GeolCarp_Vol70_No4_325

GEOLOGICA CARPATHICA

, AUGUST 2019, 70, 4, 325–354

doi: 10.2478/geoca-2019-0019

www.geologicacarpathica.com

Integrated bio- and cyclostratigraphy of Middle Triassic

(Anisian) ramp deposits, NW Bulgaria

GEORGE AJDANLIJSKY

, ANDRÉ STRASSER

and ANNETTE E. GÖTZ



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

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

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



annette.goetz@port.ac.uk

(Manuscript received March 18, 2019; accepted in revised form June 27, 2019)

Abstract: A cyclostratigraphic interpretation of peritidal to shallow-marine ramp deposits of the early Middle Triassic

(Anisian) Opletnya Member exposed in outcrops along the Iskar River gorge, NW Bulgaria, is presented. Based on facies

trends and bounding surfaces, depositional sequences of several orders can be identified. New biostratigraphic data

provide a time frame of the studied succession with placement of the boundaries of the Anisian substages and show that

the Aegean (early Anisian) substage lasted about 1.6 Myr. In the corresponding interval in the two studied sections,

80 elementary sequences are counted. Five elementary sequences compose a small-scale sequence. The prominent cyclic

pattern of the Opletnya Member can thus be interpreted in terms of Milankovitch cyclicity: elementary sequences repre-

sent the precession (20-kyr) cycle and small-scale sequences the short eccentricity (100-kyr) cycle in the Milankovitch

frequency band. Medium-scale sequences are defined based on lithology but only in two cases can be attributed to

the long eccentricity cycle of 405 kyr. The transgressive-regressive facies trends within the sequences of all scales imply

that they were controlled by sea-level changes, and that these were in tune with the climate changes induced by the orbital

cycles. However, the complexity of facies and sedimentary structures seen in the Opletnya Member also implies that

additional factors such as lateral migration of sediment bodies across the ramp were active. In addition, three major

sequence boundaries have been identified in the studied sections, which can be correlated with the boundaries Ol4, An1,

and An2 of the Tethyan realm.

Keywords: Sedimentary cycles, palynology, conodonts, Triassic, Western Balkanides, NW Bulgaria.

Introduction

The Anisian is a crucial time interval in Earth’s history to

understand carbonate platform reorganization in the aftermath

of the most severe extinction event at the end of the Permian

(e.g., Benton 2015) and the incipient break-up of the super-

continent Pangaea (Stampfli et al. 2013), the northwestern

Tethyan realm being best suited to study the comeback of shal-

low-marine environments at the beginning of the Mesozoic

(e.g., Feist-Burkhardt et al. 2008; Stefani et al. 2010; Haas et

al. 2012; Escudero-Mozo et al. 2015; Matysik 2016; Chatalov

2018).

Since the first detailed descriptions in the late 19

century

by Toula (1878), Middle Triassic carbonate successions of

the Western Balkanides have been studied to define lithostrati-

graphic units and detect facies types (Tronkov 1960, 1968,

1973, 1976, 1981, 1992; Tronkov et al. 1965; Chemberski et

al. 1974; Assereto & Čatalov 1983; Assereto et al. 1983;

Chatalov 1997; Benatov & Chatalov 2000; Chatalov et al.

2001). Microfossils and invertebrate groups have been used

(Tronkov 1968, 1976; 1983, 1995; Budurov & Stefanov 1972;

Budurov 1980; Budurov & Trifonova 1995; Benatov 2000,

2001) to compare and correlate these deposits referred to as

“Balkanide type” (Ganev 1974; Chatalov 1980, 1991; Zagor-

chev & Budurov 2009) lithologically and stratigraphically

with the “Germano-type” Muschelkalk successions of the

Germanic Basin, showing striking facies similarities (Chatalov

1991). However, up to date the lack of a robust biostratigraphic

framework hampers a precise age control of these deposits and

hence regional correlation with Middle Triassic ramp systems

of the northwestern Tethyan (e.g., Michalík et al. 1992; Haas

et al. 1995; Philip et al. 1996; Török 1998; Götz et al. 2003;

Rychliński & Szulc 2005; Götz & Török 2008) and Peri-

Tethyan, i.e. Germanic realms (e.g., Götz 1996; Szulc 2000;

Pöppelreiter 2002; Borkhataria et al. 2006; Matysik 2016).

The challenge of a high-resolution correlation of Anisian

Muschelkalk ramp cycles across the Tethyan shelf and the

Peri-Tethyan basin was outlined by Götz & Török (2018).

Recent studies revisited the excellent outcrops along the

Iskar River gorge (Fig. 1), focusing on the continental–marine

transition and ramp initialization (Ajdanlijsky et al. 2018), and

on the overall facies development of the ramp system

(Chatalov 2013, 2018). The onset of the Anisian ramp system,

after continental sedimentation in the Early Triassic, features

a pronounced cyclic character of peritidal and shallow-marine

carbonates (Chatalov 2000), and different hierarchical scales

can be defined for the Early to early Middle Triassic interval.

A major sequence boundary occurs at the base of the Middle

Triassic Iskar Carbonate Group (Ajdanlijsky et al. 2018).

A mid-Anisian maximum-flooding zone is inferred from

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the litho- and biofacies of the so-called “Terebratula Beds”

(Chatalov 2013), and a long-term, second-order sea-level rise

is interpreted from the Olenekian onwards, followed by a mid-

Anisian to early Carnian sea-level fall again creating a major

sequence boundary (Chatalov 2018).

The present study yields new biostratigraphic data provi-

ding a refined age control. Palynofacies analysis is used to

decipher short- and long-term sea-level fluctuations by chan-

ges of terrigenous input, preservation and sorting of phytoclasts,

and prominent phytoplankton (acritarch) peaks indi

cating

major flooding phases. Combined with the detailed analysis of

lithology and sedimentary structures, this allows a robust

sequence- and cyclostratigraphic interpretation to be proposed.

Geological setting

The study area, located approximately 35 km north of Sofia,

is part of the Western Balkanides, i.e. the Western Balkan

Tectonic Zone (Ivanov 1998) of the Alpine orogenic belt. Its

pre-Mesozoic basement includes high-grade metamorphosed

lower Paleozoic sedimentary and igneous rocks and upper

Paleozoic sedimentary, igneous and volcanic rocks (Yanev 2000).

The overlying Triassic succession forms the base of the Meso-

zoic cover and is subdivided into three units: the Petrohan Terri-

genous Group (Tronkov 1981) consisting predominantly of

fluvial deposits; the Iskar Carbonate Group (Tronkov 1981)

composed of shallow-marine carbonates and mixed siliciclas-

tic–carbonate rocks; and the Moesian Group (Chemberski et

al. 1974) represented by siliciclastic–carbonate and carbonate

rocks (Fig. 2). The Iskar River gorge exposes an excellent con-

tinuous succession of the marine Iskar Carbonate Group with

a maximum thickness of about 480 m (Chatalov 2013). The lower

part of the group is assigned to the Anisian stage (Fig. 3).

Materials and methods

Two key sections (Lakatnik and Sfrazen) of the Mogila

Formation as part of the Iskar Carbonate Group (Figs. 2, 3)

were logged in great detail. The Sfrazen section, representing

the formation’s type section, is situated north of Sfrazen ham-

let, 1.5 km west of the village of Opletnya. The Lakatnik sec-

tion is located directly north of the Lakatnik railway station,

about 3 km west of the Sfrazen hamlet. Both sections offer

excellent lateral and almost continuous vertical exposures of

the Mogila Formation. The detailed bed-by-bed documenta-

tion includes lithology, fossil content, and sedimentary struc-

tures. The microfacies and the textures (Dunham classification)

were determined on the outcrop with a hand lens and in

52 thin-sections. In the Sfrazen section, the overlying Babino

Formation and lowermost Milanovo Formation were logged in

order to obtain additional biostratigraphic tie points. Samples

for biostratigraphic analysis of palynomorphs and conodonts,

and for palynofacies analysis were taken from all three forma-

tions and different lithologies. In both sections, a small-scale

cycle within the lower Mogila Formation (Opletnya Member)

was sampled for high-resolution palynofacies analysis.

Fig. 1. Excellent outcrops along the Iskar River gorge expose the Iskar Carbonate Group overlying the Petrohan Terrigenous Group (PTG).

The Opletnya Member is ca. 135 m thick.

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the marine parts of the succession (Budurov & Stefanov 1972,

1973, 1975; Budurov 1976, 1980). They were later accompa-

nied by analyses of benthic foraminiferal assemblages, firstly

to erect a refined zonation scheme including standard cono-

dont and foraminifera zones (Budurov & Trifonova 1984,

1995; Budurov et al. 1995), and secondly to provide a tenta-

tive correlation with the Tethyan ammonite zones. Studies by

Tronkov (1960, 1968, 1981, 1983) focused on invertebrate

groups, mainly bivalves and brachiopods, and also used rare

ammonoid findings (Tronkov 1976) to overcome the limited

correlation of the Bulgarian Triassic with the Triassic of

the Tethyan and Peri-Tethyan realms. Later, Benatov (1998)

established regional bivalve and brachiopod zones for the

Middle Triassic in order to correlate them with the existing

zonation schemes. Consequently, these zones were used for

dating and regional correlation (Benatov et al. 1999; Benatov

2000, 2001). However, the facies-dependent occurrence of

bivalves and brachiopods and their generally low time resolu-

tion hampers precise dating and correlation at regional and

over-regional scales. Furthermore, conodont assemblages

obtained from olistoliths (Budurov 1976) without reference

sections, the noticeable conodont provincialism (Budurov &

Petrunova 2000) and Peri-Tethyan endemism (Chen et al.

2019), and ultimately the huge recent progress in Triassic

conodont research applying multi-element taxonomy and

revised, lineage-based zonation schemes (e.g., Chen et al.

2015, 2019), demonstrate the urgent need of a revision of

the existing conodont stratigraphy of Bulgaria.

Previous palynological investigations in the Triassic of

Bulgaria have been limited to a few attempts to date forma-

tions (Kalvacheva & Čatalov 1974; Čatalov & Visscher 1990;

Petrunova 1992a, b, 1999, 2000; Budurov et al. 1997). How-

ever, recent palynological studies on continental and marine

deposits of the Early–Middle Triassic transition interval

exposed in outcrops along the Iskar River gorge (Ajdanlijsky

et al. 2018) show the huge potential to establish a high-resolu-

tion palynostratigraphy. So far, the Olenekian–Anisian boun-

dary was palynologically identified in the uppermost fluvial

Petrohan Terrigenous Group, differing from previous interpre-

tations, which placed this boundary at different positions in

the Mogila Formation of the overlying Iskar Carbonate Group

(Tronkov 1983; Chatalov 2018). In the present study, the focus

is on palynomorphs to further develop Triassic palynostrati-

graphy. Additionally, conodonts are used to refine the existing

Anisian stratigraphy of NW Bulgaria.

Palynological key taxa allow dividing the Anisian succes-

sion into three palynoassemblages (Fig. 3). Assemblage I

identified in the lower Mogila Formation is characterized by

Anisian index taxa including Cristianisporites triangulatus,

Illinites kosankei, Illinites chitonoides, Stellapollenites thier

gartii, Tsugaepollenites oriens, and Triadispora crassa. Early

Triassic elements such as Densoisporites nejburgii and

Voltziaceaesporites heteromorphus are still present, indicating

an early Anisian (Aegean) age (Heunisch 1999, 2019). Assem-

blage II is composed of Anisian taxa as listed above, with the

last appearance of Cristianisporites triangulatus and Illinites

kosankei in the uppermost Babino Formation, indicating

a Bithynian–Pelsonian age. Assemblage III of the basal

Milanovo Formation is characterized by Illinites chitonoides,

Stellapollenites thiergartii, Tsugaepollenites oriens, Tria

dispora crassa, and the first appearance of Kraeuselisporites

wargensis, indicating a late Anisian (Illyrian) age (Kustatscher

& Roghi 2006).

In the Sfrazen section, the occurrence of conodonts (N. ger

manica / N. kockeli) in the upper Mogila Formation and the

Babino Formation enables the placement of the Bithynian/

Pelsonian boundary (Götz et al. 2019) in the lowermost

Zimevitsa Member of the Babino Formation. The identifica-

tion of Pelsonian elements such as Paragondolella bulgarica

(Chen et al. 2015), accompanied by findings of crinoid colum-

nal segments of Holocrinus dubius, an index species of the

Pelsonian (Hagdorn & Głuchowski 1993; Głuchowski &

Salamon 2005; Niedźwiedzki & Salamon 2006), in bioclastic

grainstones of the Zimevitsa Member, support this age

assignment.

