The oldest sediments recovered of middle to late Miocene age at the most northerly site (Site 1115) are mainly deep-water volcaniclastic turbidites related to the Miocene Trobriand arc (Fig. F4). This arc relates to southward subduction of oceanic crust of the Solomon Sea, located to the north (Honza et al., 1984; Davies et al., 1987). Seismic interpretation suggests that a forearc sedimentary wedge ~5000 m thick, is present beneath part of the Woodlark Basin (Fig. F3). Drilling at Sites 1109 and 1118 terminated in diabase and basalt, either as massive rock (Site 1109) or as conglomerate (Site 1118). These lithologies probably originated as part of a Paleogene ophiolite (Taylor, Huchon, Klaus, et al., 1999; Robertson et al., 2001).
During the late Miocene, the forearc underwent regional emergence as also documented by industry wells to the northwest (Harris et al., 1985). At Site 1109, fluvial channelized? conglomerates, deposited in a nonmarine paralic setting, were overlain by lagoonal then open-marine carbonates. Similar conglomerates are abruptly overlain by open-marine carbonates with abundant coralline algae at Site 1118 further south. In the north, at Site 1115, 4-m, basalt-derived conglomerates with root traces are overlain by shallow-marine to lagoonal facies. The source of these conglomerates was possibly a then-emergent forearc to the southwest or west.
Paralic to inner neritic sediments appear at Sites 1109 and 1115 at approximately the same time (>5.54 and <8.6 Ma). Shallow-water deposition was followed by Pliocene-Pleistocene deeper water hemipelagic and turbiditic deposition with discrete episodes of volcanic ash and volcaniclastic turbiditic input. In addition, the two sites (Sites 1114 and 1116) on the uplifted footwall (Moresby Seamount) document relatively proximal turbiditic and mass-flow deposition related to Pliocene rifting of the Trobriand forearc.