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BY 4.0 license Open Access Published by De Gruyter Open Access February 1, 2023

Petrography and mineralogy of the Oligocene flysch in Ionian Zone, Albania: Implications for the evolution of sediment provenance and paleoenvironment

  • Ana Fociro EMAIL logo , Oltion Fociro , Irakli Prifti , Redi Muçi and Jeton Pekmezi
From the journal Open Geosciences

Abstract

In this study, the reconstruction of the formation condition in the Oligocene flysch (Berati and Zhitomi section), in the Berati anticline structure, north-eastern part of the Ionian tectonic zone (Albania), is elucidated using petrographic–mineralogical characteristics and grain size analysis. Outcrops from the Berati and Zhitomi and the drilled wells were selected for sampling based on previous stratigraphic and paleontological studies. The mineralogical study of the flysch deposits made it possible to evidence for the first time three mineralogical zones: (a) quartz–garnet (b) serpentine, epidote with mica, and (c) feldspar zone, and three these petrographic zones: (a) quartz, (b) quartz–serpentine, and (c) quartz with mica and feldspars. The reconstruction of the depositional environment is based on the petrographic study of rock types, their textural characteristics, and grain size statistics. The granulometry coefficients like mean, sorting, skewness, and kurtosis are calculated arithmetically and the C/M diagram as well. Referring to the mineralogical composition of the sandstones and siltstones, the obtained data were used for the correlation of the sections and the delineation of the leaching area and the direction of the sediment movement. The mineralogic and petrographic characteristics show that during the Oligocene, the region has been under continuous paleogeographic change and under intensive orogenic activity, which has influenced the character of the mineralogical–petrographic composition of these deposits. Based on the pebbles petrography present in the slump horizons and on heavy mineral assemblages, it was evidenced that the eastern tectonic zones of Kruja, Krasta, and Mirdita (Albania) were the main suppliers of sedimentary material. The Passega C/M diagram suggests suspension and saltation as the main mode of sediment transport prior to deposition.

1 Introduction

From a regional geological point of view, the sedimentary and tectonic events in Albania indicate three main stages in the evolution of this Alpine–Mediterranean area. The Albanides are part of the Hellenides–Dinarides chain and as such of the Peri Adriatic Alpine chain system [1,2]. The three major stages are the result of the subduction of thinned continental crust at the margin of the Adria plate, consequent to the convergence of Africa and Eurasia since Late Cretaceous, with the rotation of the thrust belts, and the related amount of tectonic shortening. Several studies have been published in the past describing sedimentary environments, facies, paleo-geography, and perspective for exploration in the Ionian zone [3,4,5,6,7].

In a regional framework, the provenance of flysch sediments and the Palaeogene-Early Miocene geodynamic evolution of the Hellenides and Dinarides have been characterized by different authors mainly based on petrographic, geochemical, and heavy mineral assemblage distribution [8,9,10,11,12,13].

The aim of this work is to provide more detailed information to decipher the provenance, the evolution, and the source area geology of the Oligocene flysch, Ionian zone, Albanides. This study presents for the first time the mineralogical and petrographic zonation of these two sections and discusses implications for the provenance of sediments, formation conditions, and depositional processes. In accordance with different studies, it was established that the characteristics of a depositional agent are directly reflected in the texture of the sediment [14,15]. This relationship is particularly evident if the texture is represented by two parameters of the grain size distribution: C the one percentile and M the median diameter. CM pattern analysis carried out using the C/M diagram in Passega and Byramjee’s modification (1969), based on the C/M ratio of the sediment sample grain size, determines its predispositions to be subject to one of the nine types of transport before deposition erns formed by sample points off a deposit are characteristic of the agent of deposition.

The obtained results are of great significance for reconstructing the evolution of sediment provenance and paleoenvironment on a regional scale and for correlating these successions with other areas (e.g., southward to the Ionian zone of Greece). However, sedimentological, petrographic, and mineralogical studies focusing on the characterization and reconstruction of the conditions of the depositional environments of Oligocene flysch in the Ionian zone, based on new methodologies used by other authors are still relatively poor [16,17,18,19].

The present article approaches three main aspects petrography, mineralogy, and relationships between texture and depositional environment. The excellent exposure and drilled wells in the Berati and Zhitomi sections make them the key areas to reveal the aspects (Figure 1a and b).

Figure 1 
               (a) Geographic location of Albania (left) and study area (right). (b) Schematic map of tectonics units in peri Adriatic area, Albanides are located within the black rectangle (modified according to refs. [5,27].
Figure 1

(a) Geographic location of Albania (left) and study area (right). (b) Schematic map of tectonics units in peri Adriatic area, Albanides are located within the black rectangle (modified according to refs. [5,27].

