Some remarks on the biostratigraphy and paleoecology of the Middle Miocene Machów Formation (Carpathian Foredeep, Poland)

The Polish Carpathian Foredeep Basin (PCFB) is the northern part of a large foreland basin system that surrounds the Carpathian orogenic belt. Like other foreland basins, the PCFB is asymetric and filled mostly with clastic sediments of the Miocene age up to 3,0 km thick at the Carpathian front and to few hundred meters in the northern marginal part. Molasse deposits of the PCFB, underlain by the platform basement, dips southward underneath the Outer Carpathian napes to a distance at least 50 km. The PCFB is subdivided into two sub-basins: the inner and outer ones, located respectively south and north of the Carpathian frontal thrust. The outer sub-basin is composed of Middle Miocene autochthonous marine strata. The Miocene succession is subdivided into three formations: the Skawina Fmsub- evaporitic, Wieliczka/Krzyżanowice Fms.-evaporitic and the Machów Fm-supraevaporitic. The last one is predominantly represented by siliciclastics sandstones and shales couplets. The age of this formation traditionally was assigned as Late Badenian and Early Sarmatian on the basis of foraminiferal research mainly. Our studies of the Machów Fm were concentrated in the eastern part of the PCFB, north of the Rzeszów. In this area we collected samples from five boreholes, in following depth intervals: S-2 (Stobierna): 1016-1338 m; S-3: 715-1669 m; S-4: 1016-1238 m; SB-1(Stara Brzóza): 350-356 m and 1043-1667 m; P-2 (Pogwizdów): 1161-1390m. The uppermost (above 350 m) and lowermost (beneath 1669) part of Machów Fm. was not studied because of the lack of core material. The aim of the study was to provide the biostratigraphic and paleoecological analyses for the Machów Fm. For this purpose smear slides from all collected samples were prepared using the standard method, and analyzed under light microscope Nikon Eclipse E600POL (LM, 1000x magnification) at normal and crossed nicols. The qualitative analysis were carried out for all the samples whereas the quantitative analysis only for the chosen boreholes S-3 and S-4.The obtained biostratigraphic data gave evidence for the upper part of the NN6 (the Early Sarmatian) and for the NN7 (the lowermost part of the Late Sarmatian) Zones. The whole sections investigated in S-2, S-4 and P-2 were classified to NN6 Zone. In S-3 interval 1669-1113 m was assigned to NN6, whereas section 843-715 m to NN7 Zone. In SB-1 interval 1667-1043 m belongs to NN6 Zone, interval 350-356 m was classified to NN7 Zone. The Discoaster exilis Zone (NN6) was defined by the presence of Reticulofenestra pseudoumbilica, Sphenolithus abies, Helicosphaera walbersdorfensis and absence of Discoaster kugleri. Besides the listed species, the typical association of this zone was also represented by freguently occuring Coccolithus pelagicus, Cyclicargolithus floridanus, Helicosphaera carteri, Sphenolithus moriformis and Umbilicosphaera rotula and sporadically observed Calcidiscus leptoporus and Calcidiscus premacintyrei. The Discoaster kugleri Zone (NN7) assignment was based on the abundance of Coccolithus miopelagicus (>10μ), used as an alternative species essentially confined to that interval, and absence of Catinaster coalithus. The NN7 Zone is difficult to distinguish because of absence or scarce abundance of significant marker species such as discoasterids and C. coalithus. The paleoecological preferences of nannoplankton species in S-3 and S-4 were considered in regard to temperature and nutrient availibility (trophy).The enrichment of C. pelagicus and C. floridanus in sediments could indicate the nearshore eutrophic environment with high nutrient levels in surface water and upwelling paleoconditions. To upwelling-prefering group belong numerous H. carteri and small Reticulofenestra. The scarcity of discoasterids, which are more common in open ocean assemblages, could confirm shallow and coastal paleoenvironment as a negative indicator however its distribution depends on paleogeographical settings. It occurs much more often in Mediterranean area than in Paratethys. Deposition near the coast and relatively shallow water depth could result in high percentage of reworked specimens, which prevails over autochthonous ones in most samples from S-3 and S-4 boreholes. The percentage of autochthonous specimens is less than 50% and fluctuates between 40-50%. Reworked material, of the Cretaceous and the Paleogene age, comes from the south, from eroded Carpathian orogene.
   Acknowledgements: The research was financially supported by the Jagiellonian University grant DS/INGUJ/2010/808. PGNiG documentary: CRZ/146/DN/05.