Sedimentology

In the studied area, the Anisian succession starts with the

Svidol Formation that demonstrates a wide variety of silici-

clastic–terrigenous, siliciclastic–carbonate, and carbonate

rocks (Ajdanlijsky et al. 2018), followed upwards by the

mainly carbonate successions of the Mogila, Babino and

Milanovo formations.

Opletnya Member

In the Opletnya Member (lower part of the Mogila

Formation), carbonates prevail while the mixed siliciclastic–

terrigenous and siliciclastic–carbonate rocks, presented mainly

by thin beds, form only an insignificant part of the succession.

Among the carbonates, where limestones prevail over dolo-

mitic limestones and dolomites, wacke- and packstones are

more common than mud- and grainstones.

Wackestones and mudstones

Wacke- and mudstones are medium- to thick-bedded

(15–80 cm), rarely thin-bedded (5–8 cm), massive, laminated

or nodular (Fig. 4b), commonly bioturbated (Figs. 4a, 5a), and

contain benthic foraminifera and ostracods. The laminations

are more often thick (3–4 mm) than thin (1–1.5 mm). In some

cases, scale and intensity of the bioturbation allows for lateral

correlation between sections. Pebble-sized intraclasts, mainly

with lithologies similar to those of the hosting bed, occur very

rarely. Solitary small fragments of gagate (jet) are observed in

several levels (Zdravkov et al. 2019). Birdseyes and different

degrees of dolomitization occur occasionally both in wacke-

stones and mudstones. These facies and sedimentary struc-

tures point to a peritidal to shallow-marine environment (e.g.,

Flügel 2004).

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Synsedimentary deformation, some of it with sigmoidal

geometry, is among the often observed features in wacke- and

mudstones (Figs. 6a, 7), and beds with this texture are com-

mon in the lower part of the Opletnya Member. Its amplitude

commonly is in the range of 5–15 cm, but in some beds may

reach or even exceed 40 cm. Several internally deformed beds

may be vertically stacked (Fig. 6b). The upper bounding sur-

face of the deformation structures may be flat but more often

is slightly concave-up. These structures can be traced laterally

over several tens of meters, whereby their amplitude gradually

decreases and finally disappears (Fig. 6d). Small- and meso-

scale slump folding is another type of synsedimentary defor-

mation common in wacke- and mudstones, observed in almost

the whole Opletnya Member. In some places, direct contact

between sigmoidal and slump folding structures can be

observed (Fig. 6c). The geometry of these structures varies

Fig. 5. Dolomites in the Opletnya Member: a — dolo-wackestone with chaotic soft-sediment deformation and subsequent bioturbation,

Lakatnik section, 82.90 m, elementary sequence 42; b — dome-shape stromatolite (below arrows), developed in the uppermost part of

the fourth medium-scale sequence, Lakatnik section, 110.3 m, elementary sequence 60; c — symmetric small ripples of packstone (arrows),

surrounded by mud- to wackstone in a dolomitized bed-set. Sfrazen section, 109.5 m, elementary sequence 60; d — tepee structure from the

upper part of small-scale sequence 12, Sfrazen section, 49.1 m, elementary sequence 23; e — matrix-supported floatstone rich in pebble-sized,

subrounded intraclasts, Sfrazen section, 49.3 m, elementary sequence 24; f — clast-supported lag deposit of conglomerate, Sfrazen section,

same level as (e). The position in meters refers to the base of the Opletnya Member, the elementary and small-scale sequence numbering is

according to Fig. 7.

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from quite symmetrical folding (Fig. 6f) to chaotic (Fig. 5a).

Most commonly these slump folds have amplitudes in the

range of 30 to 50 cm, but in some cases may exceed 2 m

(Fig. 6e). These features are interpreted as having formed by

sliding of soft but cohesive sediment, possibly by instabilities

on channel margins (Hardie & Garrett 1977), or induced by

the occasional impact of storm waves or earthquakes (seismi-

tes; Montenat et al. 2007).

Although wacke- and mudstones are common in the stu-

died sections, this lithology is dominant in the lower part

of the Opletnya Member. Mudstones mark the deepest

and/or most quiet sedimentary environments, with restricted

water circulation. The intense bioturbation supports this

interpretation. The nodularity often is connected with

increased clay content as a result of increased terrigenous

supply.

Fig. 6. Synsedimentary deformations in the Opletnya Member: a — 40 cm thick bed with sigmoidal structure from the middle part of

the medium-scale sequence, Lakatnik section, 15.3 m, elementary sequence 7; b — two stacked beds (between dashed lines and arrows)

with sigmoidal structure and another one above them. Sfrazen section, 13.1 m, elementary sequence 6; c — wackestone bed with sigmoidal

structure (between arrows) overlain by a mudstone bed (between dashed lines) with slumps, Sfrazen section, 24.1 m, elementary sequence 12;

d — 12 cm thick bed with sigmoidal structure (between dashed lines) with concave top surface that pinches out laterally (arrow). Hammer for

scale, Lakatnik section, 12.0 m, elementary sequence 6; e — about 2 m thick wacke- to mudstone bed with slumps (between arrows) in its lower

part (hammer for scale), Sfrazen section, 28.2 m, elementary sequence 14; f — detail of the same level shown in (e). The position in meters is

from the base of the Opletnya Member, the elementary sequence numbering is according to Fig. 7.

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Packstones, grainstones, rudstones, and floatstones

Bioclasts, mudstone lithoclasts, ooids as well as a high

amount of peloids are the typical components of the pack- and

grainstones. Rudstones and floatstones are very rare; they con-

tain pebble- to cobble-sized intraclasts, the roundness of which

varies in a wide range. The intraclasts are either chaotically

distributed (Fig. 4e), or they occur in the bottom set (Fig. 4f)

or on the foreset surfaces of cross-beds (Fig. 4d). Lag deposits

and imbrication structures are also observed. In some cases,

the surfaces of the intraclasts are not sharp and show a gradual

transition to the hosting material, which could be the result of

re-sedimentation of semi-lithified material, or incipient disin-

tegration of the semi-consolidated beds (Fig. 4e). Some beds

are strongly bioturbated (Fig. 4f) and may show overpacking

due to early compaction. Partial dolomitization is more com-

mon in packstones than in grainstones.

Packstones, grainstones, and rudstones/floatstones com-

monly form massive beds. The most prominent sedimentary

structure is the well-developed cross-bedding — low-angle,

planar and trough type — commonly with thick lamination

(Fig. 4c, d, f). Small-scale cross-bedding displays both wavy

bedding (Fig. 5c) and current ripples (Fig. 4d). Reactivation

surfaces are also common (Fig. 4d). The set thickness ranges

between a few cm to a few tens of cm, while that of the cosets

may reach several meters (Fig. 7). Such sediment bodies can

form regionally traceable bed packages, which have been con-

sidered as stratigraphic markers (Tronkov 1968). The cross-

bedding direction varies in a wide range — within one coset as

well as along the same level in both sections (Fig. 7).

The pack- and grainstone beds indicate an active hydrody-

namic sedimentary environment that led to the development

and lateral migration of different types of bars and shoals.

The variety of morphology, scale, and orientation of the cross-

bedding, the reactivation and accretion surfaces, and the inter-

calation of these features indicate highly variable high-energy

settings typical of shallow-water environments. The presence

of intraclasts, some of them semi-lithified, implies an almost

permanent subaqueous erosion by waves and currents. Wavy

bedding and the variable orientation of the cross-beds suggest

a tidal influence (e.g., Reineck & Singh 1975; Gonzales &

Eberli 1997).

Dolomites

The partially or completely dolomitized packstones, wacke-

stones, and mudstones have an uneven distribution in the stu-

died sections. They can be observed as thin to thick beds, but

may also stack into packages with thicknesses of 2.65–2.80 m

or even 3.25–3.55 m that intercalate with several very thin

beds of mixed carbonate-terrigenous rocks (Fig. 7). In the

middle and upper parts of the Opletnya Member, almost com-

pletely dolomitized sets of 9.50 to 14.80 m can be observed.

Although massive and laminated structures are most common

for these rocks, they also are associated with tepees and flat

pebbles that form local lags (Fig. 5d). Nodular structures are

also present. Lenses with chaotically oriented lithoclasts

(Fig. 5e, f), some of them with imbrication structures, are com-

mon. In some beds, most often dolo-packstones and rare

dolo-grainstones, different types of cross-bedding can be

observed (Fig. 5c), and intraclasts may form short bands lying

on the foreset lamina. Stromatolites with different morpholo-

gies, developed in dolomitized mud- and wackestone beds, are

observed at several levels in the studied sections (Figs. 5b, 7).

The dolomite-dominated intervals are interpreted as the shal-

lowest, high-salinity sedimentary environment in the Opletnya

Member. Desiccation cracks and tepee structures mark epi-

sodes of subaerial exposure, indicating the establishment of

tidal flats under a semi-arid climate. These climatic conditions

are favorable for microbially-mediated dolomitizaton of

microbial mats (e.g., Petrash et al. 2017) and/or early-diage-

netic reflux dolomitization (e.g., Adams et al. 2018). However,

no detailed geochemical studies were preformed and the pos-

sibility of late-diagenetic dolomitization cannot be excluded

either (e.g., Lukoczki et al. 2019). Storm events and/or strong

tidal currents ripped up cohesive sediment, incipient hard-

grounds, or microbial mats to form flat-pebble conglomerates

and lag deposits (e.g., Hardie & Ginsburg 1977; Hillgärtner et

al. 2002).

Erosion surfaces

Other common features of the Opletnya Member are the

erosional surfaces. Often they demonstrate channel morpho-

logy (Fig. 8a–c). In many cases, the flanks of these channels

are very steep to almost vertical, indicating a relatively

advanced degree of cohesion and early lithification of the

sedi ment into which the channel was cut (Fig. 8b). The erosion

surfaces either separate beds of different lithology (Fig. 8a, b

and d), or they occur within a set of lithologically monotonous

beds and are underlain by thin layers or lenses of mixed silici-

clastic–carbonate or fully siliciclastic material (Fig. 8c).

Stacking of multiple erosional surfaces is documented at

several levels of the profile. The erosional depth varies

between 5–15 cm and several tens of centimeters, but in some

cases reaches even 60–80 cm (Figs. 7 and 8a). Lag deposits

draping the erosional surfaces are observed in some cases.

Firmgrounds and hardgrounds

Firm- and hardgrounds on top of dolomitic limestone and

dolomite beds are also observed (Figs. 7 and 8d). They are

irregular, with amplitudes of several centimeters. Borings

belong to Balanoglossites and Trypanites, characteristic

ichno taxa of Middle Triassic ramp settings (Knaust 1998,

2007; Knaust et al. 2012; Chrząstek 2013).

Lakatnik Member

Limestones, thick-bedded pack- and grainstones, are also

dominant in the Lakatnik Member (upper Mogila Formation).

This member is furthermore characterized by levels rich in

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crinoids. Lenses of dolomitic limestones and dolomites are

intercalated.

Babino Formation

The lower part of the Babino Formation (Zimevitsa

Member) is dominated by nodular, clayey, rare massive and

laminated, often bioturbated wacke- to mudstones (Fig. 9c)

that alternate with mainly bioclastic, massive to cross-bedded

packstones (Fig. 9a, b, d). Along the base of the unit, as well

as in several distinct beds above, levels rich in brachiopods

and crinoids are observed (Figs. 9b, d, 10). The volume of

mixed siliciclastic-carbonate rocks is minor. Synsedimentary

deformation structures are common. At around 10 m from

the base of the unit, an erosional, karstified surface with

evidence for subaerial exposure is observed (Fig. 9a).

The upper half of the Babino Formation (Zgorigrad Member)

is formed by mainly nodular, thin- to medium-bedded wacke-

and packstones that contain conodonts, bivalves, and

brachiopods.

The Babino Formation is covered by the massive to thick-

bedded dolomites with crinoids of the Milanovo Formation.

