Synthesis of Literature and Field Work Data Leading to the Compilation of a New Geological Map—A Review of Geology of Northwestern Greece

This paper presents the geological structure of the entire region of northwestern Greece (Epirus). Four geotectonic zones (Subpelagonian, Pindos, Gavrovo, and Ionian) develop in this area, overthrusting one another, their compression axes trending NE-SW. Normal, reverse, and strike-slip faults with main directions NNW-SSE, NE-SW, and E-W have influenced the geological formations. In the context of this paper, the results of all previous, relevant studies were considered, summarized and reviewed, in order to provide a brief historical recursion and present some of the most important discoveries made in the area, from 1840 until present. All these results were evaluated and combined, the geological formations were grouped according to their characteristics and field work enabled the confirmation or addition of new data, which led to the compilation of a new geological map, using GIS techniques, for the improved visualization of the geological and tectonic structure of northwestern Greece. This map illustrates a lot of new data, based on detailed geological-tectonic mapping, depicting the precise boundaries of the geological formations, detecting of Neogene and Quaternary sediments and evaluating fault activity. The knowledge and illustration of an area’s geological structure constitute a dynamic tool for further scientific research and economic development.


Introduction
The knowledge of geology of an area contributes to the exploration and exploita-tion of rocks and minerals with economic benefits, prognosis of natural disasters, evaluation, and overcome of environmental problems, definition of paleo-environmental conditions, successful construction of engineering works, etc. Therefore, the good understanding and knowledge of the geological structure of an area contribute variously to the improvement of human life quality and solution of its significant problems.
The geological structure of northwestern Greece (Epirus; Figure 1) is very complicated and complex as a result of many intense tectonic events (overthrusting, thrusting, folding, faulting). Since 1840, this area has been studied by many researchers and a large number of geological formations have been identified and examined. Different views about the geological conditions have been put forward.
These views have been evaluated, summarized, synthesized, and reviewed in this paper. New data from field work observations have been added, resulting in modification of geological boundaries, identification of their main characteristics, Epirus is made up of the geological formations belonging to the Subpelagonian, Pindos, Gavrovo, and Ionian Geotectonic Zones as well as of post-alpine formations [1]. The Ionian Zone dominates (percentage by 78%) in the northwestern Greece area. The geotectonic Zones of Pindos, Gavrovo, and Subpelagonian occupy 12%, 5.6%, and 4.4% of the total area respectively ( Figure 1).

Historical Recursionand Summary of Previous Geological Researches
During 19 th century, Boué [2], Viquesnel [3], and Neumayr [4] were the first geologists who studied the geological conditions in Epirus. Philippson [5] [6] [7] [8] [9] examined the whole structure of Epirus identifying the thrusted Pindos Zone over the Ionian Zone and the vertical axis level of folds in the eastern half part of Epirus.
During the first half of 20 th century, Renz [10]- [15] established the stratigraphic regime in western Greece confirming the Pindos ovethrust and identifying the overthrust sheets towards the west. Brunn [16] showed the inclination of the Mitsikeli anticline eastwards.
Aubouin [17] [18] [19] compiled the first geological study for the whole Ionian   [20]. These faults were created by an extended extensional stress field resulted in the first tectonic deformation on the Early Lias limestones (known as Pantokratoras limestones; Figure 3 and Figure   4) in that period. In 1966, the same scientific team studied in details the tectonic setting for the entire Epirus compiling an excellent map at scale 1:100,000 and distinguished three successive compressional periods: Aquitanian-Burdigalian, Late Burdigalian (main tectonic phase) and Mio-Pliocene-Quaternary.  Bernoulli and Renz [21] presented new sedimentary and stratigraphy data regarding the Jurassic formations in Epirus. In the context of hydrocarbon exploration in western Greece, the scientific team of British Petroleum-B.P. Co [22] [25] noted that the Triassic breccias were formed from the dissolution of the subsurface Ionian evaporites.
Guzzetta [26] controverted the tectonic interpretation after IGRS-IFP [20] and proposed a thick-skinned deformation which is characterized by rootless faults and high dip thrusts up risen from a main detachment surface located at the evaporite level. Similar interpretation was attributed by B.P. Co [22], Jenkins [27], and Sorel [28].
King [29] examined a micro-earthquake series whose focal mechanisms are associated with the complicated deformation due to local heterogeneities in the stress regime and small scale changes since the tectonic movement of Apulian  [39] leading to the creation of sedimentary basins.
Karakitsios [40] [41] determined the formation of the Louros limestones (Middle Lias), provided new data about the stratigraphy of the Foustapidima limestones(Upper Triassic)and noted that many normal faults re-activated as reverse faults or thrust surfaces affected by alpine orogenic-compressional forces during Oligocene. Clews [42]  Karakitsios [44]- [50] defined in details the stratigraphy of the Ionian Zone with scientific publications during 1988-1995. He signalized the contribution of diapirism to the sedimentation and tectonics, studied its evolution and considered that the Ionian Zone constitutes an example of reverse graben with evaporites as substratum. During 1990, 1992, and 1995, he noted that the pre-existing normal fault systems of Jurassic age re-activated as reverse faults during Burdigalian. Listric faults of Jurassic age were transformed to reserve faults, thrusts or strike-slip faults. A detachment at the evaporite level was identified. Due to halokinesis, the reverse movement was carried out only in the upper part of the faults and therefore, the re-activation partially followed the typical reverse tectonics.
Rondoyanni [51] provided new data about the stratigraphy and tectonics of Plio-Pleistocene deposits in the Preveza area. Waters [52] studied the deformation and tectonic evolution of northwestern Greece and considered the induced movement as a result of deep invisible tectonic structures (sinking thrusts and folds) based on geophysical, stratigraphical, and tectonic data. Karakitsios [53] examined unconformities and paleokarst phenomena of Jurassic age in the Ionian Zone.
Nikolaou [54] correlated the surface manifestations of hydrocarbons with the dominated subsurface tectonic structures in Epirus. Paschos [55] studied the geodynamic evolution of south Epirus since Miocene, focusing on the stratigraphy and tectonics of Neogene and Quaternary sediments.

