PETROGRAPHY AND PROVENANCE OF VOLCANICLASTIC SANDS AND SANDSTONES RECOVERED FROM THE WOODLARK RIFT BASIN AND TROBRIAND FOREARC BASIN, LEG 180
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Date
2002Author
Sharp, Timothy R
Robertson, Alastair H F
Metadata
Abstract
Modal analysis of middle Miocene to Pleistocene volcaniclastic sands
and sandstones recovered from Sites 1108, 1109, 1118, 1112, 1115,
1116, and 1114 within the Woodlark Basin during Leg 180 of the Ocean
Drilling Program indicates a complex source history for sand-sized detritus
deposited within the basin. Volcaniclastic detritus (i.e., feldspar,
ferromagnesian minerals, and volcanic rock fragments) varies substantially
throughout the Woodlark Basin. Miocene sandstones of the inferred
Trobriand forearc succession contain mafic and subordinate
silicic volcanic grains, probably derived from the contemporary Trobriand
arc. During the late Miocene, the Trobriand outerarc/forearc (including
Paleogene ophiolitic rocks) was subaerially exposed and eroded,
yielding sandstones of dominantly mafic composition.
Rift-related extension during the late Miocene-late Pliocene led to a
transition from terrestrial to neritic and finally bathyal deposition. The
sandstones deposited during this period are composed dominantly of
silicic volcanic detritus, probably derived from the Amphlett Islands
and surrounding areas where volcanic rocks of Pliocene-Pleistocene age
occur. During this time terrigenous and metamorphic detritus derived
from the Papua New Guinea mainland reached the single turbiditic
Woodlark rift basin (or several subbasins) as fine-grained sediments. At
Sites 1108, 1109, 1118, 1116, and 1114, serpentinite and metamorphic grains (schist and gneiss) appear as detritus in sandstones younger than
~3 Ma. This is thought to reflect a major pulse of rifting that resulted in
the deepening of the Woodlark rift basin and the prevention of terrigenous
and metamorphic detritus from reaching the northern rift margin
(Site 1115). The Paleogene Papuan ophiolite belt and the Owen Stanley
metamorphics were unroofed as the southern margin of the rift was exhumed
(e.g., Moresby Seamount) and, in places, subaerially exposed
(e.g., D'Entrecasteaux Islands and onshore Cape Vogel Basin), resulting
in new and more proximal sources of metamorphic, igneous, and ophiolitic
detritus. Continued emergence of the Moresby Seamount during
the late Pliocene-early Pleistocene bounded by a major inclined fault
scarp yielded talus deposits of similar composition to the above sandstones.
Upper Pliocene-Pleistocene sandstones were deposited at
bathyal depths by turbidity currents and as subordinate air-fall ash. Silicic
glassy (high-K calc-alkaline) volcanic fragments, probably derived
from volcanic centers located in Dawson and Moresby Straits, dominated
these sandstones.