GeolCarp_Vol69_No2_129

GEOLOGICA CARPATHICA

, APRIL 2018, 69, 2, 129–148

doi: 10.1515/geoca-2018-0008

www.geologicacarpathica.com

The Early to Middle Triassic continental–marine transition

of NW Bulgaria: sedimentology, palynology

and sequence stratigraphy

GEORGE AJDANLIJSKY

, ANNETTE E. GÖTZ



and ANDRÉ STRASSER

University of Mining and Geology “St. Ivan Rilski”, Department of Geology and Geoinformatics, Sofia 1700, Bulgaria; g.ajdanlijsky@mgu.bg

University of Portsmouth, School of Earth and Environmental Sciences, Portsmouth PO1 3QL, United Kingdom;



annette.goetz@port.ac.uk

University of Fribourg, Department of Geosciences, Geology-Palaeontology, 1700 Fribourg, Switzerland; andreas.strasser@unifr.ch

(Manuscript received September 11, 2017; accepted in revised form February 13, 2018)

Abstract: Sedimentary facies and cycles of the Triassic continental–marine transition of NW Bulgaria are documented

in detail from reference sections along the Iskar river gorge between the villages of Tserovo and Opletnya.

The depositional environments evolved from anastomosing and meandering river systems in the Petrohan Terrigenous

Group to mixed fluvial and tidal settings in the Svidol Formation, and to peritidal and shallow-marine conditions

in the Opletnya Member of the Mogila Formation. For the first time, the palynostratigraphic data presented here allow for

dating the transitional interval and for the precise identification of a major sequence boundary between the Petrohan

Terrigenous Group and the Svidol Formation (Iskar Carbonate Group). This boundary most probably corresponds to

the major sequence boundary Ol4 occurring in the upper Olenekian of the Tethyan realm and thus enables interregional

correlation. The identification of regionally traceable sequence boundaries based on biostratigraphic age control is a first

step towards a more accurate stratigraphic correlation and palaeogeographic interpretation of the Early to early Middle

Triassic in NW Bulgaria.

Keywords: Lithofacies, sedimentary cycles, palynology, continental–marine transition, sequence stratigraphy, Triassic,

NW Bulgaria.

Introduction

Among the prominent features of the Triassic continental–

marine transition in NW Bulgaria is the pronounced cyclic

character of its sedimentation, recorded at different hierar-

chical scales. Although this stratigraphic interval has been

the focus of many previous lithological and lithofacies studies

(Tronkov 1983; Mader & Čatalov 1992; Ajdanlijsky 2002,

2010a, b; El-Ghali et al. 2006, 2009; Stefanov & Chatalov

2015; Chatalov et al. 2015; Chatalov 2018), the lack of bio-

stratigraphic dating hampers the genetic interpretation of

the deposits and their time range.

The good exposure and only minor tectonic disturbance of

the Lower–Middle Triassic succession along the central and

northern parts of the Iskar river gorge provide excellent condi-

tions for detailed lithological and stratigraphical studies.

The study area includes the Iskar river valley between the

villages of Tserovo and Opletnya (Fig. 1) where the complete

Triassic succession is well exposed and where some of the

reference sections of the Lower and Middle Triassic series

in Western Bulgaria are located. The current study covers

the transitional interval from the upper parts of the entirely

continental facies to the lowermost shallow-marine parts of

the succession. The here presented sections were selected on

the basis of stratigraphic significance, number of lithological

and lithofacies studies previously done, and last but not least

the accessibility allowing for the establishment of regional

reference sections.

The new palynological data, combined with the well-recog-

nizable lithological levels and surfaces and the characteriza-

tion of sedimentary cyclicity of different orders are a good

basis for developing an improved, high-resolution strati-

graphic scheme and better regional correlations of this strati-

graphic interval.

Geological setting

The study area belongs to the central-eastern part of the

alpine Western Balkan Tectonic zone (Western Balkanides) of

Ivanov (1998). The Triassic succession forms the base of its

Mesozoic cover and lies over pre-Mesozoic basement, inclu-

ding high-grade metamorphosed lower Palaeozoic sedimentary

and igneous rocks and upper Palaeozoic sedimentary, igneous

and volcanic rocks (Fig. 1). Here, the Triassic succession is

referred to as “Balkanide type” (Ganev 1974; Zagorchev &

Budurov 2009) and is subdivided into three parts. The lower

part is dominated by continental terrigenous red beds repre-

senting mainly fluvial and rare alluvial deposits that litho-

stratigraphically are referred to as Petrohan Terrigenous Group

(Tronkov 1981). The middle part consists of carbonate and

mixed siliciclastic-carbonate rocks of the Iskar Carbonate

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fluvial systems (Ajdanlijsky 2001a, 2005, 2009; Ajdanlijsky

et al. 2004). Because of the very irregular terrain on which

the Petrohan Terrigenous Group rests in some parts of the

Iskar river gorge (Tronkov 1960, 1963; Rashkov 1962; Tronkov

et al. 1965; Ajdanlijsky 2010a) its thickness varies from 32 to

over 145 m, but most of the sections measured reveal a range

from 90 to 110 m (Ajdanlijsky 2005).

