GeolCarp_Vol68_No6_543

GEOLOGICA CARPATHICA

, DECEMBER 2017, 68, 6, 543–561

doi: 10.1515/geoca-2017-0036

www.geologicacarpathica.com

Sedimentary record of subsidence pulse at the

Triassic/Jurassic boundary interval in the

Slovenian Basin (eastern Southern Alps)

BOŠTJAN ROŽIČ

, TEA KOLAR JURKOVŠEK

, PETRA ŽVAB ROŽIČ

and LUKA GALE

1,2

Department of Geology, Faculty of Natural Sciences and Engineering, University of Ljubljana, Aškerčeva 12, 1000 Ljubljana, Slovenia;

bostjan.rozic@ntf.uni-lj.si

Geological Survey of Slovenia, Dimičeva 14, 1000 Ljubljana, Slovenia

(Manuscript received January 19, 2017; accepted in revised form September 28, 2017)

Abstract: In the Alpine Realm the Early Jurassic is characterized by the disintegration and partial drowning of vast

platform areas. In the eastern part of the Southern Alps (present-day NW Slovenia), the Julian Carbonate Platform and

the adjacent, E–W extending Slovenian Basin underwent partial disintegration, drowning and deepening from

the Pliensbachian on, whereas only nominal environmental changes developed on the large Dinaric (Friuli, Adriatic)

Carbonate Platform to the south (structurally part of the Dinarides). These events, however, were preceded by an earlier

— and as yet undocumented extensional event — that took place near the Triassic/Jurassic boundary. This paper provides

evidence of an accelerated subsidence from four selected areas within the Slovenian Basin, which show a trend of

eastwardly-decreasing deformation. In the westernmost (Mrzli vrh) section — the Upper Triassic platform-margin —

massive dolomite is overlain by the earliest Jurassic toe-of-slope carbonate resediments and further, by basin-plain

micritic limestone. Further east (Perbla and Liščak sections) the Triassic–Jurassic transition interval is marked by

an increase in resedimented carbonates. We relate this to the increasing inclination and segmentation of the slope and

adjacent basin floor. The easternmost (Mt. Porezen) area shows a rather monotonous, latest Triassic–Early Jurassic

basinal sedimentation. However, changes in the thickness of the Hettangian–Pliensbachian Krikov Formation point to

a tilting of tectonic blocks within the basin area. Lateral facies changes at the base of the formation indicate that the tilting

occurred at and/or shortly after the Triassic/Jurassic boundary.

Keywords: Southern Alps, Slovenian Basin, rifting, Triassic/Jurassic boundary, Sinnemurian, resedimented limestones,

block tilting.

Introduction

The opening of the Central Atlantic and the related marginal

oceanic basins (e.g., Piemont–Liguria Ocean) brought about

a major reorganization of paleogeographic units in the western

Neotethys area (Schmid et al. 2008; de Graciansky et al. 2011;

Masini et al. 2013). Although crustal extension has been

documented for the interval extending from the Late Triassic

to the Middle Jurassic, the main paleogeographic changes tend

to be concentrated in a relatively short period postdating the

Triassic–Jurassic boundary. On the European rifted margin,

the extension resulted in an intense block-tilting along listric

faults, which is reflected in pronounced lateral changes within

Lower Jurassic deposits (Lemoine et al. 2000; Chevalier et al.

2003; de Graciansky et al. 2011). The entire southern Tethian

rifted margin, situated on the Apulian (Adriatic) microplate, is

likewise marked by the disintegration and partial drowning of

the vast Late Triassic/earliest Jurassic carbonate platform.

In the Austroalpine domain this resulted in a significantly

reduced extension of the Hauptdolomit–Dachstein Platform

(Mandl 2000; Böhm 2003; Gawlick et al. 2009, 2012), which

was followed by the formation of horst and graben structure

(Bernoulli & Jenkyns 1974; Eberli 1988; Krainer et al. 1994).

Prominent, latest Triassic–early Lower Jurassic differentiation

of the sedimentary environments is reported also from the

Central and Inner Carpathian units and the Transdanubian

Range unit (Vörös & Galácz 1998; Plašienka 2002, 2003;

Haas et al. 2014).

In the Southern Alps, the earliest Jurassic (Late Hettangian–

Sinemurian) was influenced by a diffuse rifting phase (Berra

et al. 2009), with the extension resulting in the formation of

four large-scale sedimentary units (Fig. 1): the internally

highly-dissected Lombardian Basin to the west, the inter-

mediate Trento Platform, the Belluno Basin, and the Friuli

Platform to the east (Winterer & Bosellini 1981; Bertotti et al.

1993; Sarti et al. 1993). The latter continues to the SE as the

vast Dinaric (Adriatic) Carbonate Platform (Vlahović et al.

2005). In the easternmost part of the Southern Alps (present -

day northern Slovenia), however, a prominent pre-Jurassic

paleotopography existed. This originated in the Middle

Triassic (Buser 1989, 1996; Šmuc & Čar 2002) and was

related to the opening of the Neotethys Ocean (Vrabec et al.

2009). The central paleogeographic unit was the Slovenian

Basin (SB), bounded by the Julian Carbonate Platform (JCP)

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to the present north, and the Dinaric Carbonate Platform

(DCP) to the present south (Cousin 1981; Buser 1989, 1996;

Rožič 2016). Because this region was paleogeographically

quite distant from the main rifting center of the Piemont-

Liguria Ocean, and owing to the inherited pre-Jurassic relief,

the aforementioned large-scale earliest Jurassic paleogeo-

graphic perturbations are not observed in the eastern sector

of the Southern Alps. However, all of the previously described

structural and paleogeographic changes can be recognized

on a smaller scale. This paper presents the evidence of such

events as recorded in the successions of the SB. Four areas

were selected where sedimentary reflection of crustal defor-

mation is best recognized: A) the Mrzli vrh section docu-

ments the drowning of the carbonate platform margins,

B) the internal deformation of the basin floor is recorded at

Perbla Village and Liščak Gorge, and C) the block tilting is

evident from the Mt. Porezen sections. The paper presents

new data related to bed-to-bed section-logging, microfacies

and lithoclast ana lysis, and foraminiferal and conodont

bio strati graphy.

Geological setting

General overview

Structure: The studied sections are located in the foothills of

the Julian Alps in NW Slovenia, from the town of Tolmin in

the west to the town of Cerkno in the east. The rocks of the

three main paleogeographic units, namely the JCP, SB and

DCP, are in thrust contacts (Fig. 2a). The DCP successions

belong structurally to the External Dinarides, which were

affected by post-Eocene SW-directed thrusting, whereas

successions of the JCP and the SB belong to the Southern

Alps and are characterized by the Miocene S-directed thrus-

ting (Placer 1999, 2008; Vrabec & Fodor 2006). Within the

Southern Alps, the Julian Nappe is made up of the formations

of the JCP. It is in thrust contact with the structurally-lower

Tolmin Nappe of the Southern Alps, comprising the SB

successions (Placer 1999). The Tolmin Nappe is further

divided into three lower-order thrust units: the lowest

Podmelec Nappe, the middle Rut Nappe, and the upper Kobla

Fig. 1. a — Position in Europe (boxed area marks part of Alpine chain presented in Fig. 1b); b — Present-day position of Early Jurassic basins

of the Southern Alps within the general structure of the Alps (please note that partly emerged areas west of the Lombardian Basin are not out-

lined) and schematic cross-section across the Southern Alps rifted margin of Adria after the first Early Jurassic extensional stage (modified

from Bosellini et al. 1981; Channell & Kozur 1997; Placer 2008; Berra et al. 2009; Rožič 2016).

