Stratigraphy of Neogene deposits in the Rethymnon Province, Crete, with special reference to the phylogeny of uniserial Uvigerina from the Mediterranean region

Abstract

Stratigraphical and micropaleontological investigations were performed on Neogene deposits of the Rethymnon Province, Western Crete. The Neogene succession includes marine as well as brackish and fresh-water sediments. Eight formations are recognized, several of which have to be regarded as lateral equivalents. From the sediment types and their distribution along the north coast it is concluded that to-day's topography still reflects the paleogeographic conditions existing during the Neogene. The micropaleontological part of the paper deals with the phylogeny of two groups of Uvigerinids, the U. melitensis group and the U. cretensis group, both showing a main trend to uniserial chamber arrangement. Apart from our Cretan material samples are studied from localities elsewhere in the Mediterranean region. These samples are derived from Italy, Spain, France and Malta, and are partly taken from the stratotypes of the Late Neogene stages. The representatives of the U. melitensis and the U. cretensis groups can be distinguished by external characters such as the presence or absence of a distinct, regular, biserial stage in between the triserial and uniserial parts of the test, the more or less recurved character of the sutures, and the regular or irregular arrangement of the early uniserial chambers. The U. cretensis group corresponds to the U. gaudryinoides group in the literature, whose phylogeny was first emphasized by PAPP(1963). Representatives of this group are common in younger Neogene deposits throughout the Mediterranean region. So far the U. melitensis group is restricted to the Maltese Neogene and to the basal part of the Cretan Neogene succession. The evolutionary trend is the same in both lineages and comprises among other things an increase in the average number of uniserial chambers, an increase in the relative length of the uniserial part of the test, and the development toward an arrangement of the uniserial chambers in more regular series. Besides, a general trend is observed to relatively more slender tests. No development is found in the length or in the breadth of the test. The striate ornamentation appears to be strongly environment controlled; specimens from deposits close to evaporites show an aberrant ornamentation. Since a typological approach of the observed features proves to give unsatisfactory results, counts and measurements are made on a number of parameters. The latter are based on the number of uniseral chambers, the character of the uniserial chambers, the length of the uniserial part of the test, and the length and the breadth of the test. The uniserial chambers can be divided into primitive uniserial chambers and fully developed ones, depending on the angle between the upper and lower sutural planes. In fully developed uniserial chambers these planes are more or less parallel, whereas they form an angle of 20° and more in primitive uniserial chambers. In ontogeny, the fully developed uniserial chambers are the younger ones. In the course of time the relative number of primitive uniserial chambers decreases, i.e., the uniserial chambers become arranged in more regular series. By comparing the number of primitive unserial chambers present at the same growth stage - for instance in all individuals with two uniserial chambers, or in those with three, or with four - parameters become available that are independent of the final ontogenetic stage the specimens reached. Apparently these parameters show the most constant development, evidently not influenced by external circumstances. In the course of phylogeny, most of the parameters and their means become more strongly correlated. In the higher part of the phylogenetic sequences random fluctuations in test size thus cause significant set-backs of the means in all other parameters, except in those based on the number of primitive uniserial chambers in specimens at the same ontogenetic stage. Both lineages are not synchronous. The melitensis lineage starts approximately at the Orbulina surface (base Serravallian?). Uvigerina (Hopkinsina) bononiensis compressa is considered to be the forerunner. The last representatives of the lineage are found in deposits of Early-Middle Tortonian Age. The cretensis lineage probably starts in the Early Tortonian, and it ranges up into the Pleistocene. Its ancestor is as yet unknown. By statistical methods four morphologically adjoining species, successive in time, could be distinguished within each lineage. Seven of our eight species are new, because no meaningful contents could be given in terms of our biometrical analysis to the existing species names. Biostratigraphic correlations of our Cretan marine formations with the Late Neogene stratotypes indicate that marine sedimentation in the Rethymnon region started in the Early-Middle Tortonian and that it persisted up into the Pliocene without interruption. An interval of less than one Uvigerina Range zone, if any, seems to be present between the uppermost part of the type Tortonian (sensu GIANOTTI 1953, not CITA et aI1965) and the base of the Tabianian stratotype. If we accept GIANOTTI'S definition of the upper limit of the Tortonian stratotype, there seems to be no need of a separate stage between the Tortonian and the Pliocene. The Andalusian stratotype is younger than the type Tortonian; actually it can be correlated with the Tabianian and Piacenzian stratotypes, which correlation points to its Pliocene Age. The Maltese Blue Clay and Greensand are considered older than the Cretan Neogene; the Upper Coralline Limestone may be contemporaneous with the basal marine deposits of Crete.