These represent our current hypotheses as to what the

(Figures 6,7.)

marginal sediments of the Atlantic may look like., In fact, we know none of this for sure. A considerable amount of geophysical surveying has been done and much more is planned as a part of the project, but only by drilling a hole into the marginal sediments can we begin to test these hypotheses.

The passive continental margins contain a very significant portion, at least 1/3, of all the sedimentary material on the surface of the earth and it has been implaced in the last 200,000,000 years. The central goal of our project is to investigate this large body of sediment and rock deposited under unique conditions and having a unique history. It is the missing link connecting the geology of the continents, which has been intensively investigated for almost 200 years, and the geology of the ocean basins which has been intensively studied for the past two decades.

There is no way to test our hypotheses except by actual recovery of samples from the column of sediments and rock which form the continent/ocean transition for the ancient analogs found on the continents have suffered late deformations and hence provide no basis upon which to extrapolate. To do this properly, the holes will need to be located where the sediments are very thick and they will need to be extensively cored. Many scientific objectives must be investigated by drilling at a single site. This is a fundamentally different approach from that we have had during the DSDP where many of the sites were drilled to answer a single question.

For the purposes of discussions with the petroleum industry and for preparing engineering cost estimates for passive margin drilling, three deep sites on the east coast of the United States and one in the Gulf of Mexico have been suggested.

Each would

use the full proposed drill string length, 10 kilometers (33,000) feet, and the riser lengths between 2500-4000 meters (8,00013,000) feet. As the cost estimates are prepared the scientists will learn about factors which affect the time required for drilling and the costs. These factors include not only water depth, penetration depth, coring, logging and instrumentation programs, but also the oceanographic conditions at the site, the expected nature of the sedimentary column, and a number of other variables. Once we have an understanding of how the factors interact and costs are generated, the scientists will be able to select the sites that will actually be drilled on the most costeffective basis. The sites that have been proposed to date are designed to establish boundary conditions within which the platform and drilling equipment must operate in order to meet the scientific objectives of the program.

In addition to drilling on the thick passive margins, the FUSOD report emphasizes the need for drilling on active margins. These are areas where the margins of a continent coincide with the edge of one of the great oceanic plates which forms the earth's surface, and where the plate is being overridden by the continent.

The western margins of Central and South America are classic examples of this type of feature. The geology of active margins may be much more complex than that of the passive margins, and may also be a site of oil, gas and ore accumulations. The oceanic slab with its cover of soft pelagic sediment is sliding beneath the continent and one would expect that this sediment would be scraped off and added to the continental margin--yet this is not the case. Recent drilling on Leg 66 of the Deep Sea Drilling Project has shown that one deep trench bordering the continent is actually filled with sediments derived from the continent which appear to have buried the pelagic sediments to

great depth. (Figure 8.)

Only by deep drilling can we gain insight into the nature of the deeper parts of this sedimentary wedge. Although the amount of sediment entering them is great, the active margins appear to contain but a relatively small proportion of the sediment on earth, only a few percent. The rest of the sediments have been subducted into the earth's mantle or transformed into other kinds of rock. Deep drilling into active margins also requires complete well control for both safety and technical reasons.

If the GLOMAR EXPLORER is converted and fitted with a riser on the west coast of the United States, it is likely that drilling on an active margin in the Pacific would be undertaken early in the program. In the Atlantic, the Lesser Antilles form a special kind of active margin area, and drilling east of the Antilles may solve some of the scientific problems. However, if