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The Cretaceous Equatorial Atlantic Gateway between the Central and South Atlantic basins is of interest not only for palaeoceanographic and palaeoclimatic studies, but also because it provided particularly favourable conditions for the accumulation and preservation of organic-rich sediments. Deposition of carbonaceous sediments along the Côte d’Ivoire-Ghana Transform Margin (Ocean Drilling Program Leg 159) was intimately linked to the plate tectonic and palaeoceanographic evolution of this gateway. Notably, the formation of a marginal basement ridge on the southeastern border of the transform margin provided an efficient shelter of the landward Deep Ivorian Basin against erosive and potentially oxidizing currents. Different subsidence histories across the transform margin were responsible for the development of distinct depositional settings on the crest and on both sides of the basement ridge. Whereas the southern, oceanward flank of the basement ridge was characterized by rapid, continuous deepening since the latest Albian-early Cenomanian, marine sedimentation on the northern, landward flank was interrupted by a period of uplift and erosion in the late Albian, and rapid subsidence started after the early Coniacian. Organic-rich sediments occur throughout almost the entire Cretaceous section, but hydrogen-rich marine black shales were exclusively recovered from core sections above an uplift-related unconformity. These black shales formed when separation of Africa and South America was sufficient to allow permanent oceanic midwater exchange after the late Albian. Four periods of black shale accumulation are recorded, some of them are correlated with the global oceanic anoxic events: in the latest Albian-earliest Cenomanian, at the Cenomanian-Turonian boundary, during the middle Coniacian-early Campanian, and in the mid-Maastrichtian. These periods were characterized by increased carbon fluxes to the seafloor, induced by enhanced palaeoproductivity and intensified supply of terrestrial organic matter. Black shale deposition appears to be intimately linked to periods of rising or maximum eustatic sea level and to the expansion of the oxygen minimum zone, as indicated by foraminiferal biofacies. Intervals between black shale units, in contrast, indicate a shrinking oxygen minimum zone and enhanced detrital flux rates, probably related to lowering sea level. Upper Cretaceous detrital limestones with high porosities may provide excellent hydrocarbon reservoirs, although their areal extent appears to be limited. Palaeogene porcellanites, capped by Neogene pelagic marls and clays, extend over a wider area and may provide another target for hydrocarbon exploration.

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Palygorskite has been identified as a major constituent of the clay fraction in midCretaceous deposits of the Meseta, the Middle Atlas, and the Rif in central and northern Morocco. Two types of palygorskite are differentiated, based on the morphology of crystallites, bio-and lithofacies associations of the containing sediments. Type 1 occurs in sections of the Meseta and the Middle Atlas, where the mineral displays bundles of long fibres under the electron microscope and is associated with shallow marine carbonate and evaporite facies. Type 2 was found in the Rif, where palygorskite occurs mostly as bundles of shorter fibres in turbiditic sediments of a supposed deepmarine environment. Associated microfossils indicate reworking of the palygorskite-bearing sediment from shallower parts of the basin. Palygorskite apparently grew authigenically in the chemically restricted environments of the Meseta and the Middle Atlas, whereas it is of a detrital origin in the Rif section. We consider that sediments containing authigenically formed palygorskite (type 1) could be the source of detrital palygorskite (type 2). Type 1 deposits on the North African shelf were probably partly reworked and responsible for the supply of palygorskite to midCretaceous Atlantic basins, where the mineral is well known from several DSDP sites. As in the case of various Recent deep-sea palygorskite occurrences, aeolian transport may have significantly contributed to palygorskite supply to the ocean basins. RESUME: L'6tude de la min6ralogie des argiles des s6diments albo-c6nomaniens du Rif, de la Meseta et du Moyen-Atlas (Maroc) montre la pr6sence fr6quente de fortes proportions de palygorskite. Dans la Meseta et le Moyen-Atlas, ce min6ral s'est form6 sur place dans des milieux chimiquement confin6s (origine autochtone = type 1). En revanche, dans le Rif, la palygorskite associ6e ~ des d6p6ts gravitaires semble remani6e depuis des milieux de plates-formes peu profonds (origine allochtone = type 2). Sous des conditions climatiques favorables, la pr6sence de min6raux fibreux dans les bassins marocains parai't principalement contr616e par les pal6omorphologies continentales, l'activit6 tectonique des marges, les vitesses d'6rosion et s6dimentation, et le confinement plus ou moins prononc6 des environnements de d6pft. Les nouvelles donn6es pr6sent6es montrent que les palygorskites des bassins c6tiers marocains constituent une des sources potentielles des palygorskites oc6aniques qui, comme dans diverses s6ries r6centes, r6sultent parfois d'un transport 6olien.Palygorskite, a fibrous Mg-rich clay mineral, is often considered to be easily destroyed by transportation (e.g. as stated by Millot, 1970). It is, therefore, still widely held that palygorskite occurrences represent sites of authigenic formation of this mineral and that detrital accumulation is 9 1996 The Mineralogical Society