Mięczaki plejstoceńskie z Horodła nad Bugiem

Leopold Dolecki, Sylwester Skompski


 Na podstawie gatunków mięczaków wydzielono chłodniejszą fazę i cieplejszą interfazę w stadiale mazowiecko-podlaskim (Warty) zlodowacenia środkowopolskiego. Określono środowiska w jakich żyły małże i ślimaki w czasie akumulacji lessu starszego górnego (LSg) w Horodle and Bugiem, wykształconego w facji bagiennej. Wśród małżoraczków oznaczono 5 gatunków nieznanych dotychczas z osadów czwartorzędowych Polski: Sclerocypris? clavata prisca Diebel et Pietrzeniuk, Candona kondrateneae Zubowicz, Candoniella subelipsoida Sharapova, Limnocythere falcata Diebel i Herpetocypris brevicaudata Kaufmann.



The paper presents results of studies on molluscans found in the section III at Horodlo (Figs. 1, 2). The studies on material obtained from samples of the Older Loess, 2,000 - 3,000 c. cm in volume, made possible identification of 20 gastropod species, 10 bivalve species, and 14 ostracod species. Moreover, there were found redeposited Cretaceous foraminifera assignable to about 20 species. A special attention should be paid to the record of Boreal-Alpine and Arctic species: Pisidium lilljeborgi Clessin, P. obtusale lapponicum Clessin, Valvata piscinalis antiqua Sowerby, and Vertigo parcedentata (Braun), indicating periglacial character of sedimentary environment of the layers bearing these organic remains. Some of the recorded ostracod species were hitherto unknown from the Quaternary of Poland: Candona kondrateneae Zubowicz, Candoniella subelipsoida Sharapova, Limnocythere falcata Diebel, Herpetocypris brevicaudata Kaufmann, and Sclerocypris? clavata prisca Diebel et Pietrzeniuk.

The studied locality is situated in area of Pleistocene terraces (Fig. 3) built of fluvial, lacustrine and loess sediments. The latter are fairly thick here, up to 38 m thick in the Grzęda Horodelska loess upland. Quaternary sediments are underlain by Upper Cretaceous chalk, limestones and marls. Stratigraphic subdivision of the Quaternary is here made taking into account a marker horizon of Masovian Interglacial fluvial sands with molluscan fauna, fossil soil horizons in loesses, and results of datings by FCl/P, Collagen and thermoluminescence methods (Fig. 4).

A paleoecological analysis, based on the recorded fauna, shows that the molluscan assemblage lived in a shallow basin of stagnant water, with a tendency to change into a swampy basin. The identified representatives of land fauna (Table 1) were presumably brought by rain water flowing to the basin from its direct neighbourhood. This is further supported by the presence of redeposited Cretaceous foraminifers. However, the presence of land species may be also explained as due to intermittent drying of the basin. Some conclusions concerning paleoclimatic conditions were also drawn. Despite of the presence of periglacial species in the molluscan assemblage, e.g. Pisidium lilljeborgi Clessin and P. obtusale lapponicum Clessin, the climate could not be Artic as the assemblage also comprises species with higher thermic requirements such as Armiger crista (Linnaeus), Anisus vortex (Linnaeus), Planorbis planorbis (Linnaeus), and Bithynia leachi (Sheppard). Moreover, there were traced some differences in composition of molluscans in individual samples. Samples 1 and 2 appear similar to one another, whereas sample 3 differs from them in presence of Vertigo parcedentata (Braun) and Pisidium lilljeborgi Clessin, predominance of species of the family Sphaeriidae, and a general decrease in number of species. Therefore, it may be concluded that in times of origin of sediments with molluscan fauna climate was initially cool (sample 3), to ameliorate somewhat afterwards (samples 1 and 2). Thus we may speak about a phase and interphase of the Warta Stadial (Mid-Polish Glaciation). Correlation of the Upper Older Loess (from which the samples have been taken) with the Warta Stadial appears consistent with current views on age of this loess in SE Poland (H. Maruszczak, 1976, 1980a, b), which is further supported by datings of sediments underlaying the loess at Horodło at 230,000-250,000 and 232,000 y. B.P. (Fig. 4).


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