9. ALTERATION OF UPPERMOST LAVAS AND VOLCANICLASTICS RECOVERED DURING LEG 152 TO THE EAST GREENLAND MARGIN1Mary Anne Holmes2 |
ABSTRACTDuring the last stages of the eruption of syn-rift basalts along the East Greenland Margin, debris flows and/or pyroclastic deposits were emplaced at Ocean Drilling Program Sites 915 and 916. The deposits and the tops of the lava flows at Site 918 were altered by subaerial weathering processes as indicated by downhole changes in mineral and chemical composition, and by the mineral paragenesis. Kaolinite and goethite, which form in acidic waters, are abundant at the tops of the weathering profiles and decrease in abundance downward. They are replaced by a smectite-hematite-opal assemblage at the bases of the weathered profiles. Gibbsite is a minor component in the upper parts of the profiles. Good preservation of parent structure and stratification indicate that only the bases of paleosols are preserved at all sites. The upper parts were probably eroded when these sites subsided below sea level. Abundant gibbsite in marine sediment overlying the paleosols and of middle to late Eocene age is probably derived from the erosion of highly weathered soils formed in a subtropical to tropical climate. A highly weathered basaltic terrane supplied abundant iron oxides in addition to gibbsite, kaolinite, illite/mica, and quartz through fluvially dominated deltaic systems on the shelf through at least the late Eocene. Only small amounts of sediment spilled over into the adjacent Irminger Basin through the Eocene, as indicated by the presence of felsic terrane-derived minerals (quartz, illite, and/or mica). Sedimentation rates were low enough in the Irminger Basin to allow Mn oxide crusts to develop until quartzose turbidites spilled over in the late Oligocene. Gibbsite was not detected in sediment of early Oligocene age and younger, suggesting a regional cooling, increased aridity, and/or leveling of the source area at this time. |
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