of Gilbert

delta foresets is connected with the formation of a significant

basinward dip of the depositional slope (Postma 1990). So,

especially KSS 1 and partly also KSS 8 are interpreted as the

result of a significant fall in base level. Further Gilbert delta

growth requires: (i) a high sediment supply; (ii) high water

flux; and (iii) high creation of accommodation space (Postma

1990). Deposition of both D 1 and D 2 reveals a general increase

of bathymetry. The sedimentary record of D 1 and D 2 is domi-

nated by large-scale prograding and aggrading topset and fore-

set packages which mainly record the creation and fill of

available accommodation space. Such stratigraphic architec-

ture reflects a low and gradual rate of creation of accommoda-

tion space. As the delta records essentially vertical stacking of

gravel beds, and as the proportion of fine-grained facies is

very low, the distal part of the basin was never filled by deltaic

deposition and the basin gradually deepened with time, thus

allowing foreset heights to increase (Backert et al. 2010). Sea-

level rise corresponds to rapid transgression across the D 1

delta top (KSS 7) and termination of deltaic deposition in the

study area. Similarly also the termination of D 2 (KSS 10)

could be connected with transgression as a transgressive sur-

face, however it could also represent the flooding surface due

to smaller scale of D 2 delta. Hypothetical reconstruction of

the depositional setting during the evolution of both the lower

delta D 1 and upper delta D 2 are presented in Fig. 13.

The dimensions of the studied Gilbert deltas can be partly

compared with the data in literature, taking into account dating

of their development. Backert et al. (2010) estimated that

a giant Gilbert-type delta (thickness > 600 m) was deposited

during a period of 500 to 800 ky. Breda et al. (2007) supposed

that steep dipping clinoforms 50 to 250 m thick were formed

during 4

-order (0.2–0.5 Ma) cycles. Similarly Benvenuti

(2003) estimates that the evolution of a 20 m coarse-grained

delta took about 100 ky. According to Gobo et al. (2015)

a Gilbert delta about 100 m thick formed during approx. 50 ky.

Significantly more rapid constructions of deltas were docu-

mented by Corner (2006) and Eilertsen et al. (2011) who

reported coarse-grained deltas about 60 m thick formed during

less than 10 ky. Similarly Postma & Cruickshank (1988) docu-

mented prograding of Gilbert type delta about 15 m thick

during approx. 2 ky. Likewise Rhine & Smith (1988) described

approx. 20 m thick coarse-grained delta deposited during

a time period of 1700 years. Even more rapid formation was

documented by Plink-Björklund & Ronnert (1999), who

described a coarse deltaic clastic wedge 20–80 m thick depo-

sited during a period of about 100 years. Similarly Nehyba et

al. (2017) documented progradation of about 4 m thick Gilbert

delta during less than 20 years. Although there is no simple

positive correlation between the time period of delta formation

and volumetry/scale of deltaic body it is obvious that D 1 and

D 2 have different scales. Similarly, although the evidence of

two laterally and stratigraphically separated coarse-grained

Gilbert deltas indicate two cycles of sea-level change, these

cycles could have different regional/basin extent/significance

and could be of different orders (3

vs. 4

order). However,

different paleoslope or variations in the ratio of water/sedi-

ment discharge can also affect the scale and distribution of the

coarse grained deltaic deposits. With a high ratio of water/

sediment discharge, the feeder system is likely to be a sheet

flow that can aggrade more uniformly along the shoreline.

As the ratio of water/sediment discharge decreases, the stream

channel sedimentation becomes dominant, and also the lateral

mobility of the feeder flow is reduced (Muto & Steel 2001).

The deep scours/KSS 1 and 8 that occur at the bases of the

Gilbert deltas D 1 and D 2 successions are interpreted as

sequence boundaries/regressive surfaces of fluvial erosion.

They are results of forced regression and relative sea level fall

(downstepping, sediment by-pass, basinward shift of facies

belt, negative accommodation). More extended basinward

progradation, deeper incision and larger thickness point to

more pronounced sea-level fall during the formation of KSS 1

then KSS 8.

The sedimentary infill (or its substantial part) of studied

Gilbert deltas is connected with ongoing normal regression,

which is typified by the rate of accommodation lower than

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the sediment supply. In such circumstances prograding and

aggrading foresets thicken downdip, dominant progradation

driven by sediment supply is followed with some aggradation.

Moreover, whereas the rates of progradation decrease with

time, the rates of aggradation increase with time (as a result of

accelerating relative sea level rise). This situation is reflected

by the stacked retrograding pattern of deltaic clinoforms in the

northern branch of D1. The sedimentary infills are capped by

transgressive surfaces.

