Sedimentary Geology 299 (2014) 42–59
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Sedimentary Geology
journal homepage: www.elsevier.com/locate/sedgeo
Genetic significance of an Albian conglomerate clastic wedge, Eastern
Carpathians (Romania)
Cornel Olariu a,⁎, Dan C. Jipa b, Ronald J. Steel a, Mihaela C. Melinte-Dobrinescu b
a
b
Jackson School of Geosciences, The University of Texas at Austin, 2275 Speedway Stop C9000, Austin, TX 78712, USA
National Institute for Marine Geology and Geo-ecology (GEOECOMAR), 23-25 Dimitrie Onciul Street, RO-024053 Bucharest, Romania
a r t i c l e
i n f o
Article history:
Received 12 July 2013
Received in revised form 4 October 2013
Accepted 5 October 2013
Available online 16 October 2013
Editor: J. Knight
Keywords:
Eastern Carpathians
Albian Bucegi Formation
Conglomerate
Shelf-slope margin
Clinoform
Debris flows
a b s t r a c t
The impressive 2000 m thick conglomerates of the Bucegi Formation exposed in the southernmost part of the
Eastern Carpathians were interpreted initially as large alluvial fans, and later suggested to be deposited as
deepwater submarine-slope deposits. However, the routing system of the coarse sediment transfer from the
source area to the deepwater slope (source-to-sink analysis) has not been explained and the mechanisms
involved in the shelf sediment storage and bypass onto the slope have not been discussed.
The present research on the Albian Bucegi Formation has provided the following new insights on their
source-to-sink aspect: (1) that the Upper Member of the Bucegi Formation, with its frequent channelized and
sheet like fine conglomerates and sandstones, contrasts greatly with the Middle and Lower members of
deepwater slope and basin-floor origin. The Upper Member is interpreted as fluvial and shallow-marine deposits
that were temporarily stored and reworked on a ‘shelf’, albeit a narrow one, bridging the area between the
deforming hinterland and the deepwater slope deposits; (2) the Upper and Middle members are genetically
linked and developed through the basinward migration of a large-scale (hundreds of metres in amplitude)
clinoform with relative flat-lying topsets and slightly steeper (few degrees), coarser grained slopes that built
out to the south and southeast; a configuration that is common along continental margins and also generally
along all types of deepwater basin margins; (3) the Middle Member contains a range of submarine, sediment
density flows that vary from high-density, mobile debris flows to lower-density sandy turbidites. The sediment
textures (sorted grain populations) inherited from the shelf ‘sorting factory’ can to some extent still be
recognised in the slope stratigraphy; and (4) the large (10–20 m diameter) carbonate and metamorphic
olistoliths that are ubiquitous on the shelf and (to a lesser extent) slope, reflect the steep gradients and very
active tectonic setting of the fractured and thrusted hinterland, from which these outsized blocks were
transported onto the adjacent shelf.
The now-proposed, narrow shelf platform of the Albian Bucegi basin margin thus functioned to temporarily store
sands and gravels, to distinctly sort some of this sediment, and to eventually bypass both sorted and new floodgenerated, unsorted materials onto the slope. Compared with other basin margins, this Albian Bucegi margin was
extremely coarse grained because of its proximity to the actively deforming mountain range, to a fractured
basement that produced more gravel than sand, to the great sediment flux from steep short rivers, and to the
narrowness (10–20 km) of the shelf.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
The Albian conglomerates of the Eastern Carpathians, firstly described
as the ‘Bucegi Conglomerates’ (Popescu-Voiteşti, 1918) represent an
extremely thick (up to 2000 m) accumulation of bedded conglomerates
intercalated with some sandstone and olistoliths (Murgeanu and
Patrulius, 1963; Patrulius, 1969). The Albian conglomerate outcrops
extend from the northern part of the Eastern Carpathians, up to their
southernmost end (i.e., the Romanian Carpathian bend area), being
described under different names, i.e., Bucegi, Ciucaş–Zăganu, Piatra
Mare and Ceahlău conglomerates (Murgeanu and Patrulius, 1963;
Săndulescu, 1984, 1994). An Albian age was established by the dating of
the underlying and the overlying deposits based on their macrofaunal
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http://dx.doi.org/10.1016/j.sedgeo.2013.10.004
assemblages (Murgeanu and Patrulius, 1957; Murgeanu et al., 1963;
Patrulius, 1969) and calcareous nannofloral associations (Melinte and
Jipa, 2007).
The conglomerates of the Bucegi Formation were interpreted as
post-tectonic “molasse” deposits (Panin et al., 1963; Contescu, 1974)
relating to the Ceahlău Nappe. The latter reflects the most widespread
and main Cretaceous tectonic movement (Murgeanu et al., 1963;
Dumitrescu and Sãndulescu, 1968; Săndulescu, 1984) of the Outer
Dacides nappe system. The Early Mid Cretaceous “Austrian” tectonic
phase started in the Late Barremian (ca. 130 Ma) and ended at around
100 Ma (Aptian to Early Cenomanian), and was followed by an
extensional collapse (Săndulescu, 1988, 1994; Ştefãnescu and Melinte,
1996; Cloetingh et al., 2006).
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
The depositional environment of the Albian conglomerates of the
Eastern Carpathians was interpreted as fluvial derived with minimum
marine reworking of the sediments (Panin et al., 1963) based on a
morphometric study of the clasts, or as a shallow water fan delta
(Mihăilescu et al., 1967; Patrulius, 1969). The Bucegi Formation has
been conventionally interpreted as ‘post-tectonic’ (Panin et al., 1963;
Contescu, 1974; Săndulescu, 1984; Schmid et al., 2008), because it
overlies traditional ‘flysch’ deposits, and because of its coarse-grained
and supposed non-marine character.
Stanley and Hall (1978) suggested that the conglomerates of the
Bucegi Formation formed on a deepwater depositional slope through
the analogy with modern continental slopes and with the modern Var
River system in southern France. The deep water depositional setting,
if correct, challenges the post-tectonic interpretation of the Bucegi
Formation, as such steep, coarse-grained, deepwater slopes were more
likely to be syntectonic. Jipa (1979, 1981, 1984) also suggested that
O
O
27 E
23 E
O
conglomerates of the Bucegi Formation represent large (hundreds of
metre thick) accretion deposits of a basin margin or geosynclinal
(deep basin tectonically active) instead of typical “molasse”.
