Deep drilling on Midway atoll

Abstract

Two deep drill holes through reef limestones and underlying clays into basalts establish a geologic section for the Hawaiian Islands that extends back through the Miocene. Chemical analyses indicate that tholeiitic basalts were covered by flows of alkalic basalts, and aa and pahoehoe structures indicate that the volcanic mound was built above the sea. The exposed part of the volcanic mound was partly truncated by wave action in pre-Miocene time, after which subsidence began. As the mound sank, weathered clays were reworked in shallow water and eventually were covered by reef limestones. Overall submergence was interrupted three or more times by emergence during the middle Miocene, at the end of the Miocene, and at least once during the Pleistocene. During periods of emergence, the limestones were leached and recrystallized. The Miocene limestone is irregularly dolomitized. This alteration may have occurred at the time the lower part of the post-Miocene section was deposited and extensively dolomitized. The altered limestones were covered by 200 feet of younger reef limestones. The last-recorded event was the growth of a now emergent reef, eroded remnants of which are found on most of Midway's rim.