Tronkov (1995) proposed a lithostratigraphic subdivision of

the Petrohan Terrigenous Group distinguishing three units:

a lower conglomerate, a middle sandstone, and an upper sand-

stone–siltstone unit. Along with Tronkov’s subdivision of the

continental Lower Triassic red beds, a complex stratigraphic

subdivision of sequences was proposed by Mader & Čatalov

(1992). They established in the section named by them the

“Buntsandstein in Bulgaria” four informal units that formed

during two tectonic and palaeoenvironmental megacycles.

Later, based on well-developed, regional bounding surfaces

with erosional amplitudes of 30–35 m, lithofacies architecture,

changes in regional pattern of fluvial palaeotransport and

the degree of development and the position of the palaeosol

levels, Ajdanlijsky (2005, 2010a) subdivided the clastic suc-

cession of the Petrohan Terrigenous Group into three main

units (MC-0, MC-1, MC-2; Fig. 2). Within these units, repre-

senting medium-scale sedimentary cycles, Ajdanlijsky (2005)

recognized small-scale cyclic patterns, thus introducing

a hierarchic nomenclature of cyclicity: elementary fluvial

cycles (EFC) representing the basic building blocks of meso-

cycles (MC), correspond to the third-order cycles (sequences)

of Miall (1997, 2010) and Wright & Marriott (1993). Based on

similarity in lithofacies of the elementary fluvial cycles, the

second and third mesocycle of the Petrohan Terrigenous

Group are subdivided into sub-mesocycles, reflecting the

stages of development of the alluvial system from the ero-

sional base-level change to the re-establishment of the river

equilibrium profile. Later, El-Ghali et al. (2009) interpreted

these mesocycles as sequence units and the sub-mesocycles as

sequence systems tracts.

The lowermost part of the Iskar Carbonate Group is repre-

sented by a transitional continental to marine, mixed silici-

clastic–carbonate, tide-dominated succession referred to as

Svidol Formation (Čatalov 1974) that is overlain by the shal-

low-marine Mogila Formation (Assereto & Čatalov 1983;

Assereto et al. 1983). The unit is comprised of sandstones,

silt- and mudstones, dolomitic to clayey limestones and dolo-

mites. According to Čatalov (1975) its origin is connected

with sedimentation in a low-relief coastal sandy to silty plain,

a supratidal evaporite clayey-carbonate setting and an inter-

tidal to shallow subtidal carbonate flat. Its thickness ranges

from 27 m in the southern part to 46 m in the northern part of

the study area. Based on bivalve, gastropod and ammonoid

findings, Tronkov (1968, 1976, 1995) placed the Svidol

Formation in the Spathian.

The Opletnya Member of the Mogila Formation, showing

thickness ranges from 134 m to over 180 m, is dominated by

micritic and clayey limestones and dolostones that form

a well-pronounced cyclicity described by Tronkov (1983) as

uniform hemirhythms, bounded by transgressive surfaces.

Another feature of the lower part of the Opletnya Member is

the occurrence of hardgrounds in parts of the Western

Balkanides. In the Iskar river gorge section and the Vratza

Mountains, the same author distinguished four beds that can

be traced over a long distance: the Tenuis, Zhitolub, Sfrazen

and Sedmochislenitzi beds (Fig. 2). Additionally, Asseretto &

Čatalov (1983) defined the Prebointitza Bed. Later, Tronkov

(1993) recognized this bed as part of the Sedmochislenitzi

Bed. Ajdanlijsky et al. (2004) subdivided the Opletnya Member

into five medium-scale sedimentary cycles (I–V; Fig. 2).

These medium-scale cycles are subdivided into small-scale

cycles (1–14; Fig. 2), which in turn are subdivided into elemen-

tary cycles (or parasequences) bounded by transgressive

surfaces. The palaeogeography of the study area during the

deposition of the Opletnya Member is interpreted as part of

a carbonate platform (Chatalov 1998, 2000a) or ramp (Čatalov

1988; Chatalov 2002, 2007, 2011) that Tronkov (1993) named

as Opletnya Carbonate Ramp and Chatalov (2013) defined as

homoclinal ramp.

Materials and methods

The sections studied, situated along the Iskar river gorge

between the villages Tserovo and Opletnya (Fig. 1; Opletnya:

N 43°06’01” E 23°25’42”, Sfrazen: N 43°05’39” E 23°25’32”,

Tserovo: N 43°00’19” E 23°21’26”), are among the most

representative for the Triassic continental–marine transition of

NW Bulgaria and provide continuous vertical and satisfactory

lateral exposure.

The lithological characteristics of the uppermost part of the

Petrohan Terrigenous Group are documented in nine detailed

logged sections. The section description is based on lithofacies

logging following the scheme of Miall (1977, 1978, 2006),

adapted and modified to the features of the study area

(Ajdanlijsky 2012, 2013a, b) and developed for the needs of

carbonate and mixed clastic-carbonate systems. A total of 216

samples and 141 thin-sections has been analyzed for this

Group. The fluvial style is interpreted on the base of archi-

tectural-element analysis (Ajdanlijsky 2014, 2015a, b) and

measurement of the sedimentary palaeotransport indicators

(Ajdanlijsky 2009).

Lithofacies documentation of the Svidol Formation is based

on seven sections (Fig. 1), all of them sampled and studied in

detail (142 samples and 92 thin-sections). Lithology and facies

studies of the Opletnya Member of the Mogila Formation

were performed in three complete sections near Tserovo and

Oletnya villages and north of the Lakatnik railway station

(406 samples and 138 thin-sections).