2 Geological setting of the study area

The Albanides are composed of eight litho-tectonic units or so-called tectonic zones integrated into the Albanian fold-and-thrust belt which extends along the country following an NNE-SSW orientation [20,21] (Figure 1). This structural framework attests to a general continuity northward in the Dinarides (Montenegro, Croatia, and Bosnia) and southward in the Hellenides (Greece; Figure 1). This structuration is inherited from several phases of deformation that affected the deposits during the Alpine Orogeny from the Late Eocene to Pliocene, following a westward propagation of the thrust front [22]. Based on several criteria like facial, structural, tectogenesis age, intensity of magmatism, relationship between different tectonic units, etc., the Albanides are commonly divided into two distinctive parts, i.e., the internal Albanides to the East and the external Albanides to the West.

In the Ionian zone, three subzones are distinguished: Çika, Kurveleshi, and Berati. In literature, it is known as the anticline carbonate structure of Berati and lies in the eastern part of the Ionian zone (Figure 2). It represents a wide platform with shallow water evaporites and carbonates during the Triassic to the beginning of the Liassic and a trough with carbonate and siliceous pelagic sediments until the late Eocene (Figure 3). Flysch was deposited during the Oligocene and early Miocene. It is characterized by a great thickness of carbonates and flysch. In the latter, the numerous presences of slump horizons are characteristic. The Ionian zone is the main oil-gas-bearing zone of Albania. The Berati anticlinal belt is the most eastern in the Ionian zone, and it is spread partly in Albanian and in Greek territory and overthrust considerably toward the west (20–30 km), thereby hiding perspective structures that occur in the subthrust. The evolution of evaporite tectonics is very important in the determination of the principal features of the structural model in the Berati anticlinal belt. The diapir eruption has been a continuous process, starting from the rifting stage (T3 to J3-Cr1), with its maximal development achieved during the compression stage (Cr2-Serravallian). Its final features were created in the post-collision stage (Serravallian to Pliocene). In the beginning, evaporite tectonics was characterized by gravitational movements; later, the orogenic movements dominated. According to previous studies, the oil fields in carbonates are two types: thrust and subthrust oil fields. There have been limited explorations of the subthrust oil fields. The advances in three-dimensional technology and deep drilling have created possibilities for the exploration of the Albania subthrust. The complexity and diversity of the subthrust is a challenge to future exploration of oil and gas in Albania. Nowadays, the more interesting zone is the subthrust beneath the overthrust of the Berati anticlinal belt, which has already experienced intensive activity (drilling and seismic works) by different oil companies. At the same time, the subthrust beneath the overthrust of the other anticlinal belts (Kurveleshi and Cika) and that beneath the overthrust of the External Albanides (Ionian and Kruja Zones; Figure 2) are the main objects of interest for the immediate future [23,24,25,26,27]. Recent studies have identified favorable zones of gas accumulation via fault distribution and sedimentary facies using 3D seismic grid, well logs, and several cores using seismic stratigraphy, geological modeling, seismic attribute analysis, acoustic impedance inversion-based reservoir quality prediction, and well logging for the delineation of gas accumulation zones [28,29,30]. Such methodologies can be extended in future studies in the entire Ionian zone.

Figure 2 
               Çika, Kurveleshi, and Berati subzones of Ionian tectonic zone and location of Berati and Zhitomi section.
Figure 2

Çika, Kurveleshi, and Berati subzones of Ionian tectonic zone and location of Berati and Zhitomi section.

Figure 3 
               Generalized lithostratigraphic column of the Ionian zone.
Figure 3

Generalized lithostratigraphic column of the Ionian zone.

Berati and Zhitomi sections lie in the upper part of the Berati anticline structure. Our study is focused only on the Oligocene flysch.

3 Materials and methods

Geological mapping of the Berati and Zhitomi sections was carried out during several field investigations from March to June 2021, where the outcrops were logged and studied bed by bed. The main types of sandstones were classified using the scheme of Pettijohn (1975). Hand-size specimens (46 in total) were collected from each section for petrographic and mineralogical characterization (using Nikon Eclipse 50i polarizing microscope). To achieve the separation of the clay mineral fraction for XRD analysis, several bulk samples taken at different levels of the Berati and Zhitomi sections were treated with 10% acetic acid for 1 h to dissolve any possible carbonate constituent present. After this, the acid-insoluble fraction was dispersed in an ultrasonic bath for 10 min. Subsequent to Atterberg sedimentation, the <2 µm size fraction was collected, following separation by 0.45 µm cellulose acetate membrane (Sartorius) filters using a suction filtration unit and drying at 40°C. Udden (1914) and Wentworth’s (1922) classification was applied for grain-size classification, based on this results granulometry coefficients like mean, sorting, skewness, and kurtosis were determined. The distribution of sediment samples is depicted using statistical approaches. Based on other studies, adopting the graphical method is the most suitable for delivering statistical analysis [31,32,33]. The grain-size parameters that will be determined in this research are mean (M), sortation (So), skewness (Sk), and kurtosis (K G). These parameters are commonly utilized for constructing depositional processes and identifying the mechanism of transportation based on the CM diagram [14,15,32,33,34].

