Abstract
Latest Miocene-Pleistocene synrift sediments at Ocean Drilling Program sites 1109, 1115, and 1118 (Leg 180), located on the hanging wall margin north of the
Moresby fault in the Woodlark Basin, showed variations in magnetic parameters carried by magnetite and maghemite related to sedimentation process in the basin. At sites
1109 and 1115, an increase in the sedimentation rate at 3.8 Ma was accompanied by the deposition of sediments with low ferrimagnetic mineral concentrations. An increase in
the ferrimagnetic mineral concentrations occurred between 3.4 Ma and 3.2 Ma at the three sites. The onset age of the change became younger with distance from the subsidence
center of the basin near the Moresby fault: 3.4 Ma at Site 1118, 3.3 Ma at Site 1109, and 3.2 Ma at Site 1115, which implies a northward onlapping of sediments with high
ferrimagnetic mineral concentration. Sediments with finer-grained ferrimagnetic minerals were deposited between 2.3 and 2.0 Ma at sites 1118 and 1109 and later, 2.8 Ma at
Site 1115 during a period of a low sedimentation rate. The upper parts of sites 1109 and 1115 had a diamagnetic contribution, which is attributed to relatively high
concentrations of diamagnetic pelagic materials at a low sedimentation rate associated with the low frequency of turbidites.
Reprinted with permission from Terra Scientific Publishing Company.
Abstract
The results of drilling during Ocean Drilling Program Leg 180 provide insights into fundamental processes of continental break-up, because rifting can be related to westward propagation of a spreading centre into continental crust. A generally north-south transect of holes was drilled across the Woodlark Rift on the uplifted northern rift margin on the Moresby Seamount (Sites 1114 and 1116),
on the hanging wall of the low-angle (25-30°) extensional Moresby Detachment Fault (Sites 1108, 1110-1113 and 1117) and across the downflexed northern rift margin (Sites 1118, 1109 and 1115). The results, when placed in
the regional tectonic context, document a history of Palaeogene ophiolite emplacement, followed by Miocene arc-related sedimentation. Regional uplift and
emergence of the forearc area took place in Late Miocene time. Submergence to form the Woodlark Rift began in latest Miocene time, marked by widespread
marine transgression and shallow-water deposition, accompanied by input of
air-fall tephra and volcaniclastic sediments. During Pliocene time, deposition
within the rift basin was dominated by deep-water turbidites, including high-density turbidites in the south. Strong extension along the north-dipping
Moresby Detachment Fault was active during Pleistocene time, associated with uplift of the Moresby Seamount and shedding of fault-derived talus, mainly
of meta-ophiolitic origin. During Pleistocene time, a carbonate platform was
constructed to the NW, trapping clastic sediment and resulting in a switch to slower, more pelagic and hemipelagic deposition within the Woodlark Rift Basin.
The marked change in rift basin configuration during Pleistocene time may relate
to westward propagation of the Woodlark oceanic spreading centre at c. 2 Ma.
Reprinted with permission from the Geological Society of London.