Detachments and mass transports along low-angle slopes: the onset of the Tithonian carbonate ramp progradation along the Eastern Sardinia Jurassic passive margin (Italy)

Carbonate megabreccias associated with submarine unconformities are documented on several passive margin slopes and, recently, they have been recognized on carbonate platform marginal slopes. Their depositional architecture and regional correlation are poorly described in the geological record, due to the lack of well-exposed outcrops. The geodynamic and stratigraphic evolution of the Tithonian (Late Jurassic) Southern European passive margin was characterized by frequent mass transport events with the deposition of carbonate megabreccias and the development of large-scale submarine unconformities with different genetic interpretations. This study investigates the depositional facies and stratigraphic architecture of upper Tithonian carbonate megabreccias, which were deposited on top of a laterally continuous, submarine erosional unconformity. This unconformity overlies outer ramp calci-mudstones (Pedra Longa Fm., up to 30 m thick), accumulated in a protected intraplatform basin (Baunei Basin) following an early Tithonian transgression. These calci-mudstones are partly coeval and separate two progradational eastward gently dipping carbonate ramp systems (Mt. Tului and Mt. Bardia Fms). The studied megabreccias are likely associated with a regressive trend marked by the sharp contact between the thin-bedded Pedra Longa strata and massive shallow-water coarse-grained Mt. Bardia Fm. carbonates. The erosional unconformity is documented over an area exceeding 6.5 x 2.5 km and it is characterized by a gentle incision in the most proximal part of the basin, while basinward (eastward) it comprises narrow (3-6 m wide) and 2-4 m deep, erosional incisions filled by thick chaotic megabreccia lenses (up to 20 m thick). The macro and microfacies of these breccias include both clasts and matrix deriving from prevalent shallow-water inner ramp, lagoonal oo-bioclastic grainstones/packstones and Pedra Longa Fm. intraformational calci-mudstones. Minor erosional events, with prevalent intraformational coarse to fine grained breccia lenses and slumping, are locally present in the lower Pedra Longa Fm... Locally, the amalgamation of the upper unconformity (main stratigraphic event) with the underlying local erosional surfaces can remove almost all the Pedra Longa succession (thickness reduced to a few metres). In these locations, the submarine erosions and the top unconformity present more differentiated features: strata-bound erosional surfaces, soft sediment deformations, intra-formational breccia pockets or a sharp boundary without mass transport events. The possible mechanisms for the megabreccia emplacement include: 1) catastrophic mass transport due to liquefaction processes, failures of shallow-water carbonate sands, along an intraplatform gentle slope, during fast carbonate progradation (sea level lowering associated to tsunami events, and/or tectonic block tilting along the Baunei Basin); 2) polyphasic amalgamated mass transport events related to the fast basinward progradation of proximal ramp carbonates with frequent gravitational failures of marginal oo-bioclastic shoals, and lithified patch reef boulders above still poorly consolidated marly calci-mudstones. Regardless the dominant processes, the lateral continuity of the unconformity, the deposition of megabreccias and their landward correlation with sedimentary dikes and polygenic carbonate megabreccias lenses, cropping outside the Baunei Basin, might be consistent with a syn-sedimentary tectonic control.