Deep Sea Drilling Project Leg 60: Cruise Objectives, Principal Results, and Explanatory Notes

Abstract

PRINCIPAL RESULTS Pacific PlateSite 452 was planned as a Pacific plate reference section to identify old ocean basin sediments and rocks which our later drilling might uncover in the inner (western) trench wall.At two holes at this site, we encountered hard impenetrable cherts of probable Campanian age after coring only a few tens of meters of mixed Neogene pelagic sediments.These thin Neogene sediments gave insufficient support to the drill collars to allow spudding into the chert.The site, one of 16 in the far western Pacific with a profound Cretaceous-Neogene disconformity, established that only a very trivial, if any, post-Cretaceous pelagic sediment component from the oceanic side would be expected to be accreted to the inner Mariana Trench wall. Mariana TroughTo test the rate and geometry of back-arc basin opening and the evolution of the new crust formed during opening, we planned one hole in the supposedly oldest part of the trough on the west side, one hole near the center on very young crust, and one hole through the Mariana arc sediment apron on the east side of the trough (Fig. 2).If Mariana Trough spreading is symmetric, then the eastern site should be of intermediate crustal age.Our first attempt at the eastern side of the Mariana Trough proved unsuccessful (coarse sandy turbidites caved into the hole at Site 455 after only 104 m of penetration), so we shifted this target to another site on the distal edge of the volcaniclastic sediment apron, nearer the center of the trough.Drilling at this revised target, Site 456, was successful, as were the other back-arc basin sites.At all three of the successful sites (Sites 453, 454, and 456), turbiditic sequences of volcaniclastic sediments derived from the Mariana Arc overlie igneous and metamorphic rocks (Fig. 3).At all three sites the sediments contain substantial hemipelagic and biogenic components.Furthermore, all sites have a transition midway through the Pleistocene from non-siliceous sediments low in CaCO 3 , indicating sedimentation very close to the carbonate compensation depth, to overlying siliceous oozes.At Site 453, the westernmost site, more recent sediments have a decreasing volcaniclastic component.We attribute this change to the site's increasing distance from the arc volcanoes as the trough opened.The ages of the oldest cored sediments, bathymetric -\ L. cnstatus £. ovata Subzone G. caribbeanica Sub -' E. annula Subzoπ -^C.maciniyrei Subz "\θ.pentaradiatus ~^•D.surculus Subzo D.