Field Trip 2: Tectonics and depositional systems of a shallow-marine, intra-continental strike-slip basin: exposures of the Český Ráj region, Bohemian Cretaceous Basin




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Field Trip 2: Tectonics and depositional systems of a shallow-marine, intra-continental strike-slip basin: exposures of the Český Ráj region, Bohemian Cretaceous Basin


David ULIČNÝ, Stanislav ČECH, and Radomír GRYGAR

1Geophysical Institute, Academy of Sciences of the Czech republic, Boční II/1401, 141 31 Praha 4; e-mail ulicny@ig.cas.cz

2Czech Geological Survey, Klárov 3, 118 21 Praha 1, Czech Republic

3Insitute of Geological Engineering, VŠB- Technical University Ostrava, 17. listopadu, 708 33 Ostrava, Czech Republic


INTRODUCTION: FOCUS OF THE FIELD TRIP


The aim of this field trip is to demonstrate the relationships between tectonic evolution and sedimentary infill of the Bohemian Cretaceous Basin, an intracontinental strike-slip basin system which formed by reactivation of Palaeozoic basement fault zones of the Bohemian Massif. Although the basin system was short-lived (<11 My) and the thickness of its preserved infill is mostly less than 1 km, good exposures and abundant borehole data allow to study in detail the record of tectonic events and other processes in depositional systems. The Český Ráj area of northeastern Bohemia is famous for its „rock cities“, areas of excellent exposure of late Turonian-Coniacian - age deltaic sandstone bodies (Uličný 2001). These coarse-grained deltas represent the most typical basin-fill style developed during the lifetime of the Bohemian Cretaceous Basin. Outcrop features examined during the trip provide insights into the hydraulic processes that operated in the depositional setting, as well as into the genetic sequence-stratigraphic aspects of the deltaic systems. This information is combined with regional correlation, based on well logs and cores, which allows to appreciate the relationships between longer-term geological processes that operated at the basinal scale: basin-floor subsidence, sediment supply, and eustasy. The trip also demonstrates the post-depositional deformation at the northern basin margin, related to widespread inversion of Central European intra-continental basins during Late Cretaceous-Paleogene times.


TECTONIC AND PALAEOGEOGRAPHIC FRAMEWORK


The Bohemian Cretaceous Basin and its source areas formed during the mid-Cretaceous, within an anastomosed system of strike-slip faults inherited from the structural pattern of the Variscan basement of the Bohemian Massif (Fig. 1; Uličný, 1997, 2001; Voigt, 1996). There is a general agreement that the basin system of the Bohemian Cretaceous formed in response to far-field stresses transmitted through the Alpine-Carpathian foreland during the early phase of eo-Alpine collision (e.g. Malkovský 1987; Ziegler 1990, Pfiffner 1992, Faupl and Wagreich, 2000), which led to reactivation of the pre-existing basement shear zones at upper crustal levels. Individual pull-apart sub-basins and adjacent source areas formed at WNW to NW-trending, principal displacement zones having a dextral sense of slip. These overlapping fault zones belong to the Elbe Zone as interpreted by Arthaud and Matte (1977), Matte et al. (1990) and other authors, or to the southern part of the Elbe Fault System sensu Scheck et al. (2002), which includes also the Sudetic Fault system (e.g. Aleksandrowski et al., 1997). Numerous subordinate NW-NNW-directed faults (Fig. 1B) probably functioned as Riedel shears related to the principal Elbe Zone faults. Several NNE-directed fault zones, such as the Rodl Line (Brandmayr et al., 1995) were reactivated as conjugate, antithetic fault zones and locally significantly influenced the basin palaeotopography.

Because of the number of sub-basins and intrabasinal uplifts active during the basin lifetime, the Bohemian Cretaceous Basin should not be seen as a single basin, and it is more useful to refer to it as a „basin system“. For instance, in the “Police Basin”, a Cretaceous successor to the Palaeozoic Intra-Sudetic Basin, as well as in the Kraliky Graben (Nysa Trough), many features in stratigraphic evolution and palaeogeography show that these Sudetic sub-basins were closely linked to the larger sub-basins formed within the Elbe Zone proper.

