15. TEPHRA EVENT STRATIGRAPHY AND EMPLACEMENT OF VOLCANICLASTIC SEDIMENTS, MOGÁN AND FATAGA STRATIGRAPHIC INTERVALS, PART I: MINERAL AND CHEMICAL STRATIGRAPHY OF VOLCANICLASTIC UNITS AND CORRELATION TO THE SUBAERIAL RECORD 1Mari Sumita 2 and Hans-Ulrich Schmincke 2 |
ABSTRACTDuring Leg 157, middle to late Miocene (14-9 Ma) stratigraphic intervals in the clastic apron of Gran Canaria were drilled at Sites 953, 955, and 956. Several hundred volcaniclastic layers composed of silt to sand-sized volcanic material, chiefly glass shards, pumice, and crystals, are interlayered with foraminifer nannofossil ooze. These intervals are 452 m thick at Hole 953C, 193 m thick at Hole 955A, and 194 m thick at Hole 956B, and were subdivided into three (Hole 953C) and two (Holes 955A and 956B) lithologic subunits. The lowermost of these subunits at all three holes correlates with the fallout tephra layers and ignimbrite cooling units of the Mogán Group on Gran Canaria (14-13.3 Ma), whereas the volcaniclastic layers of the 1-2 upper subunits correlate with the volcanic units of the Fataga Group (13.3 to ~9 Ma). We have subdivided the cores into ~100 volcaniclastic units at each hole based on a more detailed study of the cores in the core repository. Most of the major volcaniclastic layers in the time interval between 14 and 12 Ma have been examined by petrographic analysis of 276 polished thin sections, electron microprobe (EMP) analysis of 3000 glass shards, and more than 4500 analyses of the main phenocryst minerals (feldspar, pyroxene, amphibole, and phlogopite). These data are combined with X-ray fluorescence (XRF) analyses of 40 bulk tuff samples, and reanalysis of rock powders made on board, 50 new XRF and EMP analyses of glassy vitrophyres from 12 selected ignimbrites, and microprobe analyses of feldspar phenocrysts from 23 ignimbrites to provide detailed data sets for both the subaerial and submarine tephra stratigraphies to facilitate their correlation. Nearly unique mineral phases or assemblages and compositions of glass, feldspar, amphibole, and clinopyroxene, and bulk rocks provide robust compositional criteria for unequivocally correlating formations and groups. Major changes in mineral compositions, for example, occur at equivalent stratigraphic levels on land and in the submarine sections, such as the disappearance of sodic anorthoclase, calcic amphibole, abundant hypersthene, and zircon at the boundary between the Lower and Middle Mogán Group or the appearance of abundant phlogopite at the boundary between the Mogán and Fataga Groups. We have also correlated many volcaniclastic layers in the interval between 14 and 12 Ma among the three holes and were able to identify at least 7 of the ~15 individual ignimbrite members (cooling units) of the Mogán Group present on land, notably ignimbrites P1, VI, P2, X, O, A, and D, in all three drill holes. The high-resolution correlations of several syn-ignimbritic volcaniclastic units with well-dated anorthoclase-bearing ignimbrites on the island allow more precise sedimentation rates to be calculated. Between 14 and 13.3 Ma, sedimentation rates were 97 to 142 m/m.y. at Hole 953C, 54 to 93 m/m.y. at Hole 956B, and 43 to 72 m/m.y. at Hole 955A. The high precision correlations also help to calibrate the biomagnetostratigraphic time scale, for example, during the time interval of ~12-13 Ma, where our lithological correlations suggest that the biostratigraphic ages suggested on board are significantly too high. |
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