The transition from the carbonate to the flysch facies in the Arachova sequence of the Parnassus-Ghiona Zone is represented by argillaceous limestone beds with flaser structures deposited during latest Maastrichtian-Palaeocene time in a pelagic carbonate environment with a periodic clastic influx. Deposition was continuous except for a short interruption during the K/T boundary interval and the earliest Palaeocene when the area was subaerially exposed. This interruption gave rise to the development of a brecciated carbonate horizon through soil-forming processes. The mineralogical composition of the clastic influx (i.e. quartz, feldspars, clay minerals, amorphous iron oxides, amorphous phosphatic compounds), in particular the clay mineral assemblages (i.e. illite, chlorite, irregularly interstratified illite-vermiculite), shows that the clastic supply represents erosional material that originated from a tectonically active continental setting of both carbonate and clastic rocks, presumably the Pelagonian Zone, as for the flysch of the Beotian and Sub-Pelagonian Zones. The arrival of the first clastic material in the Arachova area as early as latest Maastrichtian time, its Pelagonian origin and the persistence of pelagic conditions of sedimentation throughout the Palaeocene, indicate that the Arachova area was situated along the northeastern margin of the Parnassus platform and that it subsided into the Beotian basin. While the central areas of the platform remained tectonically stable during middle Palaeocene times and there was an extensive development of stromatolites, the northeastern marginal areas transitional to the Beotian basin continued to subside allowing pelagic carbonate sedimentation with periods of clastic influx. The total collapse of the platform in the late Palaeocene gave rise to the deposition of the flysch over the entire zone.