NDLI: Blackened bioclasts and bituminous impregnations in the Koněprusy Limestone (Lower Devonian), the Barrandian area, Czech Republic: implications for basin analysis

Content Provider	Springer Nature Link
Author	Suchý, Václav Sýkorová, Ivana Dobeš, Petr Machovič, Vladimír Filip, Jiří Zeman, Antonín Stejskal, Michal
Copyright Year	2012
Abstract	Carbonate reef talus facies of the Koněprusy Limestone (Pragian, Lower Devonian, Barrandian) locally exhibit widespread impregnation by organic matter resulting in a partial to complete blackening of the limestones. Two contrasting types of impregnation are recognized: blackening of individual carbonate fossils and bioclastic layers within the limestone originated very early during diagenesis. The blackening is due to finely dispersed organic matter and possibly some iron sulphides and clay minerals that selectively adhered to the outer layers of corals, bryozoans, and crinoid fragments, leaving other fossils unaltered. These darkened fossils are similar to black pebbles—i.e., reworked, dark to black limestone clasts and bioclasts that are known to occur exclusively in shallow-water zones of both ancient and modern carbonates. The alteration of fossil fragments may have taken place in very shallow-water environments, possibly those of saline and reducing back-reef lagoons or supratidal-intertidal zones, with organic matter being derived from decayed algae and microbes, or early vascular terrestrial plant material. Following the coloration, the blackened fossils were removed from their original position by waves or storms and transported into relatively deeper-water reef slope settings to form graded, “salt-and-pepper”-colored bioclastic beds. The presence of blackened fossils in the carbonate succession may point to episodic emergence and indicates a vanished vegetated siliciclastic hinterland that may once have existed to the west or south from the present-day erosive edge of the Barrandian Devonian strata. Subvertical veins cutting the Koněprusy Limestone and filled with black solid bitumen and blackened calcite resulted from a subsequent but substantially later diagenetic event, which is a testament of aqueous and petroleum fluid migration through the succession during deeper burial. Microthermometric characteristics of the aqueous inclusions embedded in vein calcite indicate that the veins were precipitated by brines of low to moderate salinity (0.5–9.5 wt% NaCl equiv.) with temperatures in the range of 87–116°C. The bitumen in the veins is epi-impsonite (Rr = 0.70–1.90%), which is interpreted as degraded petroleum residuum that experienced thermal alteration at around 120°C. The AFT modeling combined with fluid inclusion microthermometry and wider geological considerations indicate that the veins originated during the Variscan orogeny, most probably upon deep burial of the Lower Paleozoic strata in Carboniferous time.
Starting Page	759
Ending Page	777
Page Count	19
File Format	PDF
ISSN	01729179
Journal	Facies
Volume Number	58
Issue Number	4
e-ISSN	16124820
Language	English
Publisher	Springer-Verlag
Publisher Date	2012-02-05
Publisher Place	Berlin, Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Blackened bioclasts Bitumen Reef carbonates Subaerial exposure Burial diagenesis Barrandian basin Lower Paleozoic Fluid inclusions Apatite fission-track analysis Ecology Geochemistry Paleontology Biogeosciences Sedimentology
Content Type	Text
Resource Type	Article
Subject	Geology Stratigraphy Paleontology

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