Site 898 is situated in the Iberia Abyssal Plain over a basement ridge (Fig. 2, "Site 897" chapter, "Background and Scientific Objectives" section, this volume) within the presumed ocean/continent transition (OCT) zone off western Iberia (see "Introduction" chapter, this volume). The site is one of a transect of drill sites across the OCT designed to study the petrological changes in basement rocks within the OCT, as a means of identifying the processes that accompanied continental breakup and the onset of steady-state seafloor spreading. Site 898 is located to sample a basement high between the thin oceanic crust to the west and the weakly magnetized thinned continental crust to the east (see Fig. 4 in "Introduction" chapter, this volume). The site is located within an intermediate zone of high magnetization and smooth acoustic basement, which is located between the latter two crustal types. The basement high under the site is roughly elliptical in plan view with a steep northern slope, and its shape contrasts strongly with the linear north-south basement ridges west of Site 897 (Fig. 2, "Site 897" chapter, "Background and Scientific Objectives" section, this volume). Two APC/XCB holes (one of which obtained only a mud-line core) were drilled to test this hypothesis at 40°41.1'N, 12°07.4'W, with the primary scientific objective of penetrating basement to a depth sufficient to firmly establish its character. Cores were obtained from Holes 898A and 898B that penetrated up to 342 m of Pleistocene to late Oligocene sediments. Operations were abruptly terminated with the loss of about 3340 m of drill pipe while retrieving pipe in a storm. This meant that insufficient drill pipe remained on board the ship for us to core a significant basement section at this site.
The oldest sedimentary unit cored at this site is a 176-m-thick sequence of late Oligocene to middle Miocene claystones and siltstones (Unit II), mostly deposited as calcareous contourites or turbidites at a rate of about 10 m/m.y. The likely contourite nature of the upper part of these deposits is emphasized by the hummocky appearance of the corresponding acoustic formation within acoustic formation 1B, seen in an east-west reflection profile across the site (Fig. 1). Shipboard examination of seismic profiles around the Leg 149 drill sites revealed a deeper layer of westward/southwestward inclined reflectors within acoustic formation 1B, which passes below the site. This is also probably part of a contourite drift, the top of which may have been cored near the base of Hole 898A. The Unit II sediments have a low remanent magnetization, and consequently, no clear magnetic polarity reversals are evident. An approximately 10-m.y. hiatus in deposition began in the middle Miocene and continued until the late Pliocene. This hiatus correlates with the angular unconformity between the regional acoustic formations 1A and 1B, defined by Groupe Galice (1979) that is visible in multichannel seismic reflection profiles across the site. The unconformity represents the onlapping of horizontally bedded turbidites onto reflectors that were folded and uplifted by northwest-southeast compression on the western Iberia margin during a compressional phase in the Betic Mountains of southern Spain and structural inversion in the Lusitanian Basin of Portugal.
A change in lithology occurred at the end of the hiatus, when 163 m of late Pliocene to Pleistocene age, fine-grained terrigenous turbidites, with thin intervening layers of nannofossil clay, began to accumulate at an average of about 90 m/m.y. The sediments appear to have been deposited above the CCD. The continuous and nearly complete sediment sequence in this interval provided us with an opportunity to observe a large number of magnetic polarity intervals that might possibly reach as far back as the Gilbert Chron. During the last 1.1 Ma of this sequence, turbidites were deposited at an average rate of about one every 4000 yr. The reason for this sudden increase in sedimentation rate is not clear, but may possibly be related to changes in climate.
The principal results from this site can be summarized as follows:
A loss of drill string prevented us from achieving our primary objective at this site and led to our premature abandonment of drilling.