An Upper Jurassic Oxfordian and lower Kimmeridgian reef complex is well exposed in the area of Slovenia northwestern Yugoslavia This reef complex is thought to be a barrier reef that developed along the shelf margin of an ancient carbonate platform From basin to lagoon the following subdivisions have been delineated a fore reef area characterized by carbonate breccias and blocks of reef debris a central reef area with abundant hydro zoans and corals that can be further subdivided into actinostromariid and parastromatoporid zones and a back reef area with locally developed lagoons and patch reefs defined as the Cladocoropsis zone EXTENT OF REEF COMFLEX In the province of Slovenia coral hydrozoan reefs of Middle Devonian Upper Carboniferous Upper Triassic Upper Jurassic Lower and Up per Cretaceous and Oligocene ages are well ex posed Of these the Upper Jurassic barrier reef complex has the largest geographical extent This barrier reef complex can be traced in a more or less continuous series of outcrops from the Soca Valley across the central portion of Slovenia to Metlika into neighboring Croatia and then along the entire Yugoslav Dinaric region into Albania The reef complex also extends westward from the Seca Valley into Italy In Slovenia the reef com plex extends for approximately 140 kilometers in length is about 20 kilometers in width and ranges in thickness from 200 to 600 meters Fig I Pos tulated restored length of the reef trend possibly amounted to at least several hundred kilometers a distance comparable to that of the present Aus