Nowy, plejstoceński poziom morski oraz ślady morza holsztyńskiego na Dolnym Powiślu

Abstract

W profilach otworów wiertniczych w Krastudach i Bukowie wydzielono 21 plejstoceńskich poziomów litostratygraficznych oraz oznaczono malakofaunę występującą w niektórych osadach. Malakofauna i otwornice wskazują na obecność 5-ciu poziomów osadów morskich, którym niekiedy towarzyszą osady jeziorne. Najniższy z poziomów morskich odpowiada osadom morza holsztyńskiego, lecz występuje jedynie w porwakach glacjalnych wśród glin zwałowych zlodowacenia środkowopolskiego. Dwa poziomy wyższe stanowią odpowiedniki mórz eemskich: sztumskiego i tychnowskiego. Poziom czwarty jest jeszcze niezbyt pewny, natomiast poziom piąty, najwyższy i najmłodszy jest nowym nieznanym dotychczas na obszarze Polski poziomem morskim (poziom krastudzki). Jego osady, podobnie jak osady mórz eemskich, zawierają faunę luzytańską i występują w kompleksie zaliczanym do tej pory do zlodowacenia północnopolskiego, w serii gniewskiej oddzielonej od osadów eemskich wyraźnym, wyższym poziomem gliny zwałowej (toruńskiej). NEW PLEISTOCENE MARINE HORIZON AND TRACES OF THE HOLSTEIN SEA IN THE LOWER POWIŚLE AREA In 1983, two drillings were made in the Lower Powiśle area (Fig. 1) within the frame of works connected with compilation of the Sztum sheet of The Detailed Geological Map of Poland in the scale 1 : 50,000 ( B.J. Nowak, W. Rabek, in print). The drillings penetrated the Pleistocene to be stopped in the Tertiary (Paleocene). The Pleistocene, 181.6 and 184.0 m thick in the boreholes, infills a depression in the Tertiary basement, with top surface situated at 91.5 and 136.6 m a.s.l. The encountered section of the Quaternary comprises (from the base upwards) sediments of the Mid-Polish Glaciation, Eemian Interglacial, and North-Polish (Vistulian, Baltic) Glaciation (Fig. 2). The presence of a guide horizon for this area, i.e. the well known Eemian Interglacial horizon (A. Makowska, 1979a), made general stratigraphic subdivision of the Quaternary fairly simple. The whole Pleistocene sequence of the boreholes was divided into 21 lithostratigraphic horizons (Fig. 2, horizons 1 - 21). The Mid-Polish Glaciation is represented in these sections by fluvioglacial sands (1), sediments of ice-dammed lakes (4), and tills (2, 2a) yielding erratic bodies of sandy and silty sediments with poor marine fauna (3). Sedimentary series of the Eemian Interglacial, subdivided into 7 clearly different lithostratigraphic horizons (5 -11), comprises sediments with marine (horizons 6 and 10) and fresh-water (horizons 8 and 9) fauna. The sedimentary complex hitherto assigned as a whole to the North-Polish Glaciation comprises 4 till horizons (12, 17, 19 and 21), designated with the symbols BII - BV in the Lower Powiśle area subdivision used by the Authoress (A. Makowska, 1973a, 1979b, 1980a). The horizons are separated by two intermoraine series varying in lithology (13 -16 and 18) and a layer of fine-grained sand (20). Sediments of the lower intermoraine series yield marine molluscs (15), relics of fresh-water ones (16), and foraminifers (13a, 15). In the studied borehole sections of the Pleistocene, molluscs were recorded in five horizons of marine sediments. The horizons were correlated with the Nowiny section, in which two marine horizons of the Eemian Interglacial have been well evidenced (A. Makowska, 1976, 1979b, 1980a, b - Figs. 1, 3). In the new borehole columns the lowermost marine horizon (A) is represented by erratic bodies of sediments presumably corresponding to those of the Holstein Sea, and the two higher ones comprise sediments of the Eemian seas: Sztum (B.) and Tychnów (C).Strata of the Tychnów horizon yield mollusc assemblage also known from other localities in this area. The hitherto recorded assemblage comprises about 30 species, including Lusitanian forms (I. Brodniewicz, 1960; A. Makowska, 1979a). The underlaying strata yield fresh-water fauna. The fauna of the Sztum horizon is generally similar but poorer than that of the Tychnów horizon. The latter assemblage comprises 11 species, including Lusitanian form Eulimella nitidissima (Mont). The fourth faunal horizon, also inferred to be marine in character, is known from Bukowo. The paleontological record for this horizon comprises single foraminifers only. In turn, the fifth, the uppermost and youngest horizon, known from the Krastudy locality, is undoubtedly marine and occuppying a primary or close to primary position (as shown by its clear geological setting). It is situated in lower part of consequently developed intermoraine (Gniew) series of the North-Polish Glaciation complex. The underlaying till of the horizon BII (Toruń) separates it from the Eemian series. Mollusc assemblage of that horizon resembles the Eemian. It comprises 16 species, also including Lusitanian form Eulimella nitidissima (Mont). Sediments of the uppermost horizon were dated at 75 ± 12 ka by the termoluminescence method (A. Bluszcz, 1984). This marine horizon, hitherto unknown in Poland, is named the Krastudy horizon after the Krastudy locality.