The tectono-sedimentary history of the Bohemian Cretaceous basin system was divided into three principal parts by Uličný (1997): initial Phase I (middle-late Cenomanian), mature Phase II (late Cenomanian-early Coniacian), and terminal Phase III (mid-Coniacian through Santonian; Fig. 2). The depositional patterns of the early Phase I, reflecting the filling of pre-Cenomanian topography and early movements of reactivated structures, are described e.g. by Uličný and Špičáková (1996) and Špičáková and Uličný (2002). The tectonic-driven transition from the early fluvial-estuarine deposition of Phase I, with dominant role of NNE-directed fault systems of the Rodl Line, to the mature Phase II characterized by dominance of dextral displacement on the NW-directed Lužice and Labe Fault Zones is described by Uličný et al. (2003, this volume). This field trip focuses on the depositional patterns of Phase II which was characterized by progradation of coarse-grained deltaic systems basinwards from the marginal strike-slip faults (Fig. 3A,B). The early Coniacian deposits of the Český Ráj region probably recorded an increase in subsidence rates which later culminated during the Phase III (Fig. 2A).

During the mature phase (Phase II) of basin evolution, the structurally deepest depocentres were situated along the most active marginal strike-slip faults: the Lužice (Lausitz) Fault Zone and southwest of the Intra-Sudetic Fault Zone. The regional subsidence rate gradients, however, were relatively gentle and the cross-section of the basin (Fig. 2) shows a geometry similar to the box-grabens typical of most pull-apart basins (Dooley and McClay, 1997). The Cenomanian through late Turonian / early Coniacian subsidence regime was characterized by relatively low subsidence rates (c. 70-100 m/Ma maximum; Uličný, 1997). Such rates, unusually low for a strike-slip basin system (cf. Christie-Blick and Biddle, 1985), are attributed to strain distribution in the broad, anastomosed fracture zone and the resulting small net displacement along the principal strike-slip faults (Uličný 1997).

The largest depocentre during this stage of basin evolution was the Lužice-Jizera sub-basin located south of the Lužice Fault Zone (Fig. 1, 2). There, the sandstone bodies of the Bílá Hora, Jizera, and Teplice Formations, representing prograding deltaic shoreface depositional systems, are spectacularly exposed in the so-called "rock cities" of North Bohemia and Saxony. The siliciclastic source was an uplifted block north of the Lužice Fault (the Western Sudetic Island, cf. Skoček and Valečka, 1983). Regions devoid of direct siliciclastic input were characterized by deposition of offshore muds and marls, and, under specific circumstances, even hemipelagic limestone-marl facies (Čech et al., 1996; Svobodová et al., 2002; Laurin and Waltham, 2003, this volume)

The youngest preserved deposits are of Santonian age, and their small areal extent precludes speculations about the palaeogeography and tectonic regime at that stage of basin evolution; it is not known whether the Late Cretaceous deposition continued beyond the Santonian (cf. Valečka and Skoček, 1991). The basin-fill succession experienced uplift and deformation during the Cenozoic, notably during the Paleogene transpressional episode (e.g.Coubal, 1990). The history of Cenozoic deformation, exhumation and erosion of the basin system has not been thoroughly studied yet. The preservation of a thick successions of Late Turonian through Santonian strata in the downthrown blocks of the Oligo-Miocene Eger Graben clearly show, however, that the Cenozoic uplift, especially in the central part of the basin, led to erosion of at least 500 m of Cretaceous strata.


CONTROLS ON EVOLUTION OF THE ČESKÝ RÁJ DELTAIC SYSTEMS (Field Trip Stops 1 and 2)


Throughout the Lužice-Jizera sub-basin, deltaic bodies show a variety of internal geometries, ranging from stacks of very thin (3-15 m) deltaic packages with low-angle foresets to thick (50-75 m) packages of high-angle foresets (10-30°). The former, named descriptively Type-L packages, were recently interpreted as record of progradation of shallow-water deltas, whereas the latter (Type-H) correspond to deep-water deltas (Fig. 3; Uličný 2001). This interpretation is, in fact, close to the intuitive notion by Zahálka (1918) that the sandstone bodies near basin margins were deposited by deltas; see Uličný (2001) for extensive discussion of the “non-deltaic” sedimentological interpretations by Skoček and Valečka (1983), Jerzykiewicz and Wojewoda (1986), and Adamovič (1994).

