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.