(Table 1) Distribution of larger benthic and planktonic foraminifers in ODP Hole 135-841B

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Larger foraminifers were recovered from three levels within Hole 841B, located on the Tongan Platform. The lowest faunas are from Eocene beds (Unit V) in fault contact with an older rhyolitic volcanic sequence. These Eocene faunas are typified by two assemblages: one with Amphistegina waiareka, Astewcyclina matanzensis, Discocyclina omphala, Operculina pacifica, together with rare Heterostegina saipanensis and Sherbornina carteri; the second assemblage contains these species and, in addition, Spiroclypeus vermicularis. The lowest assemblage is associated with a middle Eocene planktonic foraminifer fauna (Acarinina, Morozovella), and the upper faunas with a late Eocene assemblage. This Eocene unit is disconformably overlain by a middle Miocene sequence (Unit IV) containing assemblages dominated by planktonic foraminifers. Two samples were found to contain shallow-water benthic foraminifers: one contained Amphistegina radiata, Cycloclypeus sp., Lepidocyclina (Nephrolepidina) ?howchini, and Sphaerogypsina globula; a second sample from this section contained only Amphistegina radiata. The third level at which larger foraminifers were found was in the lower part of the late Miocene section (Unit III), where late Eocene larger foraminifers (Asterocyclina matanzensis, Discocyclina omphala, Nummulites pengaronensis, and Pellatispira madaraszi) are present in a conglomeratic sequence with late Miocene planktonic foraminifers present in the matrix. The two larger foraminifer assemblages of Unit V are typical of the Ta3 and Tb Letter Stages, which are distinguished by the presence of Spiroclypeus vermicularis. The Ta3 Letter Stage assemblage is associated with a Zone P14 planktonic fauna, and the Tb Letter Stage assemblage with Zones PI5 to PI6 fauna. The larger foraminifers associated with middle and late Eocene planktonic foraminifers are almost certainly in situ, and indicate water depths from between 50 and 100 m in the middle Eocene, shallowing to <50 m, and then gradually deepening to >100 m by the end of the Eocene. The middle Miocene faunas have been derived from shallow-water environments adjacent to the sites of deposition (which would have been in excess of 100 m), and may have been contemporaneous or reworked from rocks no older than the Ta5 Letter Stage. Late Eocene larger foraminifers (Tb Letter Stage) were reworked into the deep-water, late Miocene environments as products of erosion from adjacent exposures of Eocene carbonates. A hiatus is probably present in the lower part of the Eocene section, separating the middle and late Eocene (Ta3 and Tb Letter Stages-Zones P14 and P15). This is probably the result of eustatic changes. Unit V almost certainly contains the Eocene/Oligocene boundary, based on the planktonic foraminifer evidence, but larger foraminifers disappear some distance below this boundary, within Zones P15 or P16. The larger foraminifer faunas from Units III and V provide evidence that water depths at this part of the Tongan Platform have increased from approximately 50 m during the late middle Eocene to below the calcium carbonate compensation depth by the late Miocene, respectively.

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