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Deep Sea Drilling Project Initial Reports Volume 25 5. SITE 242 The Shipboard Scientific Party1 SITE DATA eastern Africa and western Madagascar, which have been summarized by Dixey (1960) and Flores (1970). An important factor contributing to the confusion is the almost complete lack of published data on the bathymetry, crustal structure, and stratigraphy of the Mozambique Channel between Mozambique and Madagascar. Flores (1970) has published a concise account of the Phanerozoic geology of Mozambique and western Mada- C gascar, based upon his personal familiarity with both areas -15 and access to data from deep boreholes. He summarizes the overall similarities of Karroo and Cretaceous stratigraphy in both areas but is careful to emphasize the time and space distribution of continental/marine facies transitions. In both southeastern Africa and western Madagascar, conti- nental lacustrine, with perhaps occasional marine condi- tions of sedimentation, were prevalent during the Permo-Carboniferous and Triassic Periods. This gave way during the Jurassic to widespread volcanism in the west with contemporaneous marine sedimentation in the east (Madagascar), south (Agulhas Bank), and in coastal Kenya (Dixey, 1960). According to Flores (1970), during the Cretaceous, the marine facies of western Madagascar Location: Mozambique Channel (Davie Ridge) changed to continental, culminating in volcanic activity Position: 15°50.65's, 41°49.23'E during the middle Cretaceous (Turonian to Santonian/ Campanian). By contrast, the present wide Mozambique Water Depth: 2275 meters coastal plain was an area of marine sedimentation Total Penetration: 676 meters throughout the Cretaceous, becoming continental in the Cores: 19 cores (134 m cut, 103.1 m recovered) west. Upper Cretaceous and Tertiary marine sediments are Deepest Unit Recovered: Early late Eocene brown nanno found on both sides of the Mozambique Channel, with Late chalk Cretaceous intrusives and late Tertiary to Quaternary volcanic activity chiefly in Madagascar and in the BACKGROUND AND OBJECTIVES Mozambique Channel (Figure 1). Long before, and since development of the sea floor The general morphological features of the Mozambique spreading hypothesis and of modern plate tectonic Channel are shown in Figure 1, which is partly based upon concepts, a variety of predrift paleo positions and an unpublished compilation of bathymetric data by subsequent movements has been proposed for Madagascar. Langseth, Heezen, and Ewing. To the south, lies the 4500- Most of the authors concerned have displayed their 5000-meter-deep abyssal plain in the Mozambique Basin unfamiliarity with (or have found it convenient to which is bounded on the west and east, respectively, by the disregard) the significance of both the similarities and Mozambique and Madagascar ridges and becomes progres- differences between the structure and geological history of sively shallower northward into the Mozambique Channel. The adjacent continental slopes of Africa and Madagascar are steeply inclined down to the average depth of about 1E. S. W. Simpson, University of Capetown, Rondebosch, South 3000 meters in the channel. The floor of this channel is Africa; Roland Schlich, Institut de Physique du Globe, Saint-Maur- deeply incised by the Zambesi Canyon and several tributary des-Fossés, France; Joris Gieskes, Scripps Institution of Oceanog- canyons from Madagascar, which effectively act as channels raphy, La Jolla, California; W. A. Girdley, Mesa College, Grand Junction, Colorado; Lucien Leclaire, Museum National d'Histoire for the transport and dispersion of terrigenous sediment Naturelle, Paris, France; B. Vaughn Marshall, U. S. Geological into the northern Mozambique Basin. Survey, Menlo Park, California; Casey Moore, University of The Mozambique Channel is divided into two distinct California, Santa Cruz, California; Carla Muller, Johann-Wolfgang- basin compartments by the asymmetric (steep to the west) Goethe-Universitàt, Frankfurt, Germany; Jacques Sigal, Institut and weakly seismic Davie Ridge, which follows the 42°E Français du Pétrole, Rueil-Malmaison, France; T. L. Valuer, Scripps Institution of Oceanography, La Jolla, California; Stan M. White, meridian southward until it disappears as a positive California State University, Fresno, California; Barbara Zobel, morphological feature near 19°S where its trend is Bundesanstalt für Bodenforschung, Hannover, Germany. continued southward by the lower Zambesi Canyon. Both 139 /COMOROp ^ABYSSAL N PLAIN Marine deposits •M Volcanic intrusions ^^ and lava flows B* Deep boreholes Basement Bathymetric contours redrawn Lang se th and Heezen 1964 v> ••• •>i.) / ''..ABYSSAL PLAIN i/J MOCAMBIQUE BASIN 100 200 45 Miles 5QC Figure 1. Map of the Mozambique Channel (slightly modified after Flores, 1970) showing