Abstracts Earth and Environmental Sciences

Stratigraphy of Neogene deposits in the Khania province, Crete, with special reference to foraminifera of the family Planorbulinidae and the genus Heterostegina

by T. Freudenthal

In this paper the stratigraphy of the Neogene deposits in the Khania Province, Crete, Greece, is described. Special attention is paid to the evolution and taxonomy of foraminiferal genera assigned previously to the family Planorbulinidae. This partial revision of the Planorbulinidae is based not only on our Cretan material, but also on samples from all over the world, in age ranging from Late Cretaceous to Recent. It is shown that at least part of the genera assigned to the Planorbulinidae probably have no relationship with Planorbulina, the nominal genus. The following taxa are discussed: Hellenocyclina REICHEL (Upper Cretaceous), with the speCIes H. beotica REICHEL, H. visserae (HOFKER) and H. charentensis n.sp.; Linderina SCHLUMBERGER(Middle Eocene, Upper Eocene), with the species L. brugesi SCHLUMBERGER,L. buranensis NUTTALL & BRIGHTON (= L. bihilensis SILVESTRI = L. nuttalli SILVESTRI), L. rajasthanensis SINGH (= L. kirtharensis SINGH = L. bikanerensis SINGH = L. kolayatensis SINGH), L. paronai OSIMO (? = L. floridensis COLE); Planolinderina n.gen. (Upper Oligocene - Lower Miocene), with the species P. escornebovensis n.sp., P. plana (HERON-ALLEN & EARLAND) and P. inaequilateralis (HERON-ALLEN & EARLAND); Planorbulinella CUSHMAN (Lower? Miocene - Recent), with the species P. trinitatensis (NUTTALL), P. zelandica FINLAY, P. larvata (PARKER & JONES), P. rokae n.sp., P. astriki n.sp., and P. caneae n.sp. The H ellenocyclina species probably belong to a single evolutionary lineage. The data on the age of our Linderina populations are rather controversial if a single lineage is assumed. The species placed in Planolinderina probably belong to two, more or less synchronously evolving, lineages. In the case of Planorbulinella it is assumed that there are three lineages, two of them being more or less parallel. All of these lineages developed according to the principle of nepionic acceleration. Their evolution is shown on the basis of measurements and counts on the early chambers. The two most important parameters are the length of the early spiral and the size of the initial chambers. However, both parameters show a negative correlation, and as a consequence - if this correlation is very strong - random influences on the size of the early chambers will cause changes in the length of the spiral as well. Such influences may cause nepionic acceleration to become less distinct and less useful for stratigraphic correlation. This becomes most apparent in Planolinderina. In Planorbulinella and Hellenocyclina nepionic development is furthermore complicated by the presence of recurrences of the one-aperture stage after orbitoidal growth had started. The results of these studies indicate that the family Planorbulinidae, as generally accepted nowadays, may comprise several completely unrelated genera. For the present it seems more correct to place each genus mentioned in a separate family. Hence, the following new families are proposed: Hellenocyclinidae, Linderinidae, Planolinderinidae,…