Sequence stratigraphy and cyclostratigraphy

Concepts

The sequence- and cyclostratigraphic interpretation and the

correlation of the studied sections follow the concepts propo-

sed by Strasser et al. (1999). The sequence-stratigraphic nomen -

clature is that of Catuneanu et al. (2009), and this nomenclature

is applied independent of the scale of the sequences (Posa-

mentier et al. 1992; Catuneanu 2019). Sequence boun daries

(SB) are indicated by the shallowest facies and may in addi-

tion be expressed by an erosive surface if sea level dropped

below the previously accumulated sediment. Transgressive

sur faces (TS) may be erosive if there was ravinement, and in

any case mark the beginning of a deepening-up facies trend

(transgressive deposits). Maximum-flooding surfaces (MFS)

are expressed by the deepest facies and display bioturbation or

hardgrounds if sedimentation rate was reduced. Shallowing-up

highstand deposits then lead to the following sequence

boundary.

Elementary sequences are the smallest units in which facies

trends and sedimentary structures indicate a cycle of sea-level

Fig. 8. Erosional surfaces in the Opletnya Member: a — two erosional surfaces (dashed lines), partly developed in dolomitized limestone,

the lower one forming a channel over 50 cm deep that marks a sequence boundary, Lakatnik section, 9.2 m, elementary sequence 6; b — small-

scale channel with steep flanks (dashed line), filled by limestones developed in the uppermost part of dolomites. The channel is 12 cm deep,

Lakatnik section, 14.2 m, elementary sequence 7; c — small-scale channel (hammer for scale) that laterally corresponds to a level with sig-

moidal synsedimentary deformation, Lakatnik section, 11.9 m, elementary sequence 6; d — hardground surface at the top of highstand dolo-

mitic limestones (black arrows), covered by bioclastic grainstones. The highstand deposits contain gagate intraclasts (above white arrows).

The top of the bed appears broken, with reddish sediment infill, Sfrazen section, 25.4 m, elementary sequence 13. The position in meters refers

to the base of the Opletnya Member, the elementary sequence numbering is according to Fig. 7.

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change. Small-scale sequences are composed of several (in

many cases five) elementary sequences and generally display

first a deepening then a shallowing trend, with the shallowest

facies at the boundaries. Several (2 to 4 in the studied sections)

small-scale sequences compose a medium-scale sequence,

which again displays a general deepening-shallowing trend of

facies evolution and the relatively shallowest facies at its

boundaries. If several elementary sequences compose an inter-

val of shallowest or deepest facies, a sequence-boundary zone

respectively a maximum-flooding zone is defined (Montañez

& Osleger 1993).

Small, meter-scale depositional units are often called

“cycles” if they are stacked in the sedimentary record. Here

we use the term “sequence” because this allows better defi-

ning the facies evolution within them and interpreting the sea-

level changes that caused it. If such sequences are bounded by

prominent marine flooding surfaces, they can be compared to

the “parasequences” of van Wagoner et al. (1990), although

these were originally defined in siliciclastic systems. We use

the term “cycle” for the cyclical or periodic processes that

controlled the formation of the sequences.

If chronostratigraphic tie points allow estimating the dura-

tion of the studied sections, and if the hierarchical stacking of

the depositional sequences reflects the ratios of orbital

(Milankovitch) cyclicity, an interpretation of the evolution of

the depositional environments can be proposed at a high time

resolution: the elementary sequences would correspond to

the 20-kyr precession cycle, the small-scale sequences to

the 100-kyr short eccentricity cycle, and the medium-scale

sequences to the 405-kyr long eccentricity cycle. These orbital

cycles translated into sea-level cycles through complex atmo-

spheric and oceanic feed-back processes (Strasser 2018). How-

ever, autocyclic processes independent of orbital cycles may

have been superimposed, making the interpretation more com-

plicated. Time-series analyses are commonly applied to demon-

strate the recording of orbital cyclicity (e.g., Hinnov 2013).

In the present case, however, the complexity of the facies

changes on the shallow ramp precludes such an approach.

Elementary sequences

In both studied sections of the Opletnya Member (lower part

of the Mogila Formation), 69 elementary sequences were

identified and correlated (Fig. 7). These are generally grouped

by five to form 14 small-scale sequences. Two or four small-

scale sequences are bundled into 5 medium-scale sequences,

Fig. 9. Lithology of the lower Zimevitsa Member (lower Babino Formation) in the Sfrazen section: a — highly irregular paleokarst surface

(arrows) implying prolonged subaerial exposure, overlain by bioclastic limestone; b — low-angle cross-bedded limestone, transgressive depo-

sits of an elementary sequence; c — plan view of nodular wackestone from the middle part of an elementary sequence; d — crinoid columnal

segment of Holocrinus dubius from transgressive deposits in the lower part of the Zimevitsa Member. All photos are from elementary sequence 6

in Fig. 10.

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in which three parts can be recognized: transgressive, maxi-

mum-flooding, and highstand deposits. Elementary and small-

scale sequences are recognized also in the lowermost part of

the Zimevitsa Member (lower part of the Babino Formation).

The elementary sequences, the smallest cyclic units docu-

mented, are defined by their bounding surfaces, composition,

facies trends, and sedimentary structures that indicate a cycle

of sea-level change. The base of each elementary sequence is

marked by the shallowest facies, and/or by a laterally traceable

erosional surface. Most of the elementary sequences can be

subdivided into three parts — a lower part that represents the

transgressive stage, a middle part containing the maximum-

flooding surface, and an upper part representing the highstand

stage. The facies composition and thickness of these parts

varies depending on the position of the elementary sequence

within the small-scale and especially the medium-scale

sequence it belongs to (Strasser et al. 1999).

In the lower (transgressive) part of a medium-scale se quence,

the sequence boundary and the transgressive surface of the ele-

mentary sequences are very close to each other or even amal-

gamated because lowstand deposits are very thin or not

recorded due to limited accommodation on the shallow ramp

(Fig. 11). Intraclasts, lithologically identical to the rocks

immediately below the erosional surface, indicate reworking

during transgression of previously cemented sediment.

Massive or cross-bedded pack- and grainstones represent

Fig. 10. Cyclostratigraphic interpretation of the lower Babino Formation (Zimevitsa Member), Sfrazen section. The legend is according to Fig. 7.

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shallow bars or shoals that formed during transgression, and

they may form 30–50 % of the volume of the elementary

sequences. The maximum-flooding surface marks the top of

these beds or can be identified above by the deepest facies

and/or intense bioturbation. In the early highstand part of these

elementary sequences, massive, laminated or nodular, often

bioturbated wacke- and mudstones dominate. The amount of

terrigenous components increases and forms thin beds of

marls or of mixed siliciclastic-carbonate sediment. In some

cases, the uppermost, late highstand part is dolomitic lime-

stone or (rarely) dolomite (Fig. 11). In other cases, the late

highstand is formed by marly wacke- and mudstone beds.

In the studied sections, the average thickness of the elemen-

tary sequences from the transgressive part of the medium-scale

sequences is in range of 2.1–2.2 m. The thickest elementary

sequences (3.9 to 4.25 m) are measured in the lowermost and

uppermost medium-scale sequences of the Opletnya Member

(Fig. 7). A gradual but steady decreasing in the average thick-

ness (from 2.3 to 1.5 m) of the elementary sequences is

observed in the transgressive part of the first four medium-

scale sequences.

In the maximum-flooding to early highstand parts of the

medium-scale sequences, the elementary sequences are domi-

nated by wacke- and mudstones while pack- and grainstones

occur in lesser amounts (Fig. 12). Once again, the pack- and

grainstone beds occupy mainly the base of these units and

rarely occupy the whole transgressive part, where the amount

of the wacke- and mudstone gradually increases. Intraclasts

are rare and small in size. The transgressive surface of these

elementary sequences can be very close to the sequence

boundary or coincides with it. However, at the top of elemen-

tary sequence 7 in the Sfrazen section, the lower erosion sur-

face is interpreted as the sequence boundary, while the second

erosion surface represents the transgressive (ravinement) sur-

face. Thus, a thin lowstand deposit is present. The maximum-

flooding surface is associated with increasing intensity of

bioturbation and, in some cases, synsedimentary deformation.

The highstand interval, dominated by massive or laminated

mud- and wackestones, forms 70–80 % of the elementary

sequences. Dolomitization of their uppermost part is rare.

However, sediment containing terrigenous siliciclastics forms

thicker and laterally traceable units.

In the highstand parts of the medium-scale sequences,

the transgressive deposits of the elementary sequences are

repre sented by pack- and grainstones, and/or by mixed silici-

clastic–carbonate or even claystone beds (Fig. 13). Dolomite

and dolomitic limestones are common. The sequence boun-

dary is represented by desiccation cracks, tepee structures, or

an erosional surface. The transgressive surface, where it can

be identified, is above the sequence boundary, making room

for thin lowstand deposits. The maximum-flooding surface

cannot always be recognized. Wacke- and mudstones domi-

nate the upper part of the units. In many of these elementary

sequences, almost the whole volume is represented by dolo-

mite or partially dolomitized limestones.

The average thickness of the elementary sequences within

the highstand parts of medium-scale sequences is around 2.05 m.

This value decreases in the lower (early) stage of the highstand

part to 1.3–1.5 m, while in the upper (late) part it increases to

1.8–1.9 m.

Fig. 11. Elementary sequences from the transgressive part of the lowermost medium-scale sequences of the Opletnya Member. Note that SB

and TS are amalgamated and represent a ravinement surface. Elementary and small-scale sequence numbering and legend as in Fig. 7.

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In the Lakatnik Member, it is not possible to clearly define

elementary sequences. In the lowermost part of the Zimevitsa

Member (Babino Formation), the elementary sequences are

less complex than those in the Opletnya Member. Their base is

marked by a transgressive surface (Fig. 10). Erosional sur-

faces are rare. The sequence boundaries generally are not

developed (elementary sequences defined by transgressive

surfaces; Strasser et al. 1999), with the exception of the ones

at the base of elementary sequences 2 and 6. The transgressive

part is presented by massive, laminated to low-angle cross-bed-

ded bio- and lithoclastic pack- to grainstones (Fig. 9b). Terri-

genous fines increase from the maximum-flooding surface

into the highstand part of the units, where massive, laminated

and nodular wacke- and mudstones predominate (Fig. 9c).

Bioturbation is very common. The average thickness of the

ele mentary sequences is similar to that in the Opletnya

Member and is in the range of 2–2.2 m.

Small-scale sequences

Within the Opletnya Member, the elementary sequences,

grouped into sets of five, form a succession of 14 small-scale

sequences (Fig. 7). Their base very often is marked by a pro-

nounced erosional surface and/or a lag of intraclasts. The mea-

sured erosional amplitude of these surfaces ranges from a few

centimeters to decimeters and, in individual cases such as

at the limit between small-scale sequences 12 and 13 in the

Sfrazen section, may reach more than 70 cm (Figs. 7d and 13).

Stacking of several erosional surfaces within one elementary

sequence situated at the base of the small-scale sequences is

also common (Figs. 4f, 8a). Similar erosional stacking can be

observed along the boundary between the first two elementary

sequences in the second small-scale sequence (Fig. 8b).

From the base upwards, the small-scale sequences display

first a deepening then a shallowing trend. The boundary

between them is always marked by the shallowest facies. Most

commonly, their lower part is dominated by limestones and/or

partially dolomitized limestones — litho- and bioclastic or

ooid-dominated pack- and/or grainstones (Figs. 4e, f, 7, 8a).

Massive, planar and trough cross-bedding with lags of intra-

clasts is common (Fig. 4c). Reactivation and lateral accretion

surfaces are observed. Different types of small-scale cross-bed-

ding, indicating wave or current hydrodynamic regimes, are

also typical for this part. The paleotransport directions vary in

a wide range and in one and the same elementary sequence

almost opposite directions can be observed (elementary

sequences 21, 22, and 31 in Fig. 7b, c).

Upsection, the thickness of the pack- and grainstones in

the small-scale sequences decreases and the amount of

wackestone beds increases, marking a deepening facies trend.

The amount of nodular and finely laminated beds increases.

Also, a gradual increase in frequency and intensity of biotur-

bations is observed (Figs. 4a, 5a). The evidences for erosional

processes gradually decrease. Firm- and hardground surfaces

are developed (Figs. 7, 8d).

The turn-around to a shallowing facies trend is marked by

the gradual increase of the amount of terrigenous material and

dolomite, of the frequency of erosional features, and of syn-

sedimentary deformations (for example small-scale sequences

2, 5, 6, 8 and 12 in Fig. 7). In many cases, the amount of pack-

stone beds also increases. At such turning points, in the lower

part of the Opletnya Member, the maximum bed thickness

(40 cm) with sigmoidal structures is recorded. Commonly,

the shallowing-upwards trend is accompanied by an increase

in erosional and synsedimentary deformation structures, for ming

in some places a stacked pattern (Fig. 6e, f; Fig. 7b: elementary

Fig. 12. Elementary sequences in the maximum-flooding zone of the lowermost medium-scale sequence of the Opletnya Member. Elementary

and small-scale sequence numbering and legend as in Fig. 7.