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Jelovš

če

Zadlaš

čica

500m

Cerkno

5km

TRNOVO NAPPE

JULIAN NAPPE

RUT NAPPE

KOBLA NAPPE

roads

Bohinj lake

Bohinjska

Bistrica

Tolmin

railway with stations

railway tunnel

lake

Fig 2c

Fig 4

GEOGRAPHICAL MARKS

RIVER

ROAD

FAULT

ANTICLINE

STRIKE & DIP OF BEDS

STRUCTURE:

fault

southalpine thrust

position of studied section

QUATERNARY:

MESOZOIC:

TILLITE

SCREE

CONGLOMERATE

MEGABRECCIA

LIMESTONE BRECCIA

MARL

RADIOLARIAN CHERT

CARBONATES

DOMINANT LITHOLOGY:

BAČA DOLOMITE FM

& MASSIVE DOLOMITE

KRIKOV FM

PERBLA FM

TOLMIN FM

BIANCONE

LIMEST. FM

LOWER FLYSCHOID FM

J,K

-J

3-5

1-3

AMFICLINA BEDS

2+3

MAIN DOLOMITE FM

& DACHSTEIN LIMEST. FM

2+3

VOLČE LIMESTONE FM

UPPER FLYSCHOID FM

-K

LITHOSTRATIGRAPHY:

STRIKE & INVERSE DIP OF BEDS

Tolmin

Tolminske

Ravne

PODMELEC NAPPE

TOLMIN NAPPE

Liš

čak

eža

500m

Fig

Kneške

Ravne

Kneža

500m

Soča

Kobarid

Tolmin

Zatolmin

Mt Grmuč

1196m.a.s.l.

Mt Vodel

1053m.a.s.l.

Mt Na vrhu

1030m.a.s.l.

Nappe (Buser 1987). In the transitional zone between the

Dinarides and the Southern Alps older NW–SE-oriented struc-

tures are overprinted by W–E-oriented South Alpine deforma-

tions (Placer & Čar 1998). The thrusts are further displaced by

Pliocene to recent strike-slip faults (Placer 1999, 2008; Vrabec

& Fodor 2006; Kastelic et al. 2008; Šmuc & Rožič 2010). This

structural history, in combination with the highly deformable

basinal rocks of the Tolmin Nappe, resulted in a fragmented

Fig. 2. a — Structural subdivision of NW Slovenia (generalized after Buser 1987). The Trnovo Nappe is the highest thrust unit of the External

Dinarides composed of the DCP successions. The Tolmin Nappe forms the base of the Southern Alps, is composed of SB succession, and is

divided into three lower-order thrusts. The Julian Nappe forms the upper portion of the Southern Alps and is composed of JCP successions.

The boxed areas indicate the position of the detailed geological maps of: b — Mt. Mrzli vrh area, c — Perbla area, d — Liščak area, whereas

the geological map of the Mt. Porezen area is represented in Figure 4.

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, 2017, 68, 6, 543–561

and complex geological setting and led to the eradication

of the original spatial relationships between the JCP, SB

and DCP.

Stratigraphy: The Norian and Rhaetian stages of the SB are

dominated by the Bača Dolomite, which is largely made up of

bedded dolomite with chert nodules and dolomite-chert brec-

cias, the latter being common in the middle part of the forma-

tion (Gale 2010). In the westernmost part of the Podmelec

Nappe, the Bača Dolomite passes upwards into massive dolo-

mite (partly logged in this study). In the northern part of the

basin the Late Norian and Rhaetian Slatnik Formation, com-

posed of alternating hemipelagic limestones and calciturbi-

dites, overlies the Bača Dolomite (Rožič et al. 2009, 2013;

Gale et al. 2012).

During the Hettangian and Pliensbachian, the limestone-

dominated Krikov Formation became deposited. It is charac-

terized by alternating hemipelagic limestone and calci turbidites.

The latter are predominant in the northern part of the basin

(Kobla Nappe), but become rarer towards the central part of

the basin (Rut Nappe), and are almost entirely absent (diluted)

in the southern part of the basin (Podmelec Nappe) (Rožič

2009; Goričan et al. 2012). In the westernmost outskirts of the

basin, the base of the formation is dominated by a thick lime-

stone breccia (also presented in this paper). The contact of the

Krikov Formation with the overlying Toarcian, marl/shale-

dominated Perbla Formation is sharp (Rožič 2009; Rožič &

Šmuc 2011).

Location and tectono-stratigraphic setting of studied

sections

As a result of intense tectonic deformation, areas appro-

priate for detailed studies are rare. Detailed geological map-

ping was performed for each selected area and the results are

summarized herein.

The westernmost outcrops of the SB can be found on

Mt. Mrzli vrh, which structurally belongs to the Podmelec

Nappe (Fig. 2b) (Buser 1987). The succession is characterized

by an Upper Triassic to Cretaceous succession of basinal

facies, with the exception of a several hundred meters thick

(?Norian–Rhaetian) massive dolomite. In the northern part of

the mapped area the massive dolomite is overlain by the

basinal Jurassic Krikov Formation. South of the E–W trending

fault, however, the massive dolomite is followed by the late

Early Cretaceous Lower Flyschoid Formation. This fault is

interpreted as a reactivated Mesozoic fault (Rožič 2005).

The studied section is located on the southern ridge of

Mt. Mrzli vrh, between the Sopotnica gorge and the Soča

Valley at an altitude of 800 m, along the trenches remaining

from the First World War (E 13°42’12”, N 46°12’34”).

The Perbla area is situated within the Rut Nappe (Buser

1987), which in this area contains an entire Norian to end-

Cretaceous succession. It is exposed in a large fold (Fig. 2c)

displaced by an E–W-trending fault along its core. The dis-

placement could not be recognized in the Toarcian and younger

rocks, therefore it is presumed to be a paleofault. The section

was logged in the Jelovšček gorge in the core of the anticline

(E 13°45’28”, N 46°13’13”).

The Liščak area lies structurally within the Podmelec Nappe

(Buser 1987). In the mapped area an undisturbed Late Triassic

to Lower Cretaceous basinal succession was documented.

The Triassic–Jurassic transition was logged in two sections

close to one another (Fig. 2d). The stratigraphically lower

section is located in a tributary stream of the Kneža River

(E 13°50’29”, N 46°11’21”). It ends with a characteristic brec-

cia megabed, which was laterally followed to the base of the

second section, which is situated at the entrance to the Liščak

stream gorge (E 13°50’18”, N 46°11’24”). Above the logged

section, part of the Krikov Formation is dolomitized.

Mt. Porezen structurally belongs to the Podmelec Nappe

(Buser 1987), and exhibits what is probably the most complete

Late Triassic (maybe even Ladinian) to Cretaceous succession.

Three sections were logged near the Otavnik peak at the

mountain’s southwestern ridge. The northwestern section is

located along the Porezen stream in the Zakojška grapa

gorge and along the tributary stream towards the Vasaje farm

(E 13°56’54”, N 46°10’7”). The southern section was mea-

sured in a gorge that cuts the southern slopes of the Ritovščica

peak (E 13°58’2”, N 46°9’21”). The eastern section was logged

in the Zapoškar stream gorge (E 13°58’34”, N 46°9’36”) and

was logged in two, closely situated, stratigraphically succes-

sive sections.

Description, biostratigraphy and sedimentological

interpretation of studied sections

Mrzli vrh section

Description: The base of the studied section is made up of

bedded dolomite with chert nodules that is overlain by thick

massive dolomite several hundred meters thick. The topmost

13 m of the massive dolomite was logged in the section

(Fig. 3), where it starts to exhibits indistinct bedding. It is

overlain by 6 m of alternating bedded (5 to 35 cm) dolomite

and partially dolomitized calcarenite. This interval contains

chert nodules. Dolomite is coarsely crystalline with sub- to

euhedral crystals up to 500 µm large, and texture-obliterating.

Calcarenite is fine- to coarse-grained and partially dolomi-

tized, with a still recognizable primary composition that shows

characteristics of the overlying calcarenite.

