CONCLUSIONS

1. Fine-grained sediments (clays, claystones/muds, and mudstones) of latest Miocene-Holocene age, which volumetrically dominate the Woodlark Basin, are partly of volcanogenic origin, derived mainly from a calc-alkaline volcanic centers. In addition, terrigenous sediment was derived from the Papua New Guinea landmass, the D'Entrecasteaux Islands, the onshore Cape Vogel Basin, and adjacent now-submerged areas.
2. The most northerly sites on the northern rift flank (Site 1115) penetrated a mid-late Miocene forearc succession, including relatively Mg and K rich volcanogenic fine-grained sedimentary rocks interpreted as derived from relatively mafic calc-alkaline volcanic centers. This volcanism was related to southward subduction of oceanic crust beneath the Miocene Trobriand arc.
3. K-poor, relatively Ti-rich fine-grained sedimentary rocks associated with conglomerates that unconformably overly massive dolerite are inferred to have been derived from mafic igneous rocks, probably related to the Paleogene Papuan ophiolite.
4. Fe-rich fine-grained sedimentary rocks above basalt/diabase conglomerates at both Sites 1109 and 1118 reflect deposition of strongly weathered material from a tropically weathered landmass.
5. Common occurrence of smectite and kaolinite at all sites reflects tropical weathering of land source areas.
6. Very high absolute abundances of the trace metals Cr and Ni (also Cu and Zn, locally) occur sporadically throughout the Pliocene successions at all sites and are attributed to input from ultramafic rocks of the Paleogene Papuan ophiolites or their alternation products (e.g., crysotile, talc, and chlorite). The probable source was the internal highlands. Exclusively fine-grained terrigenous sediment reached all parts of the present western Woodlark Basin during early-middle Pliocene time, prior to development of the present deep rift basin. Coarse sediment was probably trapped in peripheral basins at this stage.
7. Fine-grained terrigenous-derived sediment occurs at all sites in the Woodlark Basin, as recorded by relatively high Al, K, Na, and minor elements, including Rb, Zr, and Y. Terrigenous input is, however, most marked at Site 1118 in the Woodlark rift.
8. The presence of mixed terrigenous and ultramafic in addition to volcanogenic constituents is interpreted to indicate that all of the sites formed part of a single turbiditic basin (or several sub-basins) during Pliocene time, until uplift of the southern rift flank to form the Moresby Seamount in late Pliocene-Pleistocene time. Low-density turbidity currents were presumably able to reach all parts of the Woodlark Basin until late Pliocene time when terrigenous input to the northern rift margin was cut off, probably as a result of deepening of the Woodlark rift to the south.
9. Complementary petrographic studies (see Sharp and Robertson, this volume) reveal input of silt-sand-sized terrigenous schist, gneiss, and serpentinite within sediments of <3 Ma at several sites in the Woodlark Basin (1109 and 1108). However, no corresponding increase in lithogenous chemical constituents is seen at this time in the composition of the fine-grained sediments. This is probably because fine-grained terrigenous sediment was derived from adjacent land areas and accumulated within the Woodlark Basin throughout its late Miocene-Holocene history. By contrast, coarser gained sediment only reached the basin after 3 Ma, following a pulse of rifting and exhumation of the southern rift margins (e.g., in the D'Entrecasteaux Islands and Cape Vogel Basin).
10. The uppermost volcanogenic muds rich in volcanic ash (Sites 1109 and 1115) were derived from a distinctive high-K calc-alkaline volcanic source, probably derived from distinctive high-K calc-alkaline volcanoes in the Dawson Straits, Moresby Strait, or Dobu Island (Lackschewitz et al., 2001).