We propose, using new outcrop observations and previous studies,
a shelf to deepwater slope model for the Bucegi Formation. This
interpretation as a coarse-grained shelf-margin sedimentary prism, and
an improved understanding of the sediment transfer mechanism across
the margin, have implications for the Carpathian evolution and also for
understanding basins with similar tectonic settings and sedimentary fill.
This paper focuses on the sedimentology on the Middle and Upper
members of the Bucegi Formation to (1) document the subaerial to
shallow water shelf-dispersal system of the Upper Member, (2) present
the arguments for a shelf-to-slope clinoform setting, (3) demonstrate
the subaqueous origin of the Middle Member bringing arguments on
what was earlier suggested by Stanley and Hall (1978), and (4) argue
that the Bucegi Formation is a syntectonic clastic wedge and not post-
25°26’24”E
C
48 N
Sir
Eas
R.
tC
et
River
A
43
ath
if
45°24’17”N
R.
Bucharest
ceg
100 km
ass
Olt
O
study area
Ialom
ita R
.
POIANA
TAPULUI
Bu
B
BUSTENI
iM
s
rpathian
South Ca
s
ian
s R.
44 N
Prahova
arp
Mure
45°21’22”N
Ialom
ita R
iver
SINAIA
2 km
Babele Sandstone and
Upper Member
Scropoasa-Laptici Sandstone
Middle Member
Fig. 1. Location of the study area. A — Map of Romania with location of the Bucegi Mountains at the southern end of the Eastern Carpathians (Romania). B — Simplified geotectonic
map. C — Detail map of the Bucegi Mountains with the lithostratigraphic units. Box (B) in (A) and (C) in (B).
Panel B is modified after Schmid et al. (2008). Panel C is modified from Patrulius (1969).
44
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
tectonic as currently supposed. We compare the conglomerates of the
Bucegi Formation with other clastic wedges or shelf-margin prisms
inactive tectonic settings.
2. Geological setting
The Eastern Carpathians, a 600 km long segment of the Carpathian
Orogen (Fig. 1), which is bordered to the west by the Transylvanian
Basin and the easternmost Pannonian Basin, to the east by the Moldavian
and Scythian platforms, and to the south and southeast by the Moesian
Platform (Fig. 1). The Eastern Carpathians are composed mainly of Jurassic
to Miocene deposits (of several basins), that were folded and thrust over
the Miocene sediments of the Carpathian Foredeep (Săndulescu, 1994;
Schmid et al., 2008). Tectonic stacking and internal deformation of the
nappes occurred from the Late Cretaceous to the Neogene (Sãndulescu,
1984; Maţenco and Bertotti, 2000; Csontos and Vörös, 2004; Maţenco
et al., 2010).
Age
Lithology
The Bucegi Formation crops out over a large area (20 km N–S and
10 km W–E) in the southern part of the Eastern Carpathians (Fig. 1A)
where thick, mostly conglomerate deposits form steep and spectacular
(over 1000 m high) cliffs along the west side of the Prahova River. The
upper part of the conglomerate succession makes a high altitude
(2000m) plateau, the Bucegi Plateau, between the Prahova and Ialomiţa
rivers (Fig. 1C).
The Bucegi Formation is over 2000 m thick and is dominated by
conglomerates, but in addition contains sandstone intercalations
throughout the succession. However, large to very large carbonate and
metamorphic blocks (olistoliths) (Patrulius, 1969) are common in the
Upper Member (also present occasionally in the Middle and Lower
members) of the succession. The Bucegi Formation is mainly clast
supported and clasts are mainly sub-rounded (Panin et al., 1963;
Briceag et al., 2009).
The Bucegi Fm. is part of the Ceahlău Nappe and overlies older
Barremian–Aptian turbidite “flysch” deposits of units such as the
Sandy–Shaley Flysch, the Piscu cu Brazi, the Comarnic and the Vârful
Lithostratigraphic units
Coniacian, Turonian
Cenomanian
Formation
Members
BABELE Sst.
BUCEGI FORMATION
UPPER MEMBER
Middle and Late
Albian
SCROPOASALAPTICI Sst.
MIDDLE MEMBER
Aptian- Early
Albian
Hemipelagic facies
Breccia
Conglomerate
Approx.
500 m
LOWER MEMBER
Microconglomerate
Wildflysch
Olistolith
Thick sandstone
Fluxoturbidite
Sandy-marly flysch
Fig. 2. Lithostratigraphy of the Aptian–Turonian deposits that crop out in the Bucegi Mountains. Note that the Bucegi Fm. is about 2000 m thick and contains three members, with the
youngest one, i.e., the Upper Member, displaying two distinct facies.
Modified after Murgeanu et al. (1963).
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
Rădăcinii formations (Murgeanu and Patrulius, 1963; Melinte and Jipa,
2007; Fig. 2). The sedimentary structures in the deposits below the
Bucegi Fm. have been used to interpret them as deep water turbidites
(Murgeanu et al., 1963; Patrulius, 1969). The Albian conglomerates
are unconformably overlain by hemipelagic deposits of latest Albian
age (Murgeanu and Patrulius, 1957).
On the basis of new as well as published data on the lithostratigraphy
of the Bucegi Formation, Jipa et al. (2013) divided the Bucegi Formation
into five members. In this study, based on genetic arguments, we will use
only three members, i.e., the Lower, Middle and Upper members with
the Babele and Scropoasa-Lãptici sandstones as lateral facies variations
within the Upper Conglomerate Member as shown in Fig. 2. The
thicknesses of different units vary greatly (Patrulius, 1969; Patrulius
et al., 1971) with the total thickness in excess of 2000 m and overall
thinning to the south. The Lower Member is about 100m thick, the Middle
Member is up to 1500m thick, and the Upper Member is over 400m with
variable thicknesses of the conglomerate and sandstone intervals of the
Babele Sandstone (up to 200 m thick) and Scropoasa-Lãptici Sandstone
(up to 200 m thick).
