9. BIOSTRATIGRAPHIC AND MAGNETOSTRATIGRAPHIC EVALUATION OF SITES 953, 954, 955, AND 956, CANARY ISLANDS 1C. Brunner,2 J. Sblendorio-Levy,3 R. Maniscalco,4 R. Howe,5 B. Herr,6 M. Fuller,7 P. Go l ds t r a nd , 8 and P. van den Bogaard 9 |
ABSTRACTSequences 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/ocean-wide 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. |
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