The oldest rocks exposed on Maio in the Cape Verde Islands (eastern central Atlantic) are MORB-type ocean-floor pillow basalts, lava breccias, and hyaloclastics generated at an inferred latest Jurassic–earliest Cretaceous spreading axis, possibly near a fossil fracture zone. Interlava sediments include distal terrigenous turbidites from the West African continental margin. Latest ocean-floor volcanism was accompanied by precipitation of ferruginous oxide sediments from hydrothermal solutions, comparable with modern spreading axes. The overlying latest Jurassic (Tithonian) and Lower Cretaceous sediments are deep–ocean nannoplankton radiolarian chalks, coeval with the similar eastern Atlantic “White Limestone” and the Tethyan Maiolica facies. By the mid-Cretaceous (Albian-Cenomanian), Maio had subsided below the calcite compensation depth with accumulation of the characteristic Atlantic anoxic black-shale facies. Thin interbeds of limestones and siltstones are pelagic calciturbidites and distal terrigenous turbidites lacking volcanic material.
After uplift, the Cretaceous pelagics are overlain apparently conformably without tilting by shallower marine to subaerial pyroclastics and epiclastics, which include tuffs, agglomerates, conglomerates, and limestones. The existence not only of rounded water-worn clasts of Mesozoic ocean-floor rocks but also of alkaline extrusive and intrusive igneous rocks points to derivation from an early plutonic complex of unknown age. The earliest preserved extrusives, which are for the most part fragmental basalts and trachytes, are possible Paleogene to Miocene in age. Further dike and sill intrusion then occurred, followed by westward-directed folding and thrusting. Diapiric doming of the Central Igneous Complex ensued, followed by deep erosion, and then eruption of now gently inclined ankaramites. By late in the Miocene, fan deltas radiated from a rapidly eroding, still volcanically active island. The Neogene saw more quiescent alkalic extrusion punctuated by peneplanation. Maio Island is thus a clue to both Mesozoic paleoenvironments in the eastern and central Atlantic and to Tertiary island building related to “hot-spot” activity.
Additional tabular matter for this article may be secured free of charge by requesting Supplementary Data 84-9 from the GSA Documents Secretary.