The section continues with a 35 m-thick interval composed

of occasionally channelized beds of limestone breccia and

calc arenite that alternate with intervals of very thin-bedded

calcarenite and micritic limestone. Limestone breccia is gene-

rally thick-bedded (up to 250 cm) and often grades to calc-

arenite. Clasts are up to 20 cm large, subangular to well

rounded, elongated, and oriented parallel to the bedding

planes. Calcarenite-supported breccia prevails, whereas clast-

supported breccia occurs in lower portions of beds exhibiting

inverse grading, locally. Breccia is rudstone with calcarenite

matrix that is identical in composition to the surrounding

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, 2017, 68, 6, 543–561

10m

15m

20m

25m

45m

30m

35m

40m

50m

55m

10m

15m

20m

25m

30m

35m

40m

LIŠČAK 2

PERBLA

clay silt

vf f m c

MUD SAND

GRAVEL

MRZLI VRH

SLUMP

10m

15m

20m

10m

15m

20m

25m

30m

35m

40m

Involutina liassica

Meandrovoluta asiagoensis

Ophthalmidium martanum

O. martanum

Involutina liassica

O. martanum

Involutina liassica

O. martanum

Involutina liassica

O. martanum

Siphovalvulina gibraltarensis

Siphovalvulina sp.

Meandrovoluta asiagoensis

Involutina liassica

Meandrovoluta asiagoensis

LIŠČAK 1

Epigondolella

ex gr

. E. bidentata

Misikella hernsteini

Misikella posthernsteini

Oncodella paucidentat

45m

50m

55m

bedded dolomite

massive dolomite

chert

dolomite - cherty breccia

limestone breccia

partly dolomitized

limestone

biomicritic limestone

partly dolomitized

limestone breccia

pebbly calcarenite

calcarenite

LITHOLOGY:

conodont fragments

D E

D E GaGb

Meandrovoluta asiagoensis

ABUNDANCE OF MOST COMMON

LITHOCLAST (in %)

ABUNDANCE OF MOST COMMON

LITHOCLAST (in %)

D E Ga I

ABUNDANCE OF MOST COMMON

LITHOCLAST (in %)

clay silt

vf f m c

MUD SAND

GRAVEL

clay silt

vf f m c

MUD SAND

GRAVEL

clay silt

vf f m c

MUD SAND

GRAVEL

Fig. 3. Studied sections with positions of biostratigraphic markers and the abundance (in %) of common lithoclasts in limestone breccia.

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calcarenites (see description below), whereas lithoclasts origi-

nated from platform, slope and basin carbonates (Fig. 4c, d; for

details see Tables 1 and 2).

Calcarenite is gray to light gray and bedded (up to 90 cm),

often graded and horizontally laminated, but textureless ver-

sions also occur. When alternating with micritic limestone, it

displays similar characteristics, but beds are very thin (mm- or

cm-sized) and contain load casts. In the upper part of the unit

(from 44

to 47

m of the section) calcarenite is indistinctly

bedded, comprised of a few thick beds (up to 180 cm) that

laterally disintegrate into a larger number of thinner beds,

which could also be fractures along the horizontal lamination.

Coarse- to medium-grained calcarenite is grain/packstone

com posed predominantly of crinoids, peloids, intraclasts and

lithoclasts, whereas with grading into fine-grained calcarenite

it turns into packstone composed of pellets, crinoids (echino-

derms) and occasional sponge spicules (Fig. 4a, b; for details

see Table 1).

Micritic limestone is gray and dark gray, and occasionally

horizontally laminated wackestone with pellets, radiolarians,

filaments and sponge spicules. In the lower part of the interval,

it is still partially dolomitized and slightly marly (for details se

Table 1).

The succession ends with thin-bedded black cherts that pass

upwards into thin-bedded silicified dark gray micritic lime-

stone that is identical in composition to the underlying facies

equivalent.

Age: The massive dolomite is presumably Norian–Rhaetian

in age (Buser 1986). In the lower part of the overlying lime-

stones Meandrovoluta asiagoensis Fugagnoli & Rettori in

association with Ophthalmidium? martanum Farinacci were

encountered in micritic limestone. Above this level, Involutina

liassica (Jones) predominates in calcarenites and a calcare-

nitic matrix of breccias, occasionally in association with

O.? martanum and Siphovalvulina sp.

Meandrovoluta asiagoensis is a relatively recently described

species, although it has commonly been figured under diffe-

rent names (Fugagnoli et al. 2003). Its stratigraphic range is

currently determined as Sinemurian to Toarcian (Fugagnoli et

al. 2003; Velić 2007), but it occurs on the Dinaric Carbonate

Fig. 4. Microfacies of the resedimented limestones from Mt. Mrzli vrh section: a — coarse grainstone with echinoderms, intraclasts and large

brachiopod; b — fine packstone with small intraclasts/peloids and bioclasts (echinoderms, calcified sponge spicules, radiolarians); c — diverse

lithoclasts of the limestone breccia: ooidal grainstone (type Gb), bioclastic wackestone (type-B) and basinal litho/intraclasts (type-A);

d — packstone lithoclast (type-E) in coarse packstone.

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Platform from at least the late Hettangian (Gale & Kelemen

2017). It is abundant in low-diversity assemblages, in various

lagoonal environments (Fugagnoli 2004 and pers. obser. of the

author) and its variable morphology corresponds to that of

an opportunistic species (Dodd & Stanton 1990, p. 288).

Ophthalmidium? martanum (determination of this species is

still considered ambiguous) first appears in upper Sinemurian,

lasting until Toarcian (Velić 2007), but earlier occurrences

cannot be excluded. Involutina liassica ranges from the

Hettan gian to the Toarcian, but is restricted to platform mar-

gins and slopes (Velić 2007). Siphovalvulina first appears in

the Hettangian, but continues to be present into the Cretaceous

(BouDagher-Fadel 2008). Based on the foraminiferal associa-

tion the succession above the massive dolomite is Sinemurian

or Hettangian in age.

Sedimentological interpretation: The original sedimentary

environment of massive dolomite is uncertain. Massive inter-

vals within the Bača Dolomite were previously documented at

other locations, and due to their sedimentary breccia structure

they were interpreted as debris-flow deposits (Gale 2010). But

the Mrzli vrh massive dolomite differs from those in its

remarkable thickness, its lack of chert clasts and lack of

primary sedimentary breccia composition. Consequently, we

interpret it as dolomitized platform limestone. It was probably

reef limestone, which rimmed the carbonate platform after it’s

progradation over marginal basinal strata, represented today

by bedded dolomites with chert at the base of massive

dolomite. Prograding platforms characterized by massive

Dachstein reef limestone are typical for the Norian–Rheatian

successions in the entire region (Reijmer et al. 1991; Mandl

2000; Gianolla et al. 2003; Krystyn et al. 2009; Gale et al.

2014, 2015). An overlying thin interval of bedded dolomites

with chert nodules (still below coarse resediments) could point

to an initial deepening of the platform in the earliest Jurassic.

The textures (Ta-b parts of the Bouma sequence) of the

overlying calcarenites indicate sedimentation dominated by

turbidites. Calcarenites that lack gradation can be interpreted

as modified grain-flow deposits or highly concentrated sandy

debris flows (Stow & Johansson 2000; Shanmugam 2000).

Thick, inversely-graded limestone breccia at the 22 m-mark of

the section is interpreted as a debris flow deposit (debrite).

Crinoid-dominated sand-sized material in resediments indi-

cates that bioclasts originated from a relatively shallow pelagic

environment. Such (Hierlatz) facies are reported from the

Austroalpine shelf above the Dachstein-type platform or the

adjacent slope (Böhm et al. 1999; Gawlick et al. 2009), and is

known also from the Lower Jurassic of the Julian Alps (Buser

1986; Šmuc & Goričan 2005; Kukoč et al. 2012; Rožič et al.

