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Age relations of Mid-Atlantic Ridge sediments were established by means of planktonic Foraminifera in several selected areas between 22° N. lat. and the equator. Sediment types containing planktonic Foraminifera include unconsolidated ooze, consolidated ooze ranging from loosely friable aggregates to hard limestone, breccia, and palagonite-rich rock.
In the 22° N. area, consolidated sediments ranging in age from late Miocene to probable middle Pliocene were dredged from the flank of the ridge. Associated with the consolidated sediments in the dredges were basalts. Where an age relation could be inferred, the basalt overlies the late Miocene. Studies of Sr/Rb isotopes in the basalts are still in progress at M.I.T. Results thus far, however, indicate that these isotopes do not permit an age assignment. On the crest of the ridge there is no evidence of the presence of sediments older than Quaternary. Materials examined from the crest of the ridge include indurated detrital tuff and palagonitic rock.
Examination of sediments collected from the 11° N. area, Romanche trench and St. Paul's Rocks, are still in progress. In the Romanche trench, late Tertiary planktonic Foraminifera mixed with Quaternary assemblages were recovered from foraminiferal ooze in a core. In the St. Paul's Rocks area, the matrix of a conglomerate containing pebbles of the St. Paul's Rocks type yielded a Pleistocene foraminiferal assemblage. In the same area a vesicular basalt was filled with limestone. The limestone in the vesicles contained a mixed assemblage of late Tertiary and Pleistocene.
It is difficult to arrive at definite conclusions on the basis of the relatively little material thus far examined. Yet, it is interesting to note that the oldest assemblages recognized are late Miocene. Though not conclusive, the present data agree with other evidence suggesting a relatively young tectonic and volcanic history for at least parts of the Mid-Atlantic Ridge.
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