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Cementacja węglanowa osadów piaszczystych klifu w Gdyni-Orłowie

Ciborowski T., Jankowski M.

Przegląd Geologiczny

2007

Vol. 55, nr 2

140-145

Carbonate cementation of sandy sediments is most common in tropical and subtropical climate, and quite rare in temperate climate. About 20 sites where carbonate cemented sandstones occur are known throughout the Polish Lowlands. A new outcrop of carbonate cemented sandstone was found near Gdynia-Orłowo (coast of the Gulf of Gdańsk). Field studies and microscopic analyses [optical microscopy and SEM] let to recognise the pattern of cementation of sandy sediments. Origin of the sandstones is connected with infiltration of rain water into glacial till (partially cracked). This till is the source of calcium carbonate transported by water into underlying fluvioglacial sands. Because of different sediments porosity and resulting degassing of CO2, CaCO3 precipitated. Additionally, elevated temperature and evaporation of water may also create favorable condition for precipitation. The process of cementation is related to the exposition of fluvioglacial sands layer caused by cliff abrasion (about 1 m per year). The best geochemical conditions for precipitation of CaCO3 are in the zone of contact with atmosphere, so this cementation seems to be recent. Carbonate cemented sandstones formed below the glacial till fall down and may be found in the colluvium of the cliff slope.

Palaeoenvironmental conditions of hardgrounds formation in the Late Turonian-Coniacian of Mangyshlak Mountains, Western Kazakhstan

Gruszczyński M.

Acta Geologica Polonica

2002

Vol. 52, nr 4

423-435

A carbon and oxygen isotope stratigraphic profile has been made, for the first time, through the Late Turonian-Coniacian sedimentary sections containing regionally widespread firm- and hardgrounds of Mangyshlak Mountains, western Kazakhstan. Generally, Turonian and Coniacian time has been considered as a transitional stage between two Oceanic Anoxic Events (OAE), because of the peculiar pattern of variation of the [delta'13C and delta'18 O] values. Unfortunately, there is no such record in the sections we examined, thus the Mangyshlak Sea behaved uniquely compared to the majority of seas and oceans at the time. The process of hardground formation is polygenetic but involved stopping deposition of calcium carbonate and initiation of the hardground over the large area of the sea floor. Normal causes of cessation of calcium carbonate seem unlikely and the expected drastic changes of hydrochemistry of the bottom waters cannot be detected in any of the minerals within the hardground sediments. Also, changes in climate, if there were any, are very difficult to estimate. Moreover, winnowing of the carbonate sediment is also not detectable from the characteristics of the hardground surface. Because the sedimentary sequence containing that regional hardground formation is transgressive, the most plausible reason for cessation and deposition of calcium carbonate is acceleration of the transgression. This might release sufficient amounts of carbon dioxide and bicarbonate to slow precipitation and deposition of calcium carbonate. Also, the greater distance from the shore might have reduced the supply of nutrients which decelerated photosynthetic activity, which in turn decreased consumption of carbon dioxide, thus enhancing precipitation of calcium carbonate. Some additional winnowing of calcium carbonate sediments would have helped in subsequent development of the harground. Finally, deceleration of the transgression renewed precipitation and deposition of calcium carbonate.