Geological Structure of Northwestern Greece
As it has been above-mentioned, Epirus is made up of geological formations belonging to the Subpelagonian, Pindos, Gavrovo, and Ionian Zones (from east to west), and sediments of Upper Eocene-Quaternary age deposited over them. A detailed description of the geological formations and lithostratigraphic structure of each zone based on previous studies (since 1840) and personal field observations follows. These formations have been grouped and the lithostratigraphical columns, which correspond to this grouping, are presented for each zone. The distribution of these formations is illustrated in the geological map of Figure 2. This map has been compiled by GIS techniquescombing new field data, produced by geological-tectonic mapping [1], with geological maps published by the Institute of Geology and Mineral Exploration-IGME at a 1:50,000 scale [65]- [88].
Particularly in terms of the correct approach of thegeological structure of northwestern Greece, apart from gathering all the relevant literature, the study focused on detailed geological-tectonic mapping, identifying the following: Notably, the background of the map is the area's terrain in 3D form, as produced by the ArcGIS software (source: www.esri.com), for the reader's improved understanding and the optimum illustration of the real picture.
Specifically, combining and synthesizing all the literature and the field work data emerged the following geological formations.

Post-Alpine Sediments
The post-alpine sediments of Epirus include: Riss, a glacier phase took place creating glacial deposits as moraines (at altitudes up to 1400 m) and glacier valleys (altitudes higher than 1900 m) forming old scree. In addition, deposits of internal basins (terra rossa-tr; Figure 6), of Pleistocene age (200,000 -250,000 years ago) [91], composed of red clays with thin layers of cherty rubbles which occur horizontal form [92]. In the Ioannina basin, thick lacustrine sediments-Pl.l (clays, marls, clayey silts, lacutrine limestones) including lignite layers [93] have been deposited during whole Pleistocene. Significant wards [16] [94]. The molassic sediments cover the boundary between the Subpelagonian and Pindos Zones and are deposited on either very thick ophiolites (mainly) or Mesozoic limestones [89] [95]. They consist of transgressive conglomerates and breccias, alternations of siltstones and sandstones, consecutive layers of marls and siltstones. In Epirus, the Mesohellenic trough formations occur in the subbasin of the Sarantaporos River (near Konitsa) and include of the following formations [65] [89] [96].
(a) Eptachorio Formation of Oligocene age. It consists of upper beds (marls with thin sandstone intercalations, intermediate sandstones, lower marls), transition beds (blue or green-yellow silty marls alternating with fine-grained to micro-conglomeratic sandstones) and base layers (alternations of polygenetic conglomerates with fine-grained to micro-conglomeratic sandstones and marls). In this formation, consecutive layers of marls-siltstones and mainly grey-blue coloured siltstones dominate, while sandstones have a smaller participation and conglomerates even smaller.

