VLADIMIR JINDRICH Department of Geology, State University of New York, Binghamton, New York, 13901

New Views in Tectonic Significance of Platform Sediments in the Bohemian Massif, Czechoslovakia

platform deposits which, because of differen- ABSTRACT tial uplift and multiple periods of erosion, are Basins filled with the continental upper developed and preserved to varying degrees. Paleozoic rocks in the Bohemian Massif rep- In the structural division of the Bohemian resent graben facies deposited in a system of Massif, the unconformity between the Lower half-grabens and grabens that follow the Permian (Autunian) and the Upper Permian Precambrian lineaments or strike-slip faults. (Saxonian) strata has been correlated with These rocks were laid down contemporane- the Saalian folding phase and interpreted as ously with an intermittent taphrogenic fault- the boundary between the Variscan basement ing over the updomed Bohemian Massif. and younger platform units (Maska, I960). Further uplift and repeated faulting in por- Intense faulting and local warping of the tions of the area of the upper Paleozoic platform cover, interpreted as "Saxonian tec- continental sedimentation are marked by tonics," is held to be causally connected with graben-rift formation on a regional scale. As the Alpine orogeny (Kodym, 196db). a result, still-active structures constitute the The purpose of this paper is to discuss the central European part of the intercontinental major structures of continental upper Paleo- belt of the world rift system of lilies (1969). zoic and younger platform sedimentary rocks The graben-rift structures of the Bohemian of the Bohemian Massif. The major concern Massif are associated with Westphalian rhyo- is to re-examine the effects of Variscan and lites, Autunian melaphyres, and Miocene- Alpine orogeny upon these structures. Pleistocene alkaline basalts. Because of the taphrogenic character of UPPER PALEOZOIC ROCKS graben-rift tectonics and volcanicity in the The upper Paleozoic sedimentary rocks of Bohemian Massif, previous views that Vari- the Bohemian Massif are developed in two scan and Alpine orogenies are responsible for major facies; paralic and continental, the these phenomena are not supported. latter associated with volcanics. The paralic facies occurs in the upper part INTRODUCTION of the thick Carboniferous molasse that over- The Bohemian Massif, including the west- lies the Upper Devonian-lower Carboniferous ern part of Czechoslovakia and bordering flysch (culm) of the Variscan eugeosyncline, parts of Austria, Germany, and Poland, is a flanking the eastern part of the Bohemian Variscan block that protrudes as a horst from Massif (Silesia). The paralic facies is rep- the adjoining younger structural units. The resented by a coal measure complex which Precambrian "Moldanubian nucleus" (Suess, was folded in the Namurian B period. 1903), gently domed to the surface, is the The continental facies was deposited after essential part of the Bohemian Massif, to the middle Namurian folding phase. Its which late Proterozoic units of the Assyntian earliest beds are of the Namurian C; the orogen are welded. The Bohemian Massif, youngest are probably of the Saxonian. The considerably consolidated and rigid as early rocks of the continental facies show similar as Precambrian times, was strongly remobil- lithologies, in that conglomerates, arkoses, ized by the Variscan orogeny. From late and feldspathic sandstones predominate over upper Paleozoic time onward, the Bohemian subordinate shales and locally developed Massif behaved as a rigid, rising cratonic workable coals. A large percentage of the block, resistant to later orogenies. Its Vari- volume of these continental strata are red scan and older basement is covered by younger beds of considerable thickness (as much as

Geological Society of America Bulletin, v. 82, p. 763-768, 2 figs., March 1971 763

