Paleo-strait Clastic Carbonate Sandbodies, Bonifacio Basin, Corsica
Excellent coastal exposures (5 km long) of the Miocene mixed carbonate-siliciclastic succession of the Bonifacio Basin (South Corsica) allow correlation of stratigraphic surfaces and evaluation of lateral variability in a paleo-strait/paleo-gulf setting. Coastal cliff photo-mosaic panels were interpreted, measured sections digitized and correlated, and over 1000 paleocurrents measured. The succession consists of three depositional sequences. During low sea level, siliciclastic systems prevailed, fed by gilbert-type deltas; during relative sea level rise, fringing reefs developed, passing basinwards to bioclastic facies evolving upward and basinward from bryomol to molechfor and rhodalgal associations.
Cross-bed dip and geometric analyses suggest forward accretion of the sediment bodies. Cross-bed sets preferentially developed during late Lowstand (LST) and Transgressive Systems Tracts (TST) as unidirectional dune fields. Late LST cross-bed sets display higher siliciclastic content, wedge-shaped geometry and thickening- thinning-upward trends. TST cross strata are bioclastic and alternate with upper flow regime plane-parallel beds, owing to current strength fluctuations or to the migration of transition slope. Unidirectional currents are interpreted to have been induced by relative sea level oscillations able to control hydraulic cross section of the basin and current strength, accomplished on an irregular topography: the basin morphology, widening to SW and narrowing to NE, suggests an evolution from gulf (closed to the North during low sea level stands) to strait (connecting the Liguro-Provençal and Thyrrhenian Basins during TST), when currents would have been accelerated. Currents were generally unidirectional, possibly due to the forcing of dominant winds. Strongly cyclic late LST deposits sometimes show lateral accretion, probably an indication of a tidal origin.
The Bonifacio Basin is a useful outcrop analog for reservoirs of palaeostrait/palaeogulf origin. The stratigraphic relationships between sandstone bodies, marginal reefs and bioclastic facies produce a strong reservoir heterogeneity; there is also remarkable variability of geometry and orientation of high-porosity cross strata.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California