2014). Clasts in limestone breccia indicate erosion of the plat-

form-margin, the slope, and subordinately also affected the

basinal carbonates, which corresponds to the toe-of-slope

sedimentary environment. Jurassic (types B, Gb, D, ?C litho-

clasts) as well as Triassic (types E and ?Ga lithoclasts) strata

were eroded. An upward-growing abundance of platform-

derived lithoclasts (Fig. 3) indicates the increasing exposure

of platform limestones. This effect could have been enhanced

by their exposure on a fault-dissected slope similar to the step-

like, i.e. terraced slope reported from the Transdanubian

Range (Galácz 1988; Haas et al. 1997, 2014). Subordinate

micritic limestone was deposited by hemipelagic sedimen-

tation. Sporadic lamination in these beds indicates resedimen-

tation by low-density turbidity currents.

Facies association of the entire limestone interval indicates

sedimentation at the toe-of-slope. The overlying strata,

Lithotype

Texture

Composition

Diagenesis

Micritic limestone

Wackestone

Pellets, calcified radiolarians, sponge spicules, foraminifera,

thin-shelled bivalves, fine bioclasts – echinoderm debris.

Partly dolomitized in lower part; minor

recrystallization and silicification of matrix, laminae

with abundant framboidal to subhedral pyrite.

Calcarenite

Coarse- to

medium-

grained:

Grainstone or

packstone

Fossils, lithoclasts (the same as in limestone breccia – see

description below), peloids, intraclasts, rare and strongly

micritized ooids.

Predominating fossils: echinoderms (crinoids) and, in the upper

part of the unit, also brachiopods (Fig. 4a).

Other fossils: calcisponges and foraminifera (common lagenids

and textulariids), ostracods and gastropods.

Non-carbonate grains are biotite, in the uppermost bed also

glauconite.

Cements: Mosaic cement in the intergranular space.

Syntaxial cement overgrows echinoderms.

Silicification: rare and selective to bioclasts, mostly

brachiopods and calcisponges.

Pyrite: fine-grained between grains; framboidal pyrite

inside micritic grains. Some grains show strong

replacement or encrustation.

Dolomitisation: intense in the lower part of the

section, decreases upwards; selective to matrix and

micritic grains.

Fine grained

Packstone or

occasionally

grainstone

Pellets and/or fine intraclasts and bioclasts, mainly

echinoderms.

Other fossils: calcified radiolarians and sponge spicules (Fig.

4b), foraminifera, mostly textulariids and Lenticulina.

Microfaulting was detected.

Limestone breccia

Rudstone

Matrix of the breccia is coarse-grained calcarenite, equal to

calcarenite described above.

Most common and largest lithoclasts (described in Table 2) are

type-B (Fig. 4c). Other common lithoclasts are type-D and

similar to them, but rarer type-C clasts. Platform derived types

E (Fig. 4d), and Ga, Gb (Fig. 4c) lithoclasts occur regularly and

become more abundant upsection (see Fig. 3.). Basinal (intra)

clasts (type-A; Fig. 4c) occur sporadically and types F and H

lithoclasts occur very rarely. Apart from lithoclasts, large

bioclasts occur: brachiopods, inozoan calcisponges, and

strongly recrystallized or silicified chaetetids.

Corresponds to those from calcarenite described

above.

Table 1: Summarized microfacies characteristics of Mt. Mrzli vrh section limestone lithotypes.

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dominated by micritic (hemipelagic), strongly silicified thin-

bedded limestone, are characteristic for a basin-plain sedimen-

tary environment (Mullins & Cook 1986).

The Perbla section

Description: The section was logged for 55 m. The under-

lying Bača Dolomite, i.e. bedded dolomite with chert nodules,

is not exposed in the logged section, but according to observa-

tions during the detailed geological mapping, the contact is

sharp. The base of the Krikov Formation (lower 36.5 m of

logged section; Fig. 3) is dominated by thick-bedded and

coarse-grained resedimented carbonates: dolomitized cherty

breccia (some beds are channelized), partly dolomitized lime-

stone breccia and calcarenite, and subordinate micritic lime-

stone (Fig. 5a). With a sharp contact this coarse-grained

interval passes into 200 m of alternating thin/medium-bedded

micritic limestone and calcarenite (calciturbidites). 19.5 m of

this succession were logged, and above the logged section

begins to be strongly silicified in the form of chert nodules in

micritic limestone and intense, often complete silicifation in

calcarenite.

In the logged section, micritic limestone occurs in two levels

within coarse resediments (40 and 160 cm thick, between the

and 19.6

m of the section, respectively) and shows indis-

tinct internal bedding. Above the coarse resediments (above

the 37.5

m of the section) it is gray to dark gray, thin-bedded

and horizontally laminated. The microfacies of the micritic

limestone correspond to those from Mt. Mrzli vrh.

Calcarenite is usually coarse- to medium-grained, sometimes

pebbly, bedded (4–100 cm) and normally graded. It often forms

the upper parts of graded limestone-breccia beds, but the tran-

sition between the two facies is usually sharp. At the 26

m of

the section, a 1 m-thick package of thin-bedded (7–12 cm),

inversely-graded pebbly calcarenite is present. In the upper

part of the section (above the 39

m), it is thin-bedded and

horizontally laminated. The composition of coarse- to medium-

grained calcarenite differs from that of the Mrzli vrh section,

as these beds in the Perbla section are mainly grainstone com-

posed of ooids, peloids intraclasts, whereas bioclasts are rare

Occurrence

Texture

Composition

Diagenesis

SMF, sedimentation and age

Type A:

Mrzli vrh

Perbla

Liščak

Wackestone, rarely

mudstone

Radiolarians, sponge spicules and rare foraminifera,

echinoderms, ostracods, thin-shelled bivalves (Fig. 4c).
Pellets occur in some clasts.

Some are strongly

dolomitized.

SMF3
Deep-water limestones (mud-chips).

Age: ?syndepositional.

Type B:

Mrzli vrh

Perbla

Liščak

Wackestone to

packstone; up to

few mm large

grains

Peloids, intraclasts and fossils: echinoderms, brachiopods,

bivalves, benthic foraminifera, and sponge spicules (Fig. 4c).

Shells are often fragmented.
Foraminifera: Trocholina umbo, common Ophthalmidium

sp. and/or Vidalina sp.

Occasional

recrystallization of

matrix to microsparite.

SMF8
Fossils assemblage indicates sedimentation

on outer shelf or slope.

Age: ?Lower Jurassic (syndepositional).

Type C:

Mrzli vrh

Perbla

Liščak

Wackestone

Pellets, rare unrecognizable small bioclasts and small

benthic foraminifera: Earlandia sp.

Occasional

recrystallization of

matrix to microsparite.

SMF?
Either of a shallow-water or open marine

origin.

Age: ?Lower Jurassic.

Type D:

Mrzli vrh

Perbla

Liščak

Packstone, rarely

grainstone

Pellets, rare foraminifera (Fig. 5e ): Meandrovoluta

asiagoensis, Earlandia sp. (Fig. 6c), Vidalina sp.

Some clasts are

recrystallized.

SMF16 or SMF 2
Either of a shallow-water or open marine

origin.

Age: Lower Jurassic.

Type E:

Mrzli vrh

Perbla

Liščak

Packstone;

poorly sorted

Dominant pellets, but with additional larger grains,

mostly foraminifera or intraclasts (Fig. 4d). In some clasts

micristised ooids, echinoderms, and shells, encrusted by

foraminifera.
Foraminifera ?Galeanella tollmanni

Common

recrystallization of

matrix to microsparite.

SMF16
Most probably of a shallow-water, inner

platform origin.

Age: Norian-Rhaetian.

Type F:

Mrzli vrh

Perbla

Wackestone

Micritisized ooids, intraclasts, echinoderms, small benthic

foraminifera and unrecognizable bioclasts.

Intense micritic rims

around bioclasts.

SMF ?
Low-energy environment close to ooidal

shoals.

Age: not determined.

