Southward subduction along the Trobriand Trough produced arc magmatism in Papua since at least the early Miocene (Davies and Smith, 1971; Davies et al., 1984). During the early and middle Miocene, the Trobriand forearc was extensional and a forearc basin developed that filled with sediments up to 5-7 km thick (Tjhin, 1976; Pinchin and Bembrick, 1985; Francis et al., 1987). The basin was inverted in the west, and the outer forearc high was uplifted in the middle-late Miocene. Multiple reverse thrusts occur in the west, but the basin is more open north of the D'Entrecasteaux Islands where the depocenter shifted southward as the outer forearc was up-tilted (Tjhin, 1976; Pinchin and Bembrick, 1985; Francis et al., 1987). Farther east, from ~151°E to the Leg 180 drilling transect, the middle-late Miocene sediments prograded from the emergent Papuan Peninsula northward across the forearc to a depocenter just south of the outer forearc high (Davies et al., 1984; Taylor, 1999; Fang, 2000). The forearc basin filled to sea level in the late Miocene.
This Miocene forearc basin history is amply documented west of 151°E by commercial wells (Goodenough 1 and Nubiam 1) and seismic surveys (Tjhin, 1976; Pinchin and Bembrick, 1985; Francis et al., 1987). Of note, the early Miocene (and late Oligocene?) Iauga volcanics (tuffs and tuffaceous conglomerates) at the base of the Goodenough 1 well attest to the early Trobriand arc volcanism (Fig. F4). The middle Miocene was a time of deepwater deposition of (silty) shales with tuffaceous interbeds that in the late Miocene shoaled upward during deposition of the thick Nubiam shale (Fig. F5). Within the regional grid of seismic profiles tied to the two wells, horizon "A" marks an important late Miocene unconformity (or correlative conformity near the basin depocenter at Goodenough 1 well) (Tjhin, 1976; Pinchin and Bembrick, 1985; Francis et al., 1987) beneath the pro-delta Ruaba unit.
More recent seismic surveys (Mutter et al., 1996; Goodliffe et al., 1999; Fang, 2000) and Leg 180 drilling at Site 1115 now confirm and extend this history to the eastern end of the forearc basin. A rapidly deposited (>375 m/m.y.) sequence of middle Miocene (~14-13 Ma) volcaniclastic clays, silts, and sands was recovered at Site 1115 beneath a late Miocene unconformity (correlative to horizon "A") (Figs. F6, F7) (Shipboard Scientific Party, 1999; Lackschewitz et al., 2001; Takahashi et al., this volume). Paleowater depths, initially >500 m, shoal upward, accompanied by an increasing component of redeposited shallow-water carbonates. Lithic fragments and geochemical analyses of the volcaniclastics indicate source terranes of calc-alkaline extrusives, dominantly basic with lesser rhyolitic and rare alkalic (Sharp and Robertson; Robertson and Sharp; Cortesogno et al., all this volume). Two rhyolitic clasts (in Pleistocene talus from Sites 1110 and 1111) gave 238U/206Pb zircon ages of 15.7 ± 0.4 Ma and provide further evidence for middle Miocene Trobriand arc volcanism (Monteleone et al., this volume). Sporadic appearances of high Cr and Ni contents in fine-grained sediments at Leg 180 sites suggest that ultramafic source rocks were exposed/weathering since at least the middle Miocene (Robertson and Sharp, this volume).
Although the record is clear, the cause of the late Miocene forearc emergence is uncertain. The compression in the west may be related to early phases of the collision of the Huon-Finisterre forearc with the Trobriand forearc, resulting from the progressive closure of the Solomon Sea by doubly vergent subduction (Fig. F1) (Jaques and Robinson, 1977; Cooper and Taylor, 1987; Pegler et al., 1995). However, the extent of the late Miocene unconformity around the Trobriand forearc basin begs a more regional cause. It may be subduction related and/or it may be related to the onset of rifting. But it certainly reflects increased sediment supply from increasingly emergent margins (Smith and Davies, 1976), in part the result of the largest fall in global sea level during the Tertiary (~225 m between 15 and 10 Ma) (Haq et al., 1988).
The age of the unconformity surface is best defined by the correlative conformity at the Goodenough 1 well. Our reexamination of the reported biostratigraphy there shows that horizon "A" (at the base of the Ruaba unit at 1445 meters below seafloor [mbsf]) is located between the first and last occurrences of Minylitha convallis (i.e., between 9.3 and 7.8 Ma) (Berggren et al., 1995, p. 190) at 1865 and 1180 mbsf. Direct interpolation (equivalent to assuming a constant sedimentation rate for this interval of 457 m/m.y.) places horizon "A" at 8.4 Ma (Fig. F5). This result fits well with the age constraints from the other drill holes, namely that there are sediments beneath the unconformity at the Nubiam 1 well that are younger than 9.3 and older than 8.3 Ma and that sediments above it at Site 1115 are younger than 8.6 and older than 5.54 Ma (Figs. F5, F6, F7).
The extent of the late Miocene erosion varies, from none at Goodenough 1 well near the contemporary basin depocenter to 4-5 m.y. of missing section at Site 1115 near the eastern basin edge. That the amount of actual uplift of much of the eastern forearc basin may have been quite small (<50 m) is attested by seismic reflection profiles that show the preservation of the topsets of prograding clinoforms immediately beneath the unconformity (Taylor, 1999; Fang, 2000). Furthermore, the angular change in bedding orientation across the unconformity at Site 1115 is small, from dipping 5°-10° to the northwest (toward the forearc basin depocenter) to horizontal (Célérier et al., this volume). This stratal geometry also confirms that the 8.4-Ma unconformity surface approximates paleo-sea level, which provides an important horizon for tracking subsequent vertical tectonics.