COEVAL GENESIS OF PILLOW LAVA ON THE SEA-FLOOR AND UNDER A THIN COVEROF UNLITHIFIED SEDIMENTS (AND ASSOCIATED FORMATION OF PEPERITES)

The Guardia Marina beach (Sardinia island, Western Mediterranean, Ital y) contains outcrops of subalkaline basaltic pillow lava within a Mioc ene sedimentary sequence of shallow marine facies. The northern sector of the beach is characterized by the presence of feeder dikes termina ting at their upper ends in antigravitative toothpaste-like massive pi llows and lateral expansions of highly vesiculated and cupola-like hol low pillows. The central sector of the beach shows partially eroded pi llows and pillowed dikes, as well as sandwiched layers of sediments be tween laterally expanded pillows. The southern sector of the beach con tains well-developed pillows (intrusive with respect to the sedimentar y sequence) with a clear development of peperitic lithofacies at the m agma-sediment interface. The lateral continuity of the sedimentary bed s suggests a coeval growth of pillows at the magma-water and magma-sed iment interfaces, as well as a recurrent process of pillow-growth from feeder dikes. The multiple-rind structure in the pillows in the south ern sector of the beach confirms the very shallow marine environment i nferred from fossil fauna and sedimentary lithofacies. The early erosi on of the pillows in the central and northern sector of the beach acco unts for the proximal character of crystal-rich epiclastic layers with in the calcarenitic sequence. The Guardia Marina outcrops show that pi llow lava can correspond both to a subaqueous environment and to a gro wth of pillows under a thin layer of poorly lithified sediments. The g enerally accepted concept that pillow lava indicates a subaqueous envi ronment must therefore be tested through an accurate study of the pill ow-sediment interface.