Is the Lacustrine to Marine Transition Paradigm an Obligatory Trajectory in Carbonate Rift Systems? The Case History from the Oligo-Miocene Sardinia Rift Basin (Italy)

Due to the intense tectonic phases that affected the Iberian-European Region about 34 Ma, extensional rift-related basins were formed as the Corsica-Sardinia microplate rifted and rotated from Eurasia. The Oligo-Miocene sub-basins of the Sardinia Rift were loci for the deposition of continental sequences laid down on top of the fractured crysralline Palaeozoic basement, and locally on its Mesozoic-Eocene sedimentary cover. In many areas the continental Oligo-Miocene sequences are capped by full marine limestone lacking evidence for gradual vertical transition. Detailed analyses of facies and the 3D sedimentary architecture of the Miocene carbonates suggest a younger, transgressive cycle deposited during a later stage of rift evolution.

The carbonate deposits of the Sardinia Rift Basin, were deposited in small tectonically-driven sub-basins. They consist of up to a few hundred metres of red algae-dominated bioclastic limestone, which directly overlie the basement, or follow thin and discontinuous siliciclastic continental-transitional deposits. These latter acted as a passive substratum for pioneer rhodalgal/bryomol assemblages (mainly bryozoans, red algae, filter-feeding molluscs and benthic forams) that only locally evolved into coral-bearing benthic communities. Abundant bryozoans remains in the basal sequence and the following sciaphile bryo-rhodalgal communities strongly suggest rapid sea-level rise and the early development of dimly lit, relatively deep marine settings. Later on, and only locally, carbonate production was able to keep up with the tectonic subsidence thus resulting in limited open carbonate factories on the higher portion of tilted fault-blocks. This vertical sequence does not fit with classic models for carbonate sedimentation in rift-basins which commonly document vertical trends characterized by gradual transition from lacustrine to paralic and then to open marine limestone.

From the production areas, loose skeletal debris were periodically removed and re-sedimented in deeper areas via a complex network of submarine channels. Resedimentation built submarine lobes and minor fan-systems in the deeper basin areas. As a consequence, a large portion of the rift-related limestone in the study area, pertains to channelized slope- and fan-systems which directly overlie continental, transitional and occasionally shallow marine siliciclastic deposits laid down during the early phases of rifting.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California