TECTONOSEDIMENTARY DEVELOPMENT OF AN EXTENSIONAL BASIN - THE NEOGENE MEGARA BASIN, GREECE

The ?Miocene/Pliocene to Pleistocene succession exposed in the Megara Basin comprises a grossly coarsening-upwards sequence at least 1 km thick. The oldest strata, exposed in the NW, record the establishment of alluvial fans and axial streams. These drained into a lacustrine environment subject to minor wave and, possibly, tidal action. The lower part of the central basin fill is comprised of marls, siltstones, sandstones and conglomerates. They record the pulsed infill of the predominantly freshwater 'Lake Megara' by progradation of a network of shallow multichannel streams, which drained a western hinterland containing ultrabasic rocks, cherts and limestones. Frequent examples of large-scale soft sediment deformation structures and stratigraphic thickening into hangingwall roll-overs indicate active fault control of sedimentation. The lake/river interface featured Gilbert deltas, which were subject to periodic transgression by ?climatically-controlled lake expansions. The upper part of the ?Pliocene basin fill largely comprises fluviatile gravels, but with two important marine transgressional horizons and renewed calcareous marl deposition, including a basinwide tufa horizon, towards the top. The overlying ?Pleistocene alluvial clastics were deposited by a NW-flowing system and thicken markedly into a major basin-bounding fault. Deposition of these fluvial deposits was interrupted towards the top of the sequence by the rapid deposition of monomict limestone breccias of debris flow origins, which are believed to mark a catastrophic event. At some time in.the late Pleistocene (c. 1 Ma), the basin bounding faults became inactive and the entire basin fill was inverted in response to uplift in the footwall of the active major fault strand that bounds the SE margin of the Alkyonides Gulf.