32. COMMENTS ON LEG 25 SITES IN RELATION TO THE CRETACEOUS AND PALEOGENE STRATIGRAPHY IN THE EASTERN AND SOUTHEASTERN AFRICA COAST AND MADAGASCAR REGIONAL SETTING Jacques Sigal, Institut François du Pétrole, Rueü-Malmaison, France INTRODUCTION The biostratigraphic columns have been given for each site (Chapters 2-10). The distribution (and the recovery The holes drilled during Leg 25 show a large degree of rates) of the cores, the paleontologic zones which have been diversity. Each had its own objectives and revealed its own recognized (planktonic and benthonic foraminifera and stratigraphic findings. They provided some understanding calcareous nannofossils), and the geochronologic scale are of the nature and age of the deposits and, possibly of the given in these columns. substratum. It is the purpose of this paper to place them in It was not possible, because of the time allowed, to write a regional setting and to compare them by enlarging the a paleontologic-taxonomic paper on the foraminifera and comparative framework. This wide geologic framework can ostracod assemblages to be included in the Initial Report. provide useful data for the interpretation of seismic However, many results are new because some drilled profiles. sequences were originally deposited in a particular realm Holes will be compared in three ways, depending on the (very austral for Site 245 for instance) and their available data: (1) with data from onshore field monographic study would require much more time. observations (sedimentary fringe of African and Madagascar Plates 1 through 11 illustrate the more important coastal "basins," and coastal boreholes) or from offshore chronostratigraphic species which support the recognized observations (offshore boreholes); (2) with considerations conventional zones or chronostratigraphic determinations. linked to hypotheses about plate tectonics; and (3) with Their purpose is essentially to justify the bio- major geological or diastrophic events. chronostratigraphic data on which the delimitations of the This comparison covers a large area over which a great drilled sequences and intercalated hiatuses are based. Also, variety of sedimentary facies is represented. In most areas, they support the paleogeographic frame in which the sites published data are sparse. Also, the data which are available and the surrounding sedimentary areas can be compared. do not all have the same comparative value because they The identified parts and the hiatuses of the stratigraphic not only were obtained progressively as years went by, but columns are given in Figure 1, based on their actual range in also by different techniques. Within the limited space the scale of "absolute" ages. The figure attempts to show, available, I have tried to avoid the two major pitfalls that for each site, those parts of the stratigraphic column which can waylay any such undertaking, i.e., altering the data were not recognized. They may thus correspond either to through diagrammatic formulating, and writing a report on gaps, i.e., nondeposition or erosion, or they may correspond regional geology. An attempt to avoid these dangers was to condensed sequences, as it has been emphasized by Le- made by limiting the comparisons to a certain number of claire in the aforementioned chapter. Very probably both crucial problems that concern the drilled sites. explanations must be considered, depending on the site, as I have tried to overcome the handicap of not having it is evident that these "hiatuses" actually occur in very dif- direct personal knowledge about the regions involved by ferent paleogeographic environments. exchanging opinions and publications with various The hiatuses are included in a scale of measured ages in colleagues who have been kind enough to assist me and to order to give the most accurate presentation of data whom I willingly extend my thanks. These include H. possible. Moreover, the "absolute" age of the lower and Besairie (Tananarive), G. Flores (Florence), G. Blant upper limits of the hiatuses, as they can be deduced from (Entreprise de Recherches et d'Activités Pétrolières, Paris), the biochronologic data, is given. These determinations are Compagnie Francaise des Pétroles (P. Burollet, Ph. Dufaure, the basic elements which enable sedimentation rates to be Cl.de Lapparent, Ph. Trouvé), M Collignon (Gières, France), calculated. and A. Balduzzi (A.G.I.P., Milano). BIOSTRATIGRAPHIC RESULTS LEG 25 SITES IN A REGIONAL SETTING Discussions of the major stratigraphic problems which have been selected for this chapter are based on three kinds Introduction of data: (l)the biostratigraphic columns of the different sites; (2) the photographic plates showing the most During stratigraphic and paleontologic studies of the Leg important species of foraminifera; and (3) a table of the 25 sites, several problems became apparent. This part of the sequences which have been dated and the hiatuses which paper isolates some of the more important ones and are either demonstrated or suspected. The following discusses each in a regional setting. These settings have been discussion is based on these data. reconstructed for the various periods under discussion. 