Upper Carnian Clastites from the Lesno Brdo Area ( Dinarides , Central Slovenia )

This paper presents a detailed study of the Tuvalian clastic member in the Lesno Brdo Area in Central Slovenia (External Dinarides). The member represents the uppermost part of the mixed siliciclastic – carbonate Carnian succesion that overlays the carstified emersion surface on top of the “Cordevolian” limestone and dolomite. The Tuvalian member is composed of red and violet sandy mudstone and marlstone that are interbedded with sandstone and gravelly sandstone. Calcite concretions are common, particularly in mudstone and marlstone. The succession was deposited on the flood plain, where fine-grained flood sediments are interrupted by mediumto coarse-grained crevasse splay sandstone and cross-stratified gravely sandstone of the small river channels. Alternatively, this succession could have been deposited in the distal zone – terminal splay/fan sediments. According to paleogeographic subdivison we suggest that the provenance area was located to the south, where carbonates, volcanoclastites/vulcanites as well as clastites were eroded.


Introduction
The Middle and Upper Triassic in the External Dinarides is dominated by thick dolomite and subordinate limestone successions that are, following the Carnian regional emersion and the formation of bauxite, interrupted by mixed clastic-carbonate series (Dozet, 1979;Jelen, 1990;Dozet, 2004;CelarC, 2004CelarC, , 2008;;Dozet, 2009;Čar, 2010).In the Lesno Brdo Area, located 15 km southwest of Ljubljana, the succession is composed of the lower clastic member, a middle limestone member and an upper clastic member Julian in age, overlain by carbonate and siliciclastic members that are Tuvalian in age.So far, only the Julian beds with rich bivalve macrofauna and microfossils have been investigated in this part of the succession (Jelen, 1990).oblak (2001) also analysed the foraminiferal assemblage in the Julian limestone member.Up above, the Carnian succession passes into the Norian-Rhaetian Main Dolomite (Grad & FerjanČiČ, 1974;jelen, 1990).
With the exception of the paleontological work in the Julian carbonate rocks, the thick succession of younger clastics was poorly studied.In this paper, we present a sedimentological analysis of the Tuvalian clastic member, the succession of which is well exposed along the road that connects the villages of Lesno Brdo and Drenov Grič with the village of Zaklanec.

Geological Setting
Structurally, the Lesno brdo area belongs to the External Dinarides (Fig. 1), more precisely to the Hrušica Nappe, which are characterized by a post-Eocene Dinaric thrusting phase with compression running in the northeast-southwest direction (PlaCer, 1999(PlaCer, , 2008)).In central and western Slovenia, the External Dinarides are composed predominantly of thick carbonate successions interbedded by Ladinian volcanoclastics and Carnian siliciclastics (buser, 2010).The Carnian succession in the Lesno Brdo -Drenov grič area was investigated by Jelen (1990) (Figs. 2 and 3).According to his stratigraphic subdivision, the Carnian succession of the area starts with "Cordevolian" grey coarse-sparry dolomite and micritic limestone with an erosional unconformity on top.Above follow the Julian lower clastic member, Julian middle limestone member, and Julian upper clastic member (Jelen, 1990).The lowermost member is deposited on the macro-paleorelief; bauxite is locally present on its base (Dozet, 1979;CelarC, 2008;ogoreleC, 2011).The Tuvalian succession starts with the carbonate member composed of various limestones and terminates with the Tuvalian clastic member, which was investigated and presented herein.Jelen (1990) described this member as massive and shaley mudstone, violet-red multi-coloured sandstone and conglomerate with quartz pebbles.Upwards, it passes through transitional beds into the Norian-Rhaetian Main Dolomite.

