Recognition of massive Upper Cretaceous carbonate bodies as olistoliths using rudist bivalves as internal bedding indicators (Campanian Merfeg Formation, Central Tunisia)

The Merfeg Formation (upper Campanian) of Central Tunisia crops out around the southwestern periclinal termination of Jebel el Kébar, near Sidi Bouzid. At its base is a massively bedded unit of locally dolomitized, sparsely fossiliferous micritic to microbioclastic limestone that contains several discrete, plurimetric mound-like bodies (lithosomes) of micritic limestone containing locally abundant rudists and corals. The lithosomes are separated laterally from one another by megabreccias and conglomerates containing clasts of similar lithology and are overlain, with sharp contact, by onlapping argillaceous pelagic limestones, within which are intercalated at least two more, somewhat thinner rudist/coral limestone units. This complex of facies is laterally equivalent to thicker, deep platform limestones of the Abiod Formation to the north and east, and to restricted carbonate platform facies of the Berda Formation to the south and west. The lithosomes have previously been interpreted as in situ downslope mudmounds that became capped by rudist and coral formations, cemented, and then surrounded by erosively emplaced debris flows. However, our detailed studies of rudist orientations imply variable and in some cases relatively high angles of bedding within the lithosomes with respect to the regional dip of the host strata. Such steep inclinations of internal bedding are unlikely to have been primary. Accordingly, we propose an alternative interpretation that the lithosomes were platform-derived olistoliths, emplaced along with the associated debris flow deposits. Micritic beds, neighbouring the olistoliths are of variable thickness and contain rare large inoceramids and randomly oriented rudists, as well as locally developed microbioclastic beds with planar and small-scale swaley cross stratification. These micritic and microbioclastic beds are, by contrast, interpreted as primary (i.e., non-olistostromal) slope deposits. Whether the proposed catastrophic collapses of the original platform margin were induced by sea-level fall or seismically triggered (or a combination of the two) remains uncertain

Cretaceous Continental Bridges, Insularity, and Vicariance in the Southern Hemisphere: Which Route Did Dinosaurs Take?

The history of biota distribution in the southern hemisphere represents a fascinating and strongly debated argument. The sequential break-up of Gondwana was indeed responsible for physical barriers that precluded dispersal event, and therefore phylogenies? Did different organism had the opportunity for dispersal episodes through recently claimed continental bridges? When geographic and environmental conditions were suitable for active diffusion during the Mesozoic? Among the most relevant taxa that inhabitant the southern landmasses in the Cretaceous period are dinosaurs. Their vast stratigraphic and geographic occurrence is intimately linked to the evolution of Gondwana and recent discoveries from all southern landmasses greatly challenged several vicariant models. In this study are summarised the most significant geological, palaeogeographic, palaeontological and phylogenetic data on Cretaceous Gondwanan dinosaur evolution, with particular emphasis on the ephemeral land bridges that constitute the bulk of the recently developed biogeographic models. A comparison between different datasets suggest that a complex and sequential mix of vicariance and dispersal patterns characterized the distribution of dinosaurian faunas in the Cretaceous. In particular, this study dispute the role of ephemeral intercontinental connections for biota dispersal in the Late Cretaceous and support an earlier peak in dinosaur distribution