New palaeoenvironmental model for the Komen (Slovenia) Cenomanian (Upper Cretaceous) fossil lagerstنtte

Three sections which crop out near Tomačevica (2 km east of Komen), Slovenia, were measured and described. Thirty-seven rock samples were collected for thin section analysis. A database on all the fossils from the Komen area was compiled from collections of the Museo Civico di Storia Naturale in Trieste, Italy, the Museo Geologico Paleontologico G. Capellini in Bologna, Italy, and the Slovenian Museum of Natural History in Ljubljana, Slovenia. The data collected on the stratigraphy, sedimentology and palaeontology of the Komen platy limestones were used to build a new depositional model for the area. This new hypothesis is proposed in place of the “lagoonal model”, too often invoked in the past in order to explain the occurrence of dark platy limestones occurring worldwide in platform settings. Given the absence of any evidence in support of a true “lagoonal system” in the examined area (e.g. channel fills, herring bone cross stratification, sand bars or bioherms delimiting the area of interest) the latter has been examined in search of an alternative sedimentological model. New data regarding facies succession and faunal composition have been gathered. The typical facies succession of the Komen Limestone comprises: (a) flat pebble breccias that grade up-section into packstones and wackestones containing progressively rarer, and smaller, flat micritic intraclasts; (b) well bedded to platy limestones; and (c) stromatolitic limestones. All of these lithologies contain scattered chert nodules and sheets. The available evidence suggests that the depositional environment of the Komen Limestone was in an intraplatform basin, in the proximity of an exposed area (tidal flat plus probably other supratidal environments populated by conifers and where karstification could occur). The bottom water conditions were periodically dysoxic to anoxic, and the stacking of different facies was due to a combination of small scale sea-level fluctuations and local subsidence of the area.