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sequence 25). Laterally, the intensity of these features may

change rapidly: while in one section they are quite prominent,

in the other they are attenuated (e.g., elementary sequences 14,

15, and 30 in Fig. 7a, b, c). In dolomite-dominated small-scale

sequences, different types of lag deposits (Fig. 5e, f) are com-

monly associated with tepees (Fig. 5d) and desiccation cracks.

In this shallowing part, the finely laminated and nodular struc-

tures still dominate (Fig. 4b), but small-scale cross-bedding

also is common (Figs. 4d, 5c).

The pattern described above varies within the studied sec-

tions. For example, bioclastic to intraclastic packstones may

predominate almost entirely in one small-scale sequence

(Fig. 7c: small-scale sequence 7) while in other cases (Fig. 7c, d:

small-scale sequences 9 and 12) the dolomitic lithology is

more prominent.

The thickness of the small-scale sequences within the Oplet-

nya Member varies in the range of 6.7–14.4 m (average 9.6 m).

Only in small-scale sequences 3, 5, and 13, the thickness is

over 11 m (Fig. 7a, b, d, e). Depending on to which part of

a medium-scale sequence the small-scale sequences belong to,

a persistent trend in the thickness of the elementary sequences

is observed. For example, within the transgressive part of the

medium-scale sequences, the small-scale sequences demon-

strate a symmetrical pattern with the thinnest elementary

sequences in the middle (Fig. 7). Around the maximum-floo-

ding and early highstand part of the medium-scale sequences,

the thickness of the elementary sequences within the small-

scale sequences decreases, while in their late highstand parts

of the medium-scale sequences the small-scale sequences

contain thicker elementary sequences in their upper part (for

example small-scale sequence 8; Fig. 7c).

One small-scale sequence, bounded by erosional surfaces,

was identified in the lowermost Babino Formation (Zimevitsa

Member). The thickness of the elementary sequences within it

decreases upwards from 3.3 m to 1.15 m (Fig. 10). The total

thickness of this small-scale sequence is 11.65 m.

Medium-scale sequences

In both studied sections of the Opletnya Member, five

medium- scale sequences have been identified, each subdi-

vided into three parts — transgressive, maximum-flooding,

and highstand deposits. The number of small-scale sequences

within them, however, is not equal and varies from two to four

(Fig. 7).

In all cases, the base of a medium-scale sequence is a promi-

nent, laterally correlatable erosional surface. In the lowermost

part, lithoclastic, bioclastic and/or ooid-dominated medium-

scale transgressive deposits predominate. The small-scale

sequences within this lower part are also developed mainly in

transgressive facies, where limestones, often cross-bedded,

predominate but dolomitic limestones are also present.

Fig. 13. Elementary sequences from the highstand part of the fourth medium-scale sequence of the Opletnya Member. Elementary and small-

scale sequence numbering and legend as in Fig. 7.

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Intraclasts are also common and occupy the foreset laminae in

cross-bedded structures but also form isolated lags. In almost

all medium-scale sequences, the measured paleotransport

indicators (planar and trough cross-bedding, reactivation and

accretion surfaces) show a unidirectional pattern (towards

the north-northeast; Fig. 7). Only in one case, at the base of

the uppermost (fifth) medium-scale sequence, there are indi-

cations about short-term (and probably local) south-south-

westward directions.

In the upper parts of the transgressive intervals of the medium-

scale sequences, the amount of wacke- to mudstone beds and

of beds with nodular structure and bioturbation increases. This

trend reaches its maximum in the maximum-flooding zone.

At the same time, an increase of the intensity and the scale of

synsedimentary deformations is observed. Firm- and hard-

ground development is also documented. From here upwards,

medium-scale sequences commonly display an increasing

diversification of the sedimentary paleotransport directions,

and in many cases almost opposite directions can be observed

in one and the same stratigraphic level.

In the highstand part of the medium-scale sequences, there

commonly is an increase in the amount of dolomite and eva-

porites. Desiccation cracks and tepee structure are also com-

mon. The amount and the size of the intraclasts increases as

well. Their roundness varies in a wide range, both vertically in

a section and laterally. Occasionally, rudstone fills small-scale

channels.

In the studied sections, the first medium-scale sequence is

well defined and includes small-scale sequences 1 to 4, and

elementary sequences 1 to 20. Small-scale sequences 5 and 6

compose a second medium-scale sequence. The thick high-

energy deposits in small-scale sequences 7 and 8 then suggest

the transgressive part of a third medium-scale sequence. Its

top is difficult to place, but the relatively thin and complex

elementary sequence 50 with an erosion surface at its base

(Fig. 7d) may be interpreted as the limit to a fourth medium-

scale sequence. This fourth sequence comprises small-scale

sequences 11 and 12, and its top is defined by the prominent

erosion surface An1. The top of the fifth medium-scale

sequence cannot be defined in the studied sections and may be

within the Lakatnik Member.

Cyclostratigraphic interpretation

The newly obtained biostratigraphical data (Ajdanlijsky et

al. 2018; this study) allow estimating the time range of the

sedi

mentary cycles documented in the studied sections.

It mostly concerns the boundaries of the Aegean substage in

the study area. Its base is defined in the uppermost part of

the fluvial succession of the Petrohan Terrigenous Group, just

below the base of the Svidol Formation by palynological data

(Ajdanlijsky et al. 2018). The top of the substage is located in

the uppermost part of the Opletnya Member of the Mogila

Formation, as inferred from the last appearance of early

Anisian palynomorphs (Fig. 3). The upper boundary of the

Aegean is situated at the top of elementary sequence 60 of

small-scale sequence 12 (Fig. 7d). The total number of ele-

mentary sequences recognized for the Aegean substage

(Ajdanlijsky et al. 2018; this study) is 80 (1 in the uppermost

Petrohan Terrigenous Group, 19 in the Svidol Formation, 60 in

the Opletnya Member). Erosion certainly occurred at the

boundaries of some elementary sequences as indicated by

the irregular surfaces and the reworking (Fig. 7), but the regu-

lar stacking pattern does not suggest that entire sequences are

missing.

Comparing this result to the new Triassic chart by Haq (2018)

that proposes a duration of about 1.7 Myr for the Aegean,

the average time duration of a single elementary sequence

would be approximately 21.25 kyr. However, according to

Ogg et al. (2016), the Aegean had a duration of 1.5 Myr, sug-

gesting a duration of 18.75 kyr per elementary sequence. In

the Middle Triassic, the periodicities of the orbital precession

cycle had peaks at ca. 18 and 22 kyr (Berger et al. 1989), with

an average of 20 kyr (Hinnov 2018). This is close to the esti-

mated duration of the elementary sequences recorded in the

Opletnya Member, which are consequently interpreted as

being related to the precession cycle. The fact that 5 elemen-

tary cycles compose a small-scale sequence suggests that

these were controlled by the short eccentricity cycle of 100 kyr

(Hinnov 2018).

In the studied sections, the medium-scale sequences have

been defined based on their lithology. The first one is well

defined with 4 small-scale (100-kyr) and 20 elementary

(20-kyr) sequences. Medium-scale sequences are in many

cases induced by the long eccentricity cycle of 405 kyr (e.g.,

Strasser et al. 2000; Boulila et al. 2008). However, the inter-

pretation of the other medium-scale sequences, comprising

two or four small-scale sequences, is less clear: they may have

resulted from a combination of allocyclic and autocyclic pro-

cesses, obscuring a clear signal of the long eccentricity cycle.

The transgressive-regressive facies trends within the sequen-

ces of all scales imply that these were — at least partly — con-

trolled by sea-level changes. Furthermore, the stacking of

these sequences reflecting the hierarchy and durations of

the orbital (Milankovitch) cycles suggests that the sea-level

changes were in tune with the climate changes induced by

the orbital cycles (e.g., Strasser 2018). However, the comple-

xity of facies and sedimentary structures seen in the Opletnya

Member also implies that additional factors such as lateral

migration of sediment bodies were active.

Palynofacies

Concept

The term palynofacies was first introduced by Combaz in

1964 to describe the total acid-resistant organic matter content

of sedimentary rocks within a specific depositional environ-

ment (Combaz 1964, 1980). Later, Tyson (1993, 1995) defined

palynofacies analysis as a methodology involving the iden-

tification of individual palynomorphs, plant debris, and

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amorphous components, their absolute and relative propor-

tions, size spectra, and preservation states.

A number of sedimentary organic matter classifications and

parameters are used in palynofacies analysis, reviewed in

Tyson (1987, 1993, 1995). In this study, sedimentary organic

matter is divided into a marine (autochthonous) fraction

including marine phytoplankton and foraminiferal test linings,

and a continental (allochthonous) fraction composed of pollen

grains, spores, and phytoclasts (Rameil et al. 2000). The here

used palynofacies parameters to decipher transgressive-re-

gressive trends within the studied succession are: (1) the ratio

of continental to marine constituents (CONT/MAR); (2) the ratio

of opaque to translucent phytoclasts (OP/TR); (3) the phyto-

clast particle size and shape (equidimensional to blade-shaped;

ED/BS); and (4) the relative proportion and species diversity

of marine phytoplankton.

Palynofacies analysis

Within the studied Anisian succession, long-term transgres-

sive–regressive trends are clearly documented in the CONT/

MAR ratio and phytoplankton abundance with three distinct

acritarch events (Fig. 3). A first marine pulse during the early

Anisian (Aegean) was recognized by an acritarch peak in

the lowermost Opletnya Member (basal part of the Mogila

Formation) below the Tenuis Bed (Ajdanlijsky et al. 2018),

characterized by a low-diversity marine invertebrate fauna

(Tronkov 1968). A second acritarch peak occurs in the upper

part of the Mogila Formation (base of the Lakatnik Member).

The third acritarch peak in the Zimevitsa Member (lower part

of the Babino Formation) is the most prominent signal accom-

panied by the highest diversity of marine invertebrates, inclu-

ding brachiopods and crinoids.

Short-term changes of sea level are documented in the chan-

ges of sedimentary organic matter content within sedimentary

sequences: changes of terrestrial input, preservation and sor-

ting of phytoclasts, and prominent phytoplankton peaks indi-

cating major flooding phases. In the Lakatnik section (Fig. 14),

a 4.5 m thick elementary sequence shows marine plankton

percentages between 5.1 and 10.7 % in the basal grainstones

(samples 1–3), the highest percentage occurring in the basal

lithoclast bed. Translucent phytoclasts of different sizes and

shapes are common. Upsection, a marked increase in phyto-

plankton is observed (samples 4, 5), with peak abundance

(23.5 %) in sample 4, also characterized by the highest ratios

of opaque to translucent (OP/TR) and equidimensional to

blade-shaped (ED/BS) phytoclasts. High percentages of

marine plankton and high OP/TR and ED/BS ratios continue

in samples 6 and 7, while the uppermost part of the sequence

(samples 8–9) shows low plankton percentages (3.6 to 6.5 %).

Within the phytoplankton group, acritarchs are most abundant

in samples 4 and 5, while prasinophytes are dominant in sam-

ples 8 and 9, and they are the only plankton group present in

sample 10. Foraminiferal test linings are recorded in samples

3, 4, 5 and 7. Bisaccate pollen grains are the dominant group

within the terrestrial particles, and spores are rare.

The basal grainstones (samples 1–3) are interpreted as trans-

gressive deposits, showing a high amount of “fresh” translu-

cent phytoclasts with a huge variety in sizes and shapes. A first

plankton peak in the basal lithoclast bed marks the initial

transgressive pulse. The sequence boundary might be directly

overlain by the transgressive surface, which explains the lack

of lowstand deposits. The level of sample 4 seems to indicate

the maximum-flooding surface on top of the transgressive

deposits with the most prominent plankton peak and the lowest

ratio in continental to marine particles. Alternatively, the inter-

val including sample 4 and 5 can be interpreted as maximum-

flooding zone since plankton percentages are the highest,

accompanied by the lowest ratio of continental to marine par-

ticles and the highest amount of equidimensional, opaque phy-

toclasts. The interval spanning samples 6 and 7 is interpreted

as early highstand deposits where the percentages of marine

plankton and equidimensional, opaque phytoclasts are still

high and foraminiferal test linings are present but the influx of

terrestrial particles is increasing. The change from an acri-

tarch-dominated to a prasinophyte-dominated plankton assem-

blage recorded in the uppermost part of the succession

(samples 8–10), as well as the switch to high terrestrial influx

with blade-shaped and mixed opaque and translucent phyto-

clasts is interpreted as indicative of late highstand deposits.

