Bruce C. Heezen

Shaping of the Continental Rise by Deep Geostrophic Contour Currents

Geostrophic contour-following bottom currents involved in the deep thermohaline circulation of the world ocean appear to be the principal agents which control the shape of the continental rise and other sediment bodies.

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.

Paleomagnetic stratigraphy, rates of deposition and tephrachronology in North Pacific deep-sea sediments

The paleomagnetic stratigraphy of 12 North Pacific deep-sea sediment cores has been investigated and has been used to date volcanic eruptions and to determine rates of deposition of pelagic sediments. Only four of the cores penetrated sediments deposited before the last reversal of the earths magnetic field (0.7 m.y.). Of these, one penetrated to the Gauss series, two to sediments deposited during the Olduvai event and one penetrated to the middle of the Matuyama series. Eight other cores, 10–16 meters long, taken within 1000 km of the Japan-Kuril-Kamtchatka arc failed to reach the Matuyama series. The rate of deposition in North Pacific pelagic sediments vary from2cm/1000y in the area east of the Asiatic continent to< 0.8cm/1000y in the mid Pacific. Assuming continuous deposition, the length of the Jaramillo event can be established as 50 000 y and the Olduvai event as 14 000 y. The apparent length of time during which the dipole field of the earth was reduced during reversals of the earths magnetic field is approximately 20 000 y. In one of the cores the top of the Olduvai event is split. This may represent the Gilsa event. The brown volcanic ash present in three of the cores apparently originated in an eruption 1.2 m.y. ago in the Aleutian Arc near the Andreanof Islands.

Deep-sea current evidence from abyssal sediments

Abstract Relatively strong bottom currents in the deep-sea can be inferred from sedimentary structures observed in bottom photographs and cores. The majority of photographs of high and steep topographic prominences, such as seamounts, escarpments and the crests of major ridges, show ripples, scour and rock outcrops. Although photographic current evidence is uncommon in the deeper waters of the ocean basin floor, striking and significant examples do occur. Current scour and ripples are observed beneath the Antarctic bottom current in the western South Atlantic, below the Gulf Stream in the Florida Straits and on the Blake Plateau, beneath the outflowing Mediterranean water west of Gibraltar and beneath the deep currents in the Drake Passage. The three types of deep-sea sands and silts are turbidite, accretionary, and residual. The latter two types are always associated with currents and in some areas turbidites are reworked by bottom currents. Traction velocities necessary for the transport of deep-sea sediment particles probably range from 4–60 cm/sec, velocities similar to those found through dynamic computations of geostrophic currents and observed by recent deep-sea direct current measurements.

The geology and evolution of the Cayman Trench

The geology and stratigraphic relationships of the Cayman Ridge, Nicaraguan Plateau, and mid-Cayman spreading center, which have been determined from the succession of rocks recovered at 80 dredge stations in the Cayman Trench, are consistent with the inferred crustal structure of the region deduced from published geophysical data. The Cayman Ridge and Nicaraguan Plateau are composed of metamorphic, plutonic, volcanic, sedimentary, and carbonate rock units that typically crop out along continental margins and in island arcs. In contrast, the trench floor is composed of mafic and ultramafic rocks identical to those recovered from the major ocean basins. Our petrographic, radiometric, and paleontologic data are correlated with the regional geology of Central America and the Greater Antilles and suggest that the Cayman Ridge and Nicaraguan Plateau developed as a single, broad island arc during the Laramide Orogeny. By late Eocene time, volcanism had greatly diminished, uplift and erosion had exposed the underlying igneous rock, thick clastic sequences had been deposited in newly formed grabens, and a zone of east-west axial accretion had been created between the rifted ridge and plateau. An average spreading rate of .4 cm/yr across the mid-Cayman spreading center since the Eocene accounts for approximately 200 km of left-lateral displacement between the Cayman Ridge and Nicaraguan Plateau. The geologic histories of the ridge and plateau differ slightly after the Eocene, but general subsidence, at an average rate of 6 cm/1,000 yr, caused progressive restriction of carbonate banks and reefs to a few isolated islands and algal pinnacles. The pre-Cretaceous history of the Cayman Trench has been extrapolated from the Paleozoic-Mesozoic geology and structure of Central America and Cuba, while the Cretaceous to Holocene evolution of the trench has been related to the relative motions between the North American, South American, and Caribbean plates. Plate convergence during the Cretaceous caused southerly subduction of oceanic lithosphere beneath the ancestral Cayman Ridge–Nicaraguan Plateau, which subsequently led to the formation of a chain of volcanic islands to the south, along the North American–Caribbean plate boundary. Numerous ophiolite-like outcrops that fringe the north Caribbean margin are believed to reflect a Late Cretaceous closure of this subduction zone. Early Tertiary left-lateral shear and tensional stresses along this plate boundary split the Cayman Ridge from the Nicaraguan Plateau, and a small spreading center was created in the rift. Declining isotherms beneath the ridge and plateau caused general subsidence and local tectonic re-equilibration in late Tertiary time.