4 Results

4.1 Petrographic characteristics of Zhitomi and Berati sections

Based on the field campaign and well logging, the following deposits were evidenced. Oligocene deposits in these regions consist of flysch, representing intercalation of clays, siltstones, and sandstones, with horizontal slump horizons of conglomerates and rarely other rocks [35]. At the bottom of the section, the clay constituent predominates with intercalation of siltstones and sandstones occurring in thin layers (Figure 4). Upward the section, the sandstone component is added, forming thicker layers. Starting from the Middle and Upper Oligocene deposits, we notice the appearance of slump horizons consisting of sandstone intersections with different textures and chaotic structures, within which conglomerate rocks can be evidenced. In rare cases, biogenic limestone horizons are encountered, in the Berati section, sample nos. 76–78 (Pg3 1) and at the top of the section, sample nos. 295–297 (Pg3 3), while in the Zhitomi section, the sample no. 100 (Pg3 1). From the macro and microscopic investigation done in the field, laboratory, and under the microscope, it results that the Oligocene deposits in both sections consist of the following types of rocks:

  1. conglomerate, sandstones, and siltstone,

  2. clay, and

  3. marls and biogenic limestones.

Figure 4 
                  Lithological column of Berati and Zhitomi Oligocene deposits: (a) mineralogical and biostratigraphic zones evidenced in the Berati section, (b) general view of the Berati outcrop, and (c) mineralogical and biostratigraphic zones evidenced in the Zhitomi section.
Figure 4

Lithological column of Berati and Zhitomi Oligocene deposits: (a) mineralogical and biostratigraphic zones evidenced in the Berati section, (b) general view of the Berati outcrop, and (c) mineralogical and biostratigraphic zones evidenced in the Zhitomi section.

Conglomerates appear within slump horizons, which are present in the middle and upper parts of the section (Pg3 2–Pg3 3; Figure 4).

The conglomerate clasts have a variable petrographic composition forming a polymict. conglomerate. Sedimentary rocks like limestone, sandstone, marl, and shale were evidenced. Metamorphic quartz shales and effusive magmatic rocks such as diabase, spilite, and intrusive rocks such as gabbro and gabbro-diorites are found, but these represent only a small quantity. In general, the clasts are well to very well rounded. This is more visible for the limestones.

Limestones are of different ages, partially recrystallized with biogenic debris from the neritic areas, mudstones, and bioclastic grainstones with terrigenous material formed within the flysch deposits of the eastern tectonic zones. According to paleontological data, the age of these clasts is Cretaceous–Paleogene. Sandstones are relatively rare, mostly quartz sandstones with basal carbonate cement of the Krasta zone (east of the Ionian zone). The petrographic study of the conglomerate clasts results that most of them have come from the eastern zones like Kruja, Krasta, and Mirdita. But it has also been affected by the Ionian zone itself, where limestones, cherts, and marls are found as clasts within slump horizons of the Upper Oligocene [36]. Sandstones and siltstones are widespread especially in the Middle and Upper Oligocene deposits. They appear with thin to medium layering in the lower Oligocene, while upward they become thick to massive. They are gray to beige and on the eroded surface beige to brownish. The layers of sandstones and siltstones have sharp boundaries especially on the lower part of the section, while in the top of the section, we notice a gradual transition from sandstones in siltstones to siltstones and carbonate clays [37].

Thus, the vertical differentiation of the clastic material is observed. Often, at the bottom of the sandstone layers, we notice different sedimentary figures. Also, at the top of the layers, there are undulated, convolute textures, etc. These textural forms, as well as conglomerate clast orientations or oblique textures, were measured during the field study and were used in conjunction with other analytical data for the paleogeographic interpretations. From the microscopic study of the thin sections, it has been noticed that the sandstones have a small grain structure at the bottom of the section, while passing from the bottom toward the top of the layers, they increase the size of the clastic material, which is also noted in the mean diameter values of the clasts (M) (Table 1; Figure 5).

Table 1

Granulometry coefficient values and the light fraction minerals present in both sections

Section Mineralogical zones CaCO3% Granulometry coefficients Light fraction minerals
M So Sk K G Quartz Feldspars Micas Serpentine
Berat I 39.0 0.661 1.17 0.653 1.233 67.3 12.3 1.7 1.9
II 35.3 0.087 1.24 0.524 1.288 60.8 11.8 1.4 7.7
III 34.5 0.111 1.56 1.568 1.066 60.0 16.3 2.9 4.8
Zhitom I 40.9 0.066 1.31 0.572 1.247 71.8 12.0 0.0 4.5
II 35.2 0.119 1.48 0.48 1.779 54.7 11.8 0.4 17.3
Figure 5 
                  (a and b) Quartz, mainly angular, monocrystalline, set in microcrystalline calcite and lithic fragments such as limestone, quartz shale, and serpentine. (c and d) Calcite crystal, mica flake, and plagioclase grains surrounded by microcrystalline calcite. (e and f) Heavy opaque minerals oriented according to the stratification.
Figure 5

(a and b) Quartz, mainly angular, monocrystalline, set in microcrystalline calcite and lithic fragments such as limestone, quartz shale, and serpentine. (c and d) Calcite crystal, mica flake, and plagioclase grains surrounded by microcrystalline calcite. (e and f) Heavy opaque minerals oriented according to the stratification.

