Date occupied: 22 May 1993
Date departed: 24 May 1993
Time on hole: 1 day, 18 hr, 14 min
Position: 40°40.477'N, 11°3.587'W
Bottom felt (drill-pipe measurement at rig floor, m): 4730.0
Distance between rig floor and sea level (m): 11.54
Water depth (drill-pipe measurement from sea level, m): 4718.5
Total depth (rig floor, m): 4977.80
Total Penetration (m): 247.80
Number of cores (including cores with no recovery): 7
Total length of cored section (m): 46.8
Total length of washed section (m): 201.0
Total recovery while coring (m): 4.89
Total recovery while washing (m): 2.78
Core recovery while coring (%): 10
Total core recovered (m): 7.67
Principal results: Site 901 is situated in the Iberia Abyssal Plain within the ocean/continent transition (OCT) zone over a basement high that has an angular shape in a single east-west seismic-reflection profile and that appears to be a tilted fault block. The block has a west-dipping fault scarp and is capped by an acoustically transparent layer, several hundred meters thick. Geophysical modeling had indicated that this basement high lay within a part of the OCT having no linear magnetic anomalies and a weak magnetization and that therefore it probably is underlain by thinned continental crust. The site was one of a transect of sites across the OCT designed to study the petrologic changes in the basement rocks within the OCT to identify the processes that accompanied continental breakup and the onset of steady-state seafloor spreading. Site 901 was drilled quickly during the last 48 hr of the leg. We drilled and washed down to 182.0 mbsf and then cored intermittently down to 247.8 mbsf. Coring was terminated when we had to depart for Lisbon.
We recovered (1) a section of Pliocene nannofossil clay or ooze, apparently unconformably overlying (2) a thin film of Early Cretaceous clay, and (3) early Tithonian black clay, silt, and sandstone containing significant terrestrial plant debris. Intervals of sandstone make up about 10% of the early Tithonian section, and the sediments are black, probably because they contain a significant amount of organic carbon.
The Tithonian (152-146 Ma) is substantially older than the time Whitmarsh, Miles, and Mauffret (1990) have interpreted for the initiation of seafloor spreading on this margin (130 Ma). If this interpretation is correct, and because we do not know the time at which rifting began here, the Tithonian sediments would have been deposited over extended or ex tending continental crust. These sediments could only have been deposited over oceanic crust if seafloor spreading had begun during or before the
Tithonian, as proposed in the Tagus Abyssal Plain (Mauffret et al., 1989) and proceeded very slowly (about a factor of 10 slower than later spreading) until 130 Ma. Furthermore, the magnetic anomaly pattern around Site 901 shows neither lineated nor large anomalies, such as those found over oceanic crust (although very slow spreading might also explain these observations), and the crustal velocity structure there is unlike oceanic crust (Whitmarsh, Miles, and Mauffret, 1990). We conclude that Site 901 probably lies over extended continental crust but could possibly lie over oceanic crust formed at a very slow spreading rate.
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Reproduced online: 15 October 2004