Methods
The sedimentological section at 1:50 scale was logged along the road that connects the villages of Lesno Brdo and Drenov grič with the village of Zaklanec (E 14°19'25", N 46°0'15").During the logging, samples of coarse-grained clastites were taken with the aim of making thin sections and samples of fine-grained clastites for the XRD and geochemical analysis.Apart from host-rock samples, several samples of carbonate concretions were taken, from which two thin sections were made.On one such sample XRD and geochemical analysis was performed.
The nomenclature of clastics is based on their structure (for example: gravelly sandstone) and the naming of their composition-based classification (for example: quartz-polilithic gravelly sandstone).
The mineral compositions of three samples of fine-grained clastites and one sample of carbonate concretions were determined using a Panalytical PW 3830/40 XRD device, which uses a PW 1820 goniometer and a PW3830 X-ray generator, with copper tube PW 2273/20 under an electric current of 30 mA and a voltage of 40 kV.Difractograms were analysed with use of X'pert HighScore Plus software, together with the PAN -ICSD database for mineral determinations.For the geochemical analysis we used a Thermo Niton XL3t 900S-He Series Analyzer XRF device.For the elemental analysis a "mining" filter was used.

Description of Lesno Brdo section
The overall thickness of the section is 39 meters, with minor covered intervals between the 12 th and 14 th meter of the section (Fig. 3).The beds dip 20 degrees south-southwest.Through the entire section slightly sandy mudstone and marlstone prevail, which are coloured red and violet, but sporadic grey to greyish-green intervals also occur.The lower 15 meters of the section contain interbedded sandstone and gravelly sandstone, up to 60 cm thick (Fig. 4a).These beds are also coloured, mostly red and violet.Sandstone is deposited mostly in channels several meters wide and often exhibits cross lamination.An interbed between the 10 th and 11 th meter of the section shows clear normal grading.The number of gravelly-sandy interbeds decreases upwards along the section.Similarly, a lower sand content in the marlstone/mudstone is observed.Carbonate concretions are common largely in the lower 8 meters of the section.They appear in fine-grained as well as coarser-grained rocks.Upwards they occur only sporadically -locally between the 15 th and 22 th meter, and at the 26 th and 38 th meter of the section.

Microfacies of the clastites
From the coarse-grained clastites we provide description of gravelly sandstones and fine-to medium-grained sandstones , and sandy mudstones/marlstones are described separately.Herein, we emphasise that the transitions between the described varieties are gradual.
(Quartz) carbonate lithic and poly-lithic gravelly to slightly gravelly sandstone Most samples contain up to 60 % of grains, which are bounded by a fine-grained matrix (Fig. 4b) and in some rare cases, also by drusy mosaic and bladed calcite cements.The grain-size varies between 0.1 and 6 mm, with up to 30 % of grains larger than 2 mm.Two grain-size classes prevail -the first between 0.2 and 0.5 mm, and the second approximately 1.5 mm.Rocks are medium-to well-sorted, in some cases also poorly sorted.Grains are predominantly subrounded to subangular, and are in point, concavo-convex and occasionally sutured contacts and immature to semi-mature in texture.Beds are often normally graded.
The composition remains almost unchanged along the entire studied section, but we do find some minor variations, predominantly in the quantitative relationship between particular grain-types.The most common are carbonate lithoclasts, among which recrystallized micritic to microsparitic limestones with common partial replacement by opaque minerals, probably iron oxides, prevail.Some grains are entirely composed of opaque minerals.The surroundings of such grains (crystals) often show a red zoning coloring.Rarer but still present are darker micritic lithoclasts, which often appear in irregular shapes.Locally, the margins of these lithoclasts show silification, and some also show micritic coverings.Next in terms of abundance are quartz grains, grains of opaque mineral, feldspars and micas.Above the 10 th meter of the section lithoclasts of granitoid volcanic rocks are also common.Quartz grains can be subdivided into the following groups: A) monocrystal quartz grains, B) simple polycrystal quartz grains, and C) microcrystal quartz grains with or without mica.In most cases, such grains are likely recrystallized matrixes of granitoid volcanic rocks, whereas some grains could also be chert lithoclasts.
Among the cements, the most common are intergranular drusy mosaic and bladed calcitic cements.Corrosive carbonate cements, which substitute and obliterate the primary structure of quartz grains, are also present.
Quartz poly-lithic and carbonate lithic fine-to medium-grained sandstone Samples contain some 60 % of grains, with intergranular spaces filled largely by a finegrained matrix and in rare cases, also by drusy mosaic and bladed calcite cements.Grain-size varies between 0.05 mm and 2 mm, where most grains fall into size-classes of around 0.1 mm and 0.5 mm.Rocks are medium-to well-sorted, whereas grains are mostly subangular to subrounded.Grains are predominantly in concavo-convex and point contacts, while some grains are also floating.Rock texture is immature to semi-mature.
The composition is similar to the previously described gravelly sandstones, with a quantity of the carbonate lithoclasts.Owing to their smaller grain size the grains that can be definitely described as granitoid volcanics are also less common.