In the Scropoasa-Lăptici Sandstone, a unit mainly constituted of
sandstones interbedded with cm thin claystones and marlstones,
45
deposited between the Upper and Middle members of the Bucegi
Formation (Fig. 2), contains calcareous nannofloral assemblages that
contain among other taxa the nannofossil Axopodorhabdus albianus,
for which the first occurrence (FO) is indicative of the base of the NC9
biozone of Roth (1983), Middle Albian in age (Bown, 2005).
3. Methodology
Outcrop data include measured sections, large scale (gigapans)
photo panels and paleocurrents at multiple locations (Fig. 3). During
the measuring of the sedimentological logs the dimensions of the 10
largest clasts in conglomerate beds were registered and plotted against
bed thickness. The comparison of the average maximum clast size to
bed thickness is one criterion to distinguish conglomeratic debris flows
and in some cases subaerial from subaqueous deposits (Nemec and
Steel, 1984). Paleocurrents were measured on clast imbrication and on
cross-stratification. The paleocurrent measurements were combined
with previously published data (Mihăilescu, 1980). Large scale photo
panels were used to sketch the bedding of the conglomerate deposits
and quantify the amount of incision associated with particular beds in
the high cliffs.
N
Ialomita River
Leaota Mts.
Bucegi Massif
Prahova River
r
be
em
rM
pe
Up
er
mb
dle
Mid
Me
r
be
em
rM
we
Lo
6
km
1500 m
Measured section
Photographic panel
Fig. 3. Oblique view of the Bucegi Mountains, from SE to NW. Google terrain surface with geological map of Patrulius (1969) draped over relief indicating the measured sections.
Note that elevations are exaggerated 3 times. Note the thicker Lower and Middle members to the north. The Upper Member, with the Scropoasa-Lãptici and Babele sandstones from
the flat area (Bucegi Plateau) at the top of the Bucegi Mountains.
Source: Google Earth.
46
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
A
B
C
D
Fig. 4. Typical facies of the Upper Member. A — Alternation of structureless metre thick conglomerates with parallel laminated sandstone. Note that sandstone usually “cap”
the conglomerate beds. B — Metre thick lenticular sandstone with inversely graded conglomerates. C — Large (50 cm) clasts within structureless microconglomerate and sandstones.
D — Alternation of structureless conglomerates and sandstone beds. Note the erosion surface at the base of the top sandstone and the inclined accretion surfaces below, which suggest channels.
4. Results
4.1. Subaerial to shallow subaqueous deposits on a narrow shelf
The three members within the upper part of the Bucegi Formation
(Patrulius, 1969) interfinger with each other and form a succession,
some 400 m thick (Fig. 2). The dominant lithology in the Upper
Member is sandstone with only occasional thin (dm) mudstone
beds. Conglomerate beds occur sporadically in the upward transition
from the underlying Middle Member and can be m to dm thick
(Figs. 4, 5). Conglomerate units (up to 27 m thick) are much more
common in the upper 200 m of the Upper Member, but are laterally
discontinuous (Fig. 6A, C, D). Metre thick conglomerate beds have
lenticular (extent of tens of metre) or sheet like geometry extending
for hundreds of metres to kilometres (Jipa et al., 2013). Some rare
large limestone blocks (over 1 m in diameter) are present within the
sandstone beds (Fig. 7C, E).
4.1.1. Olistoliths
Extremely large (tens of metres in diameter) limestone blocks
(olistoliths) (Fig. 6C inset) are also present toward the northern and
the western parts of the area (Patrulius, 1969; Jipa et al., 2013). A
significant observation regarding some of the olistoliths is that despite
their large size they occur within single conglomerate beds. The
relationship between the size of the olistoliths and the great thickness
of the host bed suggests that the olistoliths are simply ‘clasts’ within
the thick beds and that the maximum clast size is about the same as
bed thickness (Nemec and Steel, 1984). The olistolith cavities have
poorly sorted sediments of the associated debris flows. However,
some of the limestone olistoliths (albeit smaller, 2–3 m diameter)
were apparently being weathered in situ in the Upper Member at the
time of deposition because they have sandstone matrix infilling
fractures (Figs. 6A, 7B). This indicates that debris flow deposits are
reworked and the larger clasts (olistoliths) are encased in finer sorted
sandstone. Some of the limestone blocks can be seen fragmented and
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
Continuation to the right
20 m
39 m
35 m
15 m
3-4 cm
1.5 m megaclasts
30 m
10 m
25 m
5m
covered
90 cm megaclast
20 m
silt
clay
pebble
grain
0
vcc m f vf
sandstone
Fig. 5. Measured section in Upper Bucegi Conglomerate Member. Note that the section is
dominated by normal graded coarse sandstone beds and structureless conglomerates
towards the top. For the location of the section see Fig. 3. For the legend see Fig. 8.
47
the “background” sandstone fills the fractures which suggest the
alteration and fragmentation “in place” or close to the source.
Sandstone (Babele Sst.), mainly medium to coarse grained, is the
dominant lithology in this upper part of the succession (Figs. 7, 8).
The sandstone is typically structureless, parallel laminated or crossstratified (Fig. 8). At some locations, in the Scropoasa-Lăptici sandstone,
cm to dm thick fine sandstone beds alternate with mudstones to form
30 m thick successions (Figs. 6B, 7G, H). The Babele Sandstone forms
10–20 m thick coarsening-upward units, although there are also some
prominent bar forms with inclined sandy strata up to 10 m thick that
show an upward fining of grain size. These large bars are erosionally
based, appear to infill channels, and change vertically from very coarsegrained sandstones and fine-grained conglomerates up to ripplelaminated fine-grained sandstone (Fig. 8). Paleocurrent measurements
(based on clast imbrication, cross-strata and ripples) have a variable
(W to N to E) orientation (Figs. 8, 9).
The sand-rich deposits of the succession occurring at the upper part of
the Bucegi Formation, in contrast to the conglomerate-rich of the Middle
Member of the Bucegi Formation, are interpreted as alluvial fan, fluvial
delta front and shelf (subaqueous) deposits based on (1) the frequent
presence of channelized erosion surfaces, (2) the abundant presence of
traction-current sedimentary structures (cross-stratification, parallel
lamination and ripple lamination), (3) the poorly sorted character and
thickness of the conglomerate beds in which the size of the largest clasts
are about the same or a little less than bed thickness, (4) presence of rare
trace fossils, and (5) wide range of paleocurrent orientation.
