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Clasts of volcanic rocks in sediments of the Klouček-Čenkov Formation in SW part of the Příbram-Jince Basin (Barrandian area)


Tomáš Vorel

Geoscience Research Reports 52, 2019, pages 129–133
Map sheets: Březnice (22-12)

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Published online: 1 September 2019

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During the new geological survey on map sheet 22-121 Mirošov (scale 1 : 25 000), the Cambrian sediments in the SW part of the Příbram-Jince Basin were revised, particularly in the area between the Mirošov town, Tři trubky Castle and the Buková village. Sediments of the Klouček-Čenkov Formation containing a large amount of volcaniclastic material were confirmed on this map sheet, in the valley of the Třítrubecký potok brook, and in the area NW of Skládaná skála.
The study of thin sections revealed some types of volcanic fragments occurring in sediments of this Formation including their description and discussion regarding the character of the former volcanic activity. The Čenkov Sandstone in the area of Tři trubky exhibits brown-purple to reddish color shades, and being mostly finely white dotted by kaolinized tuff clasts and volcanic glass (Fig. 1). The Klouček Conglomerate often contains visible fragments of volcanic rocks maximum a few cm large (Figs 2 and 3), most often around 0.5-1 cm in size. Such fine-grained conglomerates show graded bedding in places (Fig. 4).
Both types of volcanic rocks (acidic and basic) were detected by microscopic study of the Klouček-Čenkov Formation. Volcanic fragments are often subrounded, indicating redeposition and medium long transport.
Generally, the fragments of acidic volcanic rocks prevail over the basic fragments in thin sections, indicating several types of their origin. Acidic volcanic rocks are mostly fine-grained rhyolites, altered rhyolithic tuffs, pumice and fragments of acidic glass (Figs 5 to 7). Rhyolite fragments show often a well-preserved fluidal to rheomophic inner structure. Consequently, some fine and coarse ash fall deposits, lava effusions, and possibly pyroclastic flows (acidic ignimbrites) originated during the former volcanic activity.
Fragments of basic rocks (andesite to basalt) showing characteristic intersertal to hyaloophitic textures, often with fine, needle-like plagioclases in vitreous matrix were observed in thin sections (Figs 8 to 10). The inner texture of basic clasts is often fluidal, locally some needle-like plagioclases turn into vitreous, dark parts of the matrix. As for their origin, the basic clasts indicate mostly effusive character of the volcanic activity – the formation of lava flows and glassy pyroclastics.
The occurrence of basic fragments together with those of rhyolites in the same samples (thin sections) point to approximately current and probably also bimodal character of the volcanic activity.
Signs of volcanism in sedimentary sequence of the Klouček-Čenkov Formation can be observed straight in the field in form of local rhyolitic tuff interlayers, up to a few meters thick. These layers consist of strongly silicified, fine-grained, ocher or red-violet rocks (Figs 11 and 12) with finely dispersed lithic fragments. They most probably represent ash fall deposits (lapilli tuff – see Fediuk 2004), or outer parts of pyroclastic flows.
However, the possible source of these volcanic rocks and volcanic clasts is still questionable. They might have come from an area south or southwest of the central Brdy region, from the so-called Islet Zone (parts of the Teplá-Barrandian Unit affected by contact metamorphism of the Central Bohemian Plutonic Complex).


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