Quantitative allochem composition analysis of Lochkovian-Pragian boundary sections of the Barrandian area (Prague Basin, Czech Republic)

The Lochkovian-Pragian boundary Event, connected to faunal and facies overturns accompanying global sea level fall, is the first in the series of global Devonian events defined in the classical Barrandian area (Prague Basin,... [ view full abstract ]

The Lochkovian-Pragian boundary Event, connected to faunal and facies overturns accompanying global sea level fall, is the first in the series of global Devonian events defined in the classical Barrandian area (Prague Basin, Czech Republic).
Carbonate compositional variations were studied on 148 samples from sections at Čertovy schody (257.5m), Na Chlumu (96.5m) and Na Branžovech (109.5m) quarries. Raw quantitative data of determinable allochems (da) obtained by point-count method were used for correspondence, principal component and cluster analysis. The latter two were applied also on data transformed by the log-ratio methodology (clr) that was used to reduce biased patterns resulting from covariance structure of closed data.
Upper parts of the Lochkov Formation consist of grey, well-bedded, locally slightly nodular calcisiltites/fine-grained calcarenites. Shale intercalations and cherts are developed at Na Chlumu Quarry. These grainstones/packstones/wackestones are crinoid dominated (≤93%da) with contribution of peloids, trilobites, ostracods, brachiopods, bryozoans, etc.
Basal parts of the Praha Formation at Na Chlumu and Na Branžovech quarries are composed of grey/pink, thick-bedded/massive fine- to coarse-grained calcarenites with stromatactis. Trough cross-bedding is developed in the lower parts at Na Chlumu Quarry. These grainstones/packstones contain crinoids (≤96%da) accompanied by bryozoans (≤13%da), brachiopods, trilobites and ostracods. These facies gradually pass upward into succession of greyish-greenish-pink slightly nodular, red nodular and grey bedded nodular calcisiltites/fine-grained calcarenites, the latter with shale intercalations at Na Branžovech Quarry. These dacryoconarid dominated (≤83%da) wackestones/packstones contain globochaetids, molluscs, trilobites, ostracods, brachiopods, bryozoans and common traces of sponge bioerosion. The Praha Formation at Čertovy schody Quarry consists of grey/pink thick-bedded/massive fine- to coarse-grained calcarenites with trough cross-bedding in lower and upper parts. These grainstones/packstones contain common crinoids (≤75%da) and bryozoans (≤75%da) with contribution of tabulate corals (≤26%da), brachiopods, ostracods and trilobites. Green algae (≤42%da) and micritisation occur especially in upper parts. Lower parts of the Zlíchov Formation (Na Branžovech Quarry) and the Suchomasty Member (Čertovy schody Quarry) were also analyzed.
With the environmental gradient from shallow photic- to deep aphotic zones, several allochem categories were distinguished using the multivariate statistical analysis. The inferred curves of facies shifts show similar results for the raw and log-ratio transformed data. Stratigraphic distribution of the results indicates a lowstand- to transgressive- to highstand- succession in the Praha Formation with a prominent sequence boundary situated close to the Lochkovian/Pragian stage boundary. The heterotroph dominated associations with little contribution of phototrophs and peloids (supported by regional-scale facies architecture) suggest ramp topography.
Funding: GA14-18183S (& RVO:67985831).