The sequence-stratigraphic nomenclature follows that of

Catuneanu et al. (2011). The high-resolution sequence-

stratigraphic analysis is based on the concepts of Strasser

et al. (1999). Parasequences are defined as being limited by

flooding surfaces (van Wagoner et al. 1990). However, the

sequence- stratigraphic nomenclature discussed by Schlager

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(2004, 2010) introducing a scale-invariant sequence model

might overcome the challenge of future basin-wide correlation

of sequences and systems tracts, once major third-order

flooding surfaces and sequence boundaries have been

defined.

Palynological samples from siltstones and limestones of

the Opletnya and Sfrazen sections span the upper fluvial

interval of the Petrohan Terrigenous Group, the Svidol

Formation (transitional interval) and the lowermost part

of the shallow- marine Opletnya Member of the Mogila

Formation (Fig. 2). The 8 samples were prepared using

standard palynological processing techniques, including

HCl (33 %) and HF (73 %) treatment for dissolution of carbo-

nates and silicates, and saturated ZnCl

solution (D ≈ 2.2 g/ml)

for density separation. Residues were sieved at 15 μm mesh

size. Slides have been mounted in Eukitt, a commercial,

resin-based mounting medium. Sedimentary organic matter

was studied under a Leica DM2000 transmitted light

microscope.

Results

Sedimentology and facies

The uppermost 50–55 m of the Petrohan Terrigenous Group

are composed of fluvial channel and near-channel sandstone

bodies and overbank fines. The sandstones are medium- to

fine-, rare coarse-grained, poorly to moderately sorted quartz

arenites, sublitharenites and subarkoses. Detrital mono-

crystalline quartz grains are dominant (average over 50 %),

with polycrystalline quartz being present (average 6 %).

Detrital feldspar occurs in small amounts, mainly presented by

potassium feldspar. The lithic fragments are mainly volcanic

(ave rage 2.5 %), and plutonic fragments are present as single

grains. Mica is mainly muscovite with biotite being present.

Mud intraclasts are very rare.

In the lower 25–35 m of this part of the Petrohan Terrigenous

Group sandstones show trough, planar and low-angle

cross-bedding and ripple cross-lamination (Fig. 4) forming

Fig. 3. Cyclic architecture of deposits of the Triassic continental–marine transition exposed in outcrop sections along the Iskar river: a — lower

part of the Svidol Formation near Sfrazen hamlet (the bush to the left of the picture is about 1.8 m high); b — Petrohan Terrigenous Group east

of Tserovo village (the cliff is about 87 m high); c – lower part of the Mogila Formation (Opletnya Member) near Zitolub spring, north-west of

the Lakatnik railway station (the lowermost cycle shown is 4.4 m thick). Lines and arrows mark the base of elementary fluvial cycles (b) and

sequence boundaries of elementary sequences (c), respectively. Abbreviations used: SB — sequence boundary; PTG — Petrohan Terrigenous

Group; SvF — Svidol Formation.

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In the uppermost 15 to 25 m of the Petrohan Terrigenous

Group the thickness of the elementary fluvial cycles decreases

more than twice ranging from 3.2 to 5.4 m (Fig. 4). The lateral

distribution of the overbank fines is much more restricted as

well as their portion within a cycle. Multiple erosional surfaces

are typical for the channel part of the cycles. Often the lag

deposits contain much more reworked calcrete nodules than

muddy intraclasts (Fig. 5f). Flaser and lenticular bedding as

well as climbing ripple cross-lamination are common in

the upper parts of the elementary fluvial cycles (Fig. 4).

Synsedimentary deformations occur some meters to tens of

meters laterally apart of similar erosional surfaces (Fig. 5d).

Based on the facies and the sedimentary structures described

above, the lower part of the Petrohan Terrigenous Group is

inter preted as belonging to an anastomosing fluvial system,

while the upper part characterizes a meandering system

(Fig. 4).

The Svidol Formation shows a large variety of siliciclastic

terrigenous, siliciclastic-carbonate and carbonate rocks. Its

basal part is represented by an 8 to over 10 m thick interval

(unit A of Tronkov & Ajdanlijsky 1998b) of alternating tidal-

and fluvial-influenced deposits showing a distinct cyclic

pattern (Figs. 6, 7a, b). The base of the small-scale sedimen-

tary cycles is characterized by sharp-based sandstone beds

with tidal ripples that periodically overlie claystones or silt-

stones (Fig. 8). In places, this surface is developed as a shal-

low scour (Fig. 6). Mainly vertical bioturbation and small-scale

synsedimentary deformation features such as sliding and con-

volute lamination are observed in the lower parts of the cycles

(Figs. 6, 7c, 8). Upsection, sandstones become thinner and

finer, claystones predominate, and thin beds of marly dolo-

stones appear (Fig. 7e), often with evidence of prolonged

subaerial exposure resulting in intrabasinal mud- and doloclast

redeposition (Figs. 6, 7f). An increase of sand content together

with carbonate pedogenic features such as powder spots and

small nodular calcretes and cluster-like aggregates of calcite

or dolomite composition as well as desiccation cracks are

observed (Figs. 6, 8).