Sandstones have carbonate–clay cementation of basal type; sometimes, they have mixed basal cement. Massive textures predominate, rarely parallel and irregular ones. The mineral-petrographic composition of the fractional mass is mainly quartz, and then, rock fragments such as limestone, quartz shale, serpentine pieces are mostly found (Figure 5a–d). The composition of this mineral–petrographic complex in Oligocene deposits, going from bottom to top, presents some changes in their quantitative ratios.

Thus, in the lower part of the section of the studied deposits, it has been noticed that sandstones and quartz alevrolites are widespread. Above, with the appearance of thick to massive sandstones of Pg3 2, serpentine sandstones appear mainly, and even above, in the Upper Oligocene, the sandstones become more polymineral [38]. Rock fragments such as quartz and gravel shales as well as friendly and feldspar minerals are especially added here. So, from the petrographic study in thin sections, Oligocene deposits in the Berat–Elbasan region are divided by these petrographic zones (Figure 4):

  1. quartz,

  2. quartz–serpentine, and

  3. quartz with mica and feldspars.

These petrographic zones are spread throughout the studied sections, as well as in the drilled wells in the study area.

The mineralogical study of sandstones and siltstones shows that in the minerals of the light fraction, quartz and a smaller quantity of feldspar predominate, while it contains a few serpentine and mica (Figure 5a–d). Heavy fraction minerals consist of metallic minerals where the chrome-spinel’s group predominates and less pyrite/marcasite is detected (Figure 5e and f; Table 4). In relatively high content, garnet minerals are found in the lower part, while in the upper part of the deposits, those of the epidote group are also found. Very low quantities of spinel, sphene, rutile, amphibole, chloritoid and traces of pyroxene, tourmaline, barite, etc. were evidenced. This mineralogical association in terms of quality is characteristic of the whole range of Oligocene deposits in the studied region. However, the quantitative content of some of the minerals in the section is different. Based on these quantitative changes in some minerals, the deposits were grouped into three mineralogical zones:

  1. quartz–garnet mineralogical zone;

  2. serpentine mineralogical zone; and

  3. epidotes with mica and feldspar mineralogical zone.

Clay in terms of their distribution occupy the main place in the lithological composition at the bottom of the studied sections. From the microscopic study and the granulometric analysis, it results that these rocks contain mostly clastic material with silts – sandy size. This clastic material contains 20–45% of the rock mass and is irregularly distributed. The petrographic and mineralogical composition of this material is like that of sandstones and siltstones. The clays contain in the form of syngenetic mixture CaCO3 that varies in quantities of 20–40%.

They have sparse or poorly preserved biogenic fragments and autogenic pyrite, often oxidized. From the XRD analysis, the mineralogical composition of the clays from this region was determined. According to this analysis, it turns out that the main constituents of these rocks are montmorillonite, illite, chlorite, and kaolinite. It also contains mixed-layer minerals composed of montmorillonite, illite + chlorite, chlorite + vermiculite, and illite + vermiculite (Tables 2–4).

Table 2

Mineralogical composition of the clays in the Berati section based on XRD analysis

Outcrop/Section Age Sample Mineralogical composition (%)
Montmorillonite Illite Montmorillonite Illite Kaolinite Chlorite Chlorite–Vermiculite Illite–Chlorite Illite–Montmorillonite
Berat Middle Oligocene (Pg3 2) 156 36.2 23.4 29.8 10.5
150 51 14 18.2 8.4 8.4
146 49 18.1 16.1 8.4 8.4
138 56.4 11.8 12.8 12.1 6.7
134 52.2 11.6 19 12.9 4.3
130 34.2 26.9 26.6 12.3
126 39.1 22.5 27.1 11.2
122 37.5 15.8 31.8 12.1 2.7
116 52.6 7.1 26.4 11.8 2.1
112 40 7.3 32.5 20.1
108 51.8 9.3 26.7 8.9 3.2
Lower Oligocene (Pg3 1) 104 50.5 6.3 26.7 10.9 5.4
100 46.5 7.1 25.5 14.4 6.4
96 46.4 10 28.2 15.4
93 53 8.6 25.4 12.9
88 46.7 9.1 28.1 11.2 4.7
83 9.2 11.2 5 6.2 68.3
79 49.8 6.5 19.1 19.1 5.4
75 36 21.1 31.1 11.8
71 30.6 20.3 32.4 12.7 3.8
67 33.2 19 31.4 11.9 4.4
63 31.4 21 31.2 12.4 4
60 49.3 14.8 13.2 7.2 7.7 7.7
54 55 17.3 9.2 6.5 11.8
50 72 13.6 3.6 10.8
46 66.3 18.8 14.8
40 53.4 22.2 10.5 3.9 9.8
36 52 18 8 5.6 7.3 9
32 47.2 16.1 15 9 6.3 6.3
28 38 19.4 19.5 14.8 8.3
24 43.5 15 16.5 11.3 6.8 6.8
21 56.7 19.9 8.4 6 9
18 62.5 15.5 4.3 2.1 7 8.5
16 9.5 23.6 17.1 7.5 42.2
11 50 14.9 14.1 10.5 10.5
7 43.1 19.7 18.5 11.4 7.1
5 40 18.4 17 13.1 11.4
2/d 51.7 22.3 3 6.5 8.2 8.2
Table 3

