Dropstones, pseudoplanktonic forms and deep-water decapod crustaceans within a Lutetian condensed succession of Central Istria (Croatia): relation to paleoenvironmental evolution and paleogeography.

In central Istria (western Croatia) a middle Eocene unit, some tens of metres thick, marks the transition from shallow-water carbonate to deeper-water clastic sedimentation. This unit consists of a basal interval, rich in macrofossils, made of clayey fossiliferous wackestones and packstones (known as Marls with crabs) and of an overlying marly interval (Globigerina Marls). Integrated paleontological, sedimentological, and petrographical analyses of three key sections, supplemented with data from several other sites, reveal the history of the depositional environment and the changes that occurred up-section in the carbonate deposition and the faunal assemblages. Palaeobathymetric reconstruction, based on the benthic and planktonic foraminiferal assemblages, shows a rapid deepening-upward from the base (palaeodepth is about 200 m) to the top (about 1000 m) of the unit. The base of the unit represents a condensed section that contains abundant macrofossils consisting of autochthonous (mainly decapod crustaceans and deep-water mollusks) and allochthonous (macroforaminifers and shallow-water mollusks) benthic, as well as pelagic (nautiloids and nektonic remains) and pseudoplanktonic (teredinids and ostreids) forms. The abundance of decapod crustaceans, probably represented by only one species that lived in deep-water, can be correlated with the substantial arrival of plant organic detritus at the bottom. The further deepening of the environment caused the disappearance of most of the benthic autochthonous forms. Dropstones, wood-boring bivalves and other pseudoplanktonic forms, and vegetation remains are mainly found within and just above the basal fossiliferous horizon. The abundance of vegetation remains and teredinids reflects the input of wood into deep-marine environments and condensation associated with sediment starvation, both related to a phase of sea-level rise. Dropstones were transported by vegetation rafting and, probably, at a lesser rate, by kelp rafting. Petrologic studies of the dropstones provide information on the source areas (deltaic areas of the Collio region, NE Italy) and on the little known palaeogeography of the Southern Alps and Outer Dinarides during the Lutetian tectonic phase of regional relevance.