Type G:

Mrzli vrh

Perbla

Liščak

Grainstone;

mostly well sorted,

and usually up to

700 µm in size

Intraclasts, peloids, ooids and rare fossils, mostly

echinoderms and foraminifera. The content of individual

grains is variable; most commonly dominated by peloids

and intraclasts (sub-type Ga), or by ooids (sub-type Gb;

Fig. 4c).

Cements are

circumgranular fibrous

and mosaic.
Rare corrosive voids

filled with micrite.

SMF15 and ?11-16
Shallow-water, high energy limestones (sand

shoals).

Age: not determined; sub-type Gb ?Lower

Jurassic.

Type H:

Perbla

Grainstone

Intraclasts and cortoids, probably also foraminifera

(Fig. 5e).

Recrystallization

(probably prior to

resedimentation).

SMF11
Platform margin shoals.

Age: not determined.

Type I:

Perbla Liščak Boundstone

Inozoan calcisponges, gastropod and bivalve shells, rare

dasycladacean algae, and an intergranular space filled

with micrite (Fig. 5d). In Liščak section occur corrosive

voids filled by rimming bladed and mosaic cements

(Fig. 6c). Large bioclasts can be encrusted by

calcimicrobes and foraminifera.

Primary corrosion-

voids.
Partial recrystallization

of matrix to

microsparite.

SMF7

Platform margin reefs.

Age: ?Norian -Rhaetian.

Table 2: Clast types from limestone breccia beds from Mt. Mrzli Vrh, Perbla and Liščak sections. Clast types are compared to Standard

Microfacies Types (SMF; Wilson 1975; revised in Flügel & Munnecke 2010) with the goal to define their original sedimentary environments

(clasts are arranged due to increasing environmental energy).

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more abundant (Fig. 3), and boundstone (type I) and cortoid

grainstone (type H) lithoclasts were additionally recognized

(Fig. 5d, e; for details see Table 2)

Age: Siphovalvulina sp. is first encountered in thin-bedded

breccia at 26 m in the Perbla section. Nine meters higher,

Siphovalvulina gibraltarensis BouDagher-Fadel et al. in asso-

ciation with Meandrovoluta asiagoensis Fugagnoli & Rettori

were encountered. The latter species is present in large abun-

dance in the micritic limestone and fine-grained calcarenite,

forming monospecific assemblages. The first appearance of

S. gibraltarensis is reported from the early Sinemurian (Velić

2007; BouDagher-Fadel 2008), or Hettangian (BouDagher-

Fadel & Bosence 2007). Accordingly, the Perbla section is

probably early Sinemurian in age, but may also be Hettangian

considering the poorly known stratigraphic ranges of the

determined foraminifera.

Sedimentological interpretation: Thick-bedded, coarse

limestone breccia were deposited by debris-flows, whereas

graded and horizontally laminated calcarenites are attributed

to turbiditic flows (Ta-b Bouma sequences). Inversely graded,

thin-bedded pebbly calcarenite was probably deposited by

grain-flows (Stow et al. 1996). Several composite beds occur.

Their base is non-graded or slightly graded breccia deposited

by debris-flows. It is followed with sharp contact by graded

(occasionally pebbly) calcarenites that originated from

turbi dity-flows. Similar two-component gravity flows were

reported from the lower slope of the Bahamas carbonate plat-

form (Mullins & Cook 1986). The lower part of the Perbla

section was deposited in toe-of-slope and/or proximal basin-

plain environments.

The section ends with alternating hemipelagic limestone

and calciturbidites. Such alternation points to sedimentation

on a basin-plain and a shift towards more distal facies is

recorded at the top of the logged section.

Lithoclasts in breccia from the Perbla section generally

correspond to those of the Mrzli vrh section and therefore indi-

cate erosion of similar parts of the platform. The more distal

location of the Perbla section with respect to the Mrzli vrh

section is reflected in the increased rate of basinal litho/intra-

clasts (type A) and a decrease in the amount of the outer

platfrom/slope lithoclasts (type B). Otherwise, the content of

particular lithoclasts shows no significant up-section alterna-

tion (Fig. 3). As reef limestone (type I) lithoclasts, likely of

Late Triassic age, were documented solely in the upper part of

the coarse-grained interval, we suppose a progressive down-

cutting of erosion into the platform margin carbonates.

The major difference, when compared to the Mrzli vrh sec-

tion, lies in the composition of the sand-sized material, which

indicates that the source area for resediments in the Perbla

section were ooidal shoals. Accordingly, different platform-

basin architecture can be supposed for the two studied sec-

tions. Alternatively, the different composition can be attributed

to the heterochrony of the sections. Although the foraminiferal

specimens occur in turbidites, they can be considered to be

contemporaneous with the studied strata, as they are present in

the matrix and not in clasts, but sorting by size during down-

slope transport is likely. On the basis of the known strati-

graphic ranges of determined foraminifera, both studied

successions are probably Sinemurian in age. The Sinemurian

(or younger) age of the studied sections is also supported by

O. martanum and S. gibraltarensis. The difference between

Meandrovoluta-dominated assemblages of the Perbla section

and the Involutina-dominated Mrzli vrh section is probably

related to the different lithology, i.e. I. liassica appears in

coarse-grained calcarenite in the Mrzli vrh section, whereas

M. asiagoensis is present in micritic limestone and fine-

grained calcarenite. Both assemblages could well originate

from different parts of the platform, e.g., Meandrovoluta from

Lithotype

Texture

Composition

Diagenesis

Calcarenite

Coarse- to

medium-

grained:

Grainstone;

in thin beds

occasionally

packstone

Radial and tangential ooids (30% of all grains), peloids (?micritised

ooids), intraclasts and rare bioclasts (up to 10% of all grains in lower part

and slightly more abundant in upper part of the section) (Fig. 5b).
Bioclasts: predominated by echinoderms and codiaceans. Others are

fragmented brachiopods, bivalves, benthic foraminifera, such as

Lenticulina and biserial textulariids, and very rare bryozoans.
In upper part of the section gastropods appear and Meandrovoluta

asiagoensis foraminifera are numerous.
Lithoclasts additionally occur in coarser beds. Their composition is the

same as in the limestone breccia. Rare small grains of biotite and

phosphatic minerals.

Cements: Mosaic cement in the

intergranular space, syntaxial cement

around echinoderms.
Silicification: rare and selective to

bioclasts, mostly as microcrystalline

quartz replacing brachiopods. Large quartz

crystals (up to 2 mm) are observed in

intergranular space or as replacement of

large echinoderms and brachiopods.
Pyrite: fine-crystals in intergranular

spaces; framboidal pyrite inside micritic

grains.
Dolomitisation: strong in the lower part of

the section; upwards selective to matrix

and micritic grains.

Fine

grained

Grainstone and

packstone

Pellets, small intraclasts and bioclasts: predominant echinoderms, rare

benthic foraminifera (Fig. 5c).

Limestone breccia

Rudstone

Matrix of the breccia is coarse-grained calcarenite, equal to calcarenite of

surrounding beds.
Most common lithoclasts (described in Table 2) are types A, C, D

(Fig. 5e), E and also Ga and Gb. Type-B (dominant in Mt. Mrzli vrh

section) occurs sporadically. Very rarely occur type- H (Fig. 5e) and type-I

(Fig. 5d) lithoclasts, the later only in the upper part of the coarse-grained

interval.
Large bioclasts are similar to those from Mt. Mrzli vrh section.

Additionally calcimicrobes frequently occur.

Dolomitisation: strong in the lower part of

the section (Fig. 5e), gradually decreases

upwards.
Silicification: selective and bound mostly

to bioclasts.

Table 3: Summarized microfacies characteristics of the Perbla section.

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the inner part of the platform (see Fugagnoli et al. 2003), and

Involutina from a more agitated environment (see Piller 1978).

Alternatively, the two sections might be slightly different in

age, but at some sub-stage level (the current accuracy of deter-

mined foraminifera being at the stage level).

