Nowe dane o budowie geologicznej Mierzei Wiślanej

Abstract

Na podstawie ok. 600 nowych otworów wiertniczych opracowano budowę geologiczną Mierzei Wiślanej i najbardziej północnej części delty Wisły. Wyróżniono i opisano pięć jednostek stratygraficznych: podłoże czwartorzędu, plejstocen przedeemski. interglacjał eemski - morski, zlodowacenie wisły i holocen. Około 20-metrowa seria holoceńska. najlepiej zbadana,. składa się z części starszej (preborealno-atlantyckiej) i młodszej (atlantycko-subborealnej). Rozdziela je przewodni poziom mineralno-biogeniczny o miąższości 0.2-2.0 m, którego wiek określony metodą 14C (23 daty) wynosi od 812U ± 140 do 6330 ± 60 lal B.P. Najmłodsza z tych dat odpowiada początkowi maksymalnej fazy transgresji atlantyckiej (litorynowej).NEW DATA ON GEOLOGIC STRUCTURE OF THE VISTULA BAY BARWhile exploring the amber deposits on the Vistula Bay Bar, Geological Enterprise - the Gdańsk Division drilled in 1981-1985 about 600 boreholes, the ranges from 6.2 to 51 m depth: about 7500 m is total. Samples were sent to Branch of Marine Geology in Sopot along with an order to carry out lithological-stratigraphical explorations and work up the geologic structure. The following laboratory tests have been carried out: sieve analysis, sedimentation test, quartz-grain roundness test, mineral-petrographic test, heavy mineral analysis, macrofauna and palinological analysis and 27 datings with the 14C method (Figs 2-7). Results of tests are presented in 10 cross-sections through the Vistula Bay Bar between Gdańsk and the eastern state frontier (Fig. 1). Separations carried out refer to some results - obtained earlier by J.E. Mojski. Archival materials and references were also taken into consideration. The following units have been distinguished in the structure of the Vistula Bay Bar and adjoing northern part of Żuławy (Fig. 8): 1 - the Quaternary basement, 2 - the Preeemian Pleistocene series, 3 - the Eemian marine interglacial series, 4 - the Vistula glaciation series, 5 - Lower and Upper Holocene cover. The Quaternary basement (Fig. 9) in the area consists mainly of the Upper Cretaceous (Campanian) and partly of the Tertiary occurring in lobes (Eocene-Oligocene and Miocene). The top of the basement lies from 140 to 75 m below sea level The subquaternary surface was formed in different periods of Quaternary and is of erosion-exaration origin. The Preeemian Pleistocene series composes two groups of glacial, fluvioglacial and glacial lake de-posits classified earlier by J.E. Mojski as the San Glaciation (South-Polish Glaciation, Oka Glaciation, Elster Glaciation) and the Odra Glaciation (Mid-Polish Glaciation, Dniepr Glaciation. Soława Glaciation). The San Glaciation deposits are better maintained and more complexed lithologically than deposits of the Qdra Glaciation, due to substancial rate of destruction the later caused by the Eemian abrasion. The Eemian Interglacial series composes fine-grained marine sands, the top erosion surface of which lies 50-20 m below sea level and the thickness reaches 30 m. Species and sizes of crusts prove the shallow littoral zone of warm sea with a low degree of salinity. Results of the other analysis show the decreasing dynamics of waters upward. The Vistula Glaciation deposits (the North-Polish Glaciation. Valday Glaciation) and deposits of the Late Glacial remained only in a few occurrences of significantly reduced thickness, mainly as sands with gravel. A degree of their destruction increases to the east and they are identified analytically in the bottom of the Vistula Bay Bar in the Sztutowo area at 20-16 m below sea level (Fig. 3) and in the Przebrno area at 21-20.5 m below sea level (Fig. 4). The Holocene deposits form a solid cover with the thickness in mast of the are ranges about 20 m, and in the eastern part there is no criteria to its determination. Within the Vistula Delta Plain there are mainly fluvial, and biogenic deposits, partly limnic deposits, and within the bar zone also limnic, litoral and aeolian deposits. The bipartite Holocene cover consists the older part deposited from the Preboreal to Atlantic period and the younger part from the Atlantic to Subatlantic period. The upper (biogenic) series of the older cover as the key horizon is observed in almost whole area of the Vistula Delta Plain and in the bottom of the Vistula Bay Bar to Piaski. The age of the key horizon determined by 23 datings with the 14C method is between 8120 and 6330 years B.P. (Tab. 1). It is evident from the above that in almost the whole area the biogenic layer deposited in mere 1790 years over the huge plain with changes of leveI ranging 8 m with paleochannels and shallow swamps and peat hogs in depressions. The beginning of the maximum transgression and littoral marine deposits over the south share of the Gdańsk Bay is determined by the laters of these datings i.e. 6330 years B.P. This statement refers to the earlier results and evaluations (B. Rosa et al). The littoral marine sands form a shank of the bar and over it dunes occur that were formed by aeolian transformation of marine deposits sedimented here in consecutive periods of sea shore line regression. The zones of brown, yellow and white dunes (Figs. 1, 10) of different ages distinguished in the Vistula Bay Bar are the subject or further field investigations carried out by Branch of Marine Geology. The investigations have been carried out to specify opinions on the genesis and phases of forming of the bar in the youngest Holocene.