Prasinophytes are the only phytoplankton in sample 10, poin-

ting to a restricted shallow depositional environment, most pro-

bably lagoonal. However, from the palynofacies data it remains

an open question whether the dolomitic limestones captured by

sample 10 represent the latest highstand deposits or lowstand

deposits with the respective sequence boundary placed bet-

ween sample 9 and 10, and the transgressive surface recorded

by the lithoclasts at the base of the overlying grainstones.

The palynofacies analysis thus completes and refines the

sequence- and cyclostratigraphic interpretation based on litho-

facies and sedimentary structures.

Discussion

Biostratigraphy, chronostratigraphy, and large-scale corre-

lations

The new biostratigraphic data obtained enable the discrimi-

nation of the Anisian substages and placement of their boun-

daries in the studied sections. The Aegean/Bithynian boundary

is placed in the upper part of the Opletnya Member (lower

Mogila Formation), and the Bithynian/Pelsonian boundary in

the lower Zimevitsa Member (lower Babino Formation).

Ajdanlijsky et al. (2018) have already identified the base of

the Aegean substage (Olenekian/Anisian boundary) in the

upper most Petrohan Terrigenous Group (Fig. 3). Based on

these data, a first precise timing of the sedimentary cyclicity

within the lower Anisian succession in the area of the Iskar

gorge becomes possible.

Large-scale cyclicity can be detected by phytoplankton

abundances. A first marine pulse in the early Anisian

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Sedimentary cyclicity

Controversial interpretations of the prominent small-scale

cyclicity in the lower part of the Iskar Carbonate Group are

documented in publications of the last decades. Čatalov (1975)

assumed that the Svidol Formation was formed under epicon-

tinental tidal-flat conditions characterized by cyclic deposi-

tion. Later, Tronkov (1983, 1989) regarded the Opletnya

Member of the Mogila Formation as a typical rhythmic suc-

cession and described the main features of these rhythms

(hemicycles), subdividing them into three lithozones: (i) lower,

formed by oolitic-bioclastic calcarenites with intraclasts;

(ii) middle, characterized by marly limestones; and (iii) upper,

made up of dolomites. The rhythms are defined by transgres-

sive surfaces. According to the same author, the thickness

of the individual sedimentary cycles in the lower Opletnya

Member is in the range between 2 and 5 m, while upsection

they can reach and even exceed 20 m. The same author

assumed that these three lithozones represent (i) the distal off-

shore shelf bars, (ii) a calm back-bar carbonate sedimentation

with limited water circulation, and (iii) isolated lagoon envi-

ronments, respectively. The time duration and/or the rank of

the cycles were not defined. He proposed that the most com-

plete and detailed stratigraphic subdivision of the Opletnya

Member could be achieved considering its rhythmic character,

using each rhythm (hemicycle) as a distinct correlatable strati-

graphic unit.

Chatalov (1998, 2000, 2004) described peritidal cycles in

the lower Opletnya Member, discriminating a total of 17 small

(meter)-scale shallowing-upward asymmetric cycles. The upper

part of the member was not discussed. He assumed that each

shallowing-upward cycle (hemicycle) formed in a tidal-flat

environment due to sequential passage through its different

bathymetric zone. As a result, from base to top, the ideal indi-

vidual cycle is tripartite, starting with a subtidal basal lag,

followed by subtidal mudstones and bioclastic wackestones,

and ending with intertidal/supratidal dolomites. The thickness

of these hemicycles varies from 1.1 m to 12.4 m, with

an ave rage of 4.4 m. According to Chatalov (2016, 2018),

the influence of relative sea-level changes on the formation of

these cycles is debatable and he proposed an autogenic control

for the formation of these peritidal ramp cycles.

Ajdanlijsky et al. (2004) interpreted the entire succession of

the Opletnya Member as a result of hierarchical cyclic pro-

cesses. The smallest recognizable cyclic unit was defined as

elementary cycle, beginning with a transgressive surface,

often with an erosional base. The lower part of these cycles

was deposited in a high-energy shallow-marine setting, during

transgression over very shallow-marine (inter- or supratidal)

deposits forming the uppermost part of the previous cycle.

The top of the transgressive part of these cycles is marked by

the deepest lithofacies, indicating maximum flooding. The upper

part of the cycles demonstrates a shallowing-upwards trend.

The elementary cycles are grouped into submesocycles, and

these in turn into mesocycles with thicknesses of several tens

of meters. Because of the absence of reliable biostratigraphic

data, the time range of the elementary and submesocycles was

not defined, but it was assumed that the mesocycles corre-

spond to the third-order cycles of Vail et al. (1991).

Field data from both sections of the Opletnya Member

obtained during the present study allow refining the previous

interpretations by a precise definition and lateral correlation of

regional bounding surfaces and individual cycles, as well as

by an assessment of lateral lithofacies variations. The newly

obtained biostratigraphic data enable to establish a time frame-

work for the sequences of different hierarchical orders and to

reinterpret the stacking pattern.

The smallest recognizable cyclic units are here called ele-

mentary sequences (following the concepts of Strasser et al.

1999). They are symmetrical with a deepening-upward trend

in the lower and a shallowing-upward trend in the upper part,

separated by a maximum-flooding surface. Their boundaries

are represented by the shallowest lithofacies. In many cases,

sequence boundary and transgressive surface are amalga-

mated, which is explained by the low accommodation on

the shallow ramp. Lowstand deposits thus are only rarely

preserved. Combined into packages of five, these elementary

sequences form 14 larger cyclic units in the Opletnya Member,

here defined as small-scale sequences.

The number of the smallest cyclic units distinguished in

the underlying Svidol Formation, interpreted as parasequen-

ces, is 19 (Ajdanlijsky et al. 2018). Their thicknesses vary

from 0.7 m to 2.9 m (average 1.45 m), similar to those of

the elementary sequences in the Opletnya Member. These

parasequences commonly combine into packages of five, with

the maximum thickness recorded in the lower part of the trans-

gressive interval of these packages. As scale and stacking pat-

tern of the parasequences defined in the Svidol Formation and

of the elementary sequences in the Zimevitsa Member are

very similar to those of the elementary sequences in the

Opletnya Member, a similar formation is assumed. The sub-

sidence rate of the ramp must have been relatively constant

throughout this time interval and allowed for enough accom-

modation to accumulate the observed sedimentary record.

Although minor erosion and/or non-deposition certainly occur-

red at the boundaries of some elementary sequences, there is

no evidence that entire sequences are missing (Strasser 2016).

Comparing the total number of the Aegean elementary and

small-scale sequences with the updated Middle Triassic chart

(Haq 2018), it can be concluded that they are documenting

an allocyclic signal and formed in tune with the precession

(20-kyr) and short eccentricity (100-kyr) orbital cycles,

respectively. The 14 small-scale sequences identified in

the Opletnya Member thus indicate that this member was

deposited over a time period of 1.4 Myr. Furthermore, based

on the similarities in the composition and thickness, it can

be assumed that the elementary and small-scale sequences dis-

tinguished in the lowermost and uppermost Bithynian sub-

stage (i.e small-scale sequences 13 and 14 in the uppermost

Opletnya Member and those from the lowermost Zimevitsa

Member) also represent precession and short eccentricity

cycles (Figs. 7, 10).

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, 2019, 70, 4, 325–354

Concerning the medium-scale sequences, only the one

recognized in the Svidol Formation by Ajdanlijsky et al.

(2018) and the one at the base of the Opletnya Member (see

above) can be attributed to the 405-kyr long eccentricity cycle.

They are both composed of 4 small-scale sequences and

20 elementary sequences and thus reflect the hierarchical

stacking characteristic of a sedimentary system controlled by

orbital cycles (e.g., Strasser et al. 2006).

The highly variable lateral and vertical patterns of facies

and sedimentary structures (see Chapter Sedimentology)

clearly indicate that the shallow ramp hosted diverse sedimen-

tary environments: from shallow marine to supratidal, from

low to high energy, from normal marine to evaporative. These

environments were subjected not only to sea-level fluctuations

controlled by the orbital cycles but also to currents that shifted

sediment bodies and to minor tectonic movements that had

an additional control on accommodation. Consequently, the

observed sedimentary record is the result of a combination of

allocyclic as well as of autocyclic and random processes

(e.g., Pratt & James 1986; Strasser 2018). From the cyclo-

stratigraphic interpretation it appears that the amplitudes of

the sea-level changes induced by the precessional and short

eccentricity cycles were sufficient to create facies changes

that were recorded on the shallow ramp, but that the amplitude

related to the long eccentricity cycle left its traces only

in the Svidol Formation and at the base of the Opletnya

Member.

Major sequences

The interpretation of the small-scale sequences in the Oplet-

nya Member as reflecting the short (100-kyr) eccentricity

cycles allows the correlation with some of the major sequence

boundaries of the Tethyan realm. Ajdanlijsky et al. (2018) cor-

related the base of the Svidol Formation with sequence boun-

dary Ol4 in the upper Olenekian (Hardenbol et al. 1998; Ogg

2012). According to Li et al. (2018), the next major sequence

boundary An1 is located 4 long (405-kyr) eccentricity cycles

above the Ol4 boundary, which corresponds to 16 short

(100-kyr) eccentricity cycles (Fig. 15). In the study area,

the Svidol Formation contains four short eccentricity cycles

that can be correlated with the long eccentricity cycle E13 of

Li et al. (2018). Consequently, the position of sequence boun-

dary An1 has to be placed at the boundary between the 12

and

the 13

small-scale sequence (Fig. 7d), and this corresponds to

the top of cycle E16 of Li et al. (2018). In the Opletnya

Member, this boundary is marked by the relatively deepest

local erosion of over 70 cm. The surface is covered by abun-

dant lags of large intraclasts and marks the top of a 10 m thick

dolomite-dominated interval with tepees, desiccation cracks,

and abundant evaporites.

According to Haq (2018), sequence boundary Ol2 (equiva-

lent to Ol4 of Hardenbol et al. 1998) just below the boundary

between the Spathian and Aegean is dated at 246.9 Ma, and

sequence boundary An1 in the upper Aegean at 245.5 Ma. This

implies a duration of about 1.4 Myr for this major sequence.

However, according to Li et al. (2018) and our own study

(Fig. 15), the top of the Aegean coincides with sequence

boundary An1 and the 16 small-scale sequences identified

here between the two boundaries suggest a duration of

1.6 Myr (Fig. 15). This discrepancy calls for more research

in radiometric dating and in astrochronology. For example,

the date proposed for the Olenekian–Anisian (Spathian–

Aegean) boundary has shifted from 247.1 Ma (Ogg 2012) to

246.8 (Ogg et al. 2016), to 246.9 Ma (Haq 2018), and then to

247.2 Ma in the IUGS International Chronostratigraphic Chart

(2018, version 08).

The lithofacies data obtained in this study match well to

the general facies trends in the major sequence bounded by

Ol4 and An1. According to Li et al. (2018), its maximum-

flooding surface is in the middle part of cycle E14 (Fig. 15).

Based on the number of identified small-scale sequences

above sequence boundary Ol4, this surface has to be placed in

the maximum-flooding zone of the lowermost medium-scale

sequence within the Opletnya Member, marked by muddy

facies and a striking acritarch peak (Fig. 3). Upsection in the

Opletnya Member, facies indicate decreasing water depth and

the gradual establishment of peritidal environments, which led

to precipitation of evaporites and early diagenetic dolomiti-

zation in the small-scale sequences at the top of this major

sequence. The fact that the study of Li et al. (2018) is based on

a deep-water section in South China implies that the general

development of this sequence was controlled by over-regional

parameters.

The identification of the next major sequence boundary

(An2) is rather uncertain. On the one hand, the lithological and

biostratigraphical data suggest that it could be situated at the

top of the 5

elementary sequence of the Babino Formation

(Fig. 10). This boundary marks an interruption in sedimenta-

tion with signs of subaerial exposure and development of

a karstic surface that could have resulted from a prolonged

time gap. On the other hand, its confident placement requires

also reliable data for the cyclicity within the interval between

boundaries An1 and An2. Such information is available for

the uppermost Opletnya Member and lowermost Zimevitsa

Member, while data on the cyclicity within the Lakatnik

Member are not available yet. A peak abundance of marine

acritarchs occurs in the Zimevitsa Member (Fig. 3) and pro-

bably corresponds to the maximum flooding identified by Li

et al. (2018) below sequence boundary An2 (Fig. 15).

