Pliocene benthonic foraminifera from homogeneous and laminated marls on Crete

Abstract

In the Pliocene, the paleogeography of central Crete consisted of a number of basins which were filled by predominantly marly sediments. In the sedimentary sequence numerous laminated sapropelic intercalations can be observed. At a higher stratigraphic level diatomaceous laminites appear. From field data and sediment analyses we have inferred a shallowing for this sequence. Correlation of the individual strata shows that sapropelic laminites and diatomites were formed simultaneously. Turbidity-controlled differences in primary production are considered to be responsible for the simultaneous formation of both lithology types. Thickness of the sapropels is determined primarily by the amount of clastic input; in diatomites thickness depends on different rates of production of siliceous micro-organisms. The benthonic foraminifera from the sediment successions were subjected to a quantitative study. On the basis of R-mode statistical analyses we can divide the benthonic fauna into a group with preference for the homogeneous sediments, a laminite-bound group and a group with an intermediate position. The statistical analyses suggest the existence of a stagnation gradient for the depositional environments of the successive laminites. At the extreme end of this gradient the autochthonous fauna is replaced by allochthonous benthonic elements. It can be demonstrated that the degree of stagnation during each period of laminite formation differed from basin to basin and was at its maximum in relatively shallow environments. A stagnation gradient seems to be of little value for the homogeneous sediments. Here, minor faunal changes can be attributed to shallowing and the development of better nutrient conditions. From distributional, sedimen,tary and faunistic evidence it is concluded that the Pliocene periods of stagnation are best expressed in a marginal setting. Periodically increased run-off seems to provide the most likely explanation for the origin of the Pliocene laminated sediments. Low saline land-derived waters caused the termination of the vertical circulation. In addition, such waters brought nutrients into the marine realm, giving rise to enhanced production in surface waters.