2 Geological framework
2.2 Outline of the regional geology
2.2.1 Neoproterozoic
The Neoproterozoic volcano-sedimentary sequences of the TBU reach a thickness of several thousand meters and were accumulated at the northern periphery
Te
plá-Barrandian
Teplá-Barrandian Saxothuringian
Saxothuringian
Moravo-Silesian Moravo-Silesian
Moldanubian Moldanubian
Mid-German
Mid-German
150 km 150 km
Crystaline High Crystaline
High
WBSZ WBSZ
CBSZ H CBSZ
H B BS SZ Z NBSZ NBSZ
?
?
?
?
Elbe Zone Elbe
Zone
Sudetic Boundary
Fault Sudetic
Boundary
Fault
Železné Hory Železné
Hory Nové Město
unit Nové Město
unit
Wrocław Wrocław
Praha Praha
München München
Brno Brno
Fig. 2.2: Sketch map of the Bohemian Massif with the TBU in the central part (modifi ed after Dallmeyer et al. 1995; Zulauf et al.
2002a; Mazur et al. 2005). CBSZ – Central Bohemian shear zone, HBSZ – Hoher Bogen shear zone, NBSZ – North Bohemian shear zone,WBSZ – West Bohemian shear zone. Box indicates the extract shown in Fig. 2.3.
Abb. 2.2: Schematische Karte des Böhmischen Massivs mit dem Teplá-Barrandium im Zentrum (verändert nach Dallmeyer et al. 1995; Zulauf et al. 2002a; Mazur et al. 2005). CBSZ – Zentralböhmische Scherzone, HBSZ – Hoher Bogen Scherzone, NBSZ – Nordböhmische Scherzone, WBSZ – Westböhmische Scherzone. Kasten markiert den in Abb. 2.3 dargestellten Ausschnitt.
Teplá-Barrandian
Teplá-Barrandian dian Moravo-SilesianMoravo-Silesian ian OphiolitesOphiolites Saxothuringian
Saxothuringian Teplá-Barrandian
n MoldanubianMoldanubian Variscan granitoidsVariscan granitoids
Journal of Central European Geology 54 (2008) 1–168
GEOLOGICA SAXONICA
of Gondwana. Siliciclastic rocks were deposited as turbidites and gravity fl ows (ChlupáĀ 1993). The Neoproterozoic successions are lithostratigraphically subdivided into two major units: the Kralupy-Zbraslav Group and the Štėchovice Group (ChlupáĀ 1993; Fig. 2.5). Microfossils confi rm an Upper Riphaean to Vendian age (corresponding to the Edia -caran in the current terminology of Gradstein et al., 2005) for both groups and allow a correlation with the Brioverian of the Armorican Massif in NW France (Konzalová 1981, 2000; Pacltová 1990, 2000; Fatka & Gabriel 1991). The Kralupy-Zbraslav Group is composed of the Blovice and the Davle formations, which are built up by alternating shales, siltstones, and sandstones with interbedded volcanics and cherts. While basic volcanics are widespread
in the thick Blovice Formation in the lower part of the Kralupy-Zbraslav Group, intermediate to acidic volcanics and appropriate pyroclastics are typical for the overlying Davle Formation. The top of the Davle Formation is represented by silicifi ed black shales.
Volcanic rocks of the Kralupy-Zbraslav Group are arranged in NE-SW trending belts and belong to three major series with tholeiitic (oldest), transitional and alkaline (youngest) geochemistry, respectively. The Jílové Zone in the SE of the TBU comprises tholeiitic volcanics in the lower part and rocks of a calcalkaline association in the upper part. Geochemical signatures of the igneous rocks of the Kralupy-Zbraslav Group correspond to volcanic arc and back-arc geotectonic settings (Fiala 1977, 1978; Pelc & Waldhausrová 1994; Waldhausrová 1984, 1997a, 1997b).
0 10 km + Paleozoic of the Islet zone Variscan granitoides
Cambrian Granitoides
Fig. 2.3: Sketch map of the TBU (slightly modifi ed after ChlupáĀ et al. 1998). KRVC – Kʼnivoklát-Rokycany volcanic complex, SVC – Strašice volcanic complex, PJB – Pʼníbram-Jince basin, STA – Skryje-Týʼnovice area.
Abb. 2.3: Karte des Teplá-Barrandiums (leicht verändert nach ChlupáĀ et al. 1998). KRVC – Kʼnivoklát-Rokycany-Vulkanitkomplex, SVC – Strašice-Vulkanitkomplex, PJB – Pʼníbram-Jince-Becken,STA – Skryje-Týʼnovice-Gebiet.
Alternating shales, siltstones, and greywackes are characteristic for the succeeding Štėchovice Group, which was deposited concordantly above the Davle Formation. Layers of tuffs and tuffi tes are revealing of continuing magmatic activity. Intercalations of conglomerates appear in the middle part of the Štė-chovice Group (Fig. 2.5).
Due to the tectonic conditions as well as the lack of reliable marker horizons and precise geo-chro nological data, the stratigraphic subdivision of the Teplá-Barrandian Neoproterozoic is still prob lematic and, chiefl y based on the presence or absence and the character of synsedimentary vol-canic rocks (e.g., Kettner 1918, Röhlich 1965, Cháb A
A BB
C
C DD
Blovice Fm. Davle Fm.
