Timescale and General Synthesis

Abstract

Deep earth sampling beneath the floor of the Mediterranean has acquired continuous sequences of marine pelagic ooze which were deposited during an interval of time ranging from the end of the Late Miocene "crisis of salinity" to the present. A recognizable evolutionary trend in assemblages of planktonic foraminifera and calcareous nannofossils has permitted the Leg 13 scientific party to document in this volume a sequence of biostratigraphic zones covering this period of time—that is from the earliest Pliocene onward. We are indeed fortunate that a majority of the criteria used to establish the boundaries of these zones (see Chapter 47.1) can also be identified in deep-sea sediment cores from other parts of the world's ocean. Many of the biostratigraphic datums used (for example, first appearances, evolutionary changes, and extinctions) have been demonstrated prior to the Mediterranean drilling venture to be, for all purposes, time-synchronous on a worldwide scale (Glass et al, 1967; Hays et al, 1969; Hays, 1971; Kobayashi et al, 1971). Our confidence in this synchroneity is supported by paleomagnetic measurements carried out on the same sediment samples that contain the fossil record. Back at least into the Gilbert epoch the pattern of reverse and normal magnetization in the deep-sea cores is identical to the sequence of sea floor magnetic anomalies near the crests of spreading mid-oceanic ridges, as well as the reversal history of the geomagnetic field recorded in radiometrically data lava flows (see Chapter 47.2). We have combined in this chapter the results of the biostratigraphic, chronostratigraphic, and paleomagnetic investigations on the Mediterranean cores in order to arrive at an applicable absolute time scale which we believe is reliable and useful. The purpose of our paper is to document this time scale, evaluate it in terms of previous attempts at formulating a time scale for the same period, and to then use it to give a narrative of changes in the sedimentary environment of the Mediterranean Region from the Late Miocene to the present.