Flat-pebble conglomerates: a local marker for Early Jurassic seismicity related to syn-rift tectonics in the Sesimbra area (Lusitanian Basin, Portugal)

Flat-pebble conglomerates have been identified in the Lower Toarcian (Levisoni Zone) carbonates of the Sesimbra region (30 km south of Lisboa, Portugal) and related to submarine mass movements. Their origin is explained through a three-stage model based on the comparative analysis of potential generating mechanisms taking into account timing and type of geodynamic evolution in the Lusitanian Basin: (a) differential lithification of thin carbonate and non-bioturbated horizons embedded within a more argillaceous matrix; (b) disruption by seismic shocks related to active extensional faulting and block tilting; and (c) gravity sliding mixing material resulting from broken lithified horizons. This sequential process originated flat-pebble conglomerates during early Jurassic phases of syn-rift evolution in the southern Lusitanian Basin.