Soft-sediment deformation

Besides cyclicity, the sigmoidal structures resulting from

soft-sediment deformation that occur in the lower part of

the Opletnya Member can serve as a potential tool for event

stratigraphy. Michalík (1997) interpreted these deformation

structures as record of tsunamites and Chatalov (2001a, b,

2004) followed this interpretation of seismic activity during

the Anisian. Eleven seismite horizons within muddy lime-

stones were described from the Sfrazen and Lakatnik sections.

In southern, western, and northern directions the number of

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AJDANLIJSKY, STRASSER and GÖTZ

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, 2019, 70, 4, 325–354

these levels decreases, which might provide evidence of

an ancient earthquake epicenter in the area of these two sec-

tions. However, no data showing their orientation were pre-

sented. Thus, the localization of a possible seismic epicenter

remains unsettled.

In the studied sections, synsedimentary deformation is obser-

ved in mudstones, as well as in wacke- and wacke- to pack-

stones (Fig. 6). The stratigraphic position of these deformation

structures is specific: Ajdanlijsky et al. (2018) pointed out that

they occur predominantly within the highstand stage of an ele-

mentary sequence. The newly obtained data confirm this

observation and also shed new light on the facies context of

these structures. Their position in the upper part of the elemen-

tary sequences, restricted lateral distribution, concave-up mor-

phology on top of the individual beds, and their proximity and

temporal correlation with small-scale cut-and-fill structures

indicate that they most probably are connected with soft-sedi-

ment slumping activated by small-scale erosional events.

Sliding on the steep flank of a channel could produce similar

sedimentary structures. Furthermore, storm-induced loading

of non-lithified, cohesive sediment may lead to thixotropic

behavior (e.g., Chen & Lee 2013). Muddy sediment was more

abundant during a highstand phase than during transgression,

which might explain the preferential stratigraphic position of

these features of soft-sediment deformation.

It has to be mentioned that the lateral distribution and scale

of sigmoidal structures is much wider and larger than pre-

viously noted. For example, to the south of the study area,

west of Tserovo village (Fig. 2), a laterally traceable horizon

of such structures is almost half a meter thick and twice as

high above the base of the Opletnya Member than the thickest

level (40 cm) in the studied sections (Fig. 7). Again, this

horizon is developed in the upper (highstand) part of an ele-

mentary sequence.

In our opinion, the sigmoidal deformation structures in

the studied sections are paleoenvironmental indicators of

the initiation of the shallowing trend within the sequences

rather than seismites. However, a detailed lateral mapping of

these structures, including orientation measurements within

isochronous levels, is necessary to elucidate their origin.

Conclusions

The early Anisian (Aegean) ramp deposits of the Opletnya

Member in northwestern Bulgaria feature a prominent cyclical

pattern of the sedimentary record. In the two studied sections

of Lakatnik and Sfrazen, the facies are carbonate-dominated

but also include terrigenous siliciclastic material and evapo-

rites, and are interpreted as having been deposited in a variety

of environments ranging from peritidal to shallow marine.

Deepening-shallowing trends of facies evolution and promi-

nent surfaces allow identifying elementary, small-scale, and

medium-scale sequences. Palynofacies analysis complements

and confirms the lithofacies analysis within selected sequences.

The sequences are hierarchically stacked, with 5 elementary

sequences composing a small-scale one, and 2 or 4 small-scale

sequences composing a medium-scale one. Biostratigraphic

data (conodonts and palynomorphs) allow defining the Anisian

substage boundaries, and thus provide the basis for an estima-

tion of the durations of these sequences. Elementary and

small-scale sequences are interpreted to reflect the signatures

of the orbital precession and short eccentricity cycles with

periodicities of 20 and 100 kyr, respectively. Accordingly, it is

Fig. 15. Cyclostratigraphic chart of the Opletnya Member in the study area. See text for explanation.

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suggested that the Opletnya Member, which comprises 69 ele-

mentary and 14 small-scale sequences (Fig. 15), was depo-

sited within about 1.4 Myr. Medium-scale sequences that

correspond to the long eccentricity cycle of 405 kyr have been

identified only in the Svidol Formation and at the base of

the Opletnya Member. This suggests that the translation of

orbital cycles into sea-level changes that were then recorded

on the shallow ramp was not straightforward, and that other

processes inherent to the sedimentary system (such as lateral

migration of sediment bodies) and/or changes in subsidence

rate must have been at work as well.

Major sequence boundaries are identified at the base of

the Svidol Formation and within the uppermost Opletnya

Mem ber, corresponding to the sequence boundaries Ol4 and

An1 of the Tethyan realm. According to the cyclostratigraphic

interpretation presented here, there are 16 small-scale

sequences between these two sequence boundaries, implying

a duration of 1.6 Myr. Large-scale flooding events are recog-

nized by peak abundances of marine acritarchs, with the most

prominent event being identified in the Pelsonian Zimevitsa

Member. This Pelsonian maximum flooding is recorded in

carbonate ramp systems along the western Tethys shelf and

in the northern Peri-Tethys basin.

This study demonstrates that, based on detailed logging and

facies analysis, a cyclostratigraphic interpretation of shallow

ramp deposits is possible. Within a time framework based on

biostratigraphy and chronostratigraphy, the duration of indi-

vidual meter-scale depositional sequences can be estimated,

and a time resolution of 20 kyr can be achieved to better inter-

pret the evolution of the sedimentary environments.

Furthermore, astrochronologically dated correlation with

regional and over-regional events becomes possible and places

the studied Bulgarian sections in a global context.

Acknowledgements: This study is part of the Triassic Ocean

project TRIO lead by A.E. Götz. The thorough review of Janos

Haas (Budapest) and the comments of the handling editor

Jozef Michalík (Bratislava) are gratefully acknowledged.

References

Adams A., Diamond L.W. & Aschwanden L. 2018: Dolomitization by

hypersaline reflux into dense groundwaters as revealed by verti-

cal trends in strontium and oxygen isotopes: upper Muschelkalk,

Switzerland. Sedimentology 66, 362–390.

Ajdanlijsky G., Tronkov D. & Strasser A. 2004: Cyclicity in the

Lower Triassic Series between Opletnya railway station and

Sfrazen hamlet. In: Sinnyovsky D. (Ed.): Geological routes in

the northern part of the Iskar River Gorge. V. Nedkov Publ.,

90–101.

Ajdanlijsky G., Götz A.E. & Strasser A. 2018: The Early to Middle

Triassic continental–marine transition of NW Bulgaria: Sedi-

mentology, palynology and sequence stratigraphy. Geol. Carpath.

69, 2, 129–148.

Assereto R. & Čatalov G. 1983: The Mogila Formation (Lower–

Middle Triassic) and lithostratigraphical levels in the Triassic

System of NW Bulgaria. Geologica Balc. 13, 6, 29–36

(in Russian).

Assereto R., Tronkov D. & Čatalov G. 1983: The Mogila Formation

(Lower–Middle Triassic) in Western Bulgaria. Geologica Balc.

13, 6, 25–27 (in Bulgarian).

Benatov S. 1998: An attempt for biostratigraphic zonation of West

Bulgarian Middle Triassic on benthonic macrofauna. C.R. Bulg.

Acad. Sci. 51, 11–12, 69–72.

Benatov S. 2000: Macrofauna from Peri-Tethyan Middle Triassic in

the southern slopes of Western Stara Planina Mountain (Western

Bulgaria). In: Bachmann G.H. & Lerche I. (Eds.): Epicontinental

Triassic, Volume 2. Zbl. Geol. Paläont., Teil I (1998), 9/10,

1137–1144.

Benatov S. 2001: Brachiopod biostratigraphy of the Middle Triassic

in Bulgaria and comparison with elsewhere in Europe. In: Brun-

ton H., Robin L., Cocks M. & Long, S.M. (Eds.): Brachiopods

Past and Present. The Systematics Association Special Volume

Series 63, 384–393.

Benatov S. & Chatalov A. 2000: New data about the stratigraphy and

lithology of the Iskar Carbonate Group (Lower–Middle Triassic)

in the Zaburde district, Western Bulgaria. Ann. Univ. Sofia “St.

Kliment Ohridski”, Fac. Geol. Geogr. 93, vol. 1 — geology,

83–105.

Benatov S., Budurov K., Trifonova E. & Petrunova L. 1999: Parallel

biostratigraphy on micro- and megafauna and new data about the

age of the Babino Formation (Middle Triassic) in the Iskur

Gorge, Western Stara Planina Mountains. Geologica Balc. 29,

33–40.

Benton M.J. 2015: When Life Nearly Died: The Greatest Mass

Extinction of All Time. Thames & Hudson, London, 1–352.

Berger A., Loutre M.F. & Dehant V. 1989: Astronomical frequencies

for pre-Quaternary palaeoclimate studies. Terra Nova 1,

474–479.

Borkhataria R., Aigner T. & Pipping K.J.C.P. 2006: An unusual,

muddy, epeiric carbonate reservoir: The Lower Muschelkalk

(Middle Triassic) of the Netherlands. AAPG Bull. 90, 1,

61–89.

Boulila S., Galbrun B., Hinnov L.A. & Collin P.-Y. 2008: High-reso-

lution cyclostratigraphic analysis from magnetic susceptibility in

a Lower Kimmeridgian (Upper Jurassic) marl-limestone succes-

sion (La Méouge, Vocontian Basin, France). Sediment. Geol.

203, 54–63.

Budai T. & Vörös A. 2006: Middle Triassic platform and basin evolu-

tion of the Southern Bakony Mountains (Transdanubian Range,

Hungary). Riv. Ital. Paleont. S. 112, 359–371.

Budurov K. 1976: Die triassischen Conodonten des Ostbalkans.

Geologica Balc. 6, 2, 95–104.

Budurov K. 1980: Conodont Stratigraphy of the Balkanide Triassic.

Riv. Ital. Paleont. 85, 3–4, 767–780.

Budurov K. & Petrunova L. 2000: Muschelkalk conodonts as compo-

nents of Peri-Tethyan fauna. In: Bachmann G.H. & Lerche I.

(Eds.): Epicontinental Triassic, Volume 2. Zbl. Geol. Paläont.,

Teil I (1998), 9/10, 989–995.

Budurov K. & Stefanov S.A. 1972: Plattform-Conodonten und ihre

Zonen in der Mittleren Trias Bulgariens. Mitt. Ges. Geol. Berg

baustud. 21, 829–852.

Budurov K. & Stefanov S.A. 1973: Etliche neue Platform-Conodon-

ten aus der Mitteltrias Bulgariens. C. R. Acad. Bulg. Sci. 26,

803–806.

Budurov K. & Stefanov S.A. 1975: Neue Daten über die Conodon-

tenchronologie der balkaniden Mittleren Trias. C. R. Acad. Bulg.

Sci. 28, 791–794.

Budurov K. & Trifonova E. 1984: Correlation of Triassic conodont

and foraminiferal zonal standards in Bulgaria. C. R. Acad. Bulg.

Sci. 37, 625–627.

Budurov K. & Trifonova E. 1995: Conodont and foraminiferal

successions from the Triassic of Bulgaria. Geologica Balc. 25,

13–19.

352

AJDANLIJSKY, STRASSER and GÖTZ

GEOLOGICA CARPATHICA

, 2019, 70, 4, 325–354

Budurov K., Trifonova E. & Zagorčev I. 1995: The Triassic in south-

west Bulgaria. Stratigraphic correlation of key sections in the

Iskar Carbonate Group. Geologica Balc. 25, 27–59.

Budurov K., Zagorchev I., Trifonova E. & Petrunova L. 1997:

The Triassic in Eastern Stara planina Mts. Lithostratigraphic

notes. Rev. Bulg. Geol. Soc. 58, 2, 101–110.

Čatalov G. 1975: Facies analysis of the Svidol Formation (Lower

Triassic) of the Teteven anticlinorium (Central Fore-Balkan).

Geologica Balc. 5, 2, 67–86.

Čatalov G. & Visscher H. 1990: Palynomorphs from Stoilovo Forma-

tion, Veleka Group (Triassic), Strandža Mountain, Southeast

Bulgaria. Geologica Balc. 20, 2, 53–57.

Catuneanu O. 2019: Scale in sequence stratigraphy. Mar. Petrol.