Liščak sections

Description: The succession was logged in two closely

situated and correlated sections (Figs. 2d, 3). The uppermost

40 m of the Bača Dolomite consists of grey to dark grey

medium-bedded dolomite with or without chert. Despite dolo-

mitization, primary sedimentary fabric is locally still preser-

ved, and horizontal and convolute laminations, scour struc tures,

load casts, normal and inverse grading were all recognized.

Macroscopically visible lamination reflects the difference in

the size of the dolomite crystals (euhedral crystals 50 to

200 μm in size).

A single, 15 m-thick breccia megabed, which is composed

of dolomite and chert clasts, occurs at the base of the Krikov

Formation. The chert clasts are angular, while the dolomite

clasts (up to 1 m large) are often plastically deformed. In the

Liščak 2 section, the breccia is overlain by thin-bedded hori-

zontally laminated clayey dolomite, and after 2 m by micritic

limestone. An overlying mud-supported limestone breccia bed

of 1 m thickness grades into strongly silicified calcarenite

(Fig. 6a, b). Above, thin-bedded micritic limestone and subor-

dinate dolomite was logged for 10 m. This interval includes

two small-scale slumps. The microstructure of micritic lime-

stone is identical to the micritic limestone described before.

The calcarenite is packstone composed of ooids/micritized

ooids, intraclasts and bioclasts (Fig. 6d), whereas the breccia

is floatstone with lithoclasts observed also in previously

described sections (Fig. 6c; for details see Tables 2 and 4).

Age: Conodont species Epigodonella ex gr. bidentata

Mosher, Misikella hernsteini (Mostler), Misikella posthern

steini Kozur & Mock, and Oncodella paucidentata (Mostler)

were recovered from the lower 7 m of logged Bača Dolomite,

confirming the Rhaetian age of this interval (e.g., Krystyn et

al. 2009; Buser et al. 2008; Gale et al. 2012). Only fragments

of conodonts were found from the overlying bedded part of the

Bača Dolomite, up to the megabreccia. The marly and

Fig. 6. a — 1 m-thick limestone breccia/silicified calcarenite bed within thin-bedded hemipelagic limestone of the Liščak section;

b — weathered surface of the limestone breccia with silicified brachiopods; c — floatstone microfacies of the limestone breccia with bound-

stone (type-I) and pelletal packstone (type-D) lithoclasts; d — silicified packstone with ooids, intraclasts, brachiopods and echinoderms.

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laminated thin-bedded dolomite and micritic limestone of the

lowermost part of the Krikov Formation are devoid of micro-

fauna. From the 1 m-thick breccia bed, shark teeth Synechodus

sp. and Paraorthacodus sp. were determined, which first

appear in the Early Jurassic (Paleobiology Database 2016).

The Early Jurassic age is confirmed by the finding of

fora minifera Meandrovoluta asiagoensis Fugagnoli &

Rettori, whereas Cousin (1973, 1981) determined Lower

Jurassic Involutina liassica (Jones) at the confluence of the

Liščak and Kneža rivers, which corresponds to the Liščak 2

section.

Sedimentological interpretation: The sedimentary struc-

tures of the Bača Dolomite are strongly obliterated, but the

formation is interpreted to have been deposited in a basin-

plain environment (Mullins & Cook 1986). The thick dolo-

mite/cherty breccia megabed that lies at the top of the

formation was formed by debris-flow. As it contains large

clasts of plastically deformed dolomite, it developed from

slumping of the basinal strata. This different deformation of

chert and dolomite clasts indicates that the mass movement

occurred after the formation of chert nodules and prior to the

lithification of the carbonate sediment, which at the time of

redeposition could still have been calcareous.

The thin-bedded dolomite and the micritic limestone above

the breccia megabed are interpreted to be hemipelagic in

origin. The lamination might indicate their partial redeposition

by low-density turbiditic flows. The breccia with silicified

brachiopods was deposited by two-component gravity-flow:

the lower part of the bed was formed by a debris-flow, the

overlying calcarenite was deposited by a turbidity current.

The composition of the debrite indicates the erosion of

Triassic–Jurassic carbonates of basinal, slope and platform

facies, whereas the turbidite carried material from a shallow

water environment. In the upper part of the logged section,

two additional slump intervals were recognized within the

hemipelagic limestone and dolomite. Debris-flow deposits

and slumps indicate agitated paleotopography within the basin

during the Triassic–Jurassic interval.

Mt. Porezen sections

Description: On Mt. Porezen three sections were logged

within the continuous, i.e. laterally undisturbed facies belt

(Fig. 7). Some thick-bedded dolomite-cherty breccia beds are

present in the underlying Bača Dolomite (not included in sec-

tions). These suggest that the fault activity started already in

the latest Triassic. A Norian tectonic pulse was documented

within SB (Gale 2010; Oprčkal et al. 2012) as well as in the

rest of the Southern Alps (Jadoul et al. 1992; Cozzi 2000,

2002).

No dolomite-chert breccia, however, occur between the

Bača Dolomite and the Krikov Formation, as is the case in the

Liščak section. Instead, the top of the Bača Dolomite is

characterized by bedded dolomite, which is followed by

a cherty interval some several meters thick. In the Zakojška

grapa (northwestern) section black chert beds alternate with

strongly silicified dolomite and subordinate dolomitic marl in

an interval 14 m thick. One of the chert beds is silicified

calc arenite with a preserved sedimentary fabric. The cherty

interval is represented in the other two sections largely as thin-

bedded, pure black chert, some 4.5 m and 5 m thick in the

southern (Ritovščica) and northeastern (Zapoškar) sections,

respectively. In the Ritovščica section, it starts with an

80 cm-thick bed that reveals its primary calcarenite fabrics,

with grain-ghosts within completely silicified parts and some

locally preserved carbonate (Fig. 8c). A similar but thinner bed

was recognized a few meters up-section. Bivalves of the genus

Halobia were found in the lower part of this interval a few tens

of meters laterally from the section (Fig. 8b).

The overlying Krikov Formation is dominated by thin-, and

exceptionally medium-bedded, grey to dark grey, wavy and

parallel laminated micritic limestone (Fig. 8a). The micro-

facies corresponds to that recorded in the previously described

sections, but is commonly strongly recrystallized and silici-

fied. In the Zakojška grapa section, water-escape structures

were recognized in thin sections. Chert nodules occur in all

sections. Marlstone is subordinate, except for the base of the

Table 4: Summarized microfacies characteristics of limestone in the Liščak section.

Lithotype

Texture

Composition

Diagenesis

Calcarenite

Partly washed-out
packstone

Brachiopod shells, intraclasts (often mud-chips), peloids, echinoderms,
micritized ooids and rare foraminifera (Fig. 6d).

Cements: recrystallization of matrix to
microsparite, mosaic cement in washed-
out intergranular spaces, syntaxial cement
around echinoderms.
Silicification: strong and replaces grains
as well as matrix.

Limestone breccia

Floatstone

The matrix between large grains is wackestone with small fragments of
brachiopod shells, ostracods, echinoderms, thin-shelled bivalves, radiolarians,
pellets, rare sponge spicules, and foraminifera (Fig. 6c).
Predominant large grains are brachiopod shells and large A-type litho/intraclasts
(30 %) which can contain non-calcified (siliceous) radiolarians and sponge
spicules. Other lithoclasts are rarer and belong to types B (13 %), C (2 %),
D (16 %), E (21 %), Ga (8 %), Gb (3 %) and I (7 %). These lithoclasts are
smaller in respect to predominant A-type clasts.

Dissolution seams.
Strong silicification of bioclasts (mostly
brachiopod shells) with chalcedony and
lithoclasts with microcrystalline quartz.
Sporadic pyrite as framboids or small
subhedral crystals in micrite and along
dissolution seams.

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Zakojška grapa section, where it forms a laminated interval

that spans the 12

to 25

m section of the formation.

As in other sections from the SB (Rožič 2009), the upper

boundary with the marl-dominated Perbla Formation is sharp.