The first two stops of the trip will be devoted to the analysis of depositional geometries, bounding surfaces, and lithofacies of the deltaic succession belonging to the TUR 7 / CON 1 genetic sequence (Fig. 2) exposed in the Český Ráj rock cities. Several key points are summarized below, as a basis for discussions at the individual field trip stops.


Bounding surfaces

Individual foreset packages are separated from one another by various types of unconformity surfaces (Fig. 4A, B). Surfaces which truncate the upper parts of underlying foreset packages can be correlated for many kilometres in well-logs and outcrops (Fig. 3). They are commonly intensely burrowed and/or overlain by gravel lags, and commonly downlapped by an overlying foreset package. Lithofacies above each of these surfaces indicate an increase in water depth and therefore they a readily interpreted as flooding surfaces in the sequence-stratigraphic terminology. The history of many of these surfaces is probably more complicated, however, involving period(s) of subaerial erosion prior to the flooding (Fig. 5B).


Foreset facies

The grain size of the foreset facies varies from fine-grained to very coarse-grained and pebbly sandstones. The main facies, summarized in Fig. 4, are as follows:

1. Cross-bedded sandstone and conglomerate facies assemblage. Trough cross-bedding is the dominant internal structure in most of the foreset packages. Soft-sediment deformation is common in the cross-strata in the type-H packages. Palaeocurrent data (Figs. 3, 6) indicate that the Český Ráj deltas (as well as other, older deltas of the Lužice-Jizera sub-basin) were strongly affected by dominantly SE-directed currents, interpreted as tidal by Uličný (2001), which caused the formation and migration of dunes on the foreset slopes.

2. Backset facies (sensu Jopling and Richardson, 1966 and Nemec, 1990) are sets of upslope-dipping cross-lamination, typically developed in distinctly coarser sandstone than the surrounding facies. The backsets are recognized as an individual facies if they rest on foresets without distinct erosion at the base of the backset. Elsewhere, backsets form a part of the chute-fill facies assemblage (below). Caution must be applied in distinguishing between true upslope-migrating backsets and cross-strata formed by obliquely migrating bedforms.

3. Chute-fill facies assemblage. The term "chute-fill” describes sandstones which rest on undulating, concave-upward erosional surfaces or fill narrow gullies, in places over 7 m deep, cut into the foresets (or, into an underlying chute fill, causing amalgamation of the chute-fill bodies). The upper surface is normally conformable with the overlying foreset geometry. This topography is analogous to erosional troughs - chutes - described e.g. by Prior and Bornhold (1990) from modern coarse-grained deltas. The basal parts of the chute fills may or may not include concentrations of pebbles and coarse sand, locally with bivalve shells, at their base. In the upper parts of some chute fills, sub-parallel, slightly undulating laminae occur, and pass upslope into backsets. In many most chute-fill bodies, cross-sections parallel to the foreset dip reveal poorly-defined backset stratification throughout the most of the chute-fill thickness. Locally,water-escape structures occur near the top of the chute-fill bodies or in the overlying foresets (see Field Stop 1b).

The formation of chutes and their infills in steep-slope deltas is interpreted as due to liquefaction of the upper part of the unstable foreset slope and downslope movement of liquefied sand behaving essentially as a cohesionless debris flow. The internal structures of the chute fills, particularly the backsets, are interpreted as caused by the supercritical nature of the gravity flows that were prone to development of hydraulic or granular jumps at topographic breaks (e.g., base of foreset slope, irregularities of chute-fill topography; cf. Nemec, 1990 and references therein; Massari, 1996). Various types of backset and parallel lamination found in the chute fills may have been formed by upslope migration of such jumps. The lack of such phenomena in shallow-water deltas with low-angle foresets may be due to the lower potential for foreset failure as well as to the higher potential of reworking of sand by processes such as currents and waves in shallow water.