The differences in the stratigraphic architecture of the

northern and southern branch of D1 could have resulted from

predisposed paleotopography (by incision) and paleobathy-

metry of the basin floor and/or local variation of sediment

supply effecting on A/S ratio.

Two almost coeval coarse-grained Gilbert deltas docu-

mented in the depositional succession of the Western

Carpathian Foredeep basin reflect two base level cycles/

sequences probably of different scale. However, deltas D1 and

D 2 could also represent two different Early to Middle Miocene

order cycles, namely TB 2.3. and TB 2.4. (Kováč et al.

2004; Hohenegger et al. 2014). Moreover, two scales of base

level cycles were recognized in the depositional succession of

lower D1 delta. Such a complex state represents a compli cation

for the basin lithostratigraphy. Recognition of a longer- term

cycle (probably 3

order cycle TB 2.3. sensu Haq et al. 1988)

and high-frequency cycles (4

to 5

order) constitute a challange

for the further research in the Western Carpathian Foredeep

basin. Adequate biostratigraphical and litho stratigraphical

correlations of the Early Badenian sedimentary succession

within the Carpathian Foredeep and the Alpine Molasse Basin

based on facies architecture of marginal and

basinal facies are necessary to understand the

actual role of local and regional ruling factors and

the establishment of reliable sequence stratigra-

phy of Early to Middle Miocene deposits.

Two Lower Badenian gravel beds (13 and 15 m

thick) separated by an about 77 m thick interbed

of fine sands and silty shales were also recog-

nized in the Roggendorf-1 borehole (see Fig. 1)

drilled in the Early to Middle Miocene deposits

of the Alpine–Carpathian Foredeep approx.

40 km SW of the area under study (Ćorić & Rögl

2004). Both gravels contain abundant limestone

and dolomite pebbles which originated from the

Calcareous Alps and Flysch Unit. The boundary

between zones NN 4 and NN 5 was observed

within the fine-grained interbed. The basal gra-

vels are regarded as the transgressive base of the

Badenian in the Molasse Basin north of the

Danube (Ćorić & Rögl 2004). The upper gravels

are considered to be the coarse basal transgres-

sion level of the Grund Formation. Remarkable

similarity in petrography and stratigraphic posi-

tion might point to a link between these gravel

beds and D1+ D 2 deposits as a response to ade-

quate factors/processes affecting the basin archi-

tecture. However, such a simple link might be misleading.

Coarse-grained Gilbert deltas are highly prone to rapid

changes in basin configuration, sea-level, climate and sedi-

ment supply during the evolution of the system (Nemec 1990a;

Postma 1990) as a result of dynamic equilibrium between

sedi ment supply, basin energy conditions, accommodation and

the overall geological framework.

The studied Gilbert deltas represent infill of two incised-

valley systems as defined by Zaitlin et al. (1994). The confi-

guration of the alluvial feeder system has a crucial influence

on the gross geometry of deltas (Postma & Roep 1985; Kim &

Chough 2000). The scale (lateral extent and thickness) of

deltaic deposits, incised base and provenance analyses all point

to an extensive fluvial system with a large catchment basin.

Sediment and water flux in this fluvial system would therefore

reflect regional tectonics and climatic variations. The very

similar provenance of D1 and D 2 joined both deltas into

a common fluvial system. Jiříček (2002) supposed that the

partially comparable Matzen delta in the Vienna basin was

formed by a Badenian Paleo-Danube. The position of the flu-

vial entry into the basin implies incised valley/valleys formed

within the active basin margin (wedge-top) oriented perpen-

dicularly to oblique to the foredeep part of the foreland basin.

This allochthonous part of the basin is not preserved or does

not outcrop. Tectonic predisposition of such a valley (avai-

lable morphology of the piggy-back sub-basin?) is probable.

It is also possible that the fluvial system was initially oriented

in the NNE–SSW direction (the axis of the flexure and the

basin). The tectonic setting at the margin of a thrust belt and

morphology of adjacent parts of the Waschberg–Ždánice unit

6
7

Fig. 13. Schematic model of the deposition condition of the lower Gilbert delta (A)

and upper Gilbert delta (B) (1 — topset deposits, 2 — foreset deposits, 3 — bottom-

set deposits, 4 — older basin infill/mostly Karpatian in age, 5 — older basin infill/

mostly Lower Badenian in age, 6 — sea level, 7 — former position of sea level).