The thickness of the Krikov Formation (between the cherty

interval and the upper boundary), however, varies signifi-

cantly on Mt. Porezen. In the Zakojška grapa section, it is

135 m thick, in the Zapoškar section it reaches a thickness of

approximately 115 m. In the Ritovščica section, the upper

boundary is poorly exposed, but it is clear that the formation

thickness does not exceed 65 m.

Age: The age of the Mt. Porezen

sections remains poorly constrained

and is based on scarce biostrati-

graphic markers and correlation (no

conodonts and radiolarians were

found). Halobia sp. bivalves found in

the black chert interval during geolo-

gical mapping indicate that it is, at

least partly, Triassic in age. A similar

and contemporaneous lithologic change

from carbonate to siliceous pelagic

sedi men tation was reported also from

the Budva Basin in the southern

Dinarides (Črne et al. 2011). Based on

the superposition, the Krikov For ma-

tion may represent an interval from the

Hettangian to Pliensbachian. Its upper

boundary is marked by the Toarcian

Oceanic Anoxic Event at the base of

the overlying marly Perbla Formation

(Rožič 2009; Rožič & Šmuc 2011;

Goričan et al. 2012).

Sedimentological interpretation:

The Mt. Porezen sections encompass

a longer time-interval than the pre-

viously described sections. In the lower

part of the cherty interval in the

Zakojška grapa and Ritovščica sec-

tions, a few beds of coarse-grained,

almost completely silicified calcare-

nite are the only coarse grained beds

derived from high-density turbidity

currents during the Triassic-Jurassic

transition period. Horizontal and wavy

laminations in micritic limestone

above the cherty interval indicate

partial redeposition of hemipelagic

sediments. The entire latest Triassic to

Early Jurassic succession therefore

shows a very monotonous, distal

basin-plain sedimentation. However,

the significantly variable thickness of

the Krikov Formation in closely

located sections can be attributed to

a tilting of the tectonic block within

the basin. A present-day 4° of block tilting in NW (azimuth

296°) directions was calculated for the virtual plane connec-

ting the base of the Krikov Formation in the three logged

sections (after rotating the formation’s upper boundary to

horizontal level). Since we did not calculate with compaction,

the original inclination was steeper.

As mentioned above, the entire studied succession repre-

sents a greater time span, and due to poor datation it is

impossible to specify the exact period of more intense sub-

sidence within the Lower Jurassic strata. Some tectonic

activity can be attributed also to the Pliensbachian–early

Fig. 7. Geological map and schematic sections (simplified from 1:100 logs) of Mt. Porezen:

variable thickness of Krikov Formation indicates block tilting, whereas lateral facies changes

at the base of the formation indicate it occurred, at least partly, at (and shortly after)

the Triassic/Jurassic boundary.

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Toarcian subsidence pulse (Berra et al. 2009), which was

well recognized in the eastern Southern Alps (Šmuc 2005;

Rožič 2009; Rožič & Šmuc 2011, Goričan et al. 2012;

Rožič et al. 2014), but lateral lithological variations described

at the base of the Krikov Formation indicate that tilting

(at least partly) occurred at the Triassic/Jurassic boundary

interval: firstly, in the Ritovščica and Zapoškar sections,

the transition from the Bača Dolomite to the Krikov Formation

is marked by pure cherts, whereas in the Zakojška grapa

the chert-rich interval is generally thicker and pure chert

alternates with dolomite beds. Secondly, above the

chert-rich interval in the Zakojška grapa section the succes-

sion is marked by the marly interval, which was not docu-

mented in the other two sections. Both variations can be

attributed to higher latest Triassic-earliest Jurassic sedimen-

tation rates in the Zakojška grapa section at the paleotopo-

graphically deepest part of the tilted block, which is in

accordance with the largest thickness of the Krikov Formation

in this section.

Discussion

The most prominent facies change is recorded in the

Mt. Mrzli vrh area, where it is interpreted as the drowning of

the platform margin. Here, we notice that the massive dolo-

mite, i.e. dolomitized reef limestone, is underlain by the

Amphiclina beds and the Bača Dolomite (Fig. 2), both of

which formations are characteristic for the SB (Buser 1986,

1989; Gale 2010). The overlying Jurassic and Cretaceous

strata are represented entirely by basin facies as well (Buser

1986; Rožič 2005). The differential sea floor paleotopography

resulting in different sedimentary environments is probably

related to discontinuously active syndepositional faults at the

SB’s westernmost margin. During the tectonically quiet Late

Triassic, intense carbonate production led to platform progra-

dation, which is in accordance with regionally recognized

platform progradations (Gianolla et al. 2003; Krystyn et al.

2009; Gale et al. 2014, 2015). The progradation process was

interrupted by a reef crisis at the Triassic/Jurassic boundary

Fig. 8. Mt. Porezen sections: a — micritic limestone of the Krikov Formation from the Zapoškar section; b — Halobia sp. from the base of the

cherty interval found close to the Ritovščica section (determination by B. Jurkovšek); c — microfacies of silicified calcarenite from the base

of the cherty interval of the Ritovščica section (under cross-polarized light).

557

SEDIMENTARY RECORD AT THE TRIASSIC/JURASSIC BOUNDARY INTERVAL IN THE SLOVENIAN BASIN

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, 2017, 68, 6, 543–561

Dinaric (Friuli)

Carbonate

Platform

Dinaric (Friuli) Platform

Belluno

Basin

Slovenian Basin

Belluno

Basin

Julian Carbonate

Platform

Slovenian Basin

Triassic platform

this study

Jurassic platform

Triassic basin

Jurassic basin

not exposed due to thrusting and eros

ion

not exposed due to thrusting and erosion

not exposed...

GAP

?GAP

Mrzli vrh

Perbla

Liščak

Porezen

Kobla (Rožič et al., 2009, 2012)

Banjščice (Ogorelec & Rothe, 1993)

FORM

Krim (Dozet, 2009)

Batognica (Buser, 1987)

SLOVENIAN BASI

bedded

dolomite

massive

dolomite

hemipelagic

limestone

marl

chert

calcarenite

dolomitized

limestones

ooidal

calcarenite

dolomite

chert breccia

iassic

Jurassic

limestone

breccia

loferitic

limestone

lagoonal

limestone

bedded

dolomite

ooidal

limestone

breccia

loferitic

dolomite

massive

oolite

dolomite

breccia

platfrom resediments

-western source

platfrom resediments

-northern source

?GAP

MAP AND CROSS-SECTION FACIES

(Flügel 2002; Kiessling et al. 2007), which, combined with

an intensification of tectonic activity, resulted in renewed

deepening along the westernmost basin margin. In suggesting

an analogy with the western and central Southern Alps (Jadoul

et al. 1992), we assume the reactivation of pre-existing faults

at the western margin of the SB (Fig. 9).

The Perbla section is marked by a carbonate breccia interval

at the Triassic/Jurassic boundary, which indicates a prominent

intensification of resedimentation. Although various processes

can trigger the formation of breccia megabeds (Spence &

Tucker 1997), we attribute this to an increase in slope inclina-

tion as a consequence of accelerated subsidence of the basin

floor. As the composition of calcarenites in the breccia-matrix

and the interstratified calciturbidites differs from those of the

Mrzli vrh section, a different source area can be inferred for

these beds. Because the Perbla section is located in the struc-

turally higher Rut Nappe (Fig. 2a), it may have been located

not only to the east, but also to the north of the Mrzli vrh

section, which lies in the structurally lower Podmelec Nappe

(Buser 1987). Consequently, a north-lying Julian Carbonate

Platform, which was covered by ooid shoals in the Early

Jurassic (Buser 1986), seems the most probable source area of

the carbonate resediments.

The composition of the lithoclasts points to a slightly diverse

architecture of platform-basin transition along the transporta-

tion paths of gravity flows. In the Perbla section the resedi-

mentation events were sourced by ooid shoals but they also

contain lithoclasts derived from the eroded underlying succes-

sion, including (Late Triassic) marginal reefs (type I, also H).

