Field trip B26

Bulgarian part

Sofia Doupnitsa Rila Rila Monastery Padesh
Blagoevgrad (about 190 km)

Leader: Ivan Zagorchev

The Neogene section of the Sofia graben begins with a variegated terrigenous formation, referred to the Maeotian Stage. The second local cycle (Pontian) is related to a lacustrine environment, and is represented by the coal-bearing Gnilyane and Novi-Iskur formations. The cycle begins with a slight local unconformity over eroded surface. A similar unconformity is even better expressed at the bottom of the last, Lozenets Formation (Dacian Lower Romanian). It consists of well-sorted yellowish to whitish conglomerates and sands. The whole sedimentary filling of the graben is intersected and displaced by neotectonic (Quaternary) faults, often well exposed during excavation for big construction works. These faults are also proofs for seismicity during the Pleistocene and Holocene times, some of them even displacing the Quaternary river terrace sediments. The graben has a heterogeneous basement: the Neogene deposits (with a total thickness of up to 1000 m) cover with depositional unconformable contacts different rock units in small pre-Middle Miocene internal blocks: Upper Cretaceous volcanics or sediments, Jurassic and Triassic carbonate or terrigenous rocks, and even (in a very few localities), the pre-Alpine basement.South of Sofia the route crosses the southern part of the Late Cretaceous Vitosha unit of the Srednogorie zone. It consists mostly of Coniacian Santonian volcanic rocks (mainly andesites) and their tuffs intruded by the Vitosha pluton. The Vitosha pluton is built up of three consecutively intruded magmatic phases that illustrate the development of a deeper source in the upper mantle or depleted lower crust (low initial 87Sr/86Sr ratio about 0.704!) of the Srednogorie zone. The first phase consists of gabbro and gabbro-monzonite, the second, - of different varieties of monzonites, and the third one, - of granosyenite in dykes or small bodies.The mountain represents a neotectonic horst uplifted at more than 1.5 km above the Sofia neotectonic graben.

Sketch of the Late Alpine and neotectonic structure
of Southwest Bulgaria

Stop 3.2. Near Doupnitsa: panorama of the Rila horst
The Rila horst has been uplifted relative to the Blagoevgrad and Dzherman grabens with an upthrow of more than 3 km since Late Miocene times. The moderately inclined to steep bounding Klisoura normal fault strikes NE,and has an angle of dip between 30 and 70o to NW.  It has an outstanding geomorphic expression.The stop is situated over a planation surface that represents a part of the Early Middle Miocene peneplain. It gradually lowers to the South,and is subsided in the Dzherman graben to altitudes about and below sea level, for to be uplifted to more than 2500 m in the Rila horst.

Stop 3.4. Exit of the town of Rila (sign) on the road to Rila Monastery
Panorama of beautiful erosion forms: the hill "Krusta" and adjacent hills formed in the Palaeogene conglomerate and sandstone of the Padala Formation are the majestic natural gates towards Rila Monastery.The Padala graben is filled in by the 350 400 m thick Padala Formation.It consists of coarse breccia and conglomerate interbedded by sandstone with abundant coalefied plant debris (parts of leaves and stems).The pebbles in the conglomerate come from the typical basement rocks that occur in Rila Mountain: biotite gneisses and migmatites, amphibolites, marble, granite, pegmatite, quartz, diorite. The Tertiary age has been suggested as early as in 1844 by Ami Boue. G. Bonchev reported for the first time (1912) the presence of a coal lens.An Early Oligocene age has been proven by S. Chernyavska with typical palynomorphs.Thus, the Padala Formation was formed in a part of the alluvial to lacustrine system that was feeding the Early Oligocene gulf of the Piyanets and Padesh grabens. The Padala graben has a post-sedimentary character.However, some of the normal faults at its northern periphery have pre-dated the sedimentation, and others have been active during the sediment deposition. Post-sedimentary steep normal faults are observed at the eastern board (at the monastery branch "Orlitsa") and within the sediments of the graben. Moderately-inclined and low-angle normal faults are also present.

Stop 3.7. Rila Monastery; 1103 m altitude
Rila Monastery is the biggest monastery of Bulgaria and the most important national spiritual center in the Middle Ages and the National Revival in the 18th and 19th centuries.It was founded in the 10th Century by the Saint Ivan Rilski
(St. John of Rila), and has been ravaged and set to fire several times.The oldest building preserved is the Hrelyu Tower (1335) built by a local feudal lord.
The other buildings are dated from the first half of the 19th Century.Important manuscripts, treasures and frescoes can be seen in the church and the museum.

Geological Institute of the Bulgarian Academy of Sciences
Author: Ivan Zagorchev
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