Ocean Drilling Program, United States

Three serpentinite-breccia units, pre-Late Cretaceous to late Barremian in age, were recovered from Subunit IVA (369.9-484.2 m below seafloor in Hole 899B), one approximately 95 m thick, and two less than 20 m. The three are similar and are interpreted to be bedded tabular units. Each contains angular fragments, many displaying jigsaw/crackle textures, set in a matrix apparently generated by fragmentation of the clasts. Sorting is generally absent, but larger boulders tend to occur in the upper half of the thickest unit. Minor internal deformation zones or slip zones occur. All these features are common in subaerial giant landslide deposits, and the serpentinite breccias are interpreted as being equivalent submarine deposits, generated by slope failure on a nearby large serpentinite fault scarp. Giant landslides are a typical feature of regions undergoing rapid extensional deformation, and such deposits may be relatively common in strongly faulted submarine extensional environments. The basement topography at the time of formation of the serpentinite units probably differed significantly from the present topography buried beneath the Iberia Abyssal Plain. As a result, the location of the source serpentinite escarpment is unknown. However, the large size of some of the fragments included within the serpentinite indicates that it was probably within a few kilometers of Site 899. The aggregate width of the belt of mantle serpentinite exposed in the Late Cretaceous at the Iberia Margin was at least 20 km. Most of this belt is now buried beneath the sediments of the Iberia Abyssal Plain.