Upsection, the small-scale cycles show a reduction in thick-

ness. The sandy siliciclastic lithofacies are still prevailing, but

the carbonate content of the rocks is increasing. This 9 to 12 m

thick interval (unit B of Tronkov & Ajdanlijsky 1998b) is

characterized by bi-directional small-scale cross-lamination

(ripple marks), the gradual disappearance of carbonate palaeo-

pedogenic features (here represented only by powder spots),

the shift of bioturbated intervals to the middle part of the cycles

as well as occurrence of mica in the sediments (Figs. 6, 8).

Wavy, flaser, lenticular and nodular bedding are common.

The red colors are gradually replaced by ochre and whitish-

beige ones. Cycles form coarsening-upward successions, built

up predominantly of marls, dolomarls and argillaceous dolo-

mites. Limestones with single and poorly preserved fragments

of brachiopods and bivalves are also present. The trend of

cycle thickness reduction is maintained.

The following interval (unit C of Tronkov & Ajdanlijsky

1998 b) is characterized by intertidal grey limy sandstones,

beige-grey marls, and limy siltstones with intercalated micritic

limestones (Fig. 8) rich in brachiopods and bivalves. Evidence

of erosion is rarely observed and mainly associated with

small-scale scour-and-fill structures.

The uppermost part of the Svidol Formation (unit D of

Tronkov & Ajdanlijsky 1998b) consists of cycles built up by

carbonates and marls with supratidal dolomites and dolomarls

prevailing upsection (Fig. 8). Fragments of marine bivalves

and crinoids are common. The top of this unit is marked by

an erosional and transgressive surface. The depositional envi-

ronment of the Svidol Formation thus reflects alternating flu-

vial and tidal influences (Fig. 6).

The lowermost part of the Mogila Formation (Opletnya

Member) is dominated by carbonates displaying a great facies

variety. Detrital quartz and clay still occur but are limited to

discrete and thin levels. Grainstones and rudstones containing

ooids and bioclasts (Figs. 9, 10b) are most prominent, with

well-developed cross-bedding (Fig. 10d) and absence of

micritic matrix. Grain- and packstones can also contain a high

amount of peloids and mudstone lithoclasts. They are com-

monly strongly bioturbated and may show overpacking.

Wackestones are often bioturbated and contain benthic fora-

minifera and ostracods. Mudstones, often laminated and com-

prising pyrite, some of them bioturbated as well, are also

common in the lowermost part of the member. In some mud-

stone beds solitary fragments or thin lenses of gagate are

observed (Fig. 10g). Birdseyes occur occasionally both in

wacke stones and mudstones. Wackestones as well as mud-

stones may be dolomitized (Fig. 10e). Small-scale synsedi-

mentary deformation, some of it with sigmoidal texture, is

also observed in several levels (Figs. 9, 10f).

Dolomites commonly are associated with tepees and flat

pebbles that form local lags (Fig. 10b) and/or lenses with

chaotic orientation of the clasts. Some of them exhibit also

imbrication structures

or the clasts form short bands lying on

the foreset lamina in cross-stratified levels. On top of distinct

dolomite beds, hardgrounds are developed (Fig. 10c). The facies

and sedimentary structures of the Opletnya Member point to

a peritidal to shallow-marine environment (Fig. 9).

Palynology

Sedimentary organic matter is poorly preserved in the lower

part of the studied interval within the Petrohan Terrigenous

Group (samples OPL-II-004 and OPL-II-005 from MC-2/2,

and OPL-III-006 from MC-2/3; Fig. 4). Samples from the

uppermost Petrohan Terrigenous Group (sample OPL-III-007;

MC-2/3; Fig. 4), from the Svidol Formation (transitional

interval, samples SFR-IV-008, SFR-IV-009, and SFR-IV-010;

Fig. 6) and from the lowermost shallow-marine Mogila For-

ma tion (lower Opletnya Member, sample SFR-013 (TST);

Fig. 9) show well-preserved organic particles.

A palynomorph assemblage dominated by Densoisporites

nejburgii, Platysaccus leschikii and Voltziaceaesporites

hetero morphus places the lower part of the studied fluvial

Petrohan Terrigenous Group (samples OPL-II-004, OPL-II-005,

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OPL-III-006; MC-2/2-3) in the Early Triassic (Olenekian).

An early Anisian (Aegean) palynomorph assemblage is

iden tified from the uppermost fluvial Petrohan Terrigenous

Group (sample OPL-III-007; MC-2/3), the Svidol Formation

(tran

sitional interval, samples SFR-IV-008, SFR-IV-009,

and SFR-IV-010) and the lowermost shallow-marine Mogila

For

mation (lower Opletnya Member, sample SFR-013),

including Anisian index taxa such as Illinites chito noides,

Stella pollenites thiergartii, Tsugaepollenites oriens,

Cris tiani sporites triangulatus, and Triadispora crassa (Fig. 11).

There fore, the studied interval is stratigraphically placed at

the Early–Middle Triassic boundary. Marine acritarchs

(Micrhystridium spp.) were identified about 6 m above the

onset of shallow-marine limestones (sample SFR-013; Fig. 9)

in the basal part of the Opletnya Member of the Mogila

Formation.

Palynofacies is dominated by opaque phytoclasts of diffe-

rent size and shape (equidimensional and needle-shaped),

Fig. 6. Lithological column, depositional setting and stratigraphy of the lower part of the continental–marine transitional interval exposed west

of Sfrazen hamlet. Abbreviations used: TST — transgressive deposits; HST — highstand deposits; SB — third-order sequence boundary;

TS — transgressive surface; FS — flooding surface; PS — parasequence. A–B — units according to Tronkov and Ajdanlijsky (1998b).

