Maurice Ewing

Atlantic Deep-Sea Sediment Cores

Studies of lithology, particle-size distributions, and micropaleontology and chemical analyses of 221 Atlantic and Caribbean deep-sea cores lead to new conceptions of processes of sedimentation, rates of sediment accumulation, Pleistocene chronology, and pre-Pleistocene history of the Atlantic Basin. Anomalous layers of sand, silt, and lutite occur widely in the deep basins of the Atlantic. Evidence for deposition of these layers by turbidity currents is as follows: (1) the layers occur in submarine canyons, in deltalike features at the terminal ends of canyons, in basins and depressions, never on isolated rises; (2) they are interbedded with late Pleistocene sediments of abyssal facies; (3) they are well-sorted and commonly graded; and (4) they commonly contain organic remains of shallow-water origin. Late Pleistocene slumping of compacted Neogene sediments along the banks of the Hudson Submarine Canyon at depths exceeding 3000 m indicates deepening of the canyon by erosion by turbidity currents. Variations in the planktonic Foraminifera in 108 of the cores and extrapolation of rates of sediment accumulation determined by 37 radiocarbon dates in 10 cores show that the last period of climate comparable with the present ended about 60,000 years ago. A faunal change indicating climatic amelioration, probably corresponding to the beginning of postglacial time, occurred about 11,000 years ago. Cross-correlations by micropaleontological methods establish the continuity of the climatic record deduced from the planktonic Foraminifera. Study of variation in the Planktonic Foraminifera leads to a different Pleistocene chronology from that proposed by Emiliani (1955). Cross-correlations of faunal zones and radio-carbon dates show that rates of continuous sediment accumulation, as opposed to turbidity-current deposition, range from 0.5 cm to 274.4 cm in 1000 years, depending upon bottom configuration. Cross-correlations by means of changes in coiling direction of planktonic Foraminifera give relative rates of sediment accumulation beyond the range of the radiocarbon method of dating. Forty one of the cores contain pre-Pleistocene sediments. The oldest sediment is Upper Cretaceous. Foraminifera and discoasters indicate the ages. Absence of sediment older than Late Cretaceous and thickness, 800–1000 m, of sediment in the Atlantic Basin as determined by seismic methods suggest that a large-scale reorganization of the Atlantic Basin took place in the Mesozoic.

SEISMIC-REFRACTION MEASUREMENTS IN THE ATLANTIC OCEAN BASINS, IN THE MEDITERRANEAN SEA, ON THE MID-ATLANTIC RIDGE, AND IN THE NORWEGIAN SEA

Seismic-refraction measurements in the western basins of the North Atlantic Ocean show that an average crustal section consists of ½–l km of low-velocity sediments and 4–6 km of oceanic crustal rock in which the seismic velocity is about 6.5 km/sec. There is good evidence from sub-bottom reflections and shear waves that in many places there is a layer with a velocity between 4.5 and 5.5 km/sec and a thickness of 1–2 km between the low-velocity sediments and the 6.5 km/sec layer, although it is not usually detected by refracted arrivals. These layers are underlain by the mantle which has an average seismic velocity near 8 km/sec. Measurements in the eastern basins show a similar crustal section, but the velocity below the deep discontinuity appears to be lower (7.7–7.8 km/sec). Measurements in the Mediterranean Sea show only low-velocity sediments underlain by a refracting layer in which the average velocity is about 4.5 km/sec. On the Mid-Atlantic Ridge the sediments are underlain by two refracting layers with velocities averaging 5.6 and 7.4 km/sec respectively. The results indicate that the ridge has been built by the upwelling of great amounts of basalt magma along a tensional fracture zone. Presumably the extensional forces and the supply of basalt magma come from convection currents deep in the mantle. Measurements in the Norwegian and Greenland seas show results very similar to those on the Mid-Atlantic Ridge, and, from this and the extension of the belt of active seismicity, it appears that the ridge structure continues through the Norwegian and Greenland seas into the Arctic Ocean. The results of a few stations on the continental shelf of North America, Britain, and Norway are presented and compared with previously published results in these areas.