Quartz poly-lithic sandy mudstone or marlstone
The sample is composed of 35 % of grains in a fine-grained matrix.Most grains fall within the size-class of mud, and only some 30 % grains are between 0.06 and 0.4 mm in size.Grains are predominantly subrounded and floating, but rare point and concavo-convex contacts also appear.The studied sample is poorly sorted and immature in texture.
Among the grains the monocrystal quartz grains and carbonate lithoclasts prevail.Other, less frequent grains belong to polycrystal quartz, opaque mineral, feldspars micas and very rare lithoclasts of granitoid volcanic rock.

Microfacies of calcitic concretions
Concretions are composed of calcite (see also next chapter) and occur predominantly in finegrained clastites  in the lower part of the section, and rarely also present higher up.The outer margins of the concretions form irregular/nodular surfaces.Concretions consist mostly of fine-crystaline microsparite, whereas the size of crystals increases gradually and irregularly towards their interiors.These include rare, unreplaced quartz grains.Concretions are dissected by variously oriented fractures that form a complex network.They are filled predominantly by mosaic sparite, and in the marginal parts the surrounding sediment can be infiltrated in them.Sparite-filled fractures can occur also on the contact surface between concretion and sediment.Cement in this enveloping fracture is prismatic, with individual crystals oriented perpendicular to the concretion margin.This structure appears younger with respect to other fractures, as it also cuts parts where sed-Fig.4. Facies of the Tuvalian clastic member at Lesno Brdo.a) Lower part of the section in which sandy marlstone is interbedded with channels of gravelly sandstone.b) Medium-grained sandstone with quartz grains and carbonate lithoclasts.c) medium-to coarse-grained sandstone with various carbonate lithoclasts, quartz grains and lithoclasts of vulcanics (V); some carbonate grains contain opaque mineral (cross-polarized light).d) Pebbly sandstone with lithoclasts of vulcanics (V) with microcrystalline matrix and quartz and feldspar phenocrysts; various carbonate lithoclasts are also present; in the lower--right corner of the micrograph a monocrystal quartz grain is marked (Q) (cross-polarized light).e) Fine-to medium-grained sandstone composed predominantly of carbonate lithoclasts and monocrystal quartz grains; at margins of particular carbonate grains thin monocrystal quartz occurs.f) Sandy siltstone in which some grains up to 0.2 mm are visible -mostly quartz, carbonate lithoclasts and some lithoclasts of vulcanics (cross-polarized light).
iment infiltrates the fractures.The surrounding sediment is sandy mudstone, which on irregular contact with concretion shows signs of intense compaction, such as bended laminas and dissolution seams in the vicinity of the contact.
Calcite concretions occur rarely also in the more coarse-grained clastites (Figs.5c-5f) and show more complex internal composition.In such concretions the primary texture and their composition only partially are still visible.Most recognizable are quartz grains and carbonate grains impregnated with opaque mineral.Some other carbonate grains are also visible, but they are mostly recrystallized into microsparite.In parts of the thin-section where the primary packing was denser, the intergranular space is filled with mosaic cement.In parts with looser primary packing the intergranular spaces are made of very coarse-crystalline radiaxial bladed calcite cement.The transition between the described textural types of concretion is gradual.

