Petrofyzikalni zaznam kacackeho eventu na profilu Hlubocepy – zeleznicni zarez (stredni devon, prazska panev)

The Middle Devonian (latest Eifelian to earliest Givetian) Kačák Event represents one of the most prominent turnovers in the Lower Palaeozoic sedimentary sequences of the Prague Basin (Chlupáč - Kukal 1988). It abruptly terminated the carbonate-dominated deposition in the entire basin, which was replaced by dark shales as a result of significant sea-level rise. This event is recognizable at inter-regional scale (e.g., in western Europe, north Africa or North America; for more details see Ellwood et al. 2011).
This study presents physical stratigraphic (magnetic susceptibility - MS and gamma-ray spectrometry - GRS) records of one of its type sections in a railway cut at Praha-Hlubočepy (50.03891N 14.39575E). This seven-metre thick section exposes transition from the uppermost levels of the Choteč Formation (Eifelian) represented by thin-bedded limestones (mostly bioclastic wackestone to packstone) to siliciclastics of the overlying Srbsko Formation (latest Eifelian to Givetian). Its basal beds are represented by dark shales and silicites of the Kačák Member overlain by siltstones and fine-grained sandstones of the Roblín Member.
Both MS and GRS records show apparently progressive trend (Fig. 2, Table 1). High covariance of both proxies (R² = 0.88) indicates their detrital origin. Two prominent levels have been identified in the section, both recognizable in lithology and physical stratigraphic record. The first one corresponds to the base of the Kačák Member, it is related to drowning of the carbonate platform during the eustatic sea-level rise (Strnad - Hladil 2001). An apparent increase in both proxies in the uppermost levels of the Kačák Member corresponds to enhanced amount of silt and fine-grained sand fractions. This is interpreted as a result of relative sea-level drop caused by an uplift during the initial stages of the Variscan orogeny accompanied by increased erosion rates and supply of terrestrial detrital material to shallow-marine basins. Studies of the provenance of detrital zircons indicate that the majority of siliciclastic material came from uplifted margins of the Teplá-Barrandian Unit. However, some more distant sources with affinity to the present-day north-western Africa can also be detected (Strnad - Mihaljevič 2005; Drost et al. 2011).