Original paper
Worldwide discoaster ray number variability at the early/middle Eocene boundary. Implication for the neritic sequences of the Nile Valley (Egypt)
Janin, Marie-Christine; Bignot, Gérard; Strougo, Amin; Toumarkine, Monique
published: 3/10/1993
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ArtNo.: ESP026002802005
Abstract
According to the MARTINI & MÜLLER's calcareous nannoplankton biozonation, the early/middle Eocene boundary corresponds to the NP13/NP14 boundary, defined by die first occurrence of Discoaster sublodoensis. This index-species is a 5-rayed asterolith very abundant in the lower part of the NP14, where it causes an abrupt increase of the percentage of 5-rayed specimens among the discoasters. Analysis of the discoaster ray number variability during the Eocene in various areas of the world indicates that this increase is also obvious in sequences devoid of typical D. sublodoensis, and that it is thus available for recognition of the earliest middle Eocene. This result is applied to die neritic sequences of the Nile Valley, where planktonic species are rare and poorly preserved. It leads to locate the early/middle Eocene boundary (= Ypresian/Lutetian boundary) near the Minia/Samalut Formation boundary (Libyan/Mokattamian boundary).