Comparison of the Origin and Evolution of Northwest Pacific Guyots Drilled during Leg 144

Abstract

Five northwest Pacific guyots were drilled during Leg 144. Drilling results indicate that carbonate platforms grew atop volcanic edifices produced by two pulses of volcanism associated with hotspots. The first pulse of volcanism, during Barremian-Albian time, constructed the MIT and Takuyo-Daisan edifices and the pedestals of Lo-En and Wodejebato guyots, and was widespread at abyssal locations. A second pulse of volcanism, mainly across the Santonian/Campanian boundary, created emergent islands in the northern Marshall Islands and Line Islands. Eruptions during the younger portion of this second episode formed some of the southern Marshall Islands chain, including Limalok Guyot. Independent of their age of formation, most of the volcanic islands developed weathering profiles (including soils) and forests. Two to seven million years after an island formed, its edifice was flooded and a carbonate platform began. These carbon-ate platforms responded to a complex function of sea-level fluctuations and environmental changes. Shallow-water sedimentation on Leg 144 platforms terminated either in Albian, late Maastrichtian, or middle Eocene time. Their termination was not a simple mid-Cretaceous drowning event. When the northward motion of the Pacific Plate brought the platforms into the zone of equatorial upwelling, this apparently inhibited carbonate production, enhanced bioerosion, and the platforms died. Modern Pacific atolls are an inadequate analog for these ancient Pacific platforms. The Cretaceous and Eocene carbonate platforms resembled modern carbonate banks, rather than modern atolls that have a coral-algal reef framework surrounding a lagoon. These Cretaceous and Eocene platforms produced vast quantities of loose carbonate sediment in large shoal deposits. Cretaceous rudist-algal-coral boundstones formed relatively thin bioherms associated with shoals; this association built peri-platform ridges near the summit margins.