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1000 m) and of large lateral extent. In addi- major boundary faults shifted during the tion, contemporaneous lava flows or sills of development of graben structures to form a melaphyres or rhyolites (or both) associated series of parallel step faults. Subsequent up- with pyroclastics appear to be fundamentally lift of the nucleus of the Bohemian Massif characteristic of these continental succes- (Moldanubian block) tilted the southern sions, and the volcanics contribute signifi- half-graben segment of the Central Bohemian cantly to the basin filling. Cyclic sedimenta- Carboniferous Basin and reversed the original tion involves the following environmentally dip of the Stephanian sediments transported controlled facies: alluvial fan, alluvial channel, into the basin from boundary fault scarps flood plain, lacustrine, and swamp. to the north. The Krkonose Piedmont Basin and the STRUCTURES OF THE UPPER Intra-Sudetic Basin are complex half-graben PALEOZOIC BASINS WITH structures with strongly rotated individual CONTINENTAL FILL blocks where boundary faults assumed the Sedimentary rocks of the upper Paleozoic character of high-angle reverse faults in the basins are unrelated to the lower Carbonifer- postdepositional history. ous geosynclinal flysch. They directly overlie These upper Carboniferous-Permian the ancient basement in thickness, locally graben and half-graben structures have been exceeding 5000 m (Jaros, 1968). interpreted as molasse infillings of Variscan Of the largest lateral extent are the beds of intramontane depressions caused by late Stephanian-Autunian age which cover a sig- phases of the Variscan orogeny (Kodym, nificant part of the interior Bohemian Massif 1960a; Havlena, 1966; Jaros, 1968). Such a and overlap the Precambrian basement in concept is not accepted in this paper. The large areas. The more or less continuous grabens and half-grabens appear to be post- sedimentary cover underlain in places by strata Variscan structures developed upon a pene- of Westphalian and of Namurian C age was planed, slightly undulating surface which had later mostly destroyed by erosion but pre- been deeply eroded to the granitic layer and served in the following basins: the Central which forms much of the graben basement. Bohemian Carboniferous Basin, Krkonose It is suggested that the graben structures are Piedmont Basin, Intra-Sudetic Basin, Bosko- primarily due to stretching and inward col- vice Furrow, Blanice Furrow, and the Central lapse of crustal segments over the updomed Sub-Cretaceous Basin (Fig. l). Bohemian Massif, a process unrelated to the Field evidence indicates that the narrow Variscan orogeny. The continental upper Boskovice Furrow (Jaros, 1968) and the Paleozoic rocks associated with volcanics are Blanice Furrow are half-grabens bordered es- here grouped in the rift valley-graben facies sentially by one normal fault flanked by thick of Pettijohn (1957), thus contrasting their conglomeratic sequences of the alluvial fan tectonic origin with postorogenic molasse facies. containing similar sedimentary lithologies Similar marginal faults with throws of but generally lacking volcanic rocks. about 1000 m were located by deep drilling Successive development of graben struc- along the northern margins of the large tures in the Bohemian Massif indicates that Central Bohemian Carboniferous Basin and the process of updoming and faulting did in the adjacent part of the Central Sub- not operate continuously nor at a constant Cretaceous Basin (Vachtl, 1965). Detailed rate. Pulses of uplift and faulting of different mapping, drilling, and evidence from coal magnitudes were followed by periods of rela- mines demonstrate that in the latter basins, tive quiescence. This is clearly reflected in the basal Westphalian sediments containing the marked cyclicity of graben fill, numerous coal are developed in a system of local down- erosional surfaces, diastems, and the distribu- faulted blocks rimmed by coalescent alluvial tion of interfingering conglomerates. Pres- fans (basal breccia) shed from adjoining horsts ence of such phenomena, in addition to local of the ancient basement. The Westphalian tilting and occasional mild warping, were topography of filled grabens and subdued erroneously related to the effect of late horsts subsequently provided a basement of Variscan folding phases. The differentiation several major grabens and half-grabens, over- and geographic nomenclature of these phases lapped on a large scale by sedimentary rocks as defined by Stille (1925), has been carried of Stephanian and Autunian age. Some of the to extremes and has previously received some

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Continental Upper Paleozoic Tertiary volcanics

Figure 1. Major taphrogenic structures and Basin, 4. Boskovice Furrow, 5. Blanice Furrow, boundary faults in the Bohemian Massif. 1. 6. Central Bohemian Cretaceous Basin under- Central Bohemian Carboniferous Basin, lain by Central Sub-Cretaceous Basin, 7. Erzge- 2. Krkonose Piedmont Basin, 3. Intra-Sudetic birge Rift.

criticism (Gignoux, 1950). Assignment of lative and which increase with depth, are the Asturian, Saalian, and Pfalzian phases to associated with linear Tertiary extrusions of late Paleozoic graben sediments of the Bo- alkaline-basalt lavas concentrated in the north- hemian Massif, and further phase differentia- west of Czechoslovakia where they underlie tion (Intra-Stephanian phases, early and late belts of the Bohemian Mittelgebirge and Asturian phase; see Havlena, 1966) does not Duppauer Gebirge (Knorr, 1932; Kopecky, seem justified. Mild warping noted in the 1966). Here the Miocene to Pleistocene vol- rabens is explained here by sinking of canic necks, lavas, and pyroclastics, inter- tmlted blocks into more laterally constricted bedded with Tertiary fresh-water deposits, space. fill a 20- to 25-km-wide rift, which has a linear extension of 200 km. The rift, a graben MESOZOIC AND TERTIARY structure passing laterally into a series of STRUCTURES normal step faults with antithetic faults, is Intermittence of updoming in later periods bounded on the northwest by a prominent is demonstrated in the general lack of the fault scarp of the Bohemian Erzgebirge, on Triassic, Jurassic, andLower Cretaceous rocks, the southwest by the boundary fault zone of and by the restricted occurrence of Upper the Central Bohemian Carboniferous Basin Cretaceous and Tertiary deposits in down- (Figs. 1, 2). On the north, the rift crosses a faulted blocks. Rejuvenated movement on 50-km-wide, southeast-trending graben (Fig. major faults of the upper Paleozoic grabens 1) filled with Upper Cretaceous sedimentary gave rise to regional graben zones or rifts in rocks and superposed upon a system of the Tertiary times. These resurgent faults, the upper Paleozoic graben and half-graben struc- vertical displacements of which are cumu- tures (Central Sub-Cretaceous Basin) associ-