The very thick conglomerate beds (up to 27 m thick) have a poorly
sorted texture and are convincingly single beds. Their bed thickness/
maximum clast size relationship (approximately 2:1 or 3:1) indicates
not only a debris flow (as opposed to water-lain deposits) but also
subaerial debris flow origin (Nemec and Steel, 1984). These and their
interbedded finer conglomerates and sandstones in the upper 200 m
of the upper Bucegi succession are likely to be the most proximal of all
Bucegi deposits, i.e., alluvial fan deposits. The deposits of the Babele
and Scropoasa-Lăptici sandstones are commonly planar laminated or
cross-stratified, sediment transported in traction and suspension. In
some intervals the cross-stratified deposits dominate (Fig. 8) and suggest
sustained flows which formed subaqueous dune fields driven by
unidirectional currents (Allen, 1982; Ashley, 1990). The paleocurrents
measured in previous studies (Mihăilescu, 1980) and our measurements
(Figs. 8, 9) show high variability, suggesting the activity of complex
marine currents in the Babele and Scropoasa-Lăptici sandstones and a
shelf environment. At some locations the paleocurrents are opposed
(Fig. 8A) which might suggest tidal currents, but typical “herringbone”
structures were not observed. The variability of the paleocurrents could
possibly also have been generated by sinuous channels in the proximal
subaerial reaches of this platform, or by longshore or tidal currents in
the marine reaches of this system. The paleoflow paths could have
become complicated by the presence of the large limestone blocks
some of which now sit within the fluvial and shallow marine deposits.
The interpretation of the olistoliths associated with the alluvial fan,
fluvial and shallow-marine deposits as syn-depositional is based on the
observation that sandstone (matrix) is filling voids or cracks of some of
the limestone blocks. The beds of the Babele Sandstone and the
underlying Scropoasa-Lãptici Sandstone (conglomerate or sandstone)
which are structureless (Figs. 7, 8) are interpreted as subaerial river
flood deposits which eventually extend into shallow water. The
alternation of silts and cm–dm thick sandstone beds in the ScropoasaLãptici Sandstone (Figs. 6B, 7G, H) represents shallow water deposits,
probably delta deposits. The structureless and lenticular conglomerate
beds become more common into the lower part of the Upper Member
of the Bucegi Formation (Figs. 4, 5).
The interfingering of the fluvial and marine facies of the Babele and
Laptioci-Scropoasa sandstones allow us to interpret the deposits as
basin-margin to shelf (non-marine to marine) deposits formed probably
during successive transgression–regression phases.
48
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
N
A
5m
N
B
25 m
N
C
10 m
MS 1, fig. 8
D
N
2m
Fig. 6. Photo panels with the Babele (A, C and D) and Scropoasa-Lãptici (D) sandstones. A — Metre thick and metre to tens of metre wide lenticular coarse sandstones. Occasionally there are
intercalations with large dm to m clasts (left inset). Cross-stratification is also locally present (right inset). B — Tens of metre thick siltstone and dm thick very fine sandstone dominated by
combined flow ripples and parallel laminations. C — Babele Sandstone with lenticular dm thick medium and coarse sandstone beds (see also the left inset). Beds are relative flat for tens of
metres. At the base of the photomosaic large (metre) clasts are abundant in a 2–3 m interval (middle inset). Stratigraphically above the photomosaic extreme large (10–20 m) limestone
blocks (olistoliths) are encased into flat bedded, lenticular sandstones (left inset). D — Dm thick lenticular beds of the Babele Sandstone.
4.2. Submarine sediment density flows on a deepwater slope
The Middle Member of the Bucegi Formation (Fig. 2) is the thickest
unit of the formation (1000–1500 m) and has overall tabular geometries
at the scale of hundreds of metres, but these conglomerates become
clearly more channelized in the uppermost few hundred metres of the
succession (Fig. 10). At the upper part of the Middle Member, the
deposits are built of multiple inverse graded dm to m-thick poorly sorted
conglomerate and coarse sandstones that interfinger with the facies of
the Babele Sandstone. Occasionally, extremely large (up to 1m diameter)
clasts occur in the deposits (Figs. 11, 12). The paleocurrent direction was
estimated based on rare clast imbrications towards E–SE.
The Middle Member deposits have a variable composition from large
clast (cobble) supported conglomerate to very coarse sandstone with
only rare pebbles. Most of the conglomeratic deposits are poorly sorted
with clast supported or matrix supported textures. An important
characteristic of this succession is that the conglomerates are very well
bedded. In addition, many of the conglomerate beds show a slight inverse
grading at their base and they become more matrix-rich (muddy and
silty) in the upper levels of the bed. In the lowermost levels of the Middle
Member, the conglomerates and coarse grained sandstones transitionally
overlie thin bedded (cm to dm thick) sandstone and mudstone beds
(Figs. 11, 12). The gradual contact observed in the southern area, where
the lower part of the Middle Member is exposed (Fig. 3) is supported
by the presence of increasing bed thickness and grain size from mud to
cobble conglomerates (Fig. 13). Soft sediment deformation caused by
sediment loading (Fig. 13E) suggests rapid sedimentation. The “largest
clast to bed thickness” ratio has a significant linear fit (Fig. 14) suggesting
deposits of debris flows (Nemec and Steel, 1984) for the Middle Member
of the Bucegi Formation. The ratio bed thickness to maximum clast size
varies, but in general has a value of about 5, suggesting that the debris
flows were subaqueous.
Interpretation of the Middle Member of the Bucegi Formation as
dominantly subaqueous sediment density flows (Talling et al., 2012) is
based on (1) the stacked and well-bedded character of the conglomeratic
succession, quite unlike the stratified conglomerates that would
originate from streamflood or streamflow processes, (2) the inverse to
normal graded character of beds, as well as the vertical increase in matrix
percent in individual beds, (3) the general lack of lamination within
beds, and (4) the broad correlation between bed thickness and
maximum clast size, but with bed thicknesses ~5 times larger than
clast size (i.e., contrasting significantly with the beds of the Upper
Member in this respect).