Palynological samples: SFR-IV-008, SFR-IV-009, SFR-IV-010.

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show a high amount of small, equidimensional opaque phyto-

clasts, a higher amount of translucent particles, and a better

preservation of palynomorphs. The palynofacies of tidal-flat

deposits (sample SFR-IV-010; Fig. 6) is dominated by opaque

phytoclasts. The onset of the transgressive phase within the

basal Mogila Formation is documented by an acritarch peak

(sample SFR-013; Fig. 9).

So far, the studied succession was interpreted as Lower

Triassic deposits (cf. Tronkov 1981; Mader & Čatalov 1992).

The here presented new palynological data reveal an early

Middle Triassic age for the uppermost Petrohan Terrigenous

Group, and the Early–Middle Triassic boundary is placed

within the late highstand of the MC-2 sequence, about 1 m

below the base of the Svidol Formation (basal Iskar Carbonate

Group). The marine plankton peak in the basal Mogila For-

mation may represent a first transgressive pulse of the carbo-

nate ramp evolution during Anisian times.

Depositional sequences

In the study area, the Petrohan Terrigenous Group is com-

posed of three mesocycles (Ajdanlijsky et al. 2004; Ajdanlijsky

2005, 2010a) that correspond to third-order cycles (sensu

Miall 1997) or third-order sequences (sensu Miall 2010),

the base of which are marked by distinctive erosion surfaces

(Fig. 2). Only the uppermost mesocycle (MC-2) is completely

developed and comprises three parts. Its base marks a regional

sequence boundary incised a few to over 30 meters into

the underlying fluvial sequence.

The lowermost part of mesocycle MC-2 (sub-mesocycle

MC-2/1) is represented by an amalgamated braided fluvial

succession representing lowstand or early transgressive depo-

sits. They are stacked, multistory medium- to coarse-grained

sandstone channel fills with a very restricted portion of near-

channel or overbank deposits (Fig. 5a; Ajdanlijsky 2010a).

The channel-fill deposits are dominated by sandy bedforms

and show a high width/thickness ratio. The fluvial palaeo-

transport pattern is unidirectional (Ajdanlijsky 2005, 2009).

The palaeosol products are represented mainly as reworked

channel lag conglomerates.

Transgressive deposits consist of mud-rich, anastomosing

isolated fluvial channels in the middle part of the sequence

(sub-mesocycle MC-2/2), indicating base-level change. In this

part of the sequence elementary fluvial cycles (EFC) with

maximum thicknesses (over 11 m) are identified. They are

formed by channel, near-channel (levee and crevasse splay)

and overbank deposits (Fig. 4). The channel part, where down-

stream accretion sand bodies dominate over the lateral

accretion ones, forms only 25 to 30–35 % of the EFC and

the crevasse splay beds are separated by relatively thick over-

bank fines. A similar proportion leads to ribbon morphology of

the channel complex. Here, in overbank intervals, silty beds and

even pure claystones are present, suitable for palyno lo gical

sampling. In braided and meandering fluvial settings, silt- and

claystones are completely absent or relatively rare, documen-

ted as very thin isolated lenses. The palaeosol levels are in

an initial stage of development, mainly in crevasse splay beds

and in overbank fines, represented by powder nodules or spots.

The uppermost 15 to 25 m thick siliciclastic package of

the Petrohan Terrigenous Group (sub-mesocycle MC-2/3)

formed in a high-sinuosity (i.e. meandering) fluvial setting,

documenting highstand deposits. An abrupt reduction of the

overbank part of the EFC is observed, while maintaining

the thickness of the channel and levee deposits, which in turn

leads to a reduction of the EFC more than twice compared to

the underlying interval (Fig. 4). Multiple erosional surfaces

and palaeosols are typical for this part. Frequent channel

erosion, caused by restricted accommodation space, led to the

development of calcrete lags in channel and near-channel beds

(Fig. 5d, f; Ajdanlijsky 2000) or caused synsedimentary slide

and slump structures (Ajdanlijsky 2001b). The top of the

Petrohan Terrigenous Group is marked by the next third-order

sequence boundary (Fig. 6).

The Svidol Formation represents a third-order depositional

sequence. Its lower part exhibits transgressive deposits (TST)

with a prominent transgressive surface that closely follows

the sequence boundary at the top of the Petrohan Terrigenous

Group, documenting a rapid shift from terrigenous facies to

a tidally influenced depositional environment (Figs. 6, 7b, 8).

Lowstand deposits are absent or thin because accommodation

was lacking. The siliciclastic-dominated lower part of the for-

mation is formed during rising sea level that remobilized sands

and clays. Accommodation space was created but constantly

filled by sediment, thus maintaining a tidal-flat or tidally-

domi nated environment. High-frequency sea-level fluctua-

tions were superimposed on this general trend and created

a cyclic succession that is interpreted to be formed by elemen-

tary sequences (sensu Strasser et al. 1999) or parasequences

(sensu van Wagoner et al. 1990). In Figure 6, the parasequence

concept is applied, the limits of the parasequences (PS) corre-

sponding to marine flooding surfaces (FS). The lower part of

the TST records more coarsening-upward than fining-upward

parasequences, each passing from tidally to fluvially influen-

ced sedimentation. They are interpreted as corresponding to

prograding bodies in a deltaic sedimentary environment. As

a whole, however, the TST is built up of retrogradational para-

sequence sets and only in its uppermost part features of aggra-

dational stacking are present (Fig. 6).