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Figure 2. Section across the Erzgebirge Rift upper Paleozoic, 3. Upper Cretaceous, 4. Ter- and the bordering Central Bohemian Carbon- tiary, 5. Tertiary volcanics, 6. Granite, iferous Basin. 1. Precambrian, 2. Continental

ated with Permian melaphyre flows. The large widely distributed Stephanian-Autunian Cretaceous graben, pierced by scattered Ter- rocks are the earliest deposits of the post- tiary alkaline volcanics, is bounded on the Variscan platform cover. Inasmuch as these northwest by the Lusatian fault zone, and on deposits followed severe erosion and were the southwest by faults of the "Elbe-Linie" laid down upon an already peneplaned base- that separate the nuclear parts of the Bohemian ment, they are neither deposits of the Variscan Massif from younger structural units. The intermontane depressions nor the Variscan Lusatian fault appears to be a normal fault molasse infillings as held by previous authors. passing upward into Cretaceous rocks to form The continental upper Paleozoic rocks are a monocline with locally developed high- noted in a graben facies in half-graben and angle reverse faults. graben structures aligned along Precambrian The resurgent faults involved in rift-graben lineaments or strike-slip faults, and they tectonics show dominant regional trends of resulted from taphrogenic movements in- northeast, northwest, and north-northeast, duced by updoming the Bohemian Massif. subdividing the Precambrian basement of Further uplift and rejuvenated taphrogenic the Bohemian Massif into polygonal blocks faulting in segments of the upper Paleozoic which retain their structural individuality continental cover led, in later history, to (Skvor and Zeman, 1969), as demonstrated graben-rift formation on a regional scale. earlier by Cloos (1948) for the European These young graben-rift structures with their continent. Such faults displacing Upper Cre- upper Paleozoic predecessors are considered taceous and younger rocks of the Bohemian to be the central European bifurcations of the Massif customarily have been referred to as intercontinental rift belt, extending between "Saxonian," and, together with the Tertiary the Oslo graben and the East African rifts, alkaline basalts, have been attributed to pres- reviewed recently by lilies (1969). sure effects of the Alpine orogeny (Kodym, Successive graben volcanism in the 1960b; Kopecky, 1966). The term "Saxonian Bohemian Massif is demonstrated by West- tectonics" was coined by Stille (1910) to phalian rhyolitic lavas rich in pyroclastics, categorize the specific fold-faulting of the Autunian melaphyres (tholeiites), and Mio- thick north German Mesozoic successions cene-Pleistocene alkaline basalts similar to associated with mobile Permian salt intru- lavas of the Rhine graben province and the sions. Inasmuch as it is now realized that East African rifts. Recent faulting, numerous hydrostatic forces due to overburden, rather hot springs and CO2 emanations of the than Alpine orogeny, were responsible for Bohemian Erzgebirge rift are evidences of these phenomena (Trusheim, I960), the still continuing taphrogenic activity. genetic connotation of the "Saxonian tec- The great majority of rift-graben faults in tonics" loses its validity and should not be the Bohemian Massif are steep normal faults, applied to young normal faults in the many of which are associated with antithetic Bohemian Massif. faults. However, some major boundary faults appear to be high-angle reverse faults, later- CONCLUSIONS ally or vertically passing into monoclines or Distribution, lithology, and tectonics of monoclinal flexures. Such reverse faults are the continental upper Paleozoic rocks show believed to steepen at depth into normal that the Variscan orogeny in the nuclear faults. Bohemian Massif was completed by the Inasmuch as taphrogenesis and orogenesis Namurian B period at the latest. The con- are held to be antagonistic processes (lilies, tinental Namurian C, Westphalian, and the 1969), the former interpretation of rift-graben

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