The Middle Member sediments were therefore likely transported as
subaqueous debris flows (Lowe, 1982; Nemec and Steel, 1984). However,
considering (1) the persistently great thickness (up to a few metres) of
conglomerate beds, and (2) the great stratigraphic thickness (up to
1500 m) of the Middle Member of the Bucegi Formation, the question
arises as to whether the conglomerates were confined into a large canyon
or rather dispersed onto a relatively “smooth” slope with “small” gullies.
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
A
49
B
Structureless sst.
Micro-conglomerate
20 cm
Clast imbrication
C
Conglomerate
20 cm
D
Paleoflow direction (SE)
texture change around the mega-clast
Conglomerate
parallel laminated sst.
Micro-conglomerate
E Structureless sst. with coarser sst. lenses
F
Lenticular structureless and
low angle laminated sst.
Cross-stratified sst.
Sst. with mega-clast
Dm to metre thick cross-stratified sst.
H
G
Combined flow ripples
Arenicolites?
small scale hummocky cross-stratification
Fig. 7. Facies photos of the Babele (A to F) and Scropoasa-Lãptici (G and H) sandstones of the Upper Member. A — Alternation of dm-thick, structureless to flat laminated medium grained
sandstone and conglomerate beds. Note the flat laminated coarse grained sandstone below the hand, and imbrication at the base of the conglomerate bed to the left. B — Large limestone
clasts into structureless micro-conglomerate. The dashed line shows the lenticular bodies within the deposits. C — Decimetre to metre thick beds of conglomerate and coarse grained
sandstone. The sandstone beds are flat to low angle laminated and some show normal grading. Textured changes around the large clasts have been observed. D — Cross-strata in very
coarse sandstone. Arrow is pointing to the paleo-flow direction. E — Metre thick sandstone beds with mega-clasts at the base, then cross-stratified and then lenticular and coarser grained
at the top. Dashed lines represent the approximate bed boundaries. F — Lenticular beds of very coarse sandstone and micro-conglomerates with structureless and low angle lamination
(at the top) and cross-stratified coarse sandstone at the base. G — Parallel laminated to structureless fine sandstone with rare trace fossils (Arenicolites). H — Centimetre to decimetre thick
fine sandstone beds with combined flow ripples (CFR) alternating with dm thick silt to very fine sandstone CFR and hummocky cross stratification (HCS). The photographs 7G and 7H are
located on Fig. 6C.
50
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
A
25 m
B
Continuation to the right
25 m
100 cm clast
?
40 m
20 m
20 m
35 m
80 cm clast
15 m
15 m
30 m
10 m
10 m
25 m
Legend
5m
5m
Structureless sandstone
Normal graded bed
(sst. or coarser)
Flat laminated sandstone
Unidirectional ripple sst.
Trough-cross strata
Soft-sediment deformation
Lower Cretaceous
MegaClast (2.5 m)
silt
clay
0
vc c m f vf
sandstone
pebble
grain
pebble
grain
silt
clay
0
vc c m f vf
sandstone
Outsized (megaclast)
Erosion basal surface
Fig. 8. Measured sections in the Babele Sandstone. A — Measured Section 1 (MS1 in Fig. 3)
dominated by cross bedded sandstone with variable paleo-flow orientations. B — Measured
Section 2 (MS2 in Fig. 3) has alternation of cross-strata with very coarse sandstone and
micro-conglomerates with large (dm thick) clasts. MS2 is stratigraphically below MS1.
Our facies and bed observations support gullied slope model, but the
overall thickness and the coarseness of the beds suggest the possible
presence of a sizable conduit/canyon for the conglomerates.
4.3. Bucegi clastic wedge formed as a shelf to slope succession
The depositional interpretation of the Upper Member of the Bucegi
Formation (including Babele and Scropoasa-Lãptici sandstones) as
Paleocurrents measured by this study (n=42)
Babele Sandstone paleocurrent (n=44)
Upper Member paleocurrent (n=5)
Upper Member including Babele Sandstone
Scropoasa-Laptici Sandstone
Middle Member
Fig. 9. Paleocurrents in the Upper Member of the Bucegi Formation and Babele Sandstone.
Note the widespread orientation of the paleocurrents. “MS 1 to 5” are the locations of
measured sections.
Modified after Mihăilescu (1980).
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
51
A
South
Figure 10 B
Figure 10C
North
40 m
B
10 m
C
5m
Fig. 10. Photographic panels with Middle Member. A — Overview of the eastern cliff of the Bucegi Mountains with 1000 m relief. B — Detail of the cliff shown in (A) showing tabular m-thick
beds. Note that beds are relative steep deepening to the left (south). C — One to five metre-thick conglomerate tabular beds.
alluvial fan, fluvial and shallow marine deposits, and the Middle Member
as shelf edge to deep water slope deposits, suggests a basin margin with
a shelf to deepwater-slope geometry (Figs. 15–17), a common
morphology for all types of basin margins with over 200 m of fronting
water depth (Steel and Olsen, 2002; Helland-Hansen et al., 2012).
The boundaries between the depositional environments are always
likely to be transitional, irregular, and not necessarily following the
lithological boundaries of the previously defined lithostratigraphic
members (Fig. 2). However, the Bucegi basin margin is uncommon
given the persistently conglomeratic nature of the deposits on the
proposed slope, the thickness of the succession and the presence of
large carbonate blocks on the interpreted shelf (Figs. 2, 6, 7E).
