Drilling on the Madeira Abyssal Plain during Leg 157 penetrated deep into the sequence of interbedded turbidites and pelagic sediments that underlie the plain. The turbidites generally have abundant and well-preserved calcareous nannofossil assemblages, with the interbedded pelagic sediments having variable preservation and abundance of nannofossils dependent on CaCO 3 content. Poor preservation in pelagic sediments from the Paleogene and Miocene necessitated the use of nannofossil datums from turbidites to construct a consistent biostratigraphy.Site 950 records Neogene turbidite deposition of organic carbon-rich, volcaniclastic-rich, and highly calcareous turbidites from different sources around the plain with thin interbedded pelagic red clays, marls, and oozes. A disconformity separates the upper part of Zone CN3 from a single turbidite belonging to Zone CN2, below which a major disconformity separates Zone CN2 from Subzone CP19b, at the top of the Oligocene. Throughout the Oligocene and late Eocene, down to Subzone CP15a, the sequence comprises thin calcareous turbidites and thick pelagic red clays. Sites 951 and 952 only penetrated as deep as Zone CN3 and have similar sequences to the Neogene section of Site 951. Total sediment and pelagic sediment accumulation rates have varied dramatically with time, with low rates in the Paleogene section of Site 950 rising significantly throughout the Neogene. Sites 951 and 952 have much higher rates in the lower and middle Miocene than Site 950, above which all three sites have broadly similar rates.

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Sequences from the volcanic apron of Gran Canaria of the Canary Islands were evaluated for continuity, steadiness of deposition rate, and age resolution to determine their fitness for study of geologic cycles through time. An age model was constructed using regression analysis of the polarity record for Site 953, which had relatively complete magnetostratigraphy from the Brunhes Chron through the middle Miocene. Local ages for biostratigraphic datum levels were estimated from the polarity record at Site 953, and were used to construct age models for the other sites, where polarity records were highly discontinuous. The regression age models were refined using biostratigraphic and sedimentologic data to delimit hiatuses, slumps, and repeated sequences, and the accuracies of the age models were checked with radiometric dates where possible.Local ages of foraminifer and calcareous nannofossil first and last occurrences were estimated from magnetostratigraphy of the sequence. Local ages of the first and last occurrences of most species do not differ significantly from current global/oceanwide ages, except those of nine species: the last occurrences of Globoquadrina dehiscens and Discoaster loeblichii and the first occurrences of Neogloboquadrina acostaensis, Globigerina nepenthes, Discoaster berggrenii, Minylitha convallis, Discoaster hamatus, Discoaster coalitus, and Discoaster kugleri. Hiatuses at the four sites generally group within four periods: the late Fataga eruptive period, the pre-Roque Nublo eruptive period, the Roque Nublo eruptive period, and the Late Pliocene and Quaternary Epochs, when changes in sea level were large and volcanic eruptions were sporadic on Gran Canaria.Bio-and magnetostratigraphic evaluations of the Canary Island sites of Leg 157 clearly show that the section cored at Site 953 is the most continuous with the steadiest rates of sedimentation and the finest age resolution. Most bio-and magnetostratigraphic zones are present, and hiatuses last <0.4 m.y. Site 954 ranks second, but is flawed by several hiatuses >1.0 m.y. The best intervals at these sites, with standard errors of the age estimate <0.1 m.y., are adequate to resolve the broad highs and lows of volcanic activity since the early middle Miocene, third-order sea-level changes, and broad environmental cycles, possibly even 0.4 m.y. Milankovitch cycles, but they are inadequate to resolve shorter cycles, like fourth-order sea-level changes, without improvement to the age models. By contrast, stratigraphic sequences at Sites 955 and 956 were generally unsuitable for time series studies requiring anything but crude age resolution over long intervals.

Site 953 in the Northeastern Atlantic, northeast of Gran Canaria, recovered an essentially complete Pleistocene to middle Miocene biostratigraphic record. Quantitative analysis of the calcareous nannofossils identified 57 calcareous nannofossil events. Forty-seven of these events have been tied, where possible, to the paleomagnetic record (Schmincke, Weaver, Firth, et al., 1995; B. Herr, pers. comm., 1996) and to the radiometric ages generated by Bogaard (Chap. 19, this volume) for this site.

BACKGROUND Volcanic aprons A volcanic apron peripheral to an oceanic island consists of three main facies above the central or core facies and is made up of extrusive and intrusive igneous rocks (Schmincke, 1994; Fig. 1):