Upsection, the marked change from siliciclastic to carbo-

nate deposits is interpreted as being related to maximum

flooding, when reduction of clastics in the water column may

have allowed the carbonate-producing organisms to proli-

ferate, and carbonate mud accumulated in a shallow-water

environment. The maximum-flooding interval (MF in PS 12;

Fig. 8) is marked by a finely laminated, dark-gray, silty marl

layer. The upper part of the Svidol Formation, featuring dolo-

mitic limestones, is interpreted as highstand deposits (HST),

although a shallowing-up trend is not visible. The sequence

boundary is not clearly defined but must be placed below

the transgressive surface of the following sequence that

initiates the deposition of the carbonate-dominated Mogila

Formation (Fig. 9).

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In the lowermost, carbonate-dominated part of the Opletnya

Member, the depositional sequences are interpreted following

the methodology of Strasser et al. (1999). Elementary sequen-

ces commonly start with high-energy facies representing tidally

influenced oolitic and/or bioclastic bars formed during trans-

gression, when the previously very shallow, intertidal or supra-

tidal environment was flooded (Fig. 10b). The corres ponding

transgressive surface is well developed and commonly erodes

into the underlying sediment (Fig. 9). The increased water

depth and diminished current energy led to the abandonment

of the high-energy bars. Low-energy wacke- and mudstones

then predominate. Intense bioturbation may indicate tempo-

rarily low sedimentation rates. A rapid shift from high-energy

to low-energy as well as reduced sedimentation rate are

interpreted to be related to maximum flooding on the scale of

an elementary sea-level cycle. Also the preservation of gagate

(Figs. 9, 10g) around maximum-flooding surfaces seems to

reflect a change in the hydrodynamic conditions. Highstand

deposits in the elementary cycle are mud-dominated, part of

them dolomitized, with evaporite pseudomorphs and/or

tepees. If siliciclastics occur, they are more abundant in

the late highstand. This suggests that they have been washed

into the system when relative sea level dropped, contrary to

the ones in the Svidol Formation that are associated to trans-

gression. The boundaries of the elementary sequences cannot

always be placed at a discrete bedding surface and rather

define thin sequence-boundary zones (Strasser et al. 1999).

Below the following transgressive surface, thin lowstand or

Fig. 9. Lithological column, depositional setting and stratigraphy of the lowermost part of the shallow-marine interval of the Opletnya Member

(Mogila Formation) exposed between Opletnya village and Sfrazen hamlet. Fossil macrofauna distribution according to Tronkov (1968).

Abbreviations used: Te — Tenius Bed; TST — transgressive deposits; HST — highstand deposits; SB — sequence boundary of elementary

sequence; FS — flooding surface; TS — transgressive surface; MFS — maximum-flooding surface of elementary sequence. Palynological

sample: SFR-013.

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, 2018, 69, 2, 129–148

early transgressive deposits may occur. In some cases, how-

ever, a prominent transgressive surface directly overlies the

sequence boundary, implying very low accommodation.

A striking feature of highstand deposits in elementary

sequences is the development of sigmoidal and other types of

small-scale synsedimentary deformation, as recorded in the

lower 30 m of the Opletnya Member. In previous studies, they

have been interpreted as product of periodic palaeoseismic

activity (Chatalov 2001a, b), related to the onset of oblique

rifting in the Palaeo-European shelf (Michalík 1997). However,

the present study reveals that most of them developed

during a stage of decreasing accommodation space when

the weakly lithified mainly wacke- and mudstones became

unstable because of shallow channel erosion and slumped over

a very short distance, forming thin lens- and/or wedge-like

disturbed bodies. Their repeating presence in the same level of

the elementary sequences (Figs. 9, 10f) indicates a cyclic

sedimentary rather than a palaeoseismic control.

Elementary sequences are composed of several beds, and

facies reflect different sub-environments in the shallow-sub-

tidal, intertidal, and supratidal realms. Autocyclic processes

such as shifting mudbanks or tidal channels are common in

Fig. 11. Palynomorphs of the Opletnya (OPL) and Sfrazen (SFR) sections: a — Stellapollenites thiergartii (Mädler 1964) Clement-Westerhof

et al. 1974 (sample SFR-IV-008); b — Tsugaepollenites oriens Klaus 1964 (sample OPL-III-007); c — Cristianisporites triangulatus Antonescu

1969 (sample SFR-IV-009); d — Triadispora crassa Klaus 1964 (sample OPL-II-005); e — Illinites chitonoides Klaus 1964 (sample

SFR-IV-010); f — Micrhystridium sp. (sample SFR-013).

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, 2018, 69, 2, 129–148

such environments (e.g., Pratt & James 1986; Strasser 1991).

Consequently, the unequivocal definition of an elementary

sequence that is related to a sea-level cycle (i.e. that is allo-

cyclic) is not always possible.

Elementary sequences stack into larger sequences, termed

small-scale sequences that again show characteristic facies

trends: many contain ooids preferentially in their lower part

and muddy facies in their upper part (Fig. 9). This is inter-

preted as being related to the long-term transgressive-

regressive sea-level evolution, over which the higher-frequency

elementary cycles were superimposed.

