Stratigraphy, depositional environment and tectonic setting of the so-called Kozly Sandstone in northwestern part of the Bohemian Cretaceous Basin


Roland Nádaskay, Jaroslav Valečka, Stanislav Čech

Geoscience Research Reports 51, 2018, pages 167–180

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Published online: 24 October 2018

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Kozly Sandstone forms up to 59.5 m thick body situated mostly within the deepest tectonic depression of the Bohemian Cretaceous Basin (BCB), and cropping out in the SW vicinity of Česká Lípa (Fig. 1). Within this study, the Kozly Sandstone is defined as a succession of predominantly quartzose sandstones, exposed in the mentioned area, together with underlying succession of amalgamated sandstone beds. Because of complicated geological setting of the area, previous age constraints as well as interpretation of spatial extent of the Kozly Sandstone differed significantly (see Fig. 2). Recently, a complete stratigraphic section of this unit was penetrated by cored borehole 4730_B (Fig. 4). It proved that the Kozly Sandstone is separated from Turonian sandstones of the Jizera Fm. by ca. 25 m thick sequence of mudstones, containing bivalves Didymotis (Didymotis event 0), Mytiloides scupini (Heinz) as well as ammonite Prionocyclus germari (Reuss) (Fig. 7). Bivalves and ammonites allowed for the first time to determine the upper Turonian age for the mudstones underlying the Kozly Sandstone, belonging to the middle part of the Teplice Fm. On its top, an accumulation of glauconite and phosphates covering erosional truncation is interpreted as a transgressive lag comprising condensed lowermost part of the Coniacian. The overlying mudstones are the uppermost lower Coniacian in age. Based on drill core and outcrop data, individual lithofacies of the studied section (Table 1), incl. the Kozly Sandstone, are interpreted as having been deposited in deltaic environment: 1) facies S, G as delta-slope foresets, formed by alternation of downslope gravity currents and reworking by tidal currents; 2) facies Sgf/Hgf as bottomsets, deposited by gravity currents; 3) facies Ste, formed by calcareous bimodal sandstone, as a tempestite, although originally deposited as bottomset turbidite; 4) both facies Ms and Mm/Mc as offshore deposits. Alternatively, the Kozly Sandstone, as sheet like body with an extent of at least 80 km2, might be interpreted as isolated sand field detached from the nearshore sands. The sand field, often covered by ripples and dunes, migrated to the SSW under fair weather wave base due to the activity of strong bottom currents, namely ebb currents.
Well-logs allowed correlation of the Kozly Sandstone (Fig. 3) within genetic sequence TUR 7 consisting of middle-upper part of the Teplice Fm (upper Turonian). The correlation shows that foresets developed southward from borehole Vf-1, in which succession of up to 20 m coarsening-upward cycles dominate the sequence TUR 7. This enigmatic sharp contrast of depositional architectures within TUR 7 sequence may reflect basinward shift of deposition resulting from the increase of clastic supply or rapid sea-level fall; both hypotheses, however, require further investigation. Further to the S/SW, the foresets pinch out into thick sequence of Coniacian mudstones. However, thick sequence of sandstones, correlated from the Kozly area into the vicinity of Děčín, indicate possible greater extent of the interpreted Kozly Sandstone than expected. Revision of archival boreholes and brief field survey allowed re-interpretation of the tectonic setting of the study area - proposed tectonic framework and idealized structure of the area is shown in Fig. 1.


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