Bottomset facies

Bottomsets of the deltaic bodies are commonly poorly exposed, but where accessible, they fall into two groups:

1. Bioturbated sandy bottomset facies, typical of type-L packages comprises very fine-grained, intensely bioturbated, sandstones with some proportion of mud, characterized mostly by a dense ichnofabric dominated by Ophiomorpha, Thalassinoides, and Planolites ichnogenera (Fig. 4H).

2. Heterolithic bottomset facies. In outcrop, this facies shows alternation of silty mudstones with upward-thickening beds of ripple-laminated sands and abundant "pot casts" sensu Aigner and Futterer (1978); it is found associated with type-H foreset packages of deep-water deltas (Fig. 4G). This facies is interpreted as sandy turbidites, triggered by delta front collapse, and deposited in the offshore area where they interfinger with mudstone representing the “background” deposition.


General absence of topset strata

In most cases the foreset packages are truncated by erosional bounding surfaces. Topset facies which are generally indicative of aggradational conditions in a delta plain, are not preserved in outcrops. Therefore, it has been difficult to interpret in detail the physiography and processes governing the sediment transport on the upper part of the delta plain, as well as the nature of transition between the topset and foreset areas. However, the characteristics of sediment delivered to the foresets as well as the lag deposits on some of the bounding surfaces suggest that the delta-plain environment was characterized by shallow, bedload-dominated fluvial channels and probably by partial redistribution of sand and gravel by waves. The absence of fluvial topset strata led some earlier workers (Skoček and Valečka, 1983; Jerzykiewicz and Wojewoda, 1986; Adamovič, 1994) to reject the deltaic origin of the Bohemian Cretaceous sandstones and therefore it is important to understand the deposition/preservation potential of topset strata within a sequence-stratigraphic context of long-term evolution of stratigraphic geometries; this will be discussed at Stop 1c.


Depositional model

A simplified depositional model, shown in Fig. 5A, combines the main processes that operated during deposition of a typical Gilbert-type, deep-water sandy delta in the Bohemian Cretaceous. According to Uličný (2001), the most important depositional processes on the foreset slopes were (i) migration of sandy bedforms driven by mostly unidirectional currents, and (ii) gravity flows of mobilized sand interpreted to be responsible for the formation and filling of the chutes (cf. Fig.6). These gravitational processes represent a link to the heterolithic bottomset facies interpreted as sandy turbidites interbedded with offshore muds. It is, however, the marine current activity what makes the Bohemian Cretaceous deltas slightly different from many cases of Gilbert-type deltas which typically are dominated by gravitational processes and commonly contain poorly sorted debris-flow deposits in their foresets (e.g. Nemec, 1990; Ori et al. 1991).

Palaeocurrent data from the cross-bedded sandstones (Figs. 3, 6, 7) indicate that the Český Ráj deltas (as well as other, older deltas of the Lužice-Jizera sub-basin) were strongly affected by predominantly SE-directed currents which caused the formation and migration of dunes on the foreset slopes. Sustained tidal circulation resulting in unidirectional currents along the NW margin of the open seaway was suggested by Uličný (2001) as the cause of the significant current activity affecting the foresets of the sandy deltas in the Bohemian Cretaceous. The persistent current activity also helps to explain another feature of the Bohemian Cretaceous deltas: why the sandstones are so “clean” - that is, why there is the lack of fine-grained clastic material in the foreset facies, especially given the dominance of muddy to marly facies in the offshore. The strong segregation of grain sizes between the sandy deltaic bodies and muddy offshore can be explained as due to the density contrast between seawater and the suspension-laden freshwater brought to the shore by river channels. Due to the lower density of freshwater, mud-grade suspended load was carried away from the delta front as hypopycnal plumes near the sea surface by the same basinal currents that caused the dune migration along delta slopes (Fig. 5A; cf. Nemec, 1995). Evidence from basin-scale geometries of muddy clinoforms for the transport of fine-grained sediment along the basin axis is also demonstrated by Uličný (2001).

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