Diversity and origin of abyssal tholeiite from the Mid-Atlantic Ridge near 24° and 30° North latitude

On cursory examination of hand specimens and thin sections, the abyssal tholeiite in a dredge haul may appear to be uniform in composition. Chemical analyses of a considerable number of fragments, however, have always revealed the existence of regular compositional variation in them. The MgO content decreases with increasing SiO2. In abyssal tholeiites with relatively low Al2O3 contents, the SiO2, total iron, Na2O and P2O5 contents tend to increase and the MgO content tends to decrease with increasing iron/magnesia ratio, probably owing to crystallization differentiation.In a certain dredge haul, high-alumina abyssal tholeiites (with Al2O3 contents near or over 17%) occur in association with low-alumina abyssal tholeiites. The magma of high-alumina abyssal tholeiites would be generated from that of low-alumina abyssal tholeiites by differentiation at a depth around 30 km.In pillow lavas of abyssal tholeiite free from weathering and metamorphism, the chilled rim of the pillow usually has virtually the same chemical composition as the more crystalline core except for a decrease of K2O content toward the rim. On the other hand, the weathered rim of pillow lavas shows marked compositional change. The Fe2O3/FeO ratio of unweathered abyssal tholeiite is in the range of 0.1 to 0.3. This ratio and the H2O− and H2O+ contents increase with advancing weathering.

Metamorphism in the Mid-Atlantic Ridge near 24 degrees and 30 degrees N

Metabasites (metabasalts and metagabbros) occur abundantly in association with serpentinites in transverse fracture zones and on walls of the median valley. These metabasites were formed by burial metamorphism probably in deeper parts of the crust and the upper mantle beneath the Ridge crest, and were brought up to the surface of the crust probably by serpentinites rising along fracture zones and by normal faulting along the median valley. The metabasalts are in the zeolite and greenschist facies and a transitional state from the greenschist to the amphibolite facies, whereas metagabbros tend to have been recrystallized at higher temperatures, being in the greenschist and amphibolite facies. Compositionally the metabasites are divided into two groups, I and II. Group I comprises those which retain the approximate composition of the original rocks except for water content, whereas group II comprises those which underwent marked chemical migration, as regards sodium in zeolite-facies rocks and calcium and silicon in greenschist-facies rocks. In rocks of group I, calcic igneous plagioclase remains unaltered, and albite and epidote did not form. This fact, along with the absence of epidote-amphibolite facies rocks, would be due to the low rockpressure during metamorphism. In some rocks of group II, albite and epidote occur. Burial metamorphism takes place probably mainly beneath the Ridge crest where the geothermal gradient is great. The resultant metamorphic rocks are probably of the low-pressure type, and move laterally by ocean-floor spreading to form the main part of the oceanic crust. Contact metamorphic gneisses, probably derived from gabbros, have been found. Some metagabbros were subjected to cataclasis by fault movements along fracture zones and the median valley.

Crystallization and differentiation in abyssal tholeiites and gabbros from mid-oceanic ridges

Abstract Tholeiitic gabbros from the Mid-Atlantic Ridge near 24°N were found to show remarkable differentiation, producing high-iron, high-titanium gabbros and aplite in later stages. Crystallization of olivine and plagioclase from abyssal tholeiite magma approximately follows cotectic relation in the system olivine-plagioclase-pyroxene.

Suspended Matter in Deep Ocean Water

A nepheloid layer has been observed by optical means in the lower part of the water column on the continental slope and rise. By sampling it has been found to be a suspension of lutite, apparently in sufficient quantity to induce downslope flow. Sediment transported in the nepheloid layer may be a major component of deep-sea sediment bodies.

