Messinian carbonate and alluvial fan sedimentation in Alonnisos Island, Greece: sedimentary response to basement controls, inversion tectonics and climatic fluctuations

A marginal marine carbonate environment, giving away to an alluvial one, wa s established during Messinian time on Alonnisos Island, the footwall uplan d of the Southern Marginal Fault of the Sporades Basin (SMFS). Analysis of the evolving depositional systems, with emphasis on their sedimentation pro cesses, faulting patterns and palaeopedological factors, has permitted an i nterpretation of the simultaneous controls of tectonism and climate. The carbonate sediments were deposited in a shallow marine environment form ed along a faulted continental margin under warm and semi-arid climatic con ditions. Faulting consisted of NE-trending dextral reverse faults and NW-tr ending strike slip faults, produced by WNW-directed compression. The baseme nt structural elements affected the spatial distribution of the offshore an d shoreface facies, whereas fifth-order cycles of sea-level change were res ponsible for the development of metre-scale, shallowing-up cycles. The compressional structures were subsequently reactivated by NNE extension . This tectonic inversion, together with a global sea-level fall, triggered alluvial fan sedimentation. Fan sedimentation was disrupted by long period s of non-deposition and soil formation under warm climatic conditions. Thre e distinct units are recognized in the fan: a lower unit consisting of clas t-poor debris flows, attributed to semi-arid-humid periods; an intermediate unit of clast-rich sheetfloods and channel flows, deposited during arid pe riods; and an upper unit consisting of matrix-rich sheetfloods related to a return to semi-arid-humid conditions. We interpret that the water-flow pro cesses responsible for deposition were most prevalent on fans of arid and s emi-arid climates, whereas debris-flow processes were more typical of clima tes with higher rainfall. As the extension proceeded during the Plio-Quaternary time, the main tecton ic activity of the Sporades Basin was taken up by the SMFS causing signific ant footwall uplift. Due to this process, Alonnisos Island was elevated abo ve the Pliocene highstand and became an area starved of Quaternary sediment ation. (C) 1998 John Wiley & Sons, Ltd.