Mineralogical composition of the clays in the Zhitomi section based on XRD analysis

Outcrop/Section Age Sample Mineralogical composition (%)
Montmorillonite-Illite Montmorillonite Illite Kaolinite Chlorite Chlorite–Vermiculite Illite–Chlorite Illite–Montmorillonite Illite–Vermiculite
Zhitom Lower Oligocene (Pg3 1) 111 82.1 4.4 4.8 8.5
109 71.8 5 10.1 5 8
107 71.8 4.4 11.5 5.4 6.8
104 67.4 5 11 5.5 5.5 5.5
101 67.7 4.8 12.9 8.1 6.4
98 32.3 21.9 29.2 16.5
96 37.1 18.7 26.7 11 6.4
94 40.6 18.5 24.1 11 5.8
92 34 20.8 28.5 11.8 4.8
89 47.5 17.8 19.7 8.1 6.8
87 22.9 32.4 15.3 29.3
85 22.3 33.3 14.3 3.4 26.6
83 23.2 26.3 13.7 6.3 30.5
81 20.8 34.4 15 5.1 24.7
79 22.9 32.8 13.6 2.5 28.1
77 20.5 23.3 10 5.8 40.3
75 22.4 28 13.4 6.3 29.8
73 21 32.6 13.7 4.2 28.5
71 20.9 29.1 13.7 4.4 31.8
69 21.2 30.3 14.3 5.3 28.8
67 21.8 32 14.1 4.6 27.5
63 22 36.7 17.4 2.4 21.5
61 26 31.5 14.9 27.6
59 29.6 21.6 33.1 15.6
57 18.1 36.4 14.5 3.7 27.2
55 45.8 17 16.6 12.8 7.6
53 49.2 14.8 9.5 8.8 8.8 8.8
51 48.7 14.6 13.3 11.6 11.6
49 47 11.4 16.1 10.5 7.4 7.4
47 19 13.6 10 8.6 48.8
45 46.3 19.2 12 10.3 6.9 52
43 17.5 13.8 10.9 7.3 50.3
37 14 12.8 11.1 8.5 53.5
33 21.3 13.4 12.6 9.4 43.2
31 50.8 16.6 12.4 10.1 10.1
29 44.5 16.5 20.1 12.3 6.5
27 37.2 15.8 24.4 15.8 6.7
25 33.9 18.7 24.5 15.8 7
23 21.8 21.9 11.2 6.5 38.5
21 33.7 19.6 22.7 12.6 6.6 4.8
19 26 28.7 11.9 33.3
15 21.7 31.4 12.9 3.4 30.4
13 29.2 30 14 4 22.8
11 27.2 27.3 27.4 13.1 4.9
7 No information; it is represented by marls
Table 4

The average values of the mineralogical–petrographic indicators calculated based on the mineralogical zones in the studied sections

Section Mineralogical zones Heavy fraction weight Heavy minerals fraction Clay minerals Mix layer
Metallic Epidote Garnet Spinel Sphene Amphibole Pyroxene Chloritoid Montmorillonite Illite Chlorite Kaolinite M-I KI/V I/kl I/M
Berat I 0.087 61.3 0.8 27.6 2.4 2.2 0.3 0.01 0.7 10.3 17.8 8.6 15.5 35.2 7.0 3.0 2.1
II 0.132 67.1 4.1 10.5 3.6 1.3 0.4 0.1 0.2 30.0 13.0 10.7 22.0 17.7 3.2 0.2 2.8
III 0.100 64.3 8.9 15.5 1.8 2.7 0.6 0.03 0.8 13.5 17.0 9.5 17.2 35.6 5.9 0.8
Zhitom I 0.077 61.6 0.5 27.1 2.1 2.0 0.5 0.02 0.4 0.6 19.6 12.5 24.1 16.4 5.1 0.1
II 0.160 66.3 5.9 20.4 2.7 0.7 1.5 0.1 0.05 72.1 4.7 4.8 1.0 5.7 2.3

The quantity of these minerals varies from the bottom toward the top of the section, and it was considered for the mineralogical zonation of these deposits. Bioclastic limestones are rare but they were found both in Berati and in Zhitomi sections. The limestones are from a few meters thick to full junctions and in their lateral extension pass into sandstones. Based on their microscopic study, it is evidenced that they consist mainly of macrofauna-sized foraminifera fragments, well preserved, containing 1–15% sand-sized material composed of quartz, limestone fragments, rarely quartz, and serpentine. The calcite cement consists of basal type. Rarely contains authigenic pyrite and glauconite. Marls are very rare, and they are found in the form of lenses and concretions, especially in the Middle and in the Upper Oligocene deposits. From their microscopic study, it is evidenced their micritic structure, with rare microfauna fragments content and rarely silt-sized clastic material.

