On the whole, the analyzed lithic fragments provide a set of data that largely fits the two-stage model depicted for the evolution of the Woodlark Basin between the late Miocene-Pliocene and Pliocene-Holocene summarized by Robertson et al. (in press) on the grounds of onland geology, geophysical, and drill evidence. The petrographic detail of the nature and distribution of the lithic clasts and grains allows some constraints of the major tectonic and volcanic events.
The clast assemblages at the footwall sites evidence (1) the nature of the oceanic slice exhumed, including generally evolved basaltic effusives and intrusives; (2) the ultramafic rocks that likely pertain to the same ophiolitic complex are restricted to transport by turbidity currents and likely derive from onland outcrops; and (3) the origin of some lithologies by uplift from relatively deep levels, implying extensional tectonics active at least since the middle Pliocene.
Calc-alkaline volcanism is recorded in the sedimentary sequences in continuity with the arc-related products from the Papuan Peninsula and D'Entrecasteaux Islands and to the latest volcanics of the Miocene Trobrian arc. The arc volcanism shows a wide compositional range from medium- to high-K and shoshonitic products; however, no apparent temporal evolution arises from the stratigraphic setting. Lava clasts, volcanogenic grains, and glass shards derive from transport by turbidity currents, whereas in the northern margin Pliocene, the increasing frequency of tephra mostly composed of glass shards and vesicular silicic fragments suggests a transition to more explosive activity and contribution to the sediments from aerial fallout materials.
Evidence of localized alkalic volcanism of presumable early to middle Miocene age is a new finding. It could represent a rift phase earlier than or coeval to the first opening of the Woodlark Basin or, less probably, could derive by depositional trajectories diverted from an adjacent basin.