The Marshall Islands are a major morphologic and geologic feature of the western Pacific. Their origin has been ascribed to the passage of the Mesozoic Pacific plate over hotspots or, alternately, to the occurrence of widespread, mid-plate volcanism. Drilling on Bikini and Enewetak atolls, surveying and dredging on neighboring seamounts and guyots, and ocean drilling carried out during DSDP-IPOD operations, have shown that the numerous atolls and adjoining presently submerged reef-capped seamounts are the result of Eocene and Cretaceous(?) to Recent reef growth on Eocene and Cretaceous(?) volcanic foundations that were erupted atop the subsiding Pacific plate of Jurassic age. The morphology, internal facies geometry, and diagenetic history of the limestone sections below the atolls and within the submerged reef-caps have been controlled be sea-level fluctuations induced by the waxing and waning of glaciers during the Cenozoic era. Studies to date indicate that the Marshall Islands region has subsided ~1500m since latest Cretaceous or Early Eocene time. This anomalously fast rate of subsidence on a lithospheric plate of Early(?) Jurassic age has been ascribed to a mid-plate thermal rejuvenation and uplift of the plate. Research supported in this award has as its objectives the bathymetric and seismic mapping and sampling of submerged, reef- capped seamounts and guyouts in the Marshall Islands. These studies will provide data with which to determine the subsidence history of this anomalous region and to determine the facies geometry, diagenetic history, and timing of cessation of reef growth of the submerged seamounts and guyouts as these relate to sea-level fluctuations. The data will also be used in planning future drilling in this area.
