The results of drilling on Resolution and Allison guyots, in the Mid-Pacific Mountains (MPM), document a long history of volcanism, subsidence, and accumulation of Hauterivian-Albian shallow-water carbonate sediments. Mid-Cretaceous emersion was followed by subsidence and accumulation of pelagic sediments.Basement beneath Resolution Guyot is subaerial flows of alkalic basalt, with radiometric dates averaging 127.6 ±2.1 Ma, emplaced at about 14°S. Overlying shallow-water carbonates are 1620 m thick. Sediments at Site 866, 2 km inward from the platform edge, were deposited in shallow subtidal to intertidal depths, with intermittent subaerial exposure. At Site 867, 0.5 km from the platform edge, beach and storm deposits are common, and at Site 868,0.1 km from the edge, sponges and rudists in life positions indicate a platform-margin environment.At Allison Guyot, alkalic basalt sills were cored at the bottom of Hole 865A. Seismic profiles suggest as much as 600 m of sediments underlie the sills, which have 40 Ar/ 39 Ar radiometric dates averaging 110.7 ±1.2 Ma. The drilled strata, 730 m thick, extend from near the base to nearly the top of the Albian. The section begins with about 200 m of clayey limestone deposited in quiet, swampy waters. Upward, clays gradually disappear, reflecting the burial of volcanic hills as the seamount subsided. The rest of the series is wackestone deposited in subtidal to intertidal environments.Core and logging data at both guyots show shallowing-upward cycles of 3 to 10 m thick. Fourier analysis yields estimates of about 100 ka as the most common frequency. Longer-term fluctuations in sea level are suggested by facies successions at a decameter scale. Diagenesis was dominated by dissolution of aragonite, and cements are now marine, low-magnesium calcite. Pore-water data show the entire succession to be open to modern seawater. Dolomite dominates in the lower 400 m of carbonate strata at Resolution Guyot, and Sr-isotope data suggest much of it formed 15 to 20 m.y. after deposition. Compaction of limestone over buried basement topography proceeded apace with deposition and continued after drowning of the guyots.During the latest Albian, a fall in relative sea level of nearly 200 m exposed limestone strata on both guyots to subaerial and wave erosion, but whether the cause was tectonic or eustatic is not yet known. By mid-Turonian time, the guyots had re-submerged, but only pelagic sediments accumulated. Why no further shallow-water sediments accumulated on the guyots is a mystery. They were at about 8°S (Resolution) to 11°S (Allison) at the time of emergence. Upper Cretaceous pelagic sediments are preserved only in cavities within Albian limestone. Eocene and Paleocene sediments on Allison Guyot, about 120 m thick, were deposited at near-equatorial latitudes. The Lower Cretaceous platform is variably encrusted with phosphorite and ferromanganese oxides, even where buried beneath pelagic sediments.Primary control on acoustic-wave velocity is from diagenetic changes in density. Cha...
The upper surface of the Albian platform limestone on many Mid-Pacific Mountains (MPM) guyots, as shown by multibeamsonar, bathymetric data, and seismic-reflection data from where pelagic sediments have buried the Albian platform limestone, shows that the top of the limestone sequence has been sculptured by streams, by waves, and by dissolution during an episode of emergence to heights of as much as 180 m above sea level. Seismic-reflection profiles show that the surface is arched over the buried volcanic basement, reflecting progressive differential compaction of the limestone section over erosional relief on the basement. This compaction began during early stages of burial of limestone layers and continued after final drowning of the platform. On some guyots, differential erosion left a prominent perimeter rim several tens of meters high. This atoll-like erosional form was drowned in post-Albian, pre-mid-Turonian time, with no further accumulation of platform limestone. Petrographic and stable-isotopic studies of samples recovered in drill holes in the upper parts of the platform limestone on Allison and Resolution guyots show dissolution cavities tens of meters below the top of the limestone, some filled with pelagic sediments and others still open and lined with stalagmite-like calcite structures and cements showing evidence of accumulation in vadose environments.
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