Sediment Distribution on the Mid-Ocean Ridges with Respect to Spreading of the Sea Floor

An abrupt change in sediment thickness between the crests and flanks of the mid-ocean ridges can be interpreted as a major discontinuity in the rates either of spreading of the sea floor or of accumulation of sediment. The preferable interpretation of the data is that the process of spreadig of the sea floor is intermittent and that the present cycle of spreading commenced around 10 million years ago. following a long period Of quiescence during which most of the observed sediments were deposited.

Continental margins and geosynclines: The east coast of North America north of Cape Hatteras

Abstract Many geophysical measurements, including seismic refraction, gravity, magnetics, and echo soundings, have been made along the continental margin of eastern North America north of Cape Hatteras in the last twenty years. These have revealed the presence of two sedimentary troughs, one under the shelf, the other in deeper water under the continental slope and rise which are separated by a ridge in the basement near the edge of the shelf. The sediments in the shelf trough have been drilled to a depth of 10,000 ft and are of shallow water character. Cores of the upper part of the sediments of the outer trough have revealed features attributed to slumping, sliding, and turbidity current action, and, in part, sediments similar to the graywackes of Pettijohns (1949) classification. This sedimentary system is quite comparable to the Appalachian system as restored for early Paleozoic time. The sediments of the inner and outer troughs are similar to those of the Appalachian miogeosyncline and eugeosyncline ( Kay , 1951), respectively, and the basement ridge resembles the Pre-Cambrian axis which separates these two troughs in the Appalachians. While there is no active volcanism in the outer (eugeosynclinal) trough at the present time, evidence of past volcanism is present in the form of partially buried seamounts with large magnetic anomalies. Conditions in the Appalachian eugeosyncline appear to have been similar with but limited volcanism prior to the beginnings of Taconic activity. The gravity calculations reveal an abrupt change in depth of the Mohorovicic discontinuity near the edge of the shelf. There is some indication that the boundary between the crust and the mantle in this area is gradational rather than a sharp discontinuity. The major process necessary to convert the present continental margin into a mountain system is the one which thickens the crust under the outer, or eugeosynclinal trough. Since the miogeosyncline is already based on a crust of continental proportions, its deformation requires only a means of folding and thrusting the surficial sediments. This is but a minor part of the overall tectonic activity.

Composition and origin of serpentinites from the Mid-Atlantic Ridge near 24° and 30° North Latitude

Serpentinites occur in transverse fracture zones and adjacent areas in the Mid-Atlantic Ridge near 24° and 30° N. In two fracture zones, about 700 km distant from each other, serpentinites show practically the same trend and range of variation in chemical composition. Their CaO content ranges from 2.05 to 0.07% by weight. Serpentinites relatively high in CaO content contain pargasite, whereas those relatively low in CaO do not. Serpentinites relatively high in CaO are chemically similar to high-temperature peridotites which are widely believed to have been derived from the upper mantle. With a decrease in CaO, the Al2O3, TiO2, K2O and FeO contents and the Fe/Mg ratio tend to decrease, whereas the H2O+ content tends to increase. This compositional variation is probably due partly to heterogeneity of uper mantle peridotite from which the serpentinites were derived, and partly to chemical migration during serpentinization. The interior of the Mid-Atlantic Ridge may be mainly made up of serpentinites. Alternatively, the Mid-Atlantic Ridge Serpentinites may have been formed by serpentinization of peridotites that were intruded into fracture zones from a great depth.

Passive seismic experiment.

Seismometer operation for 21 days at Tranquillity Base revealed, among strong signals produced by the Apollo 11 lunar module descent stage, a small proportion of probable natural seismic signals. The latter are long-duration, emergent oscillations which lack the discrete phases and coherence of earthquake signals. From similarity with the impact signal of the Apollo 12 ascent stage, they are thought to be produced by meteoroid impacts or shallow moonquakes. This signal character may imply transmission with high Q and intense wave scattering, conditions which are mutually exclusive on earth. Natural background noise is very much smaller than on earth, and lunar tectonism may be very low.