4.2 Characteristics of mineralogical zones

Based on the quantitative changes of terrigenous minerals that constitute sandstones and siltstones as well as the changes of many other petrographic mineralogical indicators, such as granulometric coefficients, mineralogical composition of clay rocks, carbonate content, etc., the mineralogical zoning of deposits was made. The characteristics of these zones are described below:

4.2.1 Quartz–garnet mineralogical zone

It is the lower mineralogical zone of Oligocene deposits (Figures 4 and 6). In Table 4, note that it is represented by the predominance of mean values of quartz minerals and garnets, and also, this mineralogical zone has relatively lower values of the mean diameter of the clastic material (M) as well as high carbonate content. Another important indicator that distinguishes the deposits of these mineralogical zones from the overlain is the low content of montmorillonite group minerals in the composition of clay. The quartz predominance in the terrigenous material has been clearly observed in the sandstones and siltstones of this mineralogical zone from the microscopic study of the thin sections. This material is well-sorted showing maturity textural and mineralogical maturity.

Figure 6 
                     Mineralogical zonation of Berati section.
Figure 6

Mineralogical zonation of Berati section.

4.2.2 Serpentine mineralogical zone

It is located over the quartz–garnet mineralogical zone; lithologically, it is characterized by a great increase in the sandstone component and the occurrence of slump horizons with conglomerates (Figures 4 and 6). In this study, it is noticed the increase of the average diameter of the clastic material whose values vary from 0.05 to 0.066 mm that were in the quartz–garnet mineralogical zone here they go up to 0.087–0.113 mm. In this zone, there is a significant increase in heavy minerals fraction, the average values of which are 0.132–0.890 mg versus 0.033–0.037 mg that are in the lower deposits. Characteristic of this mineralogical zone is the pronounced increase of serpentine minerals, which has been noticed even earlier [39,40,41]. Due to this composition, these deposits are included in the petrographic zone with quartz–serpentine sandstones.

In the composition of clay minerals, we noticed that montmorillonite, whose average values for the lower zone are 0.5–10.8%, while in this zone 30–72.1% (Tables 1, and 4). The deposits of this zone, as it appears from the field and laboratory petrographic data, have been formed in a shorter period than the bottom ones. This is also reflected in the texture and the degree of elaboration and sorting of the clastic material. This is evidenced by the low content of syngenetic calcium carbonate, whose values are 5–10% lower than in the quartz–garnet mineralogical zone.

4.2.3 Epidote with mica and feldspar mineralogical zone

Includes the upper part of the Oligocene. From the study, it was noticed that the terrigenous components of sandstones and siltstones are more heterogeneous than in the lower part of the section. Particularly noticeable here is the pronounced content of rock fragments material represented by quartz shales, cherts and limestone, and fewer pieces of serpentinized and chloritized igneous rocks. From the minerals, we notice the high content of the epidote, which characterizes the deposits of this part of the section. Also, here, we found mostly mica and feldspar minerals, and it should be noted that mica is mainly represented by muscovite and are in the form of large flakes.

In general, the study also shows the tendency to increase the minerals of the epidote to some extent of serpentine and the sand component. Going from older deposits to younger ones the content of quartz minerals, garnets, and carbonate increases. As the values of the sorting coefficient of the grainy material tend to get low, the mineral content of the heavy fraction increases.

4.3 Formation conditions of Oligocene flysch

Oligocene flysch deposits are turbidite marine formations. The conditions of their formation have changed in terms of the sea–continent border, the marine environment where the sediment accumulated, as well as the leaching area. The characteristics of these changes are evidenced through petrographic and mineralogical indicators, from the oldest toward the youngest deposits. In the deposits of the quartz–garnet mineralogical zone, the flysch is represented by thin clayey-silty-sandy layers. The composition of terrigenous material in these rocks is quite mature. Quartz comprises very high values. This maturation is also observed in the mineral content of the heavy fraction, predominated by metallic minerals and garnets. At the Berati and Zhitomi sections, the deposits of the quartz–garnet mineralogical zone have clear flysch rhythmic interlayering, regular stratification, and often parallel textures, which reveals a relatively calm hydrodynamic environment. The composition of rocks is found syngenetic calcium carbonate, about 40%. These rocks also contain significant amounts of planktonic microfauna fragments. The reworking of sedimentary material is relatively good, while the average sorting is low.

