Change from rimmed to ramp platform forced by regional and global events in the Cretaceous of the Friuli-Adriatic Platform (Southern Alps, Italy)

Abstract

Carbonate platforms are sedimentary archives recording the evolution of the global carbon cycle. Their stratigraphic architecture depends on the regional tectonics, controlling subsidence rates and geometries, as well as the paleoceanography and evolutionary trends, controlling the different organisms thriving at their margins, such as frame-building corals or mound-building microbes. We present an integrated bio- and chemostratigraphic study of the Aptian to Santonian interval of a base-of-slope section located in the Southern Alps of northeastern Italy that we correlate with the classic section representing the Friuli-Adriatic Carbonate Platform, one of the largest isolated platforms of the low latitude Tethys. We show the effects of the end of the passive-margin stage and the interaction between foreland flexuring due to the growing Alps, to which the study area represented the retroforeland, and the approaching prowedge of the Dinarides. The Friuli-Adriatic platform margin shows an abrupt change from reef rimmed to ramp, where abundant microbial mounds provided the habitat for the rudists to thrive. This change occurred around the late Albian and likely correlates with the Oceanic Anoxic Event (OAE) 1d. The previous OAE's did not change the structure of this platform, whose margins were mostly rigid and colonized by corals and calcareous sponges. Late Albian was a time of important changes in paleoceanography in Tethys and North Atlantic Oceans. We propose that the paleooceanographic changes related to the OAE- 1d had more profound impacts on the Friuli-Adriatic Platform than the previous Cretaceous OAE's since they co-occurred with the tectonic transition from passive margin to foreland ramp. The increased subsidence rates, in conjuction with the important late Albian paleoceanographic changes, created favourable conditions for a dramatic change in the platform margin physiography and ecology.