Geol. 106, 128–159.

Catuneanu O., Abreu V., Bhattacharya J.P., Blum M.D., Dalrymple

R.W., Eriksson P.G., Fielding C.R., Fisher W.L., Galloway W.E.,

Gibling M.R., Giles K.A., Holbrook J.M., Jordan R., Kendall

C.G.St.C., Macurda B., Martinsen O.J., Miall A.D., Neal J.E.,

Nummedal D., Pomar L., Posamentier H.W., Pratt B.R., Sarg

J.F., Shanley K.W., Steel R.J., Strasser A., Tucker M.E. &

Winker C. 2009: Towards the standardization of sequence stra-

tigraphy. EarthSci. Rev. 92, 1–33.

Chatalov G. 1980: Two facies types of Triassic in Strandza Mountain,

Bulgaria. Riv. Ital. Paleont. S. 85, 3/4, 1029–1046.

Chatalov G. 1991: Triassic in Bulgaria — a review. Bull. Tech. Univ.

Istanbul 44, 1/2, 103–135.

Chatalov A. 1997: Sedimentology of the carbonate rocks from the

Mogila Formation in Western Balkanides. PhD Thesis Abstract,

Sofia University “St. Kl. Ohridski”, 1–46 (in Bulgarian).

Chatalov A. 1998: Arid carbonate tidal flat sedimentation imprinted

in the Mogila Formation (Spathian-Anisian) from the Western

Balkanides, Bulgaria. Hallesches Jahrb. Geowiss. Reihe B,

Beih. 5, 32–33.

Chatalov A. 2000: The Mogila Formation (Spathian-Anisian) in the

Western Balkanides of Bulgaria — ancient counterpart of an arid

peritidal complex. In: Bachmann G.H. & Lerche I. (Eds.):

Epicontinental Triassic, Volume 2. Zbl. Geol. Paläont., Teil I

(1998), 9/10, 1123–1135.

Chatalov A. 2001a: Deformational structures in the Iskar Carbonate

group (Lower-Upper Triassic) from the Western Balkanides.

Geologica Balc. 30, 3–4, 43–57.

Chatalov A. 2001b: Signature of paleoseismic activity in the Triassic

carbonate rocks from Northwestern Bulgaria. Sediment 2001,

Jena, Abstract Vol., 29.

Chatalov A. 2004: Route VIII. Sedimentological sites in the carbonate

Triassic between Lakatnik railway station and Opletnya village.

In: Sinnyovsky D. (Ed.): Geological routes in the northern part

of the Iskar River Gorge. V. Nedkov Publ., 102–116.

Chatalov A. 2013: A Triassic homoclinal ramp from the Western

Tethyan realm, Western Balkanides, Bulgaria: Integrated insight

with special emphasis on the Anisian outer to inner ramp facies

transition. Palaeogeogr. Palaeoclimatol. Palaeoecol. 386, 34–58.

Chatalov A. 2016: Dominant autogenic control on the formation of

peritidal cycles in the Olenekian carbonate succession of NW

Bulgaria. Geosciences 2016, Abstract Vol., 109–110.

Chatalov A. 2018: Global, regional and local controls on the develop-

ment of a Triassic carbonate ramp system, Western Balkanides,

Bulgaria. Geol. Mag. 155, 641–673.

Chatalov A., Benatov S. & Vangelov D. 2001: New data about the

holostratotype of the Babino Formation (Middle Triassic). Ann.

Univ. Sofia “St. Kliment Ohridski”, Fac. Geol. Geogr. 94, Vol. 1

– geology, 27–40.

Chemberski G., Vaptsarova A. & Monahov I. 1974: Lithostratigraphy

of the Triassic variegated terrigenous-carbonate and carbonate

sediments studied with deep drilling in Northwestern and Central

North Bulgaria. Ann. DSO Geol. Res. 20, 327–341 (in Bulgarian).

Chen J. & Lee H.S. 2013: Soft-sediment deformation structures in

Cambrian siliciclastic and carbonate storm deposits (Shan-

dong Province, China): Differential liquefaction and flui-

dization triggered by storm-wave loading. Sediment. Geol.

288, 81–94.

Chen Y.L., Krystyn L., Orchard M.J., Lai X.L. & Richoz S. 2015:

A review of the evolution, biostratigraphy, provincialism, and

diversity of Middle and early Late Triassic conodonts. Pap.

Palaeontol. 2, 235–263.

Chen Y., Scholze F., Richoz S. & Zhang Z. 2019: Middle Triassic

conodont assemblages from the Germanic Basin: implications

for multi-element taxonomy and biogeography. J. Syst. Palaeon

tol. 17, 5, 359–377.

Chrząstek A. 2013: Trace fossils from the Lower Muschelkalk of

Raciborowice Górne (North Sudetic Synclinorium, SW Poland)

and their palaeoenvironmental interpretation. Acta Geol. Polon.

63, 3, 315–353.

Combaz A. 1964: Les palynofaciès. Rev. Micropaléont. 7, 205–218.

Combaz A. 1980: Les kérogènes vus au microscope. In: Durand B.

(Ed.): Kerogen: Insoluble organic matter from sedimentary

rocks. Edition Technip, 55–111.

Escudero-Mozo M.J., Márquez-Aliaga A., Goy A., Martín-Chivelet

J., López-Gómez J., Márquez L., Arche A., Plasencia P., Pla C.,

Marzo M. & Sánchez-Fernández D. 2015: Middle Triassic car-

bonate platforms in eastern Iberia: Evolution of their fauna and

palaeogeographic significance in the western Tethys. Palaeo

geogr. Palaeoclimatol. Palaeoecol. 417, 236–260.

Feist-Burkhardt S., Götz A.E., Szulc J. (coordinators), Borkhataria R.,

Geluk M., Haas J., Hornung J., Jordan P., Kempf O., Michalík J.,

Nawrocki J., Reinhardt L., Ricken W., Röhling H.-G., Rüffer T.,

Török Á. & Zühlke R. 2008: Triassic. In: McCann T. (Ed.):

The Geology of Central Europe. Vol. 2. Geological Society,

London, 749–821.

Flügel E. 2004: Microfacies of Carbonate Rocks. Springer, Berlin,

1–976.

Ganev M. 1974: Stand der Kenntnisse über die Stratigraphie der Trias

Bulgariens. In: Zapfe H. (Ed.): Die Stratigraphie der alpin-medi-

terranen Trias. Symposium Wien, Schriftenr. Erdwissenschaftl.

Komm., Österr. Akad. d. Wissenschaften, Bd. 2, 93–96.

Głuchowski E. & Salamon M. 2005: The Lower Muschelkalk cri-

noids from Raciborowice, North-Sudetic Basin, SW Poland.

Geol. Quarterly 49, 1, 83–92.

Gonzalez R. & Eberli G.P. 1997 : Sediment transport and bedforms in

a carbonate tidal inlet; Lee Stocking Island, Exumas, Bahamas.

Sedimentology 44, 1015–1030.

Götz A.E. 1996: Fazies und Sequenzanalyse der Oolithbänke (Unte-

rer Muschelkalk, Trias) Mitteldeutschlands und angrenzender

Gebiete. Geol. Jb. Hessen 124, 67–86.

Götz A.E. & Török Á. 2008: Correlation of Tethyan and Peri-Tethyan

long-term and high-frequency eustatic signals (Anisian, Middle

Triassic). Geol. Carpath. 59, 4, 307–317.

Götz A.E. & Török Á. 2018: Muschelkalk ramp cycles revisited. In:

Montenari M. (Ed.): Stratigraphy & Timescales, vol. 3: Cyclo-

stratigraphy and Astrochronology, 265–284.

Götz A.E., Török Á., Feist-Burkhardt S. & Konrád Gy. 2003: Palyno-

facies patterns of Middle Triassic ramp deposits (Mecsek Mts.,

S Hungary): A powerful tool for high-resolution sequence stra-

tigraphy. Mitt. Ges. Geol. Bergbaustud. Österr. 46, 77–90.

Götz A.E., Luppold F.W. & Hagdorn H. 2019: Biostratigraphie und

Zonierung der Conodonten des Muschelkalks. In: Deutsche

Stratigraphische Kommission (Ed.): Stratigraphie von Deutsch-

land XIII. Muschelkalk. Schriftenr. Dt. Ges. Geowiss. 91, in

press.

Haas J., Kovács S. & Török Á. 1995: Early Alpine shelf evolution in

the Hungarian segment of the Tethys margin. Acta Geol. Hung.

38, 95–110.

353

MIDDLE TRIASSIC RAMP DEPOSITS (NW BULGARIA)

GEOLOGICA CARPATHICA

, 2019, 70, 4, 325–354

Haas J., Budai T. & Raucsik B. 2012: Climatic controls on sedimen-

tary environments in the Triassic of the Transdanubian Range

(Western Hungary). Palaeogeogr. Palaeoclimatol. Palaeoecol.

353–355, 31–44.

Hagdorn H. & Głuchowski E. 1993: Palaeobiogeography and strati-

graphy of Muschelkalk echinoderms (Crinoidea, Echinoidea)

in Upper Silesia. In: Hagdorn H. & Seilacher A. (Eds.):

Muschelkalk. Schöntaler Symposium 1991. Goldschneck, Korb,

165–176.

Hardenbol J., Thierry J., Farley M., Jacquin T., de Graciansky P.-C. &

Vail P. 1998: Mesozoic and Cenozoic sequence chronostrati-

graphic framework of European basins. SEPM Spec. Publ. 60,

1–13.

Hardie L.A. & Garrett P. 1977: General environmental setting. In:

Hardie L.A. (Ed.): Sedimentation on the Modern Carbonate

Tidal Flats of Northwest Andros Island, Bahamas. John Hopkins

University, Stud. Geol. 22, 12–49.

Hardie L.A. & Ginsburg R.N. 1977: Layering: the origin and environ-

mental significance of lamination and thin bedding. In: Hardie

L.A. (Ed.): Sedimentation on the Modern Carbonate Tidal Flats

of Northwest Andros Island, Bahamas. John Hopkins University,

Stud. Geol. 22, 50–123.

Haq B.U. 2018: Triassic eustatic variations re-examined. GSA Today

28, https://doi.org/10.1130/GSATG381A.1.

Heunisch C. 1999: Die Bedeutung der Palynologie für Biostratigra-

phie und Fazies in der Germanischen Trias. In: Hauschke N.

& Wilde V. (Eds.): Trias — eine ganz andere Welt, Europa

am Beginn des Erdmittelalters. PfeilVerlag, München,

207–220.

Heunisch C. 2019: Palynomorphs of the Muschelkalk. In: Deutsche

Stratigraphische Kommission (Ed.): Stratigraphie von Deutsch-

land XIII. Muschelkalk. Schriftenr. Dt. Ges. Geowiss. 91, in

press.

Hillgärtner H., Dupraz C. & Hug W. 2002: Microbially induced ce-

mentation of carbonate sands: are micritic meniscus cements

good indicators of vadose diagenesis? Sedimentology 48,

117–131.

Hinnov L.A. 2013: Cyclostratigraphy and its revolutionizing appli-

cations in the Earth and planetary sciences. GSA Bull. 125,

1703–1734.

Hinnov L.A. 2018: Cyclostratigraphy and astrochronology in 2018.

In: Montenari M. (Ed.): Stratigraphy & Timescales, vol. 3:

Cyclostratigraphy and Astrochronology. Acad. Press, 1–80.

Ivanov Zh. 1998: Tectonics of Bulgaria. Unpubl. Habil. Thesis. Sofia

University “St. Kliment Ohridski”, 1–634 (in Bulgarian).

IUGS International Chronostratigraphic Chart 2018: International

Commission on Stratigraphy, version 2018/08. www.strati-

graphy.org.

Kalvacheva R.K. & Čatalov G.A. 1974: Palynomorphen aus phylli-

toiden Tonschiefern des Strandza-Gebirges. C. R. Acad. Bulg.

Sci. 27, 10, 1419–1422.

Knaust D. 1998: Trace fossils and ichnofabrics on the Lower Muschel-

kalk carbonate ramp (Triassic) of Germany: tool for high-resolu-

tion sequence stratigraphy. Geol. Rundsch. 87, 21–31.

Knaust D. 2007: Invertebrate trace fossils and ichnodiversity in shal-

low-marine carbonates of the German Middle Triassic (Muschel-

kalk). In: Bromley R.G., Buatois L., Mángano G., Genise J.F. &

Melchor R.N. (Eds.): Sediment–Organism Interactions: A Multi-

faceted Ichnology. SEPM Spec. Publ. 88, 221–238.