Because of the tectonic setting and coarse grain size, the Bucegi
slope gradient was likely considerably steeper than the 1–4° of
most passive margins slope gradients (O'Grady et al., 2000; Olariu
and Steel, 2009). The change from dominantly sandstone to
dominant conglomerate lithology at the transition between Upper
and Middle members of the Bucegi Formation (Fig. 15) cannot be
explain by a single sloping gradient such as in the case of an alluvial
to fan delta (Fig. 16A). An alluvial fan to fan delta environment will
build a succession with an overall coarsening character (Figs. 16A, 17A;
see also Prior and Bornhold, 1990). The thick shallow water sandstone
deposits (of Babele and Scropoasa-Lãptici) with paleocurrents oriented
in multiple direction (even paleo-landward, see Fig. 16B) and which
overlie thick gravity flow deposits of Middle Member is best explain
by a shelf (albeit narrow) to slope morphology (Fig. 16B). This break
of gradient is also highlighted in the Bucegi Formation by the
presence of relatively coarse-grained conglomerates within the
most channelized (discontinuous conglomerate pods) morphologies
seen in this succession (Figs. 15, 16B). The deposits of the Babele and
Scropoasa-Lãptici sandstones show spectacular evidence of gravel
and sand sorting (Figs. 6A, 7E) so that well-sorted clast populations
(especially the oft-cited pebble-sized conglomerates). The contrast
between clast sorting and reworking (with m-sized blocks in crossstratified and flat laminated sandstones; Figs. 6A, 7E) on the outer
reaches of the interpreted shelf (Figs. 16, 17) compared with relative
poor sorting on the inner alluvial reaches (Jipa et al., 2013) is especially
notable. The well sorted clast populations from the shelf were then
available for transport across the shelf edge and they show up within
the slope deposits along with coarser, poorly sorted conglomerates
(flushed out directly from the alluvial reaches during large floods and
storms) (Figs. 11, 12). The production of well-sorted clast populations
thus provides good evidence of both a morphological shelf-slope
break and an active sorting ‘factory’ (currents, waves and tides) on the
shelf platform in front of the alluvial fans. The effects of this sorting
system is especially well seen because the ‘topsets’ of the Bucegi
wedge built up by repeated shelfal regressions and transgressions, this
itself enhancing the production of sorted gravels. On a larger scale, the
shelf sorting processes testify to the clinoformal morphology of the
Bucegi wedge rather than to a simple shallow-to-deepwater ramp
(Fig. 17A) as has been sometimes suggested (Panin et al., 1963;
Mihăilescu et al., 1967).
A series of large-scale (hundreds of m in height and km long)
accretion surfaces (clinoforms) was suggested by Jipa (1979, 1984),
based on the observation that different lithological units within the
Bucegi succession downlapped onto a basal surface. These are likely to
be slope clinoforms that prograded toward the present south–
southeast. The margin was probably relatively steep (N 100) and likely
prograded relatively slowly because the hundreds of metres water
depth inferred from accretion surfaces. The shelf component of this
morphology, added in the present study, had a width of less than 20 km
as judged also from the presence of the olistoliths that are mainly
confined to the inner and middle shelf reaches. The source of the clasts
are limestone (at times 10–20 m diameter) and metamorphic rocks (see
also Mihăilescu et al., 1967; Patrulius, 1969) currently cropping out
some 10–20 km to the west and north (Fig. 3).
4.4. Syn-tectonic Bucegi clastic wedge
The Bucegi Formation should be referred to as syntectonic deposits
based on the presence of (1) the interpreted steep conglomerate margin
of the basin, and (2) the extremely large limestone olistoliths that were
transported out from the uplands out onto the Bucegi shelf. The
conglomerates testify to relatively steep gradients over distance of
kilometres to tens of kilometres. In order to build a 2000 m-thick
conglomerate succession, the steep depositional gradients had to be
sustained throughout the evolution of the system by active uplift and
subsidence (tectonic activity). In addition to the implications of coarse
conglomerates transported over large distances, the presence of very
large (tens of metres) olistoliths in multiple levels of the Babele
52
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
A
B
Erosion surface
Normal graded
Structureless lenticular
m thick conglomerate
Erosion surface
Normal graded
Lenticular sst.
20 cm
20 cm
Clasts imbrication
E
D
C
normal graded sst.
inverse graded cong.
inverse graded cong.
Erosion surface
normal graded cong.
structureless cong.
normal graded sst.
50 cm
20 cm
G
F
Normal graded sst.
Conglomerate
with mega-clasts
Normal graded
very coarse sst.
20 cm
1m
Erosion surface
H
Normal graded
very coarse sst.
Normal graded
very coarse sst.
20 cm
Fig. 11. Facies photographs of the Middle Member of the Bucegi Formation. A — Structureless m-thick conglomerate beds with a few dm erosion topography. B — Dm thick conglomerate
beds overlain by normal graded sandstone beds. Note the clast imbrication at the base suggesting traction currents. C — Dm thick conglomerate beds with alternation of structureless of
normal and inverse graded beds. D — Dm to m thick conglomerate beds with relative tabular geometries. E — Dm to m thick very coarse sandstone beds. Note the upper bed has the lower
part conglomeratic and then some outsized clasts (40–50 cm diameter) and then an upper normal graded grain size trend. F — Dm to m thick very coarse sandstone beds alternating with
matrix supported poorly sorted conglomerates. G — Normal graded very coarse sandstone. H — Normal graded dm to m thick normal graded very coarse sandstone beds with erosional
base. Note some occasional large (dm) matrix supported clasts.
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
A
25 m
B
Continuation to the right
50 m
(128 m with cover)
25 m
53
Continuation to the right
50 m
Top of the
section
20 m
20 m
(37 m with cover)
45 m
45 m
65 m
15 m
15 m
40 m
(57 m with cover)
40 m
Top of the
section
10 m
10 m
35 m
60 m
35 m
(27 m with cover)
1-2 cm cm
Megaclast
(105 cm)
59 m
(140 m
with cover)
90 cm megaclast
covered
5m
5m
30 m
(47 m with cover)
55 m
55 m
30 m
25 m
50 m
(128 m
with cover)
vc c m f vf
sandstone
silt
clay
silt
clay
pebble
grain
0
pebble
grain
0
vc c m f vf
sandstone
25 m
50 m
Fig. 12. Measured sections in Middle Member of the Bucegi Formation. The section to the left is lower stratigraphically than the section to the right. Note that both sections are dominated
by m-thick conglomerates and coarse sandstone beds with no internal structure or graded (normal or inverse). For location see Fig. 3 and legend see Fig. 8.