Discussion

To date, the lack of precise age control of the studied

interval, documenting the continental–marine transition within

the Lower to Middle Triassic of NW Bulgaria, hampers its

chronostratigraphic subdivision. Previously, the chronostrati-

graphic placement of the Petrohan Terrigenous Group, the

Svidol Formation and the Opletnya Member of the Mogila

Formation in the area of the Iskar river gorge was based on

regional geological criteria and data from adjacent areas

(Ganev et al. 1965, 1970; Assereto et al. 1983; Chatalov 1994,

1997a, 1999, 2000b, 2005a, b). By analogy with the German

Triassic deposits, Tronkov (1968, 1983) used the bivalve

Costatoria costata (Zenker), which in Germany indicates the

Costatoria costata–Beneckeia tenuis zone, as an index fossil

for the uppermost parts of the Lower Triassic series, and

the lowermost parts of the Anisian stage were defined by

the presence of Plagiostoma radiatum (Goldfuss). Accordingly,

he placed the Lower–Middle Triassic boundary about 12 m

below the boundary between the Opletnya Member and the

dolo mites of the upper member of the Mogila Formation

(Lakatnik Member) (Fig. 2). Some authors (Tronkov &

Ajdanlijsky 1998 a, b; Chatalov & Stanimirova 2001;

Ajdanlijsky et al. 2004) accept and use these biostratigraphic

index fossils, while others place the boundary in the middle

(Chatalov 2000a) or even lower parts of the Opletnya Member

(Chatalov 2005a, 2007, 2013, 2018), sometimes quite

arbitrarily and without giving biostratigraphic evidence.

On the other hand, independent local studies of different

researchers within the continental, transitional and marine

intervals of the Lower–Middle Triassic led to the introduction

of different terminologies in the literature, using cyclostrati-

graphic or sequence-stratigraphic terms.

Scale, composition, lateral extent and nature of the boun-

ding surfaces of the three mesocycles within the Petrohan

Terrigenous Group (Ajdanlijsky 2005, 2009) correspond with

the third-order sequences sensu Miall (2010). However,

the identification of systems tracts of third-order sequences

following the non-marine sequence model of Wright &

Marriot (1993), Shanley & McCabe (1994), and Gibling

& Bird (1994) is still hampered by the lack of studies on

a basin scale.

Based on the here presented palynostratigraphic data, the

boun dary between the uppermost mesocycle МС-2 of the

Petrohan Terrigenous Group and the base of the Svidol

Formation (Iskar Carbonate Group) is dated and correlated

with a major sequence boundary Ol4 in the upper Olenekian

(Hardenbol et al. 1998; Ogg 2012) of the Tethyan realm.

Using this boundary as a time line, the three mesocycles of

the Petrohan Terrigenous Group (MC-0, MC-1, MC-2;

Fig. 2) may be interpreted as medium-scale sequences within

Fig. 12. Palynofacies of fluvial deposits: a — Anastomosing river systems (sample OPL-II-005) are characterized by a high variety of phyto-

clast sizes and shapes (op — opaque particles, tr — translucent particles) as well as a high proportion of degraded organic matter (DOM);

b — meandering river systems (sample OPL-III-006) show a high percentage of small, equidimensional opaque phytoclasts (ope). Scale

(100 µm) applies to (a) and (b).

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the Olenekian Stage, bounded by the Ol3 and Ol2 sequence

boundaries. On the other hand, this boundary may also be

correlated with the S2/An1 and An1/An2 sequence boundary

in the late Olenekian of the Peri-Tethys realm and the southern

Alpine basins (Rüffer & Zühlke 1995; Szulc 2000; Feist-

Burkhardt et al. 2008).

The new palynological data obtained from the Triassic

continental–marine transitional interval allow redefinition of

the age range of both the Petrohan Terrigenous Group and

the Iskar Carbonate Group (Fig. 2), which is important not

only for the intrabasinal correlation but also for the application

of sequence- and cyclostratigraphy for precise stratigraphic

sub division and interregional correlation. Previously, the Petrohan

Terrigenous Group and the continental–marine transitional

succession of the basal Iskar Carbonate Group (Svidol For-

mation and the lowermost Mogila Formation) were placed

in the Early Triassic (Tronkov 1983; Chatalov 2018; Fig. 2).

The new biostratigraphic data indicate an Anisian age for the

uppermost Petrohan Terrigenous Group, the Svidol Formation

and the lowermost Mogila Formation.

Refinement of the early Anisian age range of the Triassic

continental–marine transition interval also serves to interpret

the palaeogeography of the area studied. Recently published

data on the benthic foraminifera association of Triassic sec-

tions along an east–west transect of the Western Balkanides

(Chatalov et al. 2016; Ivanova et al. 2016) provided an Aegean/

Bithynian age for the transitional interval. This confirms

a significant E–W surface leveling at the end of the Early

Triassic in NW Bulgaria as documented in sub-mesocycle

MC 2/3 (highstand deposits) of the Petrohan Terrigenous

Group (Ajdanlijsky 2005, 2010b), which led to a meandering

fluvial sedimentation style. Later, during the continental–

marine transition interval, the relatively flat palaeotopography

enabled preservation of minor fluctuations in sea level with

high- frequency transgressions and regressions, generating

para sequences and elementary sequences with significant

lateral extent. Such conditions continued during the initial

stage of accumulation of the sediments of the Opletnya

Member.

