Coeval genesis of pillow lava on the sea floor and under a thin cover of unlithified sediments (and associated formation of peperites)

Abstract

The Guardia Marina beach (Sardinia island, Western Mediterranean, Italy) contains outcrops of subalkaline basaltic pillow lava within a Miocene sedimentary sequence of shallow marine facies. The northern sector of the beach is characterized by the presence of feeder dikes terminating at their upper ends in antigravitative toothpaste-like massive pillows and lateral expansions of highly vesiculated and cupola-like hollow pillows. The central sector of the beach shows partially eroded pillows and pillowed dikes, as well as sandwiched layers of sediments between laterally expanded pillows. The southern sector of the beach contains well-developed pillows (intrusive with respect to the sedimentary sequence) with a clear development of peperitic lithofacies at the magma-sediment interface. The lateral continuity of the sedimentary beds suggests a coeval growth of pillows at the magma-water and magma-sediment interfaces, as well as a recurrent process of pillow-growth from feeder dikes. The multiple-rind structure in the pillows in the southern sector of the beach confirms the very shallow marine environment inferred from fossil fauna and sedimentary lithofacies. The early erosion of the pillows in the central and northern sector of the beach accounts for the proximal character of crystal-rich epiclastic layers within the calcarenitic sequence. The Guardia Marina outcrops show that pillow lava can correspond both to a subaqueous environment and to a growth of pillows under a thin layer of poorly lithified sediments. The generally accepted concept that pillow lava indicates a subaqueous environment must therefore be tested through an accurate study of the pillow-sediment interface.