A number of specific drilling objectives related to all the drill sites of Leg 149 are presented below. The ways in which the individual sites were expected to contribute to these objectives are discussed in the site chapters of this volume.
The principal objective of Leg 149 was to sample the upper crust within the OCT of the Iberia Abyssal Plain to establish its nature and to test some of the predictions based on geophysical observations. Naturally, this bold objective had to be tempered by the accessibility of the crust, using current technology. To achieve significant progress within a single leg, four proposed sites (IAP-2, 3C, 4, and 5) were chosen (Fig. 4). These sites lie on basement highs situated at critical points within the OCT (Fig. 5). We expected to be able to drill three of these sites within the time allotted to Leg 149.
We planned to penetrate the upper acoustic basement to a depth of several hundred meters and, using cores and downhole logs, to determine its origin and history. This was to be done by petrological and chemical analysis of the cores, by microstructural examination of the cores, by examination of the mineralogy of the cores, by apatite fission track analysis and/or isotope dating of suitable core material, by velocity and magnetic measurements in cores, by analysis of geochemical logs, by interpretation of the Formation Microscanner (FMS) and other logs, and by whatever other means seemed appropriate.
Secondary objectives related to the sediments themselves. One aim was to determine the history of turbidite sedimentation in the Iberia Abyssal Plain. Work done in the Madeira Abyssal Plain indicates that, in general, a single turbidite was deposited each time sea level changed between a glacial and an interglacial period or vice versa (Weaver and Kuijpers, 1986). We also expected to determine to what extent the age and frequency of turbidites related to past climatic change. Another objective was to date the deformation of the sediments and to relate this to the Paleogene and Miocene deformation in Europe (mentioned above). Last, we also intended to test estimates of the depth of the ooze/chalk transition made on the basis of seismic refraction measurements in the Iberia Abyssal Plain (Whitmarsh, Miles, and Pinheiro, 1990) and to relate the velocity logs to these predictions.
We planned to estimate heat flow at each of the Leg 149 sites through measurements of thermal conductivities and thermal gradients. Thermal conductivity of the core samples was to be measured routinely on board the ship. The thermal gradient was to be determined by measuring in-situ temperatures in relatively shallow sediments at various depths (approximately the upper 300 mbsf) using the ADARA temperature and WSTP tools. Temperatures in open holes were to be measured as part of most logging runs. We planned to correct temperature logs, on the basis of the results of successive runs in the same hole, for disturbances caused by drilling and circulation.
We expected to acquire data that could be used to estimate the late post-rift subsidence history of the Iberia Abyssal Plain. We planned to observe the paleodepth, age, environment of deposition, and physical properties of each sedimentary unit. We did not expect to be able to deduce the synrift and early post-rift subsidence histories because we did not expect to encounter a continuous sequence of sediments of that age. The subsidence history was unlikely to be precise at these sites because the basin was relatively starved of sediments. Estimates of depths of deposition of continental slope sediments from paleoenvironment observations are, at best, accurate to 500 m.