Bioturbation structures of the Kropivnik Fucoid Marls (Campanian-lower Maastrichtian) of the Huwniki-Rybotycze area (Polish Carpathians)

The Kropivnik Fucoid Marls of the Polish Outer Carpathians contain numerous bioturbation structures, and comprise thin-bedded sandstones, hard and soft marlstones and muddy to clayey, mainly turbiditic shales. Comparison of three sections elucidated the stratigraphic and sedimentological controls on the distribution of the bioturbation structures, which are most common on the soles of sandstone beds, and within and on the tops of beds of hard marlstone overlain by shale. Most of the bioturbation structures are taxonomically undeterminable. Nevertheless, twenty seven ichnospecies, including one new ichnospecies (Taenidium recurvum), and seven forms of trace fossils of unknown taxonomic affinity were recognised. Most ichnotaxa were observed as individual specimens. Irregular structures apparently formed by the squashing of burrows filled with fluidised fill, called here turbidichnia, are quite common in some marlstone beds in the lower part of the succession. Planolites beverleyensis and Halopoa imbricata are the most frequent trace fossils on the lower surfaces of sandstone beds. Chondrites intricatus, Planolites beverleyensis and Thalassinoides suevicus represent the most common taxa in hard marlstones. Endoreliefs of Scolicia isp. are quite frequent in the top parts of marlstone beds in some parts of the succession. Other ichnotaxa were found either rarely or as single specimens only. The burrowing depth, reaching 15 cm below the tops of normally-graded rhythmic units, together with the relatively rich trace fossil assemblage and common occurrence of Planolites and Thalassinoides, is interpreted as indicating fully oxic conditions at the bottom of the sedimentary basin.