the location of Site 242 and of the seismic reflection profiles in Figures 2 and 3. SITE 242 features, each in its own way, act as effective barriers to the Burroughs (1971), that basement on the east side of the transport of terrigenous sediment across the channel. West Davie Ridge should be representative of the Madagascar of the Zambesi Canyon and the volcanic islands of Europa plate, and the stratigraphic section should be similar to that and Bassas da India lies the Europa Basin and abyssal plain of eastern Kenya and Somalia. (3500 m), which is underlain by stratified sediments more 2) Movement of Madagascar eastward away from Africa than 1 sec DT (double way time) thick. Between the Davie implies the presence of oceanic or thinned continental Ridge and northwestern Madagascar, extending northward crust beneath the Mozambique Channel. to the volcanic Comoro Islands (which mark the southern 3) If no lateral movement of Madagascar relative to boundary of the Somali Basin), lies the Comoro Abyssal Africa has taken place since the Paleozoic, the channel Plain (3500 m), which is similarly underlain by stratified should be underlain by a succession of terrestrial and sediments more than 1 sec DT thick. A series of airborne shallow-marine sediments and volcanics transitional magnetic profiles flown east-west across the Mozambique between the stratigraphic sections found in east southern Channel and northern Mozambique Basin has been Africa and western Madagascar. published by Green (1972). Except over volcanic islands, the magnetic anomalies over the channel between 10°S and One of the major objectives of Leg 25 of the Deep Sea 22°S (Europa Island) are below 10Oy, with wavelengths Drilling Project was to provide essential data which would between 15 and 60 km. These contrast with the much assist in the solution of this enigmatic problem and, with greater amplitude and reduced wavelength of the anomalies this end in view, two sites located in the Mozambique over the Mozambique Basin between 25°S and 30°S. Channel were selected for drilling on the basis of available data and our present meager understanding of the structure Reconstructions of Gondwanaland have variously placed and stratigraphy of this region. Madagascar in three possible positions at the end of the Paleozoic before the commencement of drift: Numerous traverses across the Davie Ridge in the 1) Adjacent to the coast of East Africa, off Somalia, northwestern Mozambique Channel between 10°S and 19°S Kenya and Tanzania, followed by drift to the south and are available from Chain Cruise 99 in 1970 (Heirtzler and slightly east. This reconstruction is favored by du Toit, Burroughs, 1971) and Thomas B. Davie Cruise 267 in 1971 1937; Fisher, Engel, and Hilde, 1968 (who deduce that it (Simpson, in preparation). Figure 1 shows only those tracks was 20° north of its present position during the Permian which pass near the proposed site. and that it has occupied its present position relative to The Davie Ridge trends north-south along the 41°30'E Africa since the Cretaceous); Dietz and Holden, 1970; meridian close to, and parallel with, the linear stretch of the McElhinny, 1970; Smith and Hallam, 1970; Heirtzler and African coast near Cape Delgado (10°S), continuing Burroughs, 1971 (who believe that the southward southward as a positive morphological feature at least as far movement began since the early Cenozoic, 42-65 m.y. ago, as 19°S from where its strike direction is continued by the and still continues at a rate of 2.9 cm/yr); and Sowerbutts, lower reaches of the Zambesi Canyon in the southeastern 1972 (who suggests the initiation of breakup during the Mozambique Channel. Cretaceous). Most, but not all, traverses across the ridge show it to be 2) Adjacent to the pre-Cretaceous continental margin strongly asymmetric with a single steep scarp facing west of Mozambique or Natal, with subsequent drift to the north (see Figures 2 and 3). Shallow-focus earthquake epicenters and east. This paleoposition is advocated by Wellington, are located near and slightly to the west of the ridge, and 1954, 1955; Flores, 1970 (commencement of drift during the wavelength of associated low-amplitude (<2007) Early Jurassic, cessation during mid-Cretaceous-Turonian); magnetic anomalies indicate deep burial of magnetic Wright and McCurry, 1970; Heirtzler, 1971 (movement basement. A thick sequence of turbidite sediments derived sometime since the Cretaceous); and Green, 1972 (who from Africa is banked up against the base of the steep believes that Madagascar separated from the Natal margin western scarp. On the eastern side of the sediment-free by east-west spreading about the Mozambique Ridge from ridge crest, the regionally smooth acoustic ridge basement the Late Triassic until Late Cretaceous/early
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