Subpelagonian Zone
The Subpelagonian Zone [18], extended in a NW-SE direction, occupy a very small part (4.4%) in the northeastern side of Epirus (Figure 1(a)). Its main characteristic is the presence of large ophiolitic masses-S.oph and the accompanied schist-chert formation-S.JC-k,sh. The ophiolites are characterized as the western (external) ophiolitic zone of Greece, and so it is suggested that the Subpelagonian and Pindos Zones originate from the same ocean area [97] (Figure 7).
The ophiolites mainly consist of peridotites and serpentinites and are accompanied by red cherts. The ophiolites of the Kastanea overthrust sheets accompanied by cherts are dated as of Jurassic age.

Pindos Zone
The Pindos Zone [18] with a NW-SE direction occupies a small part (12.2%) of the SE side of Epirus including the biggest part of its mountainous area ( Figure  1(a)). It is extended from the Greek-Albanian borders to the Pindos mountain chain including the Mts Grammos (northern Pindos), Lakmos (northern Pindos) and Athamanian (Tzoumerka). Palaeo-geographically, Pindos constituted a very large marine basin of great depth (Figure 8). The Pindos Zone is suggested as a tectonic nappe overthrusted on the Gavrovo Zone westwards (exceeding 100 km at some locations) and it is known as Tectonic nappe of Pindos [89] [98]. In the northern part, the overthrusted nappe of Pindos has fully covered the Gavrovo Zone and it is observed tectonically on the Ionian Zone directly [99]. It is characterized by the presence of overthrust sheets from east to west forming continuous repetitions of the geological formations of this zone and they are mainly caused by tangential compressional tensions [18]. The rocks of the Pindos Zone have been folded intensely forming multi-numerous close, inclined and overturned folds and many fronts of thrusts and reverse faults are observed along the tectonic nappe of Pindos [85]. The age of the Pindos Zone formations is of Triassic-Upper Eocene. The oldest stratigraphically formation of the Pindos Zone is a Triassic clastic formation-P.T-k,s consisting of (a) sandstones, cherts, marls, and limestones of Middle Triassic age, and (b) calcite turbidites, limestones (platy to thin-platy, marly, grey-black to black coloured), cherts (red to black), clayey marls, sandstones (green) and volcano-sedimentary materials (andesites, tuffs, basalts) of Middle-Upper Triassic age. These formations have been multi-folded [100].
The schist-chert formation-P.JC-k,sh of Jurassic age is composed of multicoloured cherts (radiolaritic, blue, green, brown, red and black coloured), clays, sandstones, siliceous limestones, and red cherts. The lower members consist of

Gavrovo Zone
The Gavrovo Zone is located between the Pindos Zone and the Ionian one ( Figure   1(a)). It is extended in a NNW-SSE direction and it has a limited exposure (5.6%).
In the northern part of Epirus, the tectonic nappe of the Pindos Zone is overthrusted on the Ionian Zone, covering fully the Gavrovo Zone. In this area, the Gavrozo zone occurs only as tectonic window [89]. In Epirus, this zone occurs at Valtos Mountains.   constitute clastic, light-coloured to grey, compact, bioclastic, locally dolomitized limestones with numerous fragments of Rudists in the upper part.
(c) Flysch of Upper Eocene (Priabonian)-Late Oligocene age (G.fl). The lower members consist of coarse to fine-grained and platy sandstones-G.fl-s with a few layers/alternations of clayey marls and small lenticular intercalations of conglomerates. The thickness of the sandstone beds reaches 3 m and it reduces gradually upwards. In the upper part, silty and clayey marls domimate. Sandstone layers and conglomerates of sandstone and calcareous cobbles occur in these marly beds. This flysch is differentiated lithologically from the flysch formations of the Pindos and Ionian Zones. Its main feature is the significant presence of conglomerates which often form banks having thickness more than 2 m. In addition, semi-rounded or well-rounded calcareous or cherty cobbles and gravels are observed in the siltstone mass [69] [71]. Finally, the rocks of the Gavrovo Zone were affected by mild orogenic tectonism [97] [99], which took place during Tertiary resulted in folding the formations with push westward, simultaneously with the overthrust of the Pindos Zone. In Epirus, a large anticlinic structure has been formed plunging towards northwestern and was affected by the activity of large normal faults in NW-SE and NE-SW general directions.