Knaust D., Curran H.A. & Dronov A.V. 2012: Shallow-marine car-

bonates. In: Knaust D. & Bromley R.G. (Eds.): Trace fossils as

indicators of sedimentary environments. Dev. Sedimentol. 64,

705–750.

Kustatscher E. & Roghi G. 2006: Anisian palynomorphs from the

Dont Formation of the Kühwiesenkopf/Monte Prà Della Vacca

Section (Northern Italy). Micropalaeontology 52, 3, 223–244.

Li M., Huang C., Hinnov L., Chen W., Ogg J. & Tian W. 2018: Astro-

chronology of the Anisian stage (Middle Triassic) at the Guandao

reference section, South China. Earth Planet. Sci. Lett. 482,

591–606.

Lukoczki G., Haas J., Gregg J.M., Machel H.G., Kele S. & John C.M.

2019: Multi-phase dolomitization and recrystallization of Mid-

dle Triassic shallow marine-peritidal carbonates from the

Mecsek Mts. (SW Hungary), as inferred from petrography, car-

bon, oxygen, strontium and clumped isotope data. Mar. Petrol.

Geol. 101, 440–458.

Matysik M. 2016: Facies types and depositional environments of

a morphologically diverse carbonate platform: a case study from

the Muschelkalk (Middle Triassic) of Upper Silesia, Southern

Poland. Ann. Soc. Geol. Polon. 86, 119–164.

Michalík J. 1997: Tsunamites in a storm-dominated Anisian carbo-

nate ramp (Vysoká Formation, Malé Karpaty Mts., Western

Carpathians). Geol. Carpath. 48, 4, 221–229.

Michalík J., Masaryk P., Lintnerová O., Papšová J., Jendrejáková O.

& Reháková D. 1992: Sedimentology and facies of a storm-

domi nated Middle Triassic carbonate ramp (Vysoká Formation,

Malé Karpaty Mts., Western Carpathians). Geol. Carpath. 43, 4,

213–230.

Montañez I.A. & Osleger D.A. 1993: Parasequence stacking patterns,

third-order accommodation events, and sequence stratigraphy

of Middle to Upper Cambrian platform carbonates, Bonanza

King Formation, southern Great Basin. AAPG Mem. 57,

305–326.

Montenat C., Barrier P., Ott d’Estevou P. & Hibsch C. 2007: Seismites:

An attempt at critical analysis and classification. Sediment. Geol.

196, 5–30.

Niedźwiedzki R. & Salamon M., 2006: Triassic crinoids from the

Tatra Mountains and their stratigraphic significance (Poland).

Geol. Carpath. 57, 2, 69–77.

Ogg J. 2012: Triassic. In: Gradstein F., Ogg J., Schmitz M. & Ogg G.

(Eds.): The geological time scale 2012. Elsevier, Amsterdam,

681–730.

Ogg J.G., Ogg G.M. & Gradstein F.M. 2016: The Concise Geologic

Time Scale 2016. Elsevier, Amsterdam, 1–234.

Petrash D.A., Bialik O.M., Bontognali T.R.R., Vasconcelos C.,

Roberts J.A., McKenzie J.A. & Konhauser K.O. 2017: Micro-

bially catalyzed dolomite formation: From near-surface to burial.

Earth Sci. Rev. 171, 558–582.

Petrunova L. 1992a: First palynological evidence of the Ladinian Age

of the Preslav Formation in Northwest Bulgaria. Geol. Bulgarica

22, 2, 46.

Petrunova L. 1992b: Palynological evidence of the Early Karnian Age

of the Moesian Group in Northwest Bulgaria. Geol. Bulgarica

22, 2, 94.

Petrunova L. 1999: Palynological correlations of the Preslav Forma-

tion (Iskur Carbonate Group) to the Peri-Tethyan Triassic of

Central Europe. Geol. Bulgarica 29, 1–2, 136.

Petrunova L. 2000: Palynomorphs around the boundary Lower–

Middle Triassic from an olistolith in Eastern Stara Planina

Mountains, Bulgaria. In: Proceed. Int. Workshop on the Lower–

Middle Triassic (Olenekian-Anisian) boundary. Tulcea, Roma-

nia, 53–55.

Philip J., Masse J. & Camoin G. 1996: Tethyan carbonate platforms.

In: Nairn A.E.M., Ricou L.-E., Vrielynck B. & Dercourt J.

(Eds.): The Ocean Basins and Margins. Vol. 8. The Tethys

Ocean. Plenum Press, New York–London, 239–266.

Pöppelreiter M. 2002: Facies, cyclicity and reservoir properties of the

Lower Muschelkalk (Middle Triassic) in the NE Netherlands.

Facies 46, 119–132.

Posamentier H.W., Allen G.P. & James D.P. 1992: High-resolution

sequence stratigraphy — the East Coulee delta, Alberta. J. Sedi

ment. Petrol. 62, 310–317.

354

AJDANLIJSKY, STRASSER and GÖTZ

GEOLOGICA CARPATHICA

, 2019, 70, 4, 325–354

Pratt B.R. & James N.P. 1986: The St George Group (Lower Ordovi-

cian) of western Newfoundland: tidal flat island model for car-

bonate sedimentation in shallow epeiric seas. Sedimentology 33,

313–343.

Rameil N., Götz A.E. & Feist-Burkhardt S. 2000: High-resolution se-

quence interpretation of epeiric shelf carbonates by means of

palynofacies analysis: an example from the Germanic Triassic

(Lower Muschelkalk, Anisian) of East Thuringia, Germany.

Facies 43, 123–144.

Reineck H.-E. & Singh I.B. 1975: Depositional Sedimentary Envi-

ronments. Springer, Berlin, 1–439.

Retallack G.J. 2013: Permian and Triassic greenhouse crises. Gond

wana Res. 24, 90–103.

Rychliński T. & Szulc J. 2005: Facies and sedimentary environments

of the Upper Scythian–Carnian succession from the Belanské

Tatry Mts., Slovakia. Ann. Soc. Geol. Polon. 75, 155–69.

Stampfli G.M., Hochard C., Vérard C., Wilhem C. & von Raumer J.

2013: The formation of Pangea. Tectonophysics 593, 1–19.

Stefani M., Furin S. & Gianolla P. 2010: The changing climate frame-

work and depositional dynamics of the Triassic carbonate plat-

forms from the Dolomites. Palaeogeogr. Palaeoclimatol.

Palaeoecol. 290, 43–57.

Strasser A. 2016: Hiatuses and condensation: an estimation of time

lost on a shallow carbonate platform. Depositional Record 1,

91–117.

Strasser A. 2018: Cyclostratigraphy of shallow-marine carbonates —

limitations and opportunities. In: Montenari M. (Ed.): Stratigra-

phy & Timescales, vol. 3: Cyclostratigraphy and Astrochronolo-

gy, 151–187.

Strasser A., Pittet B., Hillgärtner H. & Pasquier J.-B. 1999: Deposi-

tional sequences in shallow carbonate-dominated sedimentary

systems: concepts for a high-resolution analysis. Sediment.

Geol. 128, 201–221.

Strasser A., Hillgärtner H., Hug W. & Pittet B. 2000: Third-order

depo sitional cycles reflecting Milankovitch cyclicity. Terra

Nova 12, 303–311.

Strasser A., Hilgen F.J. & Heckel P.H. 2006: Cyclostratigraphy —

concepts, definitions and applications. Newsl. Stratigraphy 42,

75–114.

Szulc J. 2000: Middle Triassic evolution of the northern Peri-Tethys

area as influenced by early opening of the Tethys ocean. Ann.

Soc. Geol. Polon. 70, 1–48.

Török Á. 1998: Controls on development of Mid-Triassic ramps:

examples from southern Hungary. In: Wright, V.P. & Burchette

T.P. (Eds.): Carbonate ramps. Geol. Soc. London, Spec. Publ.

149, 339–367.

Toula F. 1878: Geologische Untersuchungen im westlichen Theile

des Balkan und in angrenzenden Gebieten. VII Ein geolo-

gisches Profil von Vraca an der Isker und durch die Isker-

schluchten nach Sofia. Sitzber. d. k. Akad. Wissenschaften Wien

77, 247–317.

Tronkov D. 1960: About the Triassic stratigraphy in Iskar River

Gorge. Ann. Geol. res. Direct. 10, 131–153.

Tronkov D. 1968: The Lower–Middle Triassic boundary in Bulgaria.

Comm. geol. inst. Bulg. Acad. Sci., paleont. ser. 17, 113–130

(in Bulgarian).

Tronkov D. 1973: Grundlagen der Stratigraphie der Trias im Belo-

gradčik Antiklinorium (Nordwest Bulgarien). Bull. Geol. Inst.,

ser. Strat. Lithol. 22, 73–98.

Tronkov D. 1976: Triassische Ammoniten-Sukzessionen im Westli-

chen Balkangebirge in Bulgarien. C. R. Acad. bulg. sci. 29, 9,

1325–1328.

Tronkov D. 1981: Stratigraphy of the Triassic System in part of the

Western Srednogorie (West Bulgaria). Geologica Balc. 11, 1,

3–20 (in Russian).

Tronkov D. 1983: The Mogila Formation (Lower–Middle Triassic) in

Iskar River Gorge and Vratza Mountain (Western Stara Planina

Mountain). Geologica Balc. 13, 6, 37–52 (in Russian).

Tronkov D. 1989: The carbonate Triassic in Lakatnik section. In:

Nachev I. (Ed.): Stratigraphy and sedimentology of the Phanero-

zoic in Bulgaria. XIV Congr. Carp.-Balk. Geol. Assoc., Field

guide, 27–29 (in Russian).

Tronkov D. 1992: Godec Formation and Mazgoska Formation — two

new Middle Triassic lithostratigraphic units in the Western Stara

Planina mountain. Geologica Balc. 22, 6, 62.

Tronkov D. 1995: Triassic System. In: Haydutov I. (Ed.): Explanation

notes to the geological map of Bulgaria on scale 1:100,000,

Berkovitca map sheet. Avers, Sofia, 44–59 (in Bulgarian).

Tronkov D., Encheva M. & Trifonova T. 1965: Stratigraphy of the

Triassic system in north-western Bulgaria. Proc. Geol. Inst., Bul

garian Academy of Sciences 14, 261–292.

Tyson R.V. 1987: The genesis and palynofacies characteristics of

marine petroleum source rocks. In: Brooks J. & Fleet A.J. (Eds.):

Marine Petroleum Source Rocks. Geol. Soc. Spec. Publ. 26, 47–67.

Tyson R.V. 1993: Palynofacies analysis. In: Jenkins D.G. (Ed.): Applied

Micropaleontology. Kluwer Academic Publishers, Dordrecht,

153–191.

Tyson R.V. 1995: Sedimentary organic matter: organic facies and

palynofacies. Chapman & Hall, London, 1–615.

Vail P.R., Audemard F., Bowman S.A., Eisner P.N. & Perez-Cruz C.

1991: The stratigraphic signatures of tectonics, eustasy and

sedimentology — an overview. In: Einsele G., Ricken W. &

Seilacher A. (Eds.): Cycles and Events in Stratigraphy. Springer,

Berlin, 617–659.

van Wagoner J., Mitchum R., Campion K. & Rahmanian V. 1990:

Siliciclastic Sequence Stratigraphy in Well Logs, Cores, and

Outcrops. AAPG Methods in Exploration 7, 1–55.

Wood G.D., Gabriel A.M. & Lawson J.C. 1996: Palynological tech-

niques — processing and microscopy. In: Jansonius J. & McGre-

gor D.C. (Eds.): Palynology: Principles and Applications. AASP

Foundation 1, 29–50.

Yanev S. 2000: Palaeozoic terranes of the Balkan Peninsula in the

framework of Pangea assembly. Palaeogeogr. Palaeoclimatol.

Palaeoecol. 161, 151–177.

Zagorchev I. & Budurov K. 2009: Triassic geology. In: Zagorchev I.,

Dabovski Ch. & Nikolov T. (Eds.): Geology of Bulgaria, vol. 2,

Mesozoic geology. Acad. Publ. “Prof. Marin Drinov”, Sofia,

39–131.

Zdravkov A., Ajdanlijsky G., Gross D. & Bech A. 2019: Organic

petrological and geochemical properties of jet from the middle

Triassic Mogila Formation, West Bulgaria. Geol. Carpath. 70,

62–74.