CONCLUSIONS

1. Sandstone detritus within the middle Miocene Trobriand forearc basin was derived predominantly from pyroxene basalts and from less-abundant silicic volcanics. Calc-alkaline volcanics related to the Trobriand Arc are thought to be the source for this detritus. However calc-alkaline volcanoes located within the inferred forearc (e.g., Woodlark Island) might also have provided a source. The presence of silicic and alkalic volcanic debris in rocks of this age further suggests that the contemporary Trobriand arc had a complex volcanic history (Figs. F19, F20).
2. With the onset of rifting in the late Miocene, sandstone and conglomerate deposited in the Woodlark rift basin prograded generally northward. They progressively covered and were sourced from the exposed Trobriand outerarc/forearc, which included Paleogene ophiolitic rocks. Derivation from the hinterland of Papua New Guinea, including the Paleogene Papuan ophiolite belt is unlikely, as ophiolite rocks other than metadolerite are absent from the detritus of the sandstones (Figs. F19, F20).
3. Upper Miocene-Pleistocene sandstone deposited during continuing extension and subsidence of Woodlark Basin indicates a change to more explosive silicic volcanism, although sporadic basaltic/doleritic detritus in rocks of this age indicates mafic volcanics continued supplying the basin. Assuming sediment pathways were similar to today (from the northwest), the probable source of ash and volcaniclastic turbidites were the Amphlett Islands, Moresby Strait, Dawson Strait (e.g., Dobu Seamount), and surrounding areas where Pliocene-Pleistocene volcanic rocks occur. Additional sources of volcanics could be the D'Entrecasteaux Islands, active Trobriand Arc volcanoes on the northern rift margin (i.e., the Luscany Islands, Trobriand Island, Woodlark Island, and Egum Atoll), the eastern Papua Peninsula, and sediment reworked from the Cape Vogel Basin to the northwest (Figs. F19, F20).
4. Complementary geochemical studies (Robertson and Sharp, this volume) revealed sporadic absolute abundances of trace metals Cr and Ni and, locally, Cu and Zn in Pliocene hemipelagic sediments and relatively high Al, K, Na, and minor elements Rb, Zr, and Y within lower-middle Pliocene hemipelagic sediments. These are interpreted to indicate that terrigenous and ultramafic derived sediments had access through fine-grained sediments to a single turbiditic Woodlark rift basin (or several subbasins) until late Pliocene time (Robertson and Sharp, this volume) (Figs. F19, F20).
5. The influx of sand-sized serpentinite and metamorphic detritus in the late Pliocene (~3 Ma) is thought to reflect a major change in the architecture of the Woodlark rift basin. A discrete pulse of rifting in the late Pliocene resulted in the deepening of the Woodlark rift basin, and terrigenous input to the northern rift margin was cut off. The Paleogene Papuan ophiolite belt and the Owen Stanley metamorphics were unroofed as the southern margin of the rift was exhumed (e.g., Moresby Seamount) and, in places, subaerially exposed (e.g., D'Entrecasteaux Islands and onshore Cape Vogel Basin), resulting in new and more proximal source of metamorphic, igneous, and ophiolitic detritus. Continued emergence of the Moresby Seamount during the late Pliocene-early Pleistocene bounded by a major inclined fault scarp yielded talus deposits of similar composition to the above sandstones (Figs. F19, F20).
6. Growth of a carbonate platform on the gently subsiding Trobriand Basin to the northwest (Tjhin, 1976) markedly reduces the clastic input the Woodlark rift basin throughout the Pleistocene. Silicic vitric fragments become the dominant type of detritus deposited at Sites 1109 and 1118 during this time. These glasses are indicative of high-K calc-alkaline volcanic centers, possibly located in the Dawson Strait, Moresby Strait, or Dobu Seamount area (Lackschewitz et al., 2001; Robertson and Sharp, this volume) (Figs. F19, F20).