The end-Spathian (Early Triassic) surface leveling deter-

mined the development of the boundary between the Petrohan

Terrigenous Group and the Svidol Formation and the sedimen-

tary record of the early Anisian continental–marine transition.

The lower part of the Svidol Formation documents the initial

transgression with alternating tidal and fluvial packages,

limited terrigenous supply and gradual development of shal-

low-marine environments recorded in small-scale sequences

(Figs. 6, 8). This gradual development of shallow-marine

environments is also reflected in the palaeontological record.

In the lowermost part of the Mogila Formation, Tronkov (1983)

reports well-preserved single specimens of Beneckeia tenuis

(Seebach) in the Tenuis Bed (Fig. 2), situated about 9.5 m

above the boundary with the Svidol Formation. Later, the same

author (unpublished data) reports on the presence of Beneckeia

tenuis in various stratigraphic horizons in the Mogila For-

mation exposed along the Iskar River and to the north, in

the Vratza region, within the Svidol Formation. Assuming that

these horizons are synchronous, the difference in their position

relative to the transitional interval along a south-north transect

in the eastern part of the Western Balkanides could be inter-

preted as evidence of a northwards marine transgression.

However, additional biostratigraphic control, e.g. conodont

data, has to validate the use of the existing sparse information

for further interpretation.

The stratigraphic subdivision of the lower Iskar Carbonate

Group based on lithofacies as well as on the nature and scale

of the cyclicity has also been discussed in previous studies.

Čatalov (1974) subdivided the Svidol Formation into two

cycles: a lower symmetrical transgressive-regressive cycle,

and an upper one represented only by its transgressive part.

Later, Mader & Čatalov (1992) established the informal strati-

graphical interval “Terminal Mudstones” that corresponds to

the Svidol Formation (see also Chatalov 2006). Based on

characteristic parasequence sets (sensu van Wagoner et al.

1988, 1990), Tronkov & Ajdanlijsky (1998b) subdivided the

Svidol Formation into four sets (sets A–D; Fig. 2), forming the

transgressive systems tract (sets A–C) and highstand systems

tract (set D) of a third-order depositional sequence (sensu

Miall 2010). Later, El-Ghali et al. (2006) interpreted the trans-

gressive systems tract as generated in tide-dominated deltaic

settings and the highstand systems tract representing a tidal-

flat environment. In the lower part of the Opletnya Member,

Chatalov (1998, 2000a) identified peritidal hemicycles, as

well as several well correlatable oolitic levels along the Iskar

river gorge (Chatalov 2005a), some of them partially or com-

pletely matching with the beds introduced by Tronkov (1983).

The same author (Chatalov 2005b) described two specific

oolitic levels, one in the lowermost part of the Opletnya

Member and another one in the uppermost part of the Svidol

Formation, that he proposes as well correlatable in the study

area. However, because of the lack of precise biostratigraphic

dating of the section, the time range of the different orders of

cyclicity has not been defined yet. Undoubtedly, the identifi-

cation of regionally traceable cycle boundaries based on bio-

stratigraphic age control is a first step towards more accurate

basin-wide correlation and palaeogeographic interpretation of

the Triassic successions in NW Bulgaria. However, a high-

resolution palynostratigraphic zonation scheme is needed to

further refine the existing stratigraphic framework by integra-

ting sequence stratigraphy and cyclostratigraphy. Ongoing

research aims at providing high-resolution biostratigraphic

data to perform regional and interregional correlations.

Conclusions

The detailed analysis of facies and sedimentary structures of

Triassic sections in the Iskar river gorge in NW Bulgaria

reveals that the depositional environments evolved from

anastomosing and meandering river systems (Petrohan

Terrigenous Group) to alternating fluvial and tidal influences

(Svidol Formation) and then to peritidal and shallow-marine

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, 2018, 69, 2, 129–148

conditions (Opletnya Member of the Mogila Formation).

New palyno stratigraphic data from these Triassic continental–

marine transitional deposits allow for a precise stratigraphic

placement of a prominent sequence boundary between the

fluvially dominated continental red beds of the Petrohan Terri-

genous Group and the shallow-marine deposits of the Iskar

Carbonate Group. This boundary may correlate with the major

sequence boundary Ol4 occurring in the upper Olenekian of

the Tethyan realm and the S2/An1 sequence boundary of

the northern Peri-Tethys Basin, thus enabling interregional

correlation and refined palaeogeographic interpretation of

the Early to early Middle Triassic in NW Bulgaria. Furthermore,

the pronounced cyclic character of the transitional sedimen-

tary succession, recorded at different hierarchical scales, and

its biostratigraphic age control enable pinpointing the first

marine pulse during the early Anisian and reconstructing

the evolution of depositional environments across the Early/

Middle Triassic boundary.

Acknowledgements: We would like to express our thanks to

Dimitar Tronkov for his useful comments and remarks on

some regional geological and biostratigraphical aspects, and

to Ivan Petrov for his enthusiastic technical support during

the field campaign in 2016. The reviews of János Haas

(Budapest), Carmen Heunisch (Hanover), and Joachim Szulc

(Cracow) greatly improved the manuscript.

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