Comments on Leg 25 Sites in Relation to the Cretaceous and Paleogene Stratigraphy in the Eastern and Southeastern Africa Coast and Madagascar Regional Setting

Abstract

The holes drilled during Leg 25 show a large degree of diversity. Each had its own objectives and revealed its own stratigraphic findings. They provided some understanding of the nature and age of the deposits and, possibly of the substratum. It is the purpose of this paper to place them in a regional setting and to compare them by enlarging the comparative framework. This wide geologic framework can provide useful data for the interpretation of seismic profiles. Holes will be compared in three ways, depending on the available data: (1) with data from onshore field observations (sedimentary fringe of African and Madagascar coastal "basins," and coastal boreholes) or from offshore observations (offshore boreholes); (2) with considerations linked to hypotheses about plate tectonics; and (3) with major geological or diastrophic events. This comparison covers a large area over which a great variety of sedimentary facies is represented. In most areas, published data are sparse. Also, the data which are available do not all have the same comparative value because they not only were obtained progressively as years went by, but also by different techniques. Within the limited space available, I have tried to avoid the two major pitfalls that can waylay any such undertaking, i.e., altering the data through diagrammatic formulating, and writing a report on regional geology. An attempt to avoid these dangers was made by limiting the comparisons to a certain number of crucial problems that concern the drilled sites. I have tried to overcome the handicap of not having direct personal knowledge about the regions involved by exchanging opinions and publications with various colleagues who have been kind enough to assist me and to whom I willingly extend my thanks. These include H. Besairie (Tananarive), G. Flores (Florence), G. Blant (Entreprise de Recherches et d'Activités Pétrolières, Paris), Compagnie Francaise des Pétroles (P. Burollet, Ph. Dufaure, Cl.de Lapparent, Ph. Trouvé), M Collignon (Gières, France), and A. Balduzzi (A.G.I.P., Milano).