The geological basement of the NNW Evros area, where the village of Petrota is located, consists of the metamorphic rocks of the Rhodope Massif. In the broader area of Petrota Pentalofos-Ormenio, the sedimentary series of the Evros molassic basin cover the metamorphic rocks of Rhodope Massif. Thus, mainly sandstones and breccias form the lithological background of the area. Finally, modern alluvial sediments of the Evros and Ardas rivers complete the local stratigraphy. In continuous, the aforementioned geological data, which are described in the “Ormenio” sheet, of the Geological Map of Greece (Andronopoulos B., 1978), are presented below, while Figure 1 shows a simplified map of the Petrota area.
Alluvial deposits
These alluvial deposits are located mainly along the lower topographic areas of the broader region and their formation is the result of the erosion, transport and deposition processes favored by the activity of Evros and Ardas rivers. Clay, silt, sand and gravels of various rock types are identified within these deposits.
Sedimentary and volcano-sedimentary formations
Pleio-Pleistocene loose conglomerates, sand and clayey silt as aggregates or in alternating layers characterize the youngest stratigraphic formations of the area. Pebbles of the metamorphic basement rocks and of the Eocene sedimentary and volcanic rocks, describe their lithological composition, while no fossils are present. These materials were deposited in an onshore palaeoenvironment consisting of river terraces and small basins.
Below the Pleio-Pleistocene sedimentary formations are found limestones and sandstones of the Upper Sandstone Series (U. Eocene). Limestones are yellow-white, marl, unbedded, with abundant fossils of corals, foraminitera, and algae. They were formed transgressively on the Upper Sandstone Series and appear a detrital character towards their base. Yellowish-brown or gray, fine-grained sandstones with abundant micas and thin clayey intercalations characterize the Upper Sandstone Series. Locally, thin-bedded sandstones, sandstone tuffs, and marl sandstones intercalations are located. The Upper Sandstone Series overlie comformably the Pentalofos Series.
Cohesive breccias consist of grits and cobbles of chloritized amfibolitic schist, and appear as massive, thick-bedded, subhorizontal layers, where also lenses of various sizes of fine clayey-silicic material, along with kaolinite, are found. In addition, within these layers, thin-bedded, fine grained, silicic layers with zeolitic minerals and kaolinite appear as intercalations. The underlying loose breccias are characterized by large, unconsolidated, angular, and unsorted blocks of schists, pegmatites, and volcanic to subvolcanic rocks. Towards their base they appear overlying the metamorphic basement rocks or the Komara Series. The volcanic tuffs are silicified, kaolinitized, and cohesive rocks and consist the lower formation of the Pentalofos Sequence. They appear only to the west of Petrota overlying the Komara Series sandstones. In general the above mentioned formations of the Pentalofos Series are developed in a NW direction from the Greek -Bulgarian borders up to Komara village.
Komara Series underlies the Pentalofos Series. Conglomerates and coarse-grained sandstones with local thin intercalations of yellowish-white clays and marls build up this formation. Pebbles consist of schist, gneiss, quartz and of volcanic rocks. Furthermore, silicified tree trunks, small lignitic occurrences, and lacustrine-brackish deposits are locally found. The Komara Series exhibits a continuous and thick development in depth and it overlies the metamorphic rocks of the Rhodope Massif.
Metamorphic rocks
The metamorphic rocks of the Rhodope Massif in the area consist of Mesozoic phyllites, schists and gneisses. Phyllites are sericitized, chlotitic or argillic, grayish to brownish-red in color and contain intercalations of grauwacke and chloritic quartzite. Schists and gneisses are described as amphibolitic, chloritic, and micaceous, with local occurrences of garnets. Plenty quartz veins and pegmatites crosscut them, while sporadic intercalations of white marbles are also present.
Figure 1. The Petrota area geological map (modified after Andronopoulos B. 1978).
Representative references (in English)
Andronopoulos B., 1978. Ormenio, Geological Map of Greece 1:50.000, IGME Publications, Athens.
Caracciolo, L., Von Eynatten, H., Tolosana-Delgado, R., Critelli, S., Manetti, P., & Marchev, P., 2012, Petrological, geochemical, and statistical analysis of Eocene–Oligocene sandstones of the Western Thrace Basin, Greece and Bulgaria, Journal of Sedimentary Research, v. 82(7), p. 482-498.
Kilias, A., Falalakis, G., Sfeikos, A., Papadimitriou, E., Vamvaka, A., & Gkarlaouni, C., 2013, The Thrace basin in the Rhodope province of NE Greece—A tertiary supradetachment basin and its geodynamic implications, Tectonophysics, v. 595, p. 90-105.
Kilias, A.D., Vamvaka, A., Falalakis, G., Sfeikos, A., Papadimitriou, E., Gkarlaouni, C.H., & Karakostas, B., 2015, The Mesohellenic Trough and the Paleogene Thrace Basin on the Rhodope Massif, their Structural Evolution and Geotectonic Significance in the Hellenides, Journal of Geology and Geosciences, v.4, p. 198.