Abstract
Abstract. Data is presented here on Reticulofenestra coccolith size distribution patterns from 122 Mid-Miocene to Pliocene samples from Deep Sea Drilling Project sites in the Western Indian Ocean and Red Sea. A clear pattern is revealed with a dramatic size reduction event occurring in the Late Miocene (nannofossil zone NN10). As a result of this event nannofloras from the interval above it are readily distinguishable by the absence of specimens longer than 5 microns; this interval is termed the “small Reticulofenestra interval”. Assemblages from above and below this interval contain large specimens but they can be reliably distinguished by different size distribution patterns within them. Analogous data from other studies is reviewed, possible causes of the pattern are discussed, and its biostratigraphic application described. The Neogene taxonomy of the genus Reticulofenestra is revised and four new combinations are proposed.
Highlights
There are excellent high resolution nannofossil zonations for the Neogene (Martini 1971, Okada & Bukry 1980,Theodoridis 1984) in practice they are often extremely hard to apply since many of the marker species used are rare, ecologically restricted, or vulnerable to diagenesis
In consequence a study was made of reference samples from Indian Ocean Deep SeaDrilling Project (DSDP) cores
The results from the four sites with reasonably continuous successions are given in Figures 3 and 4. These are discussed together since the results are very similar in each case; even though the sites are separated by thousands of kilometres (Fig. 2). They can be subdivided into four intervals, as discussed below, with reference to the standard nannofossil zones of Martini (1971)
Summary
There are excellent high resolution nannofossil zonations for the Neogene (Martini 1971, Okada & Bukry 1980,Theodoridis 1984) in practice they are often extremely hard to apply since many of the marker species used are rare, ecologically restricted, or vulnerable to diagenesis. In well preserved pelagic sediments these problems are not important, but in the less favourable conditions routinely encountered during applied work they can greatly reduce the biostratigraphic resolution. Reticulofenestrid coccoliths are promising since they are the most abundant and widespread elementof the nannofloras, and are diagenetically resistant. Biostratigraphic problems of this kind were encountered during a study of the Neogene of the Makran of Pakistan (Platt et a1 1985). This work provided strongevidencefor a surprisingly simple, consistent, and useful pattern of size variation, as described here
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