Sandstone (Patrulius, 1969; Jipa et al., 2013), now encased within fluvial
and shallow water sandstones, suggest periodic tectonic pulses. Thrust
faults mapped to the N and W of the study area (Patrulius et al., 1971;
Jipa et al., 2013) were active during the Albian and these (Fig. 17)
would have dislodged limestone blocks into the rivers and onto the
shelf. An alternative mechanism could have been an occasional, unusual
54
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
A
Structureless and graded conglomerates
Fig. 14B
Sharp based structureless and parallel laminated sst.
20 cm
Laminated mudstone with thin lenticular vf. sst.
Laminated mudstone
C
B
Parallel laminated sst.
Normal graded
Conglomerate
Structureless conglomerate beds
Inverse graded?
20 cm
D
Dm thick conglomerate
and micro-conglomerate
Fig. 14B
Dm thick sst. beds
(see14E for details)
E
Soft sediment deformation (loading)
Structureless sst.
Parallel laminated sst.
20 cm
Fig. 13. Transition at the base of the Middle Member of the Bucegi Formation. A — Laminated mud with cm–dm thick structureless and parallel laminated fine sandstone beds. Toward the
top (right of the figure) there are thicker sandstone beds and conglomerate beds. B — inverse to normal graded bed with the lower part conglomeratic and the upper part parallel laminated
medium sandstone. C — Alternation of dm thick conglomerate and sandstone beds above the mudstone deposits of Fig. 13A. Note again the location of Fig. 13B. D — Dm thick medium
coarse sandstone beds below bedded structureless conglomerates and pebble-size conglomerate beds (Middle Member). E — Structureless and parallel laminated dm(s) thick sandstone
beds. Note the soft sediment deformation interval in the upper half of the section.
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
55
80
70
Clast size (cm)
60
Babele Sst.
(~200 m)
R² = 0.5112
50
Jipa et al.,
2013
R² = 0.5918
40
Upper
Member
(>400 m)
R² = 0.557
30
Shelf
MS 1, Figure 8
20
MS 2, Figure 8
10
0
0
0.5
1
1.5
2
2.5
Scropoasa
-Laptici Sst.
(~200 m)
3
Bed thickness (m)
Beds of measured section 3
section 4
section 5
Fig. 14. Large clast size vs. bed thickness for the conglomerate beds. The plot show the
beds from the Sections 3 (Upper Member), 4 and 5 (Middle Member).
5. Discussion
~2000 m
forward progradation of block-bearing mega-debris flows (such as the
27 m thick bed now seen on the marginal alluvial fans; Jipa et al.,
2013), which were subsequently reworked by streams and marine
currents, leaving the olistoliths (Figs. 6A, C, 7C, E) as the only remaining
evidence of the original debris flow.
MS 3, Figure 4
Middle
Member
(~1500 m)
The interpretation of the Middle Member of the Bucegi Formation as
deepwater-slope, subaqueous debris flows and of the Upper Member
with the Babele and Lãptici-Scropoasa sandstones as alluvial fan, fluvial
and shallow marine deposits suggest a relatively deepwater basin with
a narrow shelf (feeder alluvial fan–fluvial system and a marine shoreline)
which allowed significant sorting of incoming debris flows, but also
generated bypassing debris flows directly into deeper water. The multiple
olistoliths, as well as the thick conglomerate succession, strongly suggest
a syntectonic setting.
5.1. Shelf-slope clinoform model versus alluvial or fan-delta ramp model
The presented shelf-slope model for the conglomerates of the Bucegi
Formation changes the previous interpretation of this unit (Panin et al.,
1963; Mihăilescu et al., 1967). The alluvial or fan-delta model (suggested
by previous interpretations) would have little or no change in gradient
along its profile, and as such would represent an immature ramp setting.
In such a model there should be no significant grain-size changes even at
the subaerial to subaqueous boundary, but rather an overall fining of
grain size down the ramp or the unique slope (Prior and Bornhold,
1990). The ramp model was supported by the observations of Panin
et al. (1963), who concluded that the morphometrics of the Middle
Member clasts are of fluvial origin. Panin et al. (1963) also described
the possibility of deposition in shallow water, but these deposits should
then have bypassed the “wave-fence”, the strong wave reworking
area, in a short time interval without being extensively reworked. The
alluvial/fan-delta ramp model is also supported by the apparent absence
of a large conduit (canyon) associated with the slope deposits. However,
the alluvial/fan-delta ramp model does not explain the overlying finer
deposits of the Babele and Scropoasa-Lăptici sandstones with thickness
of several hundred metres, with well-sorted lenticular conglomerate
bodies encased within finer sediments (Fig. 8).
The shelf to slope model for the Bucegi Formation (Figs. 16B, 17B, C) is
supported by the sedimentary architecture and interpretations of very
thick subaerial to subaqueous shallow-marine deposits (Figs. 6–8) of
the Upper Member (Babele and Scropoasa-Lăptici sandstones) which
overlie the Middle Member with hundreds of metre thick subaqueous
debris flow deposits (Figs. 10–12) on a deepwater slope. A change in
MS 4, Figure 12
Slope
MS 5, Figure 12
Figure 13
Lower
Member
(~100 m)
Basin
floor
Fig. 15. Summary vertical section of the Bucegi Formation with the overall depositional
settings (shelf, slope, basin floor). Note the section is not to scale, the Middle Member is
the thickest. The figures with more detail for each part of the system are also indicated.
the gradient at shelf-to-slope break as suggested by the model
(Figs. 16B, 17) can explain the very significant grain size change by
building the shelf platform (Fig. 16B) which represent the “sorting
factory” where the sediments are reworked by multi-directional basinal
paleocurrents (Fig. 9). The presence of large-scale channelized
conglomerate pods at the area of grain size changes also suggest the
presence of the shelf break zone.
