Sequence stratigraphy and Paleogene tectonic evolution of the Transylvanian Basin (Romania, eastern Europe)

The Transylvanian Basin of Romania belongs to the 800 x 400 km wide Pannonian domain of the European Alpine megasuture bordered to the east and the north by the Carpathians. It represents a digitation of the epicontinental Tethyan seaways locally connected during the Palaeogene to the peripheral foredeep troughs. During that time, it was filled up by a 500-m-thick sediment pile organized into three shallow marine and non-marine facies alternations. Each evolved from alluvial fans to restricted marine and outer marine environments. They are dated from Lutetian to Chattian times. The study is focused on the lowermost alternation onlapping the basal, post-Maastrichtian unconformity. This alternation consists of the superimposition of a thick retrogradational and a thin progradational depositional system. The retrogradational depositional system grades upwards from stacked, fault-controlled deposits of alluvial fan, ephemeral stream, salina and sabkha, and restricted marine bioclastic shales. The progradational depositional system is composed of outer marine to estuarine sandstones and shales. The two depositional systems are bounded at their tops by two baselevel change unconformities underlain by highly mobile, low relief sandstone bodies that were deposited in shoal belts. These two unconformities mark significant changes in the regime of the subsidence. These are, respectively, a baselevel rise or 'drowning' unconformity where the shoal deposits were associated with oolitic ironstones and glauconitic shales that typify basin starvation during a period of maximum basin drowning, and a baselevel fall or 'uplifting' unconformity where the bioclastic shoal deposits were buried by alluvial flood plain deposits that characterize periods of relief rejuvenation tentatively attributed to compressive events. This bimodal succession is interpreted in terms of underfilled-overfilled stages related to intraplate tectonic deformation. The underfilled stage corresponds to the fault-controlled drowning of the basin and the overfilled stage to the increasing flexural rigidity of the substrate culminating in differential uplift in the area. Three of these successions comprise the Palaeogene sedimentary record of the Transylvanian Basin. They attest to a large-scale pulsating evolution of this continental microplate during its northward migration towards the European plate.