University of Hawaii, United States

Five fallout tephra layers and 13 heterolithological volcaniclastic deposits drilled at Holes 1115A, 1115B, 1115C, 1109C, 1109D and 1118A, during Leg 180 on the downflexed northern margin of the western Woodlark Basin, have been dated by single-crystal laser (super 40) Ar/ (super 39) Ar analyses. The fallout tephra layers range in age from 0.135+ or -0.008 Ma to 2.84+ or -0.03 Ma. Sedimentation ages determined for the volcaniclastic deposits range from 1.75+ or -0.29 Ma to 3.79+ or -0.01 Ma, closely matching the nannofossil, planktonic foraminifer and palaeomagnetic chronostratigraphies of the holes. However, two volcaniclastic deposits from 516.91 m below seafloor (mbsf) and 632.5 mbsf in Hole 1118A are significantly older than indicated by biostratigraphic and palaeomagnetic data, probably because of the presence of older reworked volcanic crystals. The youngest ash layer is derived from explosive eruptions in the Dawson Strait area of the D'Entrecasteaux Islands, whereas the four older tephra layers are attributed to explosive eruptions in the Moresby Strait area of the D'Entrecasteaux Islands. The (super 40) Ar/ (super 39) Ar ages of volcaniclastic sand layers in Holes 1115C and 1118A indicate a transition from a shallow-water succession (<150 m) to a deeper-water succession (150-500 m) with rapid deposition of volcaniclastic sands, mainly by turbiditic currents, at 3.8 Ma. This transition is related to the subsidence of the margin during rifting of the Woodlark Basin. Two volcaniclastic deposits with ages of 13.84+ or -0.02 Ma and 14.04+ or -0.03 Ma, respectively, provide important time markers in the middle Miocene sedimentary sequence at Hole 1115C, where biostratigraphic ages are scarce. Our (super 40) Ar/ (super 39) Ar ages represent the first marine record of Miocene to Pleistocene volcanism in the area of eastern Papua.