Abstract
One of the most important achievements during the first two years of the Ocean Drilling Program (ODP) was the concerted effort to conduct major programs of logging and downhole measurements upon revisiting three of the deepest holes drilled into the oceanic crust by the Deep Sea Drilling Project (DSDP). This effort established holes 395A, 418A, and 504B (Table 1) as our best reference sites to “ground truth” the structure and physical properties of the upper oceanic crust. The results of logging and downhole measurements at these and other ODP holes and their relationship to regional geophysical studies were the subject of a poster symposium at the AGU 1988 Fall Meeting, from which this special section developed.
Although it was not the intent to restrict this special section to results from holes 395A, 418A, and 504B, all of the papers here deal with at least one of these holes. This emphasizes their importance as reference sites, which arises not only because of the extensive downhole measurements but simply because they are among the four deepest DSDP/ODP penetrations into genuine upper oceanic crust (i.e., crust formed at a mid‐ocean ridge; Figure 1). Holes 395A and 418A were drilled in North Atlantic crust formed at a slow spreading rate, and 504B was drilled in eastern Pacific crust formed at a moderate spreading rate. Thus the set of reference sites lacks deep holes in crust formed at fast spreading rates, which is a serious shortcoming that will hopefully be addressed by upcoming ODP drilling in the Pacific Ocean. (The set of deep reference holes has already been complemented by the drilling and logging success in 500 m of tectonically exposed gabbros in hole 735B during leg 118 to the southwest Indian Ocean in 1987 [Robinson et al., 1989]; however, interpretation of the logging results from hole 735B had not progressed enough to be included here and will appear first in the work by Robinson et al. [1990].)
