A geophysical survey employing satellite navigation was carried out over the Reykjanes submarine ridge southwest of Iceland. Water depth, sediment thickness, and the gravity and magnetic fields were continuously measured. In addition, bottom cores and measurements of sediment and water temperatures were obtained at stations. Expendable radio sonobuoys were used to make seismic refraction measurements. This paper combines these various geophysical data to obtain information about phenomena in the water layer, about details of crustal structure, and about mechanisms operating at the ridge axis. The satellite navigation results and water temperature data are used to deduce current directions and magnitude over the ridge. These currents play a role in the observed distribution of sediment. Variations in these currents are inferred from sediment temperature measurements. Magnetic profiles parallel to the ridge crest are used to demonstrate the presence of a thin, highly magnetized layer (termed layer 2A, since it constitutes the top of layer 2 of refraction seismology), as well as to directly infer the presence of normally and reversely magnetized rocks in bands on the ridge. Seismic refraction measurements reveal: (1) a 6.5‐km/sec layer under the ridge; (2) a flankward increase in seismic velocity in the crust; and (3) evidence for a surface layer of relatively low velocity (about 3 km/sec) corresponding to layer 2A. Geothermal measurements revealed two zones of low heat flow, one within 10 km of the ridge axis and the other about 75 km from the axis. The maximum values of heat flow were observed in a zone from 15 to 50 km. The over‐all average of heat flow over the ridge is not significantly different from that observed in the adjacent oceanic basins. Free‐air gravity anomalies over the Reykjanes ridge range from +25 to +60 mgal. Compared to the mid‐Atlantic ridge, the Bouguer anomalies over the Reykjanes ridge are about 60 mgal less, but the gradients are nearly the same. The narrow axial magnetic anomaly can be traced with minor offsets to the Reykjanes Peninsula. On Iceland, positive magnetic anomalies occur over much wider areas, implying that the active zone is much wider in Iceland than over the Reykjanes ridge.

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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...

Horizon beta is a subbottom reflector in the North Atlantic deep ocean sediments that extends over a large portion of the North America basin. Cores from an outcrop of beta contained shallow-water Aptian-Albian sediments and deep-water Cenomanian sediments. A core near an outcrop of a deeper horizon, horizon B, contained shallow-water Lower Cretaceous (Barremian-Hauterivian) sediments. These cores can be interpreted to support extensive subsidence of the eastern portion of the basin in early Cretaceous time. It is equally likely that the shallow-water deposits are a result of sediments slumping into an already deep basin. A reconciliation of these interpretations depends upon the JOIDES project.