687 oo m.y. ex approx. U0 239 240-A 241 O 242 > PLIOCENE-QUAT. 5.5 \ NN11 late 10 NN10 MIOCENE middle 16 N12, NN7 early- \ 22.5 NP25 NP25 A late 24.5-26 OLIGOCENE 30 I 7 • no middle I corer s èàTTv~ 35 f 37.5 f2 late NP20 43 37.5-38.5 P15 EOCENE middle t? 49 ^50 (bottom) early 52:53.5 53.5 ?P4 late 58-5-59.5 BasaltNP12 PALEOCENE 60 56-58 early NP2-3 P1 NP3 65- 61-64 B^U Ma. 71-72 Ma 70 L. Ca CaT Ca 76 Basalt ^71-73 Sa. Late 82 CRETACEOUS 38 Tu. t? 94 (bottom) Ce. 100 Alb. 106 Apt. - 112 Early Ba CRETACEOUS 118 Ha. 124 Val. 130 Be. 136 JURASSIC Basalt Figure 1. Identified parts and inferred hiatuses of the stratigraphic column from Early Cretaceous to Recent (scale in million years). CRETACEOUS AND PALEOGENE STRATIGRAPHY Figure 2 portrays the region 120 m.y. ago, during the Early Early Cretaceous. This hole encountered Late and then Cretaceous. Figure 4 is a 75 m.y. reconstruction of the Late Middle Jurassic having the particular feature, as opposed to Cretaceous and Figures 5 and 6 are reconstructions of the series of the same age outcropping in Madagascar, of middle Tertiary, 45 and 36 m.y. ago, respectively. Figure 3 containing small doleritic sills or effusive flows. is of Madagascar as it appears today with notations of cited localities. Neocomian-Early Cretaceous at Site 249 The occasional affinities and communities of microfauna Comments on the Marine Late Jurassic mentioned in relation to this site lead one first to turn The Jurassic was not reached in any of the boreholes. It toward Madagascar for our comparison. is fairly certain that it does not exist at Site 249 (at least in Generally speaking, within the regional setting with the sediment column) despite the fact that the great which we are concerned here (Figure 2), the Cretaceous thickness attributed to the Neocomian might suggest its marks a new invasion of the sea on the African craton. This presence. was in the form of a transgression which was quite In the vicinity of Site 249, the Jurassic of Mozambique extensive although less widespread than the one in the late and South Africa should be taken into consideration. In lias-Middle Jurassic time interval. Mozambique however, the late Jurassic that has been recognized is located far north of Site 249. Moreover, doubt is cast on this age by G. Flores (1972) because he Comparison with the West Coast of Madagascar does not believe that the lamellibranch macrofauna are It can be assumed that during the Early Cretaceous the conclusive proof for that age. A deeper hole at Site 242, respective depositional realms for Madagascar and Site 249 perhaps, would have penetrated such old sediments. In were geographically quite close, as suggested by the southern Tanzania, the late Kimmeridgian and Tithonian paleogeographic reconstruction maps published by various also exist. Many authors attribute the role during authors and as we mentioned in a publication several years Cretaceous time of a barrier to the Mozambique axis that is ago (Sigal et al., 1970). It is customary to distinguish the assumed to have limited the south-southwest extension of two basins of Majunga (with its northern extension toward the Lias and middle and late Jurassic deposits which are Diego) and Morondava (with its southern extension toward quite extensive in Tanzania and Madagascar. Cap Sainte Marie) (Figure 3). In South Africa, the marine Jurassic (probably In the Majunga Basin, the Early Cretaceous sequence is Callovian) was recently identified for the first time (Dingle quite different from that at Site 249, first, because of the and Klinger, 1971) west of Knysna. It was in the form of absence of the intervention of continental facies south of sandy and conglomeratic fluviatile deposits associated with the Betsiboka River, and second, because of the absence of a few shallow-marine sandy deposits. Judging from these volcanic rocks. However, several species, especially among sediments, this appears to represent a marine incursion of the ostracods, show an affinity. Hence, a biological the paralic type, perhaps leading to sediment fill in the similarity is shown even though the deposits and geological troughs which were the first signs of the great fractures that history are different. Also, from the biological standpoint, would later break up the Gondwana continent. it should be added that the foraminiferal microfauna at Site 249, like those in the Algoa Basin, apparently correspond Basement of the Neocomian: the Eruptive Rock to warmer areas. at Site 249 In the Morondava Basin, a similar comparison cannot be Based on what is known onshore in the southern basin in made from the biological standpoint because of insufficient Mozambique, three possibilities for the eruptive rock at Site data.