The Pleistocene paleogeography and environment, being similar to the present, is the easiest to envision, but it is significantly different from that of the Pliocene. A Quaternary carbonate platform developed on the Trobriand forearc west of 151°1´E (Fig. F2), replacing the formerly siliciclastic deposition there. It is represented by the Kiriwina Limestone in the Goodenough 1 and Nubiam 1 wells (Fig. F4). East of 151°10´E, the northern margin continued to subside in middle bathyal water depths (Fig. F7), outpacing any possible reef growth except along the outer forearc high (the Trobriand-Woodlark trend) and around Pliocene volcanic centers such as Egum Atoll (Fig. F2). The rift-onset erosional unconformity, which was at sea level at 8.4 Ma, is now under ~0.8 km of sediments and 2.2 km of water on the northern rift flank in the vicinity of Site 1109 and is buried beneath ~2.5 km of sediments in the ~3-km water depths of Moresby rift (Fig. F8).
That the middle and late Pliocene sediment compositions and depositional styles at Sites 1114 and 1116 on Moresby Seamount are very similar to those of Site 1108 at Moresby rift supports the inference that all three sites were in laterally connected depositional basins during Pliocene time (Robertson et al., 2001). In contrast, movement in the Pleistocene on the normal faults that bound Moresby Seamount uplifted the footwall Seamount and lowered the hanging-wall rift basins such that these sites are no longer in depositional continuity (Fig. F8). This isolation by faulting is younger than the late Pliocene deposition at Sites 1114 and 1116 (Figs. F6, F7) and so must have occurred in the Pleistocene. Furthermore, terrigenous sediments ceased to reach Sites 1109 and 1115 during the Pleistocene (Robertson and Sharp, this volume) (note, the Quaternary section was drilled without coring at Site 1118). The significant decrease in sedimentation rates seen in the sediment and seismic stratigraphies at the northern margin sites, as well as at Moresby rift Site 1108, occurred at ~1.2 Ma (Figs. F7, F8) (Goodliffe et al.; Takahashi et al., both this volume). We infer that rift faulting had sufficiently changed the depositional pathways so as to isolate these sites from all but local clastic input by this time.
Structural analysis of core and FMS borehole images from Site 1114 indicates an oblique extensional system on Moresby Seamount, with differential rotation of strata within fault-bounded blocks (Louvel et al., this volume). Nevertheless, the strata dominantly dip 10°-40° northwest, and the tectonic breccia at the faulted sediment-basement contact dips ~60° southwest, consistent with the seismic reflection data (e.g., Fig. F8) (Louvel et al., this volume; Goodliffe et al., 1999).
Given that at the longitude of Moresby Seamount nearly all seismicity is focused on Moresby rift, it is reasonable to consider the implications of focusing all the recent extension there and to compare this estimate with that derived from observed offsets on the rift-bounding faults. Using the Woodlark Basin opening poles of Taylor et al. (1999), we obtain an average extension rate of 37 mm/yr along azimuth 355° over the last 1.2 Ma at Moresby rift (specifically, at 9.7°S, 151.6°E)—a value that is well constrained by the seafloor-spreading magnetic anomalies farther east. For comparison, the total basement offset on the Moresby normal fault is ~10 km horizontally and 5 km vertically (which equates to 11 km at 27° dip), and the corresponding offsets on the northern antithetic fault of the Moresby rift are 1.5 km horizontally and vertically. One-third of the total 11.5 km of horizontal extension represented by the Moresby rift north of Moresby Seamount occurred early in the rift history (8.4-3.8 Ma) (Fig. F8). Therefore, <220 k.y. is required to account for the other 8 km of horizontal offset if the full extension rates typical of the last 1.2 Ma (37 mm/yr) are applied.
We do not propose that all the extension over this period was focused on Moresby rift alone. For example, the faults bounding the basin south of Moresby Seamount probably accounted for a similar amount of extension during this time. It is clear, however, that with most of the total opening focused ahead of the seafloor spreading tip (i.e., around Moresby Seamount), vertical motions associated with extensional faulting will have radically changed the local topography and sedimentation. Examples of these effects have been mentioned above, including the uplift and isolation of Moresby Seamount and the cessation of sediment onlap onto the northern margin.
Other than pelagics, Quaternary sedimentation in Moresby rift has been limited to talus derived from fault scarps plus sediments eroded from the northern margin, as confirmed at Sites 1108 and 1110-1113 (Shipboard Scientific Party, 1999). A detailed oxygen isotope and nannofossil taxonomic study of surface sediments at the northern margin Site 1109 revealed that the top 17 mbsf were deposited in 195 k.y. and that the relative abundance of species is consistent with the waxing and waning of the Western Pacific Warm Pool in phase with interglacial and glacial cycles (Takahashi and Okada, in press).