Palaeomagnetism and the Globorotalia truncatulinoides Datum in the Tasman Sea and Southern Ocean: Reply

Abstract

DR THEYER replies: Watkins, Kennett and Vella's critique1 requires further discussion on a baffling stratigraphic problem occurring in the far South-east Indian Ocean. In that area, Globorotalia truncatulinoides overlaps with distinctive late Miocene-Pliocene foraminifers and radiolarians in at least twelve widely separated piston cores2,3. To explain an overlap of such magnitude and stratigraphic consistency by invoking simple reworking seemed unreasonable to me and I therefore proposed a mid-Pliocene (Gauss) G. truncatulinoides datum plane for this area2. There are, of course, alternative interpretations. One can assume, for example, that the region is affected by an extensive Brunhes-Gauss disconformity (representing a removal of mostly Matuyama sediments) and that some mixing of Brunhes and Gauss sediments has occurred above and below the hiatus. This explanation is compatible with a Quaternary range of G. truncatulinoides, which now seems established (J. P. Kennett, personal communication) for at least parts of the present area and would agree with the early Pliocene evolutionary sequence of the radiolarian Lamprocyclas heteroporos4 which occurs in the Gilbert sections of cores E39-40, E39-48 and E39-56 (ref. 3). It would also be in keeping with the 230Th dates in E39-40 (ref. 3) and the internal consistency of all important radiolarian and foraminiferal ranges in the cores. Finally, it would agree with previous work in this area which describes an erosional disconformity of predominantly Gauss age3,5. This alternative explanation is now being tested with additional palaeomagnetic work, 230Th analyses and study of nannofossils. I recognize that my original interpretation, which at the time of its composition seemed adequately substantiated by my findings2,3, is only one of other possible explanations and may even be wrong. Watkins, Kennett and Vella1, however, do not really question my interpretation as such, but attempt to invalidate it by discrediting my palaeomagnetic measurements and faunal identifications.