CRETACEOUS STRATIGRAPHY OF THE IONIAN ZONE, HELLENIDES, WESTERN GREECE

The Cretaceous sedimentary successions of the Ionian Zone, Hellenides, western Greece, are composed of pelagic limestones intercalated with cherty layers. The micritic and biomicritic beds with abundant chert n odules and cherry horizons, which were deposited during late Tithonian to early Santonian times, belong to the Vigla Limestone Formation, wh ile the sediments deposited during the late Santonian to Maastrichtian , formed elastic limestone beds in which chert nodules also occur spar sely. In the Cretaceous beds calpionellids, planktonic and benthonic f oraminifera characteristic of the Tethyan realm, and radiolaria have b een recorded. The calpionellids, together with radiolaria, colonized t he entire basin during the Berriasian to early Valanginian, the latter becoming dominant during the Hauterivian to early Albian as a result of anoxia. Planktonic foraminifera first appeared in the basin during the late Albian and persisted until the Maastrichtian. The numbers dec reased, however, during the Cenomanian-early Turonian interval, when r adiolaria increased owing to anoxic conditions, and during the Campani an-Maastrichtian interval because the basin became shallow. During thi s interval larger benthonic foraminifera colonized the basin. Zonal ma rkers have been recognized in calpionellid and planktonic foraminifera l assemblages on the basis of which two calpionellid zones are disting uished, viz. the Calpionella alpina and Calpionellopsis Zones (Berrias ian-early Valanginian) along with seven planktonic foraminiferal zones , viz. the Rotalipora ticinensis, Rotalipora appenninica (late Albian) , Rotalipora brotzeni (early Cenomanian), Helvetoglobotruncana helveti ca (early to middle Turonian), Marginotruncana sigali (late Turonian t o early Coniacian), Dicarinella concavata (late Coniacian to early San tonian) and Dicarinella asymetrica (late early-late Santonian) Zones. The anoxic conditions that prevailed in the Ionian basin during the Ba rremian-early Albian, Cenomanian-early Turonian and Coniacian-Santonia n intervals probably arose as a result of (a) the accumulation of larg e amounts of organic matter because the palaeotopography of the basin periodically hindered the circulation of water from the ocean and (b) the oxygen content of the intruding oceanic waters was low. (C) 1995 A cademic Press Limited.