Congo Submarine Canyon

In May, 1957, a brief survey of the Congo Submarine Canyon was conducted from the Research Vessel VEMA. The canyon was found on the continental slope near the seaward limit of earlier surveys and traced to the west for 150 miles. Where the survey was discontinued, in depths of 2,200 fathoms, the leveed canyon was still a prominent feature. At the 550-fathom contour the canyon is about 5 miles wide at its rim and 500 fathoms deep. The canyon is V-shaped and echoes from the thalweg are usually recorded after the echoes from the steep walls. The outer parts of the canyon, in depths exceeding about 1,800 fathoms, are bounded by huge natural levees. In 1963 a 30-mile-long section of the canyon was surveyed 150 miles west of the 1957 study. Four biological trawls and ten sediment cores were collected from the canyon region. One trawl in 2,140 fathoms contained abundant tree leaves and a rich fauna. The canyon cores contain silt, sand, and organic debris. The canyon was discovered in 1886 by a cable route survey. Between 1887 and 1937 the Luanda-Sao Thome cable broke 30 times in the canyon. Breaks occurred most frequently during months of maximum river discharge and consequently at the times of greatest bed-load discharge. Cable breaks were limited to periods of years when the river channel was undergoing major changes in position and depth. The cable breaks which occurred up to 120 miles seaward of the river mouth are attributed to turbidity currents generated at the river mouth at times of maximum bed-load transport. These turbidity currents flowed down the continental slope, eroding the deep slope canyon and building the natural levees of the continental rise, and eventually spread out on the Angola Abyssal Plain. In contrast to oth r great rivers, the Congo is not building a subaerial delta; virtually its entire bed load is being carried by turbidity currents via the Congo Submarine Canyon to the great Congo Cone on the floor of the Angola Basin.

Tectonic fabric of the Atlantic and Indian oceans and continental drift

The floor of the Indian Ocean is dominated by (1) the seismically active Mid-Oceanic Ridge, (2) scattered linear micro-continents (mostly meridional), and (3) fracture zones (some displace the axis of the Mid-Oceanic Ridge and others parallel the micro-continents). The pattern suggests that movement along the Diamantina Fracture Zone has displaced Australia to the east relative to Broken Ridge. In the Arabian Sea north-northeast trending fracture zones have displaced the axis of the Carlsberg Ridge. The complex tectonic fabric of the Indian Ocean is difficult to explain in terms of a simple pattern of convection currents. The location and origin of the Mid-Oceanic Ridge, of oceanic rises, aseismic ridges and transcurrent fault systems must be accounted for in any hypothesis of continental displacement despite unique or exotic assumptions as to strength, viscosity or composition of the oceanic crust and mantle.

Chain and romanche fracture zones

Abstract A series of left-lateral faults displace the axis of the Mid-Atlantic Ridge in the equatorial Atlantic. Two of the most prominent of these faults are the Romanche and Chain Fracture Zones. The ridge crest is offset 180 miles by the Chain Fracture Zone. The Vema Depth of 4106 fm is the maximum depth observed in the Romanche Trench. The floor of the trench is locally smoothed by sediments gravity transported from adjacent areas. Allochthonous fossils occur in cores from the trench floor. Scour by bottom currents is apparent in photographs and in the character of the sediments obtained by coring.

Ionian Sea Submarine Canyons and the 1908 Messina Turbidity Current

A prominent system of submarine canyons has been discovered in the Ionian Sea south of the Strait of Messina. The canyons are from 2 to 5 miles wide and more than 100 fathoms deep. Below the base of the continental slope, the canyons widen and develop flat floors as they extend southward 150 miles across a large cone to the Messina Abyssal Plain. On December 28, 1908, the Strait of Messina was affected by a severe earthquake which devastated the city of Messina and nearby communities. Only two of the eight submarine cables which linked Italy and Sicily within the area of maximum intensity were damaged. Ten hours after the quake, a cable from Malta to Zante parted 120 miles to the south in 1800 fathoms depth at a point where the cable crossed the submarine canyon leading from the Strait to the abyssal plain. The earthquake apparently generated a slump which initiated a turbidity current which broke the two successive cables as it traveled downslope through the canyon at an average speed of 12 knots. The tsunami which swept repeatedly against the surrounding coasts appears to have been produced by the slump and the turbidity current. The cables lying in shallower parts of the Strait, although directly within the epicentral area, did not break because the strong currents through the Strait had removed essentially all the unconsolidated sediments. Multiple sub-bottom echoes, observed beneath the canyon floors and beneath the abyssal plain but absent from the cone, are reflections from coarse-grained layers of sediment deposited by turbidity currents. Cores from within the canyons contain graded beds of sands and silts. Cores from isolated topographic locations show a pelagic record of stagnations and tephra falls which may be correlated.

Geomagnetic Reversals and Pleistocene Chronology

The palaeontology and palaeomagnetic stratigraphy of several Atlantic and Pacific cores has been determined to establish the age relationship of various palaeontological boundaries which have been used to define a Pliocene–Pleistocene boundary in deep sea sediments.