110

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GEOLOGICA CARPATHICA

, 2018, 69, 1, 89–113

(accelerated by a sea-level drop) diverted the drainage into

a western direction. Further tectonic activity finally switched

the drainage system towards the east into the subsiding and

opening Vienna Basin.

Conclusions

The Lower Badenian “basal or marginal coarse clastics” in

the southernmost part of the Western Carpathian Foredeep

were interpreted as deposits of two coarse-grained Gilbert

deltas based on the study of both outcrops and boreholes.

Four facies associations/principal depositional environ-

ments have been identified in the studied deposits. Three of

them correspond to a tripartite Gilbert type delta profile.

Facies association 1 is interpreted as topset, facies association 2

as foreset and facies association 3 as bottomset. The remaining

FA 4 represents open marine pelagic deposits.

The lower delta is significantly thicker (up to 160 m), more

areally extended and reveals a more complicated stratigraphic

architecture than the upper delta. The laterally traceable

boundary/base of the lower Gilbert delta is connected with

a significant migration of basin depocentre, a new basin

physio graphy with relatively steep margins, which is inter-

preted as a consequence of a relative sea level fall, followed by

major erosion and incision several tens of m downward and

several km basinward. This surface represents a sequence

boundary and is also arbitrarily used as the Karpatian/Badenian

boundary. Formation, progradation and aggradation of the thick

coarse-grained Gilbert delta pile generally indicate a dramatic

increase of sediment supply from the hinterland, followed by

both relatively continuous sediment supply and an increase of

accommodation space over the available time (interpreted as

lowstand normal regression). Two coeval deltaic branches

were recognized in the lower delta with partly different strati-

graphic arrangements (directed by the interplay between the

available accommodation space and the sediment supply),

confirmed by identified key stratal surfaces. The northern

delta branch is typical with generally uniform lithology and

reflects delta progradation, aggradation and final retrograda-

tion. The southern branch is characterized by a more compli-

cated lithology due to multiple alternation of facies associations

and a slightly higher total thickness. Such an arrangement

points to alternation of phases of delta progradation and retro-

gradation (followed by aggradation). The differences in the

stratigraphic architecture of the branches are explained by

variations in the sediment delivery, inherited paleotopography

and by a possibly relatively more rapid formation of accom-

modation space towards the southern part of the basin.

Termination of deposition of the lower delta is connected with

relatively rapid and extended drowning of the delta plain and

is explained by a transgressive event (Lower Badenian in age).

The upper delta was recognized only in a restricted area and

its maximum total thickness reaches 33 m. The lower boun-

dary of this delta reflects a significant decrease in the ratio of

accommodation space/sediment supply, which is interpreted

as a relative sea level fall and a sequence boundary (within the

Lower Badenian). Subsequent progradation and aggradation of

coarse-grained Gilbert delta deposits is connected with a fol-

lowing increase of accommodation space and intense, spatially

localized sediment supply. The flat upper surface of this Gilbert

delta is connected with a landward shift in the topset break-

point and rapid flooding (all Lower Badenian in age).

Lateral and vertical/stratigraphical separation of both

Gilbert deltas suggests migration of the delta depocentre and

evolution of the basin margin. The evidence of two laterally

and stratigraphically separated coarse-grained Gilbert deltas

indicates two regional/basin wide sea-level cycles/deposi-

tional sequences, but not necessarily of the same order.

Provenance analysis did not recognize significant diffe-

rences between the deposits of the two Gilbert deltas, but

revealed their multiple sources and the role of basin canni-

balism. The studied gravels are polymict with a dominant role

of Mesozoic carbonates, which make them specific in the sedi-

mentary infill of the Carpathian Foredeep. Although the heavy

mineral assemblage of the studied deposits is very typical for

the Lower Badenian deposits of the Carpathian Foredeep, the

garnet spectra differ in detail from available data from the basin.

The provenance analysis identified several partial source areas

(Mesozoic carbonates of the Northern Calcareous Alps and/or

the Western Carpathians, crystalline rocks of the eastern

margin of the Bohemian Massif, older sedimentary infill of

the Carpathian Foredeep and/or Alpine Molasse Zone, sedi-

mentary rocks of the Carpathian / the Alpine Flysch Zone).

A source from an area now below the surface is highly pro-

bable. The scale (lateral extent and thickness) of deltaic depo-

sits, their deeply incised base and provenance all point to

an extensive fluvial system with large catchment basin.

Acknowledgements: The manuscript highly benefited from

thoughtful reviews of two unknown reviewers. The author

also thanks the Czech Geological Survey for giving access to

the Iváň 1 and 22-41 D Pasohlávky boreholes.

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