By the elaboration of granulometric data and the construction of the CM diagram, it is evidenced a typical pattern of turbidites. The cumulative curve analysis of the samples located in the fields assigned to the dominant deposition from traction in the C/M diagram (Fields I, II, III, IX – Dominant deposition by traction with small share of suspension) showed that the predominant type of transport prior to deposition was saltation (Figure 7).

Figure 7 
                  Samples analyzed and depositional processes derived from C/M diagram [34].
Figure 7

Samples analyzed and depositional processes derived from C/M diagram [34].

The share of saltation in the fields of the C/M diagram corresponding to the deposition of graded suspension in conditions of high (Field IV) and moderate turbulence (Field V) (Fields IV, V – Graded suspension transported in highly turbulent conditions and in moderately turbulent conditions) amounted to 72–83% (Field IV), and 19–81% (Field V).

Deposition from the suspension of 29–97% dominated in the fields assigned to the deposition of graded suspension that is transported in conditions of low turbulence and uniform suspension of varied grain size (Fields VI and VII, graded suspension transported in low turbulent conditions and uniform suspension with more complex deposition; Figure 7).

In this sedimentation environment, there were also good conditions for the chemical and biochemical sedimentation of syngenetic calcium carbonate. Sediments located in the field of the C/M diagram corresponding to the finest uniform suspension and pelagic suspension (Field VIII, finest uniform suspension, and pelagic suspension) were deposited in majority from the suspension of 87–91% and saltation of 2–5.3%. The water current velocity during deposition from traction, saltation and suspension largely overlap in the samples studied. This indicates that, prior to deposition, transport in saltation, suspension, and partially in traction occurred at similar flow velocities for both sections.

During diagenesis there were favorable chemical conditions as a result of which authigenic pyrite was formed, which is found in the form of irregular aggregate nests, as well as inside of the microfauna fragments.

For the studied region we think that during the formation of the quartz–garnet mineralogical zone they were not the same throughout the region. In the Zhitomi section we think that the incoming of sedimentary material has been abundant. In addition to suspension transport, saltation, and traction transport of coarser material at the bottom of the water flow, in comparison to other sections in the north, like Berati section. Based on mineralogical indicators we also note that the southernmost part of the study region had the source area closer. Thus, the least stable minerals are 2–3 times more abundant in Zhitomi than in other northern sections. Other least stable minerals, like pyroxene and amphibole, also show a tendency to increase their content by passing from the northern to the southern section. The occurrence of the deposits of the serpentine mineralogical zone begins with the significant addition of terrigenous material with coarse texture. Many types of sedimentary structures from irregular, wavy, convoluted, and parallel as well.

Several slump horizons with conglomerates, gravelites, and massive sandstone horizons are present. These demonstrate that during this geological time (Pg3 2) in the leaching zone and within the sedimentation basin, orogenic movements have happened, which have changed the sea-continent border and increased the incoming quantity of terrigenous material from the continent in a very short time.

Due to the high velocity of sediment deposition, it had no time to fully develop maturation processes. This is evidenced by the decrease of the coefficient of stability and the relative increase of least stable minerals such as serpentine, amphibole, and pyroxene. The sorting of the clastic material is lower comparing those of the quartz–garnet mineralogical zone. The presence of calcium carbonate solutions in the sedimentation basin is lower, and it is reflected in the content of syngenetic calcium carbonate up to 10% lower than in the zones below, this is more typical, especially the Zhitomi section. This is related to the weakening of chemical weathering in the leaching zone, due to uneven relief, as a result of orogenic movements that have occurred during that period.

The clastic material in the sandstones and siltstones of this mineralogical zone is relatively less reworked than the lower deposits. This suggests a shorter transport time due to the uneven relief but also indicates a closer distance to the leaching zone. From the petrographic study of the pebbles present in the slump horizons, it can establish that the eastern tectonic zones Kruja, Krasta, and Mirdita during this period were the leaching zones and the main supplier of sedimentary material [42]. Small quantities of sedimentary material have also come from the formations formed within the Ionian zone itself.

Based on the tectonic mineralogical coefficients derived from the zircon/mica mineral content ratios, it appears that from north to south, the last paleo relief of the sedimentation basin has had an upward trend. The coastal line was closer to the region of Zhitomi and Berati, which is reflected in the sedimentation coefficients based on the mineral quartz/feldspar, zircon/epidote + amphibole + pyroxene +, and garnet/epidote + amphibole + pyroxene minerals [43].

Based on the CM diagrams (Figure 7), we note that these deposits are formed in a marine sedimentary environment with variable dynamics, sedimentary material for the most part has come in suspension form and has sedimented in a deep sea with low hydro dynamism.

At a certain moment, the sedimentary basin is affected by pronounced seismic oscillations, which have some strong currents and turbidites with large sediment loading, bringing parts from the leaching zone, among which pebbles and gravels [44,45].