Mineral and geochemical composition
Mineralogical analysis was elaborated on three samples of fine-grained clastites and one sample of carbonate concretion.Clastites were sampled at approximately the 4 th , 24 th and 37 th meter of the sections (samples: LB3,8; LB24,0 in LB37,2).The composition of samples taken from the fine-grained clastites is uniform.Samples contain five main minerals: quartz, calcite, hematite, alkaline plagioclase, and muscovite/illite (Fig. 6).Carbonate concretion, taken at approximately the 16 th meter of the section (sample: LB15,7), is predominantly composed of calcite, whereas some quartz reflections/peaks were also detected.
Geochemical analysis was made on the same samples as were used for mineralogical analysis.Table 1 one presents concentrations of the main elements (in percentages), whereas Table 2 presents the concentrations of oxides of main elements.
The samples of fine-grained clastites (LB3,8; LB24,0 in LB37,2) show a higher volume of silicium bound to SiO 2 (quartz varieties and silicates).This is followed by calcium as a part of calcite and dolomite.The particularly lower magnesium content indicates the greater presence of calcite with respect to dolomite.Samples also contain iron as part of Fe 2 O 3 (hematite).It is visible on the macro level as well as microscopically as a typically red colour of rocks/grains.Samples also contain some aluminium, which is bound in alumosilicates and potassium that is connected to the presence of the muskovite/illite.All three samples contain similar amount-values of their main constitutive elements.
Like fine-grained clastites, the results of mineralogical and geochemical content profiles also correspond to the carbonate concretion sample (Table 1 and Table 2), where the most common elements are calcium and silicon or, written in oxide-form, CaO and SiO 2 .

Sedimentary analysis of Lesno Brdo section
The studied succession is composed exclusively of clastic rocks, where fine-grained clastics (sandy mudstones/marlstones) prevail and are interbedded with (quartz) poly-lithic sandstones and gravelly sandstones.The composition of clastics is indicative of continental sedimentation, whereas facies, structure and texture emplace the studied section in an alluvial environment, most probably a floodplain.Fine-grained clastics are suspension sediments of flood events; thin-bedded and structureless sandstones can be described as crevasse-splay deposits, whereas channelized and sometimes cross-laminated gravelly sandstones can be interpreted as deposits of either crevasse-channels or small meandering river channels (Skaberne, 1995;bridGe, 2003;Miall, 2006;aSlan, 2013).
An arid climate, which is typical for the Upper Carnian (breDa et al., 2009;kieSSlinG, 2010), is indicated in the studied section by a characteristic red to violet rock colour, which is a consequence of the presence of iron in oxide form (hematite), and by the presence of carbonate concretions (alonSo-ZarZa, 2003;khalaF, 2007;MouSSavi-haraMi, 2009;breda & Preto, 2011).Given the pronounced arid conditions, the studied section could alternatively be interpreted as terminal fan deposits of a dryland river system with a diminishing discharge down-flow, causing a transition from a channelized to an unconfined flow (nicholS & FiSher, 2003).
Like the sedimentation on terminal fans, was interpreted a comparable Travenanzes Formation of the Dolomites was interpreted (breDa et al., 2009;breda & Preto, 2011;Preto et al., 2015), where based on the overall analysis of far-better exposed Tuvalian rocks the authors presented a comprehensive sedimentological/facies model (Fig. 7).The southernmost facies zone A, in which also the Lesno Brdo studied section could be emplaced, is characterized by meandering dry-land rivers concluding in terminal fans.This zone passes through sabkhas and tidal flats of facies zone B to facies zone C with restrictive lagoons and sub-tidal marine environments.The Travenanzes Formatian is also characterized by the presence of carbonate concretions, mostly dolocretes.Preto and colleagues (2015) consider a dolomite as a primary diagenetic mineral.In contrast to carbonate concretions of the Travenanzes Formation, at Lesno brdo these are composed of calcite.In the future, it would be worthwhile making additional geochemical and mineralogical analyses with the aim of defining their formation more precisely.
The direction of transport cannot be distinguished from the data on the Lesno Brdo section, but taking into consideration the paleogeographic reconstruction of the Upper Triassic, where the deep marine Slovenian Basin was located to the north (buSer, 1989;Gale, 2010;gale at al., 2012;2016), we conclude that the sediment source-area was generally located to the south (present-day orientation).The composition of sediments indi- cates that in the hinterland, i.e. the provenance area of clastites, outcropping rocks were carbonates, volcanoclastites/vulcanites and possibly also clastites.Some grains, like those composed of coarse sparite or opaque mineral, could be of pedogenic origin.An intense pedogenesis and formation of carbonate and iron concretions occurred on the dried flood plain, which was followed by deflation, fragmentation of these into smaller grains, and redeposition by wind within a series of diverse sub-environments (breDa & Preto, 2011).

