Ages of Horizon A and the Oldest Atlantic Sediments

Abstract

Although strongly reflective interfaces within the unconsolidated sediments of the North American basin have been evident since the early seismic studies, the continuity of these reflectors was not understood before the development of deep-ocean continuous seismic-profiling techniques (1). Seismic profiles have shown that one of these reflecting horizons, horizon A, is continuous over most of the North American basin (2, 3, 4). The horizon marks an abrupt oceanwide change in sedimentation and has been proposed as an important target of deep-ocean sedimentary drilling (5). The presence of a similar reflector in South Atlantic basins has been reported (6, 7), and horizons in the Pacific Ocean and Caribbean Sea have been identified as possibly synchronous with the Atlantic horizon A (8, 9).Recent surveys have defined an outcrop of horizon A in the North American basin, and sediment cores taken from the outcrop area have identified the reflective interface as the top of a turbidite sequence of Upper Cretaceous age. This finding supports earlier speculation that the horizon is a fossil abyssal plain; the moderate amount of distortion of the horizon indicates that the major Atlantic basins have The authors are on the staff of Lamont Geological Observatory, Columbia University, Palisades, New York. 1966 been relatively stable at least through most of Cenozoic time. We now present the seismic data associated with horizon A and the outcrop area, and discuss their pertinence to the questions of stability of the basins, the possible age of the deepest sediments below horizon A, and the geologic history of the oceans. DECEMBER Horizon AHorizon A, sometimes referred to as reflector A, is commonly the strongest and most continuous coherent subbottom reflective horizon in the Atlantic Ocean basins. The horizon often defines the top of a stratified zone in the sediments, which is identified on the seismic records as a group of closely spaced and mutually conformable reflectors. This stratification varies both locally and regionally, but one cannot always be sure to what extent its appearance on the profile records may be influenced by the thickness and nature of the overlying sediments, or by the type of seismic technique involved. In general the thickness of the zone of stratified sediments immediately below the horizon rarely exceeds 500 meters (0.5-second reflection time).The sediment both above and below the stratified zone is usually acoustically transparent and may contain several weak reflectors. The transparent zones are thought to represent quiescent deposition of pelagic sediment, or deposits composed only of very fine particles transported by density currents (10).Horizon A extends from beneath the continental rise, eastward across the Bermuda rise; in some places it apparently continues to the base of the mid-Atlantic ridge. It has been mapped southward from the Kelvin seamount group to the edge of the north wall of the Puerto Rico trench. In some areas, such as the abyssalhills province and parts of the Sohm abys...

Regional Aspects of Deep Water Drilling in the Gulf of Mexico, East of the Bahama Platform and on the Bermuda Rise

“…In the central eastern part at the boundary of the Rise, the abyssal hills province occupies the interval between the Rise and the flank of the Mid-Atlantic Ridge. Reflectors A and ß have not been recognized in the areas of thin sediment cover mentioned above (Ewing et al, 1966).…”

Chert and Porcellanite from Deep Sea Drilling Project Site 436, Northwest Pacific

“…Using an average value of 2.72 x 10 5 g/cm 2 -sec for the acoustic impedance of the immediately overlying unsilicified clays (Carson and Bruns, this volume), the reflections coefficients for the boundary between chertporcellanite and clay-claystone range from 0.29 to 0.65. These high values are not surprising, because chert and porcellanite layers are strong seismic reflectors in the Pacific and Atlantic Oceans and in the Bering Sea (Ewing et al, 1966;Gartner, 1970;Saunders et al, 1973;Scholl and Creager, 1973;Hein et al, 1978).…”

Site 6

“…From the Rise, it dips beneath the young turbidites of the adjacent abyssal plains (Sohm, Hatteras and Nares), except to the south and east where the sedimentary sequence is absent. Horizons A, ß, and B become complicated in parts of the Bermuda Rise, and neither of the younger two can be positively identified east of the Rise and on parts of its eastern flank (Ewing et al, 1966).…”