Part of this load comes from the sedimentation basin itself from the erosion of turbidite currents within the basin of partially lithified sediments and formations, which we encounter in the surrounding horizons in the form of folded and irregularly distributed fragmental layers. As for the deposits of the lower mineralogical zone, the sedimentary material was transported in the form of suspension and at the bottom of the current with saltation and traction. The depth of the sedimentation surface has fluctuated, but the general trend has been the gradually shallowing since the beginning of the deposition of the serpentine mineralogical zone. Thus, the sedimentation basin from the lower to upper bathyal from the end of the epidote mineralogical zone with mica and feldspar.

According to the mineralogical indicators, it appears that at the time of the formation of this part of the section, the coastline continued to move toward the west as a result of the eastern and perhaps also the south-eastern uplifts.

This is argued by the decrease in the stability coefficient values compared to the following mineralogical zone; this approach to the leaching zone indicates also a certain increase in unstable minerals such as pyroxene, amphibole, and epidote. From the petrographic study of the rocks and the sandstones in general, it is noticed that in the deposits of this mineralogical zones, the sedimentary material coming from the Ionian zone is added, which is expressed in the limestone and chert clasts of the Cretaceous–Paleogene. This shows that at this time the anticlinal belt of Tomorri and partly also the westernmost belts, emerged and served as washing areas.

However, even at the time of the formation of deposits of the epidote mineralogical zone with mica and feldspar, the main supply of sedimentary material was from the eastern zones. The high content of quartz schists and micaceous minerals, especially epidote group minerals, testifies that at this time there was a significant influence of the washout areas rich in metamorphic rocks and the weakening of the arrival of sedimentary material from the basic and ultrabasic magmatic massifs, which in the previous mineralogical area, to some extent, provided the nature of the supply of sedimentary material.

5 Conclusions

The Oligocene sedimentary succession of the Berati and Zhitomi section in the Berati-Elbasan area (central Albania) represent turbidites, deposited in a deep-marine clastic system, with recurrent periods of poorly oxygenated conditions supported by the presence of pyrite crystals. The succession consists of conglomerates, sandstones, siltstones, and claystone accumulated during phases of high influx of siliciclastic turbidity currents and chemical and biochemical rocks such as limestones, marls, and so on, coming mainly from the adjacent tectonic zones. Going upward in both sections (from the older toward younger deposits) based on the calculated granulometric coefficients, the source areas lie closer. The paleo-relief of the sea floor had a dip direction South–North. These conditions within the depositional basin are concurrent with small- and large-scale fluctuations in paleogeography, orogenic activity, and basin configuration in the Oligocene. For the first time, based on petrographic observations of the sandstones and siltstones, there are proposed three main petrographic zones, evidenced from the bottom to the top of the both sections, and they are as follows: quartz, quartz–serpentine, and quartz–quartzite zone with mica and feldspar. While based on petrographic observations of the clastic minerals, three main mineralogical zones are proposed as follows: quartz–garnet, serpentine, epidote with mica and feldspar mineralogical zone. The Kruja, Krasta, and Mirdita tectonic zones were the main supplier of sedimentary material. Meanwhile, heavy mineral assemblages suggest the presence of two major source types as well. The first is related to the existence of ophiolitic sources, indicated by the occurrence of chrome spinel, pyroxenes, green amphiboles, and partly epidote which are related to volcanic complexes. The second is a metamorphic source shown by abundant garnet accompanied by chloritoid. Stratigraphic trends in the heavy mineral distribution of the Oligocene flysch in Ionian zone give insights into the changing tectonic situation of the source areas, and a regional east–west trend with changing ophiolitic detritus indicates a very complex feeder system.

The C/M pattern indicates also the different environmental deposition conditions due to differences in the hydrodynamic regimes prevailing in the region. The majority of all the samples collected in this study are included in fields IV, V, VI, and VII on the C/M diagram. Conditions of depositional environment based on Passega and Byrjamjee 1969 indicate graded suspension transported in highly, moderately low turbulent conditions, and suspension with more complex deposition.

In conclusion, this study illustrates the potential of the deposits of the Berati anticline structure (Berati and Zhitomi sections) to be used for regional-scale palaeoenvironmental reconstruction of the Oligocene Paratethys and for petrographic and mineralogical correlation with other similar successions present in the Ionian zone in Greece.

Acknowledgement

We would like to take this opportunity to acknowledge the time and effort devoted by the two anonymous reviewers to improving the quality of the published manuscript in OPEN GEOSCIENCES.

  1. Author contributions: A.F., O.F., and I.P. conceived and planned all the workflow of the article. A.F. and I.P. carried out the fieldwork, logging, and sampling. A.F., O.F., R.M. and J. P contributed to the sample preparation for XRD analysis. A.F. contributed to the thin section description. A.F., O.F., I.P., R.M. and J.P. contributed to the interpretation of the results. A.F. and O.F. took the lead in writing the manuscript.

  2. Conflict of interest: The authors state no conflict of interest.

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Received: 2022-08-07
Revised: 2022-11-21
Accepted: 2022-11-22
Published Online: 2023-02-01

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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