Ocean Drilling Program, United States

This paper presents an interpretation of seismic reflection profiles acquired in the southeastern part of the Iberia Abyssal Plain using lithostratigraphic and biostratigraphic information obtained from ODP Leg 149 and DSDP Site 398. Six seismic units are recognized; Units 1 to 5 span the lower Albian to Holocene, but some doubt exists concerning the age of Unit 6; it is probably Valanginian to Aptian in age. Only seismic Units 1 and 2 are not deformed significantly by a previously identified Neogene compressional episode. Unit 2 onlaps the folds, and Unit 1 truncates them in places and oversteps Unit 2 toward the fold crests. The boundary between Units 2 and 3 coincides with the late/middle Miocene boundary and so constrains the main compressional event to a very short period of time. Undulatory and inclined reflections in Units 1-3 indicate significant sediment transport by contour currents since the middle Eocene. Depressions between acoustic basement highs were filled by sediments equivalent to Units 4-6, commencing in the Early Cretaceous (probably during the Valanginian). The sedimentary fill shows simple onlapping relationships with the flanks of basement highs, most of which were completely sediment covered by Eocene time. Applying accepted criteria for the recognition of synrift packages on seismic sections, we found no unequivocal examples of synrift intervals in the study area or on previously published seismic sections from the deep Galicia Margin. Therefore, we conclude that the rift episode affecting the western Iberian Margin, from the Iberia Abyssal Plain northward, occurred over a very short interval, and/or that depositional rates were very low, so that synrift sedimentary sequences did not accumulate to produce a seismically resolvable thickness. The results of this study strongly suggest that rifting on the southeastern margin of the Iberia Abyssal Plain occurred at the latest before the Hauterivian, probably early in the Valanginian and after the Tithonian. We suggest that the synrift episode beneath the deep Galicia Margin postdates Tithonian-Berriasian shallow-water carbonates and predates Valanginian siliciclastic sediments that infill the basement topography created during rifting. Unfortunately, almost no in situ sediments were obtained from Leg 149 sites that would have enabled paleodepth estimates to be made for the Cretaceous. Therefore, it was not possible to quantify tectonically driven postrift subsidence. However, using digitized versions of the interpreted seismic lines, smoothed total tectonic subsidence (TTS) plots along these lines were produced, from which crustal thickness and extension factors (beta ) were determined. TTS of up to 6 km is consistent with the subsidence of either normal oceanic crust formed > or =100 Ma or continental crust thinned by beta of between 5 and 7 at 135 Ma).