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Previous work in antarctic regions south of Australia and New Zealand described an erosional disconformity, centered in the southern Tasman basin, that formed when bottom currents removed sediments as old as late Miocene (Gilbert). Detailed analyses of cores from Eltanin cruise 39 indicate, however, that the extent of the disconformity is considerably greater than previously reported, especially on the west, along the Southeast Indian Ridge. Sediments just below the erosional surface have a remarkably constant late Gauss age (2.6 m.y.b.p.), but no specific areal trends could be determined from observed age variations. Locally, some Brunhes-age sediments have accumulated during the last 0.3 m.y., and range in thickness from thin veneers to 2 m or more.
Late Gauss and late Brunhes sedimentary hiatuses, as observed in the present area, are easily overlooked in routine core studies. The magnetic polarity is the same, and biostratigraphic definition of the late Brunhes in these latitudes is difficult. An excellent illustration of this problem is given by core E39-40. Th230 measurements, visual inspection, evaluation of radiolarian and foraminiferal indices, and polarity changes demonstrate that sediments in the top 60-70 cm are of Brunhes age (less than 0.3 m.y.B.P.), that sediments contain certain reworked Gauss elements, and that the hiatus is marked by a layer of distinctly stained (feromanganese coatings?) foraminifers. A Gauss-Gilbert sedimentary sequence follows in the remainder of the core.
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