Ionian Zone
The Ionian Zone occupies the largest part (78%) of Epirus (western part) extended in NW-SE direction between the Zones of Gavrovo and Pindos ( Figure   1 [105]. There is no surface manifestation of the salt. Salt has been found by boreholes at various sites, as for example in Monolithi Ioannina (northeastern of Arta) [106] and Filiates [20]. Except for the original stratigraphic position, the evaporites are often found within younger rocks of the Ionian Zone due to diapirism and their movement through faults.
Primarily, the Triassic evaporites of the Ionian Zone were fractured and moved (horizontally and vertically) mainly during the orogenesis of Pindos and they are re-activated during Neogene and Quaternary providing neo-diapirisms in the corresponding sediments. The movement of the evaporites (palaeodiapirisms and neodiapiris [56] [107] ms) contributed to the geomorphological characterristics and the tectonic evolution of the area. The Triassic evaporites, which had been risen to the surface in various areas, were exposed to the erosion resulted in the removal of easy soluble salts (NaCl, KCl) by the influence of meteoric waters, Figure 13. Occurrence of Triaccic breccias in the Filiates area. the wetting of the remaining anhydrite and its change to gypsum [56]. By this way, the occurrences of gypsum and the absence of salts at surface can be explained. Despite the thickness of the primary evaporitic series is suggested to be greater than 1000 m, its real thickness reaches 3500 m due to diapirism [20] [73] [75], according to the results from deep oil exploration boreholes (e.g. Filiates-1) [75].
Next formation is the neritic Pantokratoras limestones, of Lower Jurassic age The upper/siliceous shales with Posidonia-I.J-sh, known as Radiolarites [111], of Upper Jurassic (Oxfordian-Tithonian) age follow [112] [113]. They consist of alterations of clayey-siliceous layers with cherts and are rich in Radiolaria [48]. In some areas, this formation includes calcareous intercalations. The thickness is estimated to be about 50 m. The main lithological difference between these shales with the above-mentioned lower Posidonia beds is their high silica content.
The distribution of the syn-rift formations of Ammonitico Rosso, Posidonia beds (Shales with Posidonia), limestones with filaments and upper Shales with Posidonia in combination with other tectonic or stratigraphic data show that their deposition took place in different subbasins (semi-trenches) resulted from the internal differentiation of the Ionian Zone. The extensional tectonic activity in combination with the salt movement (halokinesis of evaporites) caused this differentiation [48] [49]. In the deeper parts of the subbasins, the full sequence consisting of lower shales with Posidonia, Ammonitico Rosso, limestones with filaments, upper shales with Posidonia was deposited with transition to no full sequences and gradual decrease in their thickness (until full stratigraphic pinching-out in relatively shallow or almost emerging areas). In areas which corresponded to submarine ridges, Lower-Middle Jurassic (Toarsian-Callovian) phosphorite zones occur [114]. The upper shales with Posidonia, in contrast with the underlying formations, are presented throughout the Ionian Zone indicating that during their deposition the submarine topographical differences were mitigated tending to be eliminated [48] [49]. The transition to the siliceous formations of the upper shales with Posidonia is considered to be related to the bathymetry of the subbasins and the Calcite Compensation Depth-CCD. Therefore, the sea level rise and the increase of CCD level, due to relative lack of organisms rich in calcite material, can explain the siliceous sedimentation represented by them.
Following the upper siliceous shales with Posidonia, the next formation is the pelagic Vigla limestones-I.C-kd of Lower-Upper Cretaceous (Berriasian-Lower Senonian) age [48] [58] [59] [115] [116] (Figure 15). It is considered to be the first post-rift formation of the Ionian Zone. It consists of platy, thin-bedded, sub-lithographical limestones of a white or grey colour, with chert intercalations (0.5 -4 cm thick), lenses and nodules as well as intercalations of white-yellow clays. Towards the upper part of the formation, the upper siliceous series of Vigla or Vigla shales of Upper Cretaceous age (Cenomanian-Turonian) occur. In this series, the siliceous intercalations alternate with clayey-marly layers [48]. At the eastern margin of the Ionian Zone and more specifically in the mountainous areas of Xirovounio and Gamila, the thick grey Vigla limestone formation becomes dolomitic, bituminous and thick-bedded in its lower parts and thin-bedded with clayey intercalations and black cherts in its upper ones [68] [80] [87].
The calcareous sedimentation was mainly due to the reduction of the CCD level.
Concerning with the pre-mentioned Vigla shales of mainly siliceous composition, their deposition is attributed to the sea level rise [27]. In the Central Ionian Zone, towards its eastern margin and over upper the siliceous series of Vigla and in anticlinic areas, an Upper Cretaceous (Lower Sennonian) phos phate horizon occurs often [114] [117] [118]. The thickness of the Vigla limestones is 700 -900 m in the Internal and External Zone and smaller (100 - [88]. This differentiation and range of thickness may be attributed to halokinesis, i.e. salt movement [48] [49]. The next post-rift formation is the Upper Sennonian limestones-I.C-k dated by the presence of Foraminifera Globotruncanids [58] (Figure 16). This formation consists of pelagic micritic sub-lithographic limestones alternating with thick-bedded brecciated limestones containing Rudist fragments. The main characteristic is their frequent occurrence as thick banks in the field. The clastic material occurs at relatively high percentage in the Internal and External Ionian Zone and is reduced in the Central one. The pelagic Paleocene-Eocene limestones-I.E-kfollow and their age has been determined by the presence of abundant Foraminifera [20]. They are composed of white, well-bedded, compact; thin to medium-platy micritic limestones with rare chert intercalations and more rarely horizons of microbrecciated limestones in some places.
The youngest formation of the Ionian Zone is flysch-I.fl of Upper Eocene-Lower Miocene (Aquitanian-Burdigalian) age when the folding of this Zone took place ( Figure 17). Its upper limit varies between end of the Oligocene in the Internal Ionian Zone and Burdigalian in the External Ionian Zone [20] [119]. The flysch has a psammitic-marly composition in the lower layers and consists of alternations of siltstones, marls, marly limestones, sandy clays, clays, sandstones and rarely conglomerates towards its upper parts. From east to west, an increase in the ratio of fine-grained sediments (clay intercalations) to coarse-grained ones(sandstones) is observed [42]. This flysch can be distinguished into three additional types de-  Miocene and then, during the emergence-orogenesis of the Ionian Zone (from east to west). The molasse sediments were deposited on the western margins of the elevated parts [60].
The stratigraghy of the Ionian Zone is related closely to its evolution from carbonate platform to pelagic sedimentation basin, which began in Middle Lias (Lower Jurassic). The formations, which deposited before the differentiation of Zone as basin between the Gavrovo and Paxoi Zones, constitute the pre-rift formations [48]. Initially, the Ionian Zone was an integral part of the single platform