Outer platform/slope carbonates (type B) are subordinate,

Fig. 9. Schematic paleogeographic map and cross-section of the eastern Southern Alps and north-western External Dinarids at the beginning of

the Early Jurassic with predicted locations and simplified logs of studied and discussed successions of the Slovenian Basin and the surrounding

platforms; arrows on map indicate proposed directions of carbonate gravity flows.

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, 2017, 68, 6, 543–561

while basinal litho/intraclasts (type A) are common, which

points to a more basin-ward location of the Perbla section with

respect to the Mt. Mrzli vrh section. In the Mt. Mrzli vrh area,

the resediments originated from the shallow pelagic environ-

ment, whereas typical lithoclasts indicate the erosion of mostly

outer platform/slope carbonates (type B) and Triassic–Jurassic

platform carbonates, whereas lithoclasts of marginal reef

limestone (type I) were not detected. This is in accordance

with our interpretation, namely that Late Triassic marginal

reef limestone (completely dolomitized) lies below the resedi-

ments in the Mt. Mrzli vrh section, and that only more

inner-platform Late Triassic carbonates (type E and ?Ga) were

exposed on the newly developed slope.

The Liščak section also records resedimentation at the

Triassic/Jurassic boundary, which can be explained by the

increasingly agitated paleotopography within the basin, but

the intensity of resedimentation (and also tectonic deforma-

tion) is far smaller than in the western sections. The lack of

coarse-grained platform-derived resediments (with the excep-

tion of a singe 1 m-thick bed) is attributed to the larger dis-

tance from the main subsidence area at the western margins of

the SB, and regionally from the rifting center further to the

west. The easternmost sections, recorded on Mt. Porezen,

indicate that a minor Lower Jurassic tectonic block tilting was

present in this area, and lateral facies changes at the base of the

Krikov Formation (see previous chapter) indicate that tilting

intensified at the Triassic/Jurassic boundary interval.

The exact timing of the subsidence pulse cannot be speci-

fied due to rather poor biostratigraphic datations. It is clear,

however, that it starts at, or slightly postdates the Triassic/

Jurassic boundary. Foraminiferal assemblages from resedi-

mented limestones from the Mt. Mrzli vrh and Perbla sections

indicate that the subsidence intensified in the Sinemurian. This

is in accordance with datations of an early rifting pulse in the

western and central Southern Alps (Bertotti et al. 1993),

Transdanubian Range (Haas et al. 2014), Austroalpine domain

(Froitzheim & Manatschal 1996), as well as Western Alps

(Chevalier et al. 2003). This extensional episode was governed

by diffuse rifting and controlled by older discontinuities (Berra

et al. 2009).

Another, previously studied SB location indicates the tec-

tonic pulse discussed herein. It is situated north of Mt. Porezen

in the Kobla Nappe, which is composed of the northernmost

outcrops of the SB. In these sections the Triassic–Jurassic

transition is marked by a continuous limestone succession

from the Upper Norian/Rhaetian Slatnik Formation to the

Krikov Formation (Rožič et al. 2009; Gale et al. 2012). Both

formations are marked by alternating hemipelagic limestones

and carbonate turbidites and debrites characteristic of proxi-

mal basin plain — lower slope environments. An interval

several meters thick and dominated by distinct thin-bedded

limestones occurs just above the Triassic/Jurassic boundary,

but general succession shows no prominent facies alternations,

which is in accordance with the previously described

eastward decrease in subsidence. However, a combined

carbon-isotope study and biostratigraphic analysis indicates

a gap at the Triassic/Jurassic boundary (Gale et al. 2012; Rožič

et al. 2012), which again can be attributed to the increasing slope

inclination that resulted in erosion or by-pass of the slope area.

In the surrounding platform areas an intensified subsidence

at the Triassic/Jurassic boundary is not clearly evident, but

some sedimentary changes could be linked to it. On the JCP,

the peritidal Norian–Rhaetian Dachstein Limestone is over-

lain by Lower Jurassic ooidal limestone (Buser 1986).

The facies change described indicates a change from an inter-

tidal environment to one of marginal ooid shoals, i.e. a general

deepening of the sedimentary environment, which could be

related to accelerated subsidence. Simultaneously, on the DCP

the Triassic/Jurassic boundary transition is largely dolo-

mitized, but the litho(chrono)stratigraphic boundary lies at the

point where stromatolitic laminae, typical for the Norian–

Rhaetian Main Dolomite, disappear (Buser 1989, 1996), and

similar sedimentary change as described for the JCP can be

predicted also for this interval. Furthermore, in the northern-

most DCP outcrops, local appearances of rather thick carbo-

nate breccia intervals were reported at the Triassic/Jurassic

boundary at several locations: at Banjška planota located

south of the herein presented sections (Ogorelec & Rothe

1993), at Mt. Krim near Ljubljana (Dozet 2009), and further to

the east near the town of Trebnje (Buser 1965). No detailed

studies of these breccias have been done, yet, but we propose

that their origin is connected with the tectonic event discussed

herein, as their local appearance (rapid lateral disappearance)

could indicate sedimentation in a paletopographicaly seg-

mented (fault-dissected) environment.

Conclusions

The extensional pulse that started at the Triassic/Jurassic

boundary or slightly later and affected the entire western

Neotethys margin is recorded also in the succession of the

Slovenian Basin. In the Southern Alps, the structural unit of

the Slovenian Basin, major crustal deformations are reported

in their western segment, which was located close to the rifting

center, i.e. a precursor of the Middle Jurassic Piemont–Liguria

Ocean. Consequently, the tectonostratigraphic reflection of

this event in the east-located Slovenian Basin succession is

less dramatic, but can be still recognized. The westernmost

Mrzli vrh section records the change from the Late Triassic

massive dolomite, which is interpreted as dolomitized mar-

ginal reef, to the toe-of-slope carbonate resediments and there-

fore records a downfaulting-related drowning of the platform

margin. The Perbla and Liščak sections record an intensified

resedimentation related to the development of a segmented

paleotopography, which is indicated by the presence of car-

bonate lithoclasts derived from the platform margin, slope and

basin facies. The source areas of resediments proved diverse.

The resediments in the Mrzli vrh section originated from

crinoid-dominated shallow pelagic environment, whereas

those from the Perbla section were shed from ooid shoals.

The easternmost Mt. Porezen sections show a rather

559

SEDIMENTARY RECORD AT THE TRIASSIC/JURASSIC BOUNDARY INTERVAL IN THE SLOVENIAN BASIN

GEOLOGICA CARPATHICA

, 2017, 68, 6, 543–561

monotonous latest Triassic–Early Jurassic distal basin-plain

succession, which is with accordance with eastward-declining

tectonic deformation. However, the highly variable thickness

of the Hettangian–Pliensbachian Krikov Formation points to

block tilting and lateral facies changes concentrated at the

base of the formation indicate that tilting occurred close to the

Triassic/Jurassic boundary. Timing of the recorded tectonic

event is loosely determined, but we relate it to an episode of

diffuse rifting, the maximum subsidence of which is docu-

mented in the latest Hettangian–Sinemurian of the entire Adria

as well as European rifted margins.

Acknowledgments: The study was sponsored by the Slove-

nian Research Agency (project number Z1-9759 and financed

by funds for the Geochemical and Structural Processes, and

Regional Geology research groups). Anonymous reviewers

are acknowledged for their thorough review of the manuscript.

We would like to thank Gilles Cunyju from the Natural History

Museum of Denmark, Copenhagen, for a view on shark teeth,

Bogdan Jurkovšek from the Slovenian Geological Survey for

bivalves, and Špela Goričan for radiolarian probes. Numerous

students from the Geological department of the Univerity of

Ljubljana are sencirely thanked for their help and company

during the geological mapping and section logging. Miran

Udovč and Ema Hrovatin are acknowledged for the prepa-

ration of thin-sections.

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