Field trip B26

Bulgarian part

Lovech - Pleven - Mezdra - Sofia (about 230 km)

Leader: Marin Ivanov

Stop 2.1. Lovech,hill Stratesh
Carbonate rocks of the Stratesh Limestone Formation(Upper Barremian) are exposed. They are a part of the Lovech Urgonian Group that consists of four terrigenous and four carbonate formations. The limestones of the Stratesh Formation are biomorphic, bioconstructed, biodetritic,oolithic, etc.The principal reef-building organisms are corals, algae and rudists(genera Requienia, Toucasia, Monopleura .).The Urgonian complex is a complex biosedimentary system formed along the northern Tethyan margin. This facies type is traced at thousands of kilometer on the territories of Europe and Asia. Carbonate sequences typical both for inner and for outer carbonate platforms are found at the Stratesh Hill.In their alternation they form parasequences and parasequence within a transgressive system tract within a third-order sedimentary sequence.
The panoramic view of the town and to the North exhibits the typical relief of the Fore-Balkan and the Danube hilly plain(Moesian platform) already mentioned: planation surfaces gradually dipping North, conformable to the drainage pattern of the Neogene Quaternary fluvial system.

Stop 2.3. Quarry "Kayluka" near Pleven
The quarry is situated in the central parts of the Moesian platform. The lower parts of the quarry exhibit the limestones of the Kayluka Formation(Upper Maastrichtian). They are biomorphic and biodetritic, abundant in fossils: bivalves, echinids, gastropods, brachiopods and rarely, cephalopods(ammonites and nautiloids). Its uppermost levels are dominated by biomorphic limestones. The upper boundary of the formation is marked by an wash-out, and unconformably covered by the Komarevo Formation (Upper Palaeocene): clayey limestones interbedded with marls. A glauconite-bearing layer (20-30 cm to 1-1.2 m thick) is situated at the base of the formation. The limestone - marl interbedding in the middle and upper parts of the formation is in doublets and probably indicates a climatic Milankovic cyclicity. The Komarevo Formation is covered with a sharp boundary and wash-out by the marls of the Avren Formation ( Lower Eocene). At the base of this formation a glauconitic marl is observed, too, as well as nummulitoids.
The section illustrates a rock sequence typical of the geodynamic state of the Moesian platform, with low dips and without important tectonic disturbances.

Stop 2.4. Quarry "Yasen"
The quarry exhibits the marls of the Avren Formation dated here in the boundary interval Lower-Middle Eocene. They represent a monotonous sequence, mostly of clayey marls in irregular interbedding with silty marls. The following gray-blueish to beige and yellowish calcareous clays of the Opanets Formation (Badenian) cover them with a sharp transgressive boundary.They contain abundant fossils of different groups (mostly gastropods and bivalves).The uppermost parts of the quarry expose Quaternary sediments:loess and palaeosoils with brown color. They cover the Neogene sediments with a sharp erosional boundary. The whole section is typical for the interrelations of the Palaeogene, Neogene and Quaternary cover sediments and their interrelations in the central parts of the Moesian platform.

Stop 2.8. Lyutibrod (after "Ritlite" - with a photostop at this geomorphic geosite).Sediments from the upper parts of the Urgonian complex (Vratsa Urgonian Group, Lyutibrod Formation) are exposed. The Lyutibrod Formation (Lower Cretaceous, Lower Aptian) is built up of detritic, oolithic, orbitolina-bearing and other limestones in alternation with terrigeneous-carbonatic packets(interbedding sandy limestones, calcareous sandstones and marls).Its sediments are typical of the external carbonate platform or marginal environments.Tempestites are often present in the section. Detailed observations prove the presence of retrogradational and progradational tendencies and sequences.In combination they form 3rd order sequences with eustatic genesis. Both the limestones and marls contain abundant and various fossils (bivalves,gastropods, corals, brachiopods and orbitolinas, the latter being rock-forming in some levels).

