| Popis: |
Lochkovian redbeds in the Dnester formation in Podolia, Ukraine, record an upward transition from marginal-marine to fluvial conditions. A 60-m-thick section through transitional strata reveals a regressive stack of three facies associations laid down in: offshore lagoon, inshore lagoon or bay, and a fluvial-estuarine plain. The offshore lagoonal association shows green to variegated alternation of fossiliferous, bioturbated limestones, quartz arenites and shales. Numerous coquinas contain schizohaline ostracods, vermiform gastropods, Lingula, charophyte gyrogonites, all mixed with fragments of marine forms such as articulate brachiopods, tabulates, goniatites and leiospheres. Trace fossil assemblages contain abundant Skolithos, Spirophyton, Teichichnus and simple tubular forms. The inshore lagoon or bay association is a largely non-fossiliferous, locally bioturbated, red succession of interbedded mudstones and sandstones, which are arranged in coarsening-up units capped by thin, channelized sandstones. The presence of Lockeia, Monomorphichnus, Rusophycus and Skolithos suggests that the coarsening-up units could have originated as small bayhead deltas and washover fan-channel complexes. A new ichnospecies, Monomorphichnus podolicus, described here, occurs also in these deposits. In the overlying fluvial-estuarine association, the only trace fossil recorded is Skolithos, which occurs in dense clusters along upper levels of cross-bedded ribbon-shaped channel fills. Until now, piperock has been considered as an ichnofabric of the Skolithos ichnofacies that is typical of sandy beaches, bars and spits, whereas the findings presented here expand landwards the environmental range of Skolithos piperocks into more intracoastal marine-influenced settings. |
