Ichnological features of a marine transgression: middle Miocene rocky shores of Tarragona, Spain

Bioerosion traces from six rocky shore localities in the Miocene Tarragona basin are described. The bioerosion affects Jurassic and Miocene carbonate substrates. The ichnotaxa include borings produced by bivalves (Gastrochaenolites and Phryxichnus), sponges (Entobia), annelids (Maeandropolydora) and sipunculids (Trypanites). Phryxichnus is recorded for the first time in the Miocene. Trace fossil assemblages, although slightly different among localities, can be tipified as a Gastrochaenolites-Entobia assemblage which is typical of Neogene rocky shores and belongs to the Entobia Ichnofacies which characterizes littoral rockground environments since the Jurassic. Borings affect both the rocky substrate and the associated conglomeratic deposits. The assemblages found on wave-cut platforms are composite assemblages resulting from the overprinting of successive ichnocenoses that replace each other during a transgression. The assemblages found in the conglomerates represent less mature communities due to clast instability produced by physical disturbance characteristic of a littoral environment. The rocky shores described in this paper were originated during a single transgressive event that took place during the Langhian-Serravalian (Middle Miocene) and the surface that can be drawn correlating the sites constitutes the base of the Tarragona Depositional Sequence.