The Urgonian complex along the Iskur valley between the Cherepish Monastery and Lyutibrod is near the boundary between two tectonic units: the West-Balkan and the Central-Balkan Fore-Balkan ones. The boundary between them has a thrust or upthrust character. Consequently, the packets of the Lyutibrod Formation are in an upright position, the sof(marly or terrigeneous-carbonate)rock intervals are eroded, and the vertical limestone packets form the natural phenomenon (geomorphic geosite) "Ritlite".

Stop 2.8b: near the Cherepish Monastery The stop displays the Cherepish Monastery and the meanders of the Iskur River around cliffs built up of light-gray to white massive limestones(Brestnitsa Formation, Upper Jurassic Lower Barremian). Micritic and biomorphic varieties are dominant. Less frequent are biomorphic (rudist)limestones (Diceras sp.).  The limestones of the Brestnitsa Formation are typical of the internal carbonate platform. It is an element of the chain of carbonate platforms situated along the northern margin of the Tethys in the Callovian Vallanginian time span.

Stop 2.10b. North of Bov (panorama of the Triassic)
The section exhibits the base and the sequences of the Triassic System of the western Balkanides. The Triassic covers with angular and regional unconformity different Palaeozoic complexes. It is subdivided into three groups.The Lower Triassic Petrohan Terrigenous Group consists of clastic rocks: conglomerates and coarse- to fine-grained sandstones interbedded with siltstones and dark- to light-red shales. It is formed in fluvial systems: alluvial plain, alluvial fans and aeolic conditions, under arid to semi-arid climate.The Iskur Carbonate Group (uppermost Lower Triassic - Upper Triassic) contains carbonatic sequences formed in shallow marine environments. The lower boundary represents a fast transition from the Petrohan Terrigenous Group and marks the marine transgression. The transition interval has a terrigenous-carbonate character with presence of fine-grained clastic or clayey rocks, and gray limestones and yellowish dolomites. Upwards in the sequence follow: rhythmic rocks (alternation of bioclastic micritic limestones and dolomites) formed in sublittoral and littoral environments; massive micritic and bioclastic limestones (with a carbonate bank geometry); nodular and bioclastic limestones (formed on a carbonate ramp - submarine slumps and storm products, tempestites and tsunamites are observed). The group ends with massive dolomites.The Triassic section ends at most places with the Moesian Group: marine red beds that correspond to a regressive sequence. It consists of varicolored rocks: from boulder breccia-conglomerate and conglomerate, polymictic to oligomictic sandstones, siltstones, to shales, marls, limestones and evaporites. The lower boundary of the Moesian Group is dated at different places as lowermost Carnian, Upper Carnian, Lower or Middle Norian. It ends also diachronously, and is followed by a significant hiatus and erosion.

Stop 2.12. Tserovo, road junction to Zimevitsa The hill Cherni Kamak exhibits a panoramic view of the contact (Vidlich tectonic zone) between the Berkovitsa and Svoge units. West of the Iskur it is a steep upthrust whereas East of it passes into an overthrust. The allochthon is built up of pre-Mesozoic complexes of the Svoge unit, and the autochthon, of Triassic and Jurassic formations of the Berkovitsa unit.

The basement of the Berkovitsa unit consists of a Neoproterozoic-Cambrian diabase-phyllitoid complex intruded by Early Carboniferous granitoids.The Mesozoic cover contains Triassic (Petrohan Terrigenous Group and lower parts of the Iskur Carbonate Group - not higher than the Anisian) unconformably covered by Lower and Middle Jurassic formations. The allochthon (Svoge unit) is strongly tectonized, and consists of sandstones and siltstones of the Ordovician. Jurassic sediments from the autochthon have been also involved in thrusting as parautochthonous lamellae (thrust horses) in the frontal parts of the thrust.

Author: Marin Ivanov

Geological Institute of the Bulgarian Academy of Sciences
Authors: Ivan Zagorchev , Marin Ivanov
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