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Rhaetian/Hettangian boundary in Pomerania, Poland

Pieńkowski G., Waksmundzka M.

Volumina Jurassica

2006

Vol. 4, no. 1

295-295

Two boreholes from Pomerania, Western Poland (Kamień Pomorski IG-1 and Mechowo IG-1) yielded core material from the Triassic/Jurassic transition in continental deposits. In Mechowo IG-1 the Rhaetian/Hettangian boundary occurs within thick sandstone succession of fluvial origin and this boundary is determined based on occurrence of megaspores (Trileites pinguis and Nathorstisporites hopliticus assemblages - Marcinkiewicz 1971). In Kamień Pomorski (25 km to NW, depth 665.5-678.2 m), the grain size of sediments is significantly finer. Also, a minor marine ingression with dinoflagellate cysts can be noted (this ingression can be tentatively correlated with the "Contorta beds", known from Germany and Scandinavia). Uppermost Rhaetian deposits represent alluvial overbank subsystems, floodplain, lacustrine and crevasse splay facies dominate. This indicates a substantial limitation of depositional energy of the alluvial palaeoenvironment, which reflects both palaeoslope (Mechowo was closer to the sedimentary source area situated to the East) and probably local tectonic factor. Consequently, erosion at the Rhaetian/Hettangian boundary (sequence boundary) is inconspicuous if any, which is an exceptional case in the Early Jurassic Polish Basin, where usually Rhaetian deposits are missing or reduced from the top by erosion. Material from Kamień Pomorski IG-1 borehole gives much more abundant palynological data. In the uppermost Rhaetian, the following miospores are most characteristic: spores Semiretisporites gothae (restricted only to the Late Triassic), accompanied by Limbosporites lundblandi Nilsson, Baculatisporites wellmanii (Couper) Krutzsch, Cingulizonathes rhaeticus (Reinhardt) Schulz, Conbaculatisporites mesozoicus Klaus, Deltoidospora toralis (Leschik) Lund, Densosporites fissus (Reinhardt) Schulz, Lycopodiacidites rugulatus (Couper) Schulz, pollens Alisporites radialis (Leschik) Lund, Cuneatisporites cf. radialis Pautsch, Ovalipolis ovalis Krutzsch. This is a typical Rhaetian miospore assemblage. Interestingly, the uppermost Rhaetian deposits show a characteristic palynofacies turnover with a conspicuous "fern peak" (huge number and domination of fern-derived spores). This may point to the environmental/climatic change at the Rhaetian/Hettangian boundary and concomitant biotic crisis. The lowermost Hettangian assemblage comprises (among others): spores Contignisporites problematicus (Couper) Playford & Dettmann, Conbaculatisporites mesozoicus (Madler) Lund, Concavisporites toralis (Leschik) Nilsson, Cosmosporites elegans Nilsson, Zebrasporites interscriptus (Thiergart) Klaus, Lycopodiumsporites semimuris Danze-Corsin & Laveine and pollens Pinuspollenites minimus (Couper) Kemp. These preliminary studies are encouraging and the Kamień Pomorski profile can add to the European record of continental Rhaetian/Hettangian boundary. Furter palynological and isotope studies are planned in this section.