Holoceńska fauna mięczaków i martwica wapienna z Głowiny koło Dobrzynia nad Wisłą

Abstract

Opisano holoceńskie osady zboczowe przesycone w różnym stopniu węglanem wapnia występującym często w postaci martwicy wapiennej. W osadach wykryto 17 gatunków mięczaków, głównie ślimaków lądowych. Odtworzono warunki powstania osadów z fauną.

HOLOCENE MOLLUSCAN FAUNA AND CALCAREOUS SINTER FROM GŁOWINA NEAR DOBRZYŃ ON VISTULA

The molluscan locality is situated at slope foot east of Dobrzyń on Vistula (Fig. 1). The slope was shaped by Early Holocene river erosion and landslide-deluvial movements. Its lower part is formed of glacitectonically disturbed Pliocene clays overlain by two till horizons separated here by fluvioglacial sands. The slope is about 40.0 m high (Figs. 2, 3).

The slope is covered with deposits among which there may be differentiated:

- deposits related to surficial creep and slopewash processes, with well marked stratification (silts, sands, gravels and boulders);

- deposits formed by landslides operating to greater depths, comprising large packets of Pliocene clays and cut by numerous dislocation planes usually dipping towards the slope. The deposits are impregnated with calcium carbonate which forms typical calcareous sinter in some places.

Gastropod and bivalve shells are nonuniformly distributed throughout the deposits, being common in deluvial ones and scarcer in the colluvial. Terrestial gastropods predominate among 17 molluscan species recorded at Głowina (Table 1). Aquatic molluscans are represented by four species only: Bithynia tentaculata (L.), Planorbis planorbis (L.), Viviparus fasciatus (M ü I l.), and bivalves of the genus Unio. The faunal assemblage appears fairly inhomogeneous from the ecological point of view. It comprises representatives of 8 out of 10 ecological groups differentiated by V. Ložek (1964), that is the species confined to forests only (Cochlodina laminata), those living in forests and open space (Arianta arbustorum, Bradybaena fruticum, Cepaea hortensis, Helix pomatia), wet forests (Perforatella bidentata), steppes (Chondrula tridens), open spaces (Euomphalia strigella), wet (Laciniaria plicata) and wet and swampy places (Monachoides rubiginosa, Succinea putris), and the above listed aquatic species living in overgrown stagnant water (Planorbis planorbis), flowing water (Viviparus fasciatus, Unio sp.) or stagnant and flowing water environment (Bithynia tentaculata).

The requirements of Recent representatives of this fauna! assemblage suggest thermal conditions similar to the present.

The age of the molluscan assemblage is primarily indicated by the species Helix pomatia, the expansion of which has mainly taken place in the historical times. This suggests Late Holocene age of the assemblage.

The processes operating on the slope were mainly related to imperviousness of Pleistocene clays and their susceptibility to mass movements, as well as effluent seepage of water out of Quaternary deposits. Southward exposition of the slope and humidity of ground were favourable for the development of vegetational cover and molluscan fauna and the formation of calcareous sinter.