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2 Geological Quarterly

2 2020

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Marine vs. terrestrial environments during Early Triassic deposition on the northeastern margin of the Central European Basin : a multidisciplinary study on the Middle Buntsandstein of the Bartoszyce IG 1 borehole, NE Poland

Becker Anna, Fijałkowska-Mader Anna, Jasionowski Marek

Geological Quarterly

2020

Vol. 64, No. 4

1023--1047

The Middle Buntsandstein Subgroup of the Lower Triassic of north-eastern Poland has been investigated in the context of possible marine ingressions into the Central European Basin. To better constrain these, palynofacies analysis and mineralogical analysis of mudstones were undertaken on samples taken from the Lidzbark and Malbork formations penetrated by the Bartoszyce IG 1 borehole, serving as stratotype section of both lithostratigraphic units. Microfacies and geochemical analyses, including C and O isotope analysis, were conducted additionally on oolitic limestones of the basal Lidzbark Formation (the lowermost Middle Buntsandstein), and the boron content was measured on all mudstone samples. Seven palynofacies types are distinguished: types 1 to 3 within the Malbork Formation and types 4 to 7 within the Lidzbark Formation. Types 1 and 2 originated on a floodplain, type 3 probably in a deltaic or barrier setting, and types 4 and 5 in a brackish lagoon or a more open basin, possibly of marine origin. Palynofacies type 6 reflects long transport and reworking, whereas palynofacies type 7 is interpreted as formed due to pedogenic processes. Clay minerals and quartz, accompanied by feldspars, calcite and dolomite are the main components of the mudstones investigated. The clay mineral association consists of illite or a mixture of illite and smectite, and chlorite. The Lidzbark Formation and the lowermost part of the Malbork Formation show less variability in mineralogical composition than the upper part of the Malbork Formation. Smectite admixtures were detected only in the upper part of the Malbork Formation (the uppermost Middle Buntsandstein). The boron content, achieved after aqua regia digestion, ranges from 70 to 121 mg/kg (96 mg/kg at average), oscillating generally around 90 mg/kg. A higher boron content, bound in silicate structure, is associated with the upper part of the Malbork Formation. All major mineralogical and geochemical changes coincide more or less with the transition from the supposed marine to the terrestrial environmental realm, interpreted from lithological and sedimentological observations within the lowermost part of the Malbork Formation. However, diagenetic alteration of the clay minerals, and of the boron content, could not be ruled out. The oolitic limestones, mainly grainstones, contain admixtures of quartz grains and rare bioclasts, the ooids nuclei being peloids or unrecognizable. The carbonates are almost exclusively composed of low-Mg calcite. A high content of Mg and the presence of small amounts of dolomite suggest that the ooids were primarily composed of high-Mg calcite and are comparable with similar Early Triassic deposits in Tethyan settings. The δ13C values range from -2 to +1‰ VPDB, fitting well with the known ranges of Lower Triassic marine carbonates. An observed δ13C depletion towards the top of the oolite-bearing part of the section may reflect a local shallowing trend that led to overall salinity decrease. A possible connection with the one of the global oceanic geochemical episodes has to be tested further. The results obtained suggest a marine origin of the lower Middle Buntsandstein deposits studied and document a terrestrial origin for the upper Malbork Formation.

Integrated palynostratigraphy and magnetostratigraphy of the Middle and Upper Buntsandstein in NE Poland : an approach to correlating Lower Triassic regional isochronous horizons

Becker Anna, Fijałkowska-Mader Anna, Nawrocki Jerzy, Sobień Katarzyna

Geological Quarterly

2020

Vol. 64, No. 2

460--479

Despite years of research, Lower Triassic deposits of the epicontinental Central European Basin still lack a detailed stratigraphy that would allow regional correlation of isochronous horizons. The best chronostratigraphic results have up to now been achieved by microspore-based biostratigraphy and magnetostratigraphy. Integrated palynostratigraphic and magnetostratigraphic investigations, carried out on Buntsandstein cores from northeastern Poland representing the eastern margin of the basin, have made precise correlations with the better-explored basin centre. The Lidzbark and Malbork formations of the Bartoszyce IG 1 borehole were examined by means of palynology and palaeomagnetic studies. Further palaeomagnetic studies were applied to the Lidzbark, Malbork and Elbląg formations of the Nidzica IG 1 borehole and the Elbląg Fm. of the Pasłęk IG 1. Two spore-poll en assemblages were distinguished representing the Densoisporites nejburgii Subzone of the D. nejburgii Zone within the lower part of the Lidzbark Fm. and the lowermost part of the Malbork Fm. Mostly reversed polarity was detected within the lower part of the succession investigated, whereas normal polarity prevailed within its upper part. A normal polarity local zone was correlated with the undivided Tbn6-Tbn7 standard magnetozones of western Poland. The reversely polarized part of the succession corresponds most probably to the Tbr5 standard magnetozone. The base of the Tbn6-Tbn7 magnetozone can serve as a good correlation horizon for regional reconstructions.