Wczesnoholoceńskie zespoły mięczaków z Podgrodzia nad Wisłoką

Stefan Witold Alexandrowicz


Early Holocene molluscan assemblages from podgrodzie on the Wisłoka river, Poland

The profile of Lower Holocene sediments from Podgrodzie comprises sands, silts and clays with a peat intercalation of a total thickness of 12 m. They represent a series formed mainly in the preboreal and boreal phases of the Holcene, during a time interval determined approximately at 10 500—7000 years BP. Four lithostratigraphic units, designated as C, D, E and F, have been distinguished in the profile in question. They correspond to respective stages of the filling of a water basin of the oxbow lake type, and the formation of an alluvial fan (Mamakowa, Starkel 1977; Niedziałkowska et al., 1977). Detailed palaeontological investigations resulted in finding a great number of gastropod and pelecypod shells in the clays and silts of the member E, as well as in the underlying and overlying sediments. The malacofaunal assemblages contain 41 species of gastropods and 7 species of pelecypods, The frequency of their occurrence is presented according to the semi-quantitative method. The value of 1/2 log 10 has been assumed as a basis for the division into frequency classes. Due to this assumtion, the boundaries of these classes can be defined in the logarithmic distribution (fig. 1) which facilitates the use of statistical and taxonomic methods for the interpretation of faunal assemblages. The malacofauna found at Podgrodzie was determined in 22 samples representing the top part of the member D, the member E (layers 1—11), and the bottom part of the member F (fig. 2). It is distributed unevenly, both as regards the number of specimens and the number of taxons in respective -samples (fig. 2 — L, T). Gastropods and pelecypods are the most abundant in the lower part of the member E whereas their assemblages show greatest differentiation in the middle part of this member. Aquatic molluscs predominate quantitatively in the lower part -of the profile, their content being variable in the upper part (fig. 2 — W). The determined taxons (fig. 3) were listed according to the ecological groups denoted with symbols 1—10, adopting the method proposed by V. Lozek (1964). Malacofaunal assemblages were distinguished on the basis of taxonomic analysis made in two variants (qualitative and quantitative methods), and on the basis of malacospectra of species (MSS) and individuals (MSI). The spectra show the presence of boundaries in the profile studied, which result from 'changes in the composition of molluscan assemblages. These changes are most pronounced between the Pg-7 and Pg-8, Pg-14 and Pg-15, or Pg-2 'and Pg-3 samples (figs. 4, 5), and make it possible to distinguish three fossil assemblages : — The assemblage of aquatic fauna (Pg-3 — Pg-7 samples) is homogeneous in composition. Land gastropods are very scarce. — The assemblage of aquatic fauna with an admixture of land gastropods (Pg-8 — Pg-14, Pg-2 samples) is characterized by a great number of taxons. The aquatic fauna is markedly differentiated, and the land species, which make up 20—30% of the assemblage, are represented, e.g. by Vertigo antivertigo, which occurs only in this part of the profile. — The assemblage of mixed fauna (Pg-15 — Pg-22 samples) is heterogeneous. Land molluscs are amply represented, the fauna becoming conspicuously poorer upwards. A f eature deserving note is the appearance of forest gastropods. The molluscan assemblages from Podgrodzie contain only the gastropod and pelecypod species reported from the present-day faunal assemblages of Poland. They developed during the period of gradual warming up, in the preboreal and boreal phases of the Holocene, reflecting the successive stages of the evolution of the sedimentary basin. The complex of Holocene deposits at Podgrodzie begins with sands and stream gravels without fauna, overlain by silts and sands deposited beyond the channel line, in a flood plain or a meander. The assemblage of aquatic molluscs contains species typical of a fluvial environment, as well as of overgrown waters and waterside environments. For the most part, it is presumably allochthonous fauna (liptocenosis). being cut off from, the direct influence of the river. The oxbow lake that formed was strongly overgrown and inhabited by aquatic molluscs. Its filling with organic matter led to the formation of peat and the reduction of the free water table. The basin became dismembered, which can be inferred from the composition of aquatic fauna (the occurrence of gastropods typical of small, intensely overgrown basins), and the appearance of land gastropods. This process went on for over 1000 years, and the effective accumulation of sediments can be estimated at 7—12 cm per 100 years. The malacofauna represents autochthonous thanatocenosis (assemblages with aquatic fauna). In the next stage the deposition rate was twice as rapid, and the oxbow Lake was completely filled up. The aquatic fauna found here testifies to the existence of intensely overgrown and periodically drying up small basins, while the land fauna suggests the presence of a variety of changeable habitats (the assemblage of mixed fauna). In the upper part of the profile malacofauna gradually disappears, showing that an alluvial fan began to form in place of the overgrown and filled-up water basin. It accumulated rapidly due to floods.

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