Conclusions
The studied upper Carnian (Tuvalian) succession from the Lesno Brdo area consists of red and violet slightly sandy mudstone and marlstone, which are interbedded with sandstone and gravelly sandstone.Facies association is typical for sedimentation of flood plain deposits interrupted by medium-to coarse-grained sediments of crevasse splay or terminal fans, and less commonly of small-scale river channels.The composition indicates that in the (south-located) source area carbonate, volcanoclastic and siliciclastic rocks were eroded, while some grains could be of pedogenic origin.
Correlation with the sedimentological model of Tuvalian strata from the Dolomites in Italy (Travenanzes Formation) puts our section within facies zone A, which is characterized by sediments from flood plains, meandering river beds, crevasse splays and terminal fans.The presence of hematite and carbonate (calcite) concretions indicates an arid continental climate during the process of deposition.

Fig. 2 .
Fig. 2. Geological map and cross-section of the surrounding area and the location of the investigated section marked by a star (modified from: Jelen, 1990).

Fig. 5 .
Fig. 5. Facies of the Tuvalian clastic member at Lesno Brdo.a) Concretion from mudstone/marlstone composed of microsparite, but irregular enlargement of crystals towards the center is also visible; in the center is a fracture filled mostly with mosaic cement (1), whereas in the marginal part it is infiltrated by sorrounding sediment; a younger generation of fractures (2) runs parallel to the margin of the concretion and is filled with prismatic cement.b) Concretion from mudstone/marlstone: signs of compaction -bending of laminae in the surrounding sediment and the younger generation of fractures in the part where the sediment is embedded deeper into the concretion c) Concretion from sandstone: larger carbonate lithoclasts are still visible, whereas the fine-grained fraction is replaced by radiaxial bladed cement; mosaic cement (M) is also present d) Figure 5c under cross-polarized light.e) Concretion from sandstone with recognisable carbonate lithoclasts as well as quartz grains; (Q) grains are bounded mostly by mosaic cement.f) Figure 5e under cross-polarized light.

Fig. 7 .
Fig. 7. Schematic paleoenvironmental reconstruction of the Travenanzes Formation (modified from breDa & Preto, 2011) A = aluvial plain with river channels, terminal fans, crevasse splays and a flood plain; B = Mudflat and coastal sabkha; C = Carbonate tidal flats and shallow lagoons.The stars mark the two most likely locations of the Lesno Brdo area.

Table 1 .
Concentration (as percentages) of main-group elements of fine-grained clastics (three upper) and carbonate concretion (below).

Table 2 .
Percentages of main-group element oxides of fine-grained clastics (three upper) and carbonate concretion (below).