Palaeoenvironmental reconstruction of Bathonian (Middle Jurassic) ore-bearing clays at Gnaszyn

Abstract

Multidisciplinary studies of the Middle–Upper Bathonian ore-bearing clays at Gnaszyn revealed variable palaeoenvi- ronmental conditions during the deposition of this seemingly monotonous sequence. We interpret the conditions in the bottom environment and the photic zone, and also evaluate the influence of the adjacent land areas, based on sedi- mentology, geochemistry, sporomorphs and palynofacies composition, benthic (foraminifera, gastropods, bivalves, scaphopods, echinoderms), planktonic (calcareous nannoplankton, dinoflagellate cysts), and nektonic (sharks) fossils. The Gnaszyn succession originated relatively close to the shore, within reach of an intense supply of terrestrial fine clas- tic and organic particles. The latter are mainly of terrestrial origin and range from 1.5 to 2.5 wt.%. The precise water depth is difficult to estimate but most likely ranges from several tens of metres to a few hundred metres. All fossil groups show minor changes throughout the succession. As the climate seems to have been quite stable during this period we consider sea-level fluctuations to have been the main factor responsible for the changes. The terrestrial input, includ- ing freshwater and land-derived clastic and organic particles (sporomorphs and cuticles), increased during periods of sea-level lowstand. As a consequence, stress conditions (lower salinity, higher nutrient availability, lower water trans- parency) in the photic zone caused blooms of opportunistic planktonic taxa. Furthermore, a faster sedimentation rate led to oxygen depletion and deterioration of the living conditions in the bottom environment due to an increased accumulation of organic matter. As a result, the benthic biota became taxonomically impoverished and commonly dominated by juvenile forms. During periods of high sea level, the source areas were shifted away from the basin, resulting in a decrease in the terrestrial influx, increase in the salinity of surface waters, the appearance of more diverse phytoplank- ton assemblages, a lower sedimentation rate, and an improvement of living conditions at the bottom.