Štěchovice Group
Palaeozoic and younger cover
West Bohemian microsegment Blovice microsegment Central Bohemian microsegment Cambrian to Devonian cover
C C
Flysch facies Volcanogenic facies Monotonous facies Post-Proterozoic deposits
Rabštejn-Úslava Group Št chovice Groupě Lower Palaeozoic deposits Zvíkovec Group
Fig. 2.4: Different lithostratigraphic concepts for the Neoproterozoic volcano-sedimentary successions of the Teplá-Barrandian unit.
A: From Chaloupsky et al. (1995). B: From Holubec (1995). C: From Cháb and Pelc (1968). D: From Röhlich (2000). [For the discussion in chapter 8.1.2: The sampling localities BL and DB are separated by a major boundary (black line) in B to D.]
Abb. 2.4: Verschiedene lithostratigraphische Konzepte für die neoproterozoischen vulkano-sedimentären Einheiten des Teplá-Barran-diums.A: Nach Chaloupsky et al. (1995). B: Nach Holubec (1995). C: Nach Cháb and Pelc (1968). D: Nach Röhlich (2000). [Für die Diskussion in Kapitel 8.1.2: Die Probenahmepunkte BL und DB sind in B bis D durch eine bedeutende Grenze (schwarze Linie) voneinander getrennt.]
Journal of Central European Geology 54 (2008) 1–168
GEOLOGICA SAXONICA
The thick, mainly continental, Lower Cambrian sediments of the Pʼníbram-Jince basin are litho-str ati graphically subdivided into fi ve formations (HavlíĀek 1971, 1998a). Conglomerates of the Žitec-Hluboš Formation unconformably overlie Cadomian deformed basement (Fig. 2.5). This basal Formation of the Palaeozoic overstep sequence is characterized by relatively immature conglomerates and sandstones with pebbles that are interpreted to be derived from the adjacent Neoproterozoic units (Kukal 1971).
However, most of the Lower Cambrian sediments are highly mature but partly mixed with material derived from synsedimentary volcanism (Kukal 1971, Drost et al. 2004). The components of the Lower Cambrian sediments were transported by fl uvial and mud fl ow mechanisms, respectively, and accumulated in a fault bounded basin with internal structure (Kukal 1971, HavlíĀek 1971). Magmatic activity around 523-511 Ma is proved by U-Pb zircon datings of calc-alkaline plutons having been emplaced in dextral NE-SW- to ENE-WSW trending transtensive shear zones (Zulauf 1997, Zulauf et al. 1997a, Dörr et al. 1998, 2002; and references therein) exposed in the western and north-western parts of the TBU. Furthermore Venera et al.
(2000) reported the presence of a ~505 Ma old (Pb-Pb, zircon) granitoid within Neoproterozoic meta-sediments in the northern part of the TBU.
Continuing subsidence caused a marine trans-gression in the Middle Cambrian (Jince Formation). In the Skryje-Týʼnovice area the marine Jince Formation unconformably overlies deformed Neoproterozoic suc cessions. Lower Cambrian rocks are not present.
In the Pʼníbram-Jince basin there is a transition from the coarser-grained, continental Chumava-Baština Formation to the fi ne-grained, marine Jince For -mation. The sediments of the overlying Ohrazenice Formation have been deposited during a regression (Kukal 1971, HavlíĀek 1971). The fauna of the Jince Formation shows relations to that of southwestern Europe (e.g., Ossa-Morena Zone: Fatka et al. 1998).
The Upper Cambrian is predominantly rep re-sented by subaerial volcanism preserved in the southern Strašice Volcanic Complex (SVC) and in the northern Kʼnivoklát-Rokycany Volcanic Complex (KRVC; Fig. 2.5). The volcanic rocks of the SVC are in general more basic than those of the northern 1993). Lithostratigraphic subdivisions of the
Teplá-Barrandian Neoproterozoic successions differing from the one described above were proposed (Fig. 2.4), e.g., by Röhlich (2000), who identifi ed three micro-segments with individual litho strati graphic features separated by NE-SW-trending major faults, by Cháb
& Pelc (1968), who divide the Blovice Formation from NW to SE in fl ysch facies, volcanogenic facies and monotonous facies, or by Holubec (1995), who proposed a litho strati graphic classifi cation into the Rab štejn-Úslava (base), Zvíkovec and Štėchovice groups (top), respec tively, whereas these groups are separated by uncon formities.
Tentative geotectonic models for the Cadomian evolution of the TBU assume subduction accompanied by the formation of oceanic island arc, back-arc basin, and remnant arc basin (Cháb 1993, Kʼníbek et al. 2000). Greywacke and chert pebbles in the middle and upper parts of the Štėchovice Group, as well as radiometric age data obtained from rhyolite pebbles are interpreted to document accretion, uplift and erosion of the Cadomian arc (Zulauf et al. 1999, Dörr et al. 2002).
The rocks of the Blovice Formation underwent at least two deformation stages during Cadomian oro-genic processes (Zulauf 1997, Zulauf et al. 1999, Kʼníbek et al. 2000; and references therein): D1 with unknown kinematics and D2 with top-to-the-N shearing. The peak of a LP-HT metamorphic event accompanying D2 is dated around 550-540 Ma (Th-U-Pb model ages of metamorphic monazite) and interpreted to be related to collisional processes and slab break off (Zulauf et al. 1999). The following collapse of the thickened crust around the Pre cam-brian/Cambrian boundary is expressed by normal faulting causing exhumation of amphibolite facies rocks in the western and northwestern part of the TBU and crustal tilting before 523 Ma (Zulauf et al. 1997a, 1999).