Discussion and Conclusions
The resulting picture of the geological and tectonic structure of northwestern The geopolitical location of Epirus, along with the elements of economic interest encountered in the area, such as the oil deposits which are detected at various locations by the scientific teams of IGRS-IFP [20] and B.P. Co [22], calls for the elaboration of new, revised maps of improved accuracy, using the new available technologies. The combined data, produced by the work of numerous researchers since the 1840's, can complete the puzzle that is the geological structure of northwestern Greece.
Taking into account the views mentioned by various researchers and the field work data, a short presentation of geological structure and tectonic conditions for each zone follows.
The Subpelagonian Zone, located east of the Pindos Zone and west of Pelagonian Zone, constitutes a small part in the northeastern part of Epirus and consists of ophiolites with deep marine sediments (schist-chert formation) deposited either on carbonate rocks or not. In the area of the Subpelagonian Zone, the Me- Siniae and Louros limestones, calcareous breccias and Upper Triassiclimestones of Upper Triassic age and Triassic evaporites (gypsum and salt). Mio-Pliocene sediments and alluvial deposits have been deposited over the Late Eocene-Lower Miocene flysch. The tectonic setting of the Ionian Zone is characterized by a series of parallel mega-synclines or mega-anticlines, thrusted or overthrusted onto each other westward. Their axes are generally oriented in a NW-SE direction, while southwards their directions are changed ranging from NNW-SSE and NNE-SSW intersected by E-W strike-slip faults.
Based on the lithology and types of rocks for each geotectonic zone, the geological structure of northwestern Greece consists of the following main-indepen-dently of zones-geological formations: -