The shelf to slope model now raises the question “where are the
bottom sets, or the deep water fan deposits associated with the shelfslope Bucegi margin?” The present study did not directly link the Bucegi
slope deposits with any “flysch” deposits, but the two possibilities to
explore are the underlying deposits of the Sandy–Shaley Flysch Formation
56
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
A
Paleocurrents
Upper Member
n=5
B
Paleocurrents
Babele Sst.
n=44
Limestone
Conglomerates
Olistoliths
Sandstone
Shelf deposits (no lithology intended) Approximate trajectories
of shoreline and shelf edge
Alluvial fans (no lithology intended)
Upper slope (no lithology intended)
Figure 15
Fig. 16. Alternative models for basin margin deposits of Bucegi Formation. A — Fan delta model. Note the absence of the shallow water shelf area and the overall grain size fining trend
down the system. B — Shelf to slope model. Uplifted area dominated by limestone provided source material for alluvial fans (5–10 km long) which transported sediments through debris
flows (note paleocurrent orientation) on a narrow (5–10 km) shelf. The debris flow deposits were reworked on the shelf (note the wide orientation of the paleocurrents, see also Fig. 9).
Probably initially there was no shelf and the alluvial fans were feeding straight into deep water forming fan deltas and later the shelf widened (note the diverging trajectories of the
shoreline and shelf edge).
or the Vârful Rădăcinii Formation (Melinte and Jipa, 2007) or the similar
age (Albian) flysch deposits underlying the other tectonic unit, such as
the Bobu Nappe (Contescu, 1974; Săndulescu, 1984), belonging to the
same structure of the Eastern Carpathians, i.e. the Outer Dacidenappe
system. On the geological map of Romania (Murgeanu et al., 1968), the
Bucegi Formation is of similar age (correlatable) with the lower and
middle part of the Curbicortical Flysch (also called the Teleajen
Formation) that is Albian pro parte in age. Eastwards, a clayey–sandy
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
57
A
B
~20 km
<1 km
faults
ov
er
>500 m
all
gr
ain
siz
e
tre
nd
C
10-20 km
5-10 km
faults
ov
>500 m
er
all
gr
ain
siz
e
tre
nd
Fig. 17. Model for the Albian Bucegi basin margin. A — Fan depositional model or gravel ramp. B — Lowstand stage of the basin margin with coarse rivers feeding the break. C — Highstand
stage basin margin with the fine (sandy sediments stored on the shelf).
Panel A is modified from Reading and Richards (1994).
flysch, with limestone olistoliths, spanning the Albian–Turonian interval
was deposited. The Curbicortical Flysch (the Teleajen Formation) and
the Clayey–Sandy Flysch correlates with the upper part of the black
shale units that was deposited in the Outer Moldavides, i.e., Audia, Tarcău
and Vrancea nappes, in the outermost (or eastern) part of the Eastern
Carpathians. The black shale formations span the Valanginian–Late Albian
interval; at the end of their deposition thick sandstones interbedded with
thin, cm thick grey and black clays were deposited (Roban and MelinteDobrinescu, 2012).
5.2. Tectonic setting of the Bucegi clastic wedge
The interpretation of the Bucegi Formation as a syntectonic unit will
have some consequences for the interpretation of similar conglomerate
deposits of the Eastern Carpathians, i.e. Ciucaş–Zăganu, Postăvaru,
Piatra Mare, Baraolt and Perşani mountains, which have been
interpreted previously as “molasse” post-tectonic deposits (Murgeanu
et al., 1963; Săndulescu, 1984). However, our study concurs with Jipa
(1984), who suggested that the conglomerates of the Bucegi Formation
58
C. Olariu et al. / Sedimentary Geology 299 (2014) 42–59
are not molasse deposits, but tectonically active basin-margin deposits,
possibly coeval to deepwater Albian “flysch” deposits. Coarse
(conglomerate) deposits related to steep faults were previously
described from tectonically active settings, but mostly interpreted as
alluvial fan deltas (e.g., Collela and Prior, 1990; Frostick and Steel,
1993). Active tectonic settings have been described in the Eocene Tyee
Basin, Oregon (Santra et al., 2013), where the basin margin clinoform
model now challenges the initial “ramp” margin setting.
6. Conclusions
The Bucegi Formation was formed across a deep-water basin in
a syntectonic regime. The deposits were delivered from erosion of
the carbonate and metamorphic deposits to the west and north via
rivers and subaerial mega-debris flows, gradually building a narrow
(10–20km) shelf as a ‘bridge’ between the hinterland and the deepwater
slope fronting the basin. Large (up to tens of metres diameter) olistoliths,
along with alluvial fan deposits and huge subaerial debris flows were
emplaced onto the inner shelf (northern part of the study area). Some
of these were preserved as part of the shelf stratigraphy, but most
were systematically reworked by river currents, tides and waves on the
shelf, producing a generally well sorted shelf surface. Some of the
strongest debris flows bypassed the growing shelf and were emplaced
onto the deepwater slope beyond the shelf break. These periodic events
can now be seen as large (hundreds of metre wide, few tens of metre
thick) conglomeratic channels or gullies along the line of the outer
shelf and upper slope. The lenticular conglomerate bodies at the shelf
edge were likely during unusually high discharge pulses which could
have been tectonically or climatically driven.
The various reworked and shelf-sorted deposits were entrained and
mixed with high-discharge flood events to deliver subaqueous debris
flows over the shelf edge. The slope was dominated by thick (up to a
few metres) debris flow deposits but in intermediate periods of lower
discharge the well-sorted shelf gravels were also periodically swept
onto the slope.
This new shelf to slope conglomerate model of the Bucegi Formation
strongly suggests the presence of age equivalent (Albian) deep water
flysch deposits fed by the Bucegi slope events. Other conglomerate
units such as the Ceahlău Nappe (central and northern Eastern
Carpathians), that were previously interpreted as molasse or as slope
“ramps” deposits should be re-evaluated. Clastic “ramps” may be initially
built out from immature basin-margin systems, but through time these
inevitably build a bridging low-gradient shelf and a clinoform margin.
Acknowledgements
This work was supported by a grant of the Romanian National
Authority for Scientific Research, CNCS — UEFISCDI, project number PNII-PCE-2011-3-0162. We will also like to acknowledge to Relu-Dumitru
Roban, Rattanaporn Fong-Ngern, and Constantin (Costin) Ungureanu for
valuable help during the fieldwork in the Bucegi Mountains. We are
indebted to Sedimentary Geology editor Jasper Knight, Alfred Uchman
and an anonymous reviewer for their useful comments and suggestions.
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