LEGS 143 and 144

Northwest Pacific Atolls and Guyots

The Western Pacific is strewn with chains and clusters of seamounts, many of which are now flat- topped guyots, with summit depths of about 1500 m, capped by shallow-water carbonate platform sediments overlying volcanic substrate. Guyot sediments can monitor subsidence rates and relative changes in sea level for times when upward carbonate-platform growth paced the tectonic subsidence of their foundations and the volcanic pedestals preserve clues to the nature of their parent mantle materials and the processes of melt extraction and differentiation. Studying the origin of these guyots contributes to the concepts of seafloor spreading, hot spots, thermal subsidence and crustal thinning, and superswells and may determine why the platform, after constructing a massive carbonate cap, suddenly ceased to keep pace with subsidence and why the apparent ages of the underlying volcano and the shallow-water cap lack the coherent patterns predicted from hot-spot and subsidence theories. During Leg 143, six sites were drilled on Allison and Resolution guyots in the Mid-Pacific Mountains (MPM) (Site 865 to Site 868), on the sediment apron flanking the Wodejebato Guyot-Pikinni Atoll pair (Site 869), and within the lagoon of a modern Marshall Islands atoll (Site 870). During Leg 144, seven sites were drilled on the Marshall Islands guyot cluster (Site 871 to Site 877), three sites were drilled on the MIT (Site 878) and Japanese seamounts (Sites 879 and 880), and Leg 129 Hole 801C was reentered to conduct downhole measurements.

The carbonate platform of Allison Guyot formed in the Barremian on a rapidly subsiding edifice and accumulated 1620 m of sediments by Albian time. In contrast, the entire 731-m shallow-water limestone section capping Allison Guyot accumulated during its rapid subsidence in the late Albian. The lagoonal facies sediments indicate shallow water throughout most of the platform histories and often meter-scale facies shifts were observed, implying short-period cycles of emergence and submergence. The summits show evidence of dissolution and mineralization, indicating emergence and karsting after the late Albian and, as pelagic sediments infilling the dissolution cavities suggest, before the mid-Turonian. Drilling and geophysical data imply a relative sea-level drop of 160 m. Holes drilled into, and next to, the perimeter mound surrounding the summit of Resolution Guyot recovered no reefal material, suggesting that these mounds, common on guyot profiles, are unlike reefs on Cenozoic atolls. The volcanic pedestal below Resolution Guyot is only about 500 m higher than the top of the adjacent basaltic plateau that underlies most of the MPM; the MPM may have been an extensive shallow water platform during the Early Cretaceous. Seismic velocities had been underestimated and large acoustic impedance contrasts formed within the section, masking true basement; estimates of limestone cap thickness for other guyots may be similarly erroneous.

Little shallow-water debris was encountered on the apron of the guyot-atoll pair. Turbidity currents, grain flows, and mass flows delivered volcaniclastics to the site through late Cenomanian and Maastrichtian times, especially in the Cenomanian and Campanian, implying volcanism on the nearby edifices. A few shallow-water fragments and coalified plant remains in Cenomanian layers suggest the existence of nearby land and shallow carbonate shoals. During the Cenozoic, volcanism ceased and pelagic sedimentation, interrupted by turbidity currents, prevailed.

The five guyots investigated during Leg 144 were all initiated by volcanic activity south of the paleoequator and were carried northward by steady movement of the Pacific Plate. The carbonate caps on these emergent seamounts kept pace with subsidence rates that, in some cases, may have exceeded 100 m/m.y. Most of the platforms contain paleoecologic-assemblage and sedimentary- facies evidence of multiple relative sea-level highstands and lowstands and the ability to keep pace with rapid subsidence and recover from sea-level excursions during their lifetime of at least 10 m.y. The Cretaceous carbonate platforms were much different from the present Pacific atoll-reef systems in that a wave-resistent, framework barrier reef was of minor importance or did not exist, instead they may have produced vast quantities of loose carbonate sediment in large shoal deposits with rudist-algal-coral boundstones forming relatively thin bioherms on the exterior ridges near the margins of the guyot. The observed shallowing-upward trend and little evidence of substantial exposure prior to drowning suggest that a contributing cause to the termination of these platforms was restriction of the areal extent of the active carbonate factory during sea-level lowstands, followed by failure to reestablish enough carbonate production to keep pace with rising waters. Sea-level change possibly acted in concert with other factors. There were at least three major episodes of carbonate platform drowning within the tropical Pacific - Albian, end of the Maastrichtian, and middle Eocene. These periods are characterized by a warm climate but environmental stress on the carbonate platform ecosystems may have been caused by factors such as paleoceanographic changes in circulation and nutrient cycling and by paleogeographic position.

The guyots show considerable variability from the "ideal" thermal subsidence models for seamounts. Subsidence of seamounts is not a simple function of age - uplift and reactivation of volcanic edifices can occur millions of years after the main seamount-building stage.