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Origin of Neoproterozoic silicites with stromatolitic structure in the Brdy area


Marcela Stárková, Patricie Halodová, Štěpánka Mrázová

Geoscience Research Reports 51, 2018, pages 57–62

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Published online: June 20, 2018

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Silicites were studied in detail on map sheet 22-121 Mirošov within geological survey carried out in the Brdy Protected Landscape Area. Neoproterozoic silicites (cherts) are part of Neoproterozoic volcano-sedimentary sequences, and belong to the Blovice Complex (Hajná et al. 2017) of the Teplá-Barrandian Unit. Particular attention was paid to lenses and fragments of black and white banded silicites in the area of Mirošov, the so-called stromatolitic silicites which occur in massive to brecciated silicites located near or inside volcanic or volcaniclastic bodies. Black and white banding of these rocks reminds of lamination and has a thickness ranging from a few mm to 1-3 cm. Cross-sections of concentrically arranged banded objects, often slightly deformed during syn-sedimentary processes, were also found in the area studied. Secondary slight deformation of banded structures ascribed to possible diagenetic or postdiagenetic processes during which a metasomatic growth of silica occurred. Brittle postsedimentary deformations of small scale in silicites can be also observed. Macroscopically banded structures of silicites are under microscope seen only as a mixture of gray and black quartz and disseminated opaque carbonaceous matter and hematite or limonite grains and accumulations of amorphous substance. Most of the quartz grains underwent re-crystallization. Thin sections revealed a presence of scattered flakes of micas and common admixture of pyrite that forms single dispersed grains and spherical objects. Iron oxides with an admixture of vanadium form irregular clusters in silicate matrix. Trace elements in analyzed silicites show elevated contents of vanadium and chromium. Since the bodies of the so-called stromatolitic silicites on the Mirošov sheet map are largely accompanied by volcanic and pyroclastic rocks it is obvious that their origin is linked with hydrothermal volcanic activity. The brecciated textures of silicites enclosing blocks, layers, fragments or relics of black and white stromatolitic silicites and their geochemical characteristics indicate hydrothermal origin. Biogenic influence in hydrothermal volcanic or postvolcanic paleoenvironment is also likely.


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