Elazar Uchupi

Continental Rise off Eastern North America

During mid-1967 two cruises of the Woods Hole Oceanographic Institutions R/V Chain provided nearly continuous seismic, geomagnetic, and gravity measurements along 8,000 km of ship track. These measurements supplement earlier ones from various sources to provide a comprehensive picture of the composition and geologic history of the continental margin off eastern North America, an area that is much larger than all of the United States east of the Mississippi River. The geomagnetic profiles portray a systematic pattern of positive and negative anomalies that are in accord with the concept of sea-floor spreading, whereby North America separated from Europe and Africa at the beginning of the Permian Period, and drifted westward from the site of rifting (the Mid-Atlantic Ridge) at average rates of 0.8-1.4 cm/year. During all this time the continent has been coupled firmly with the adjacent sea floor, as though both continent and sea floor were on the same conveyor belt. Gravity information suggests that a relict structure of the original rift is preserved in the same general area as the geomagnetic slope anomaly, beneath the seaward part of the continental shelf, the continental slope, or the upper continental rise. It has the form of a complex linear ridge of crystalline rocks that rises above the zone of sharpest landward slope of the Mohorovicic discontinuity. Seismic refraction measurements support the presence of such a ridge, bordered on both sides by linear trenches. The continuous seismic reflection profiles measured during the cruises reveal shallow acoustic basement in the form of a ridge complex that is shallower, but in the same general area, and probably is related to the deep ridge. The ridge and associated trenches served as dams and b sin sinks to trap land-derived sediments during the Mesozoic Era, so that only pelagic silts and clays could reach and be deposited on the irregular oceanic basement seaward of the barrier. During Late Cretaceous to middle Eocene time one or more thick deposits of probably chemical origin formed blankets of deep-sea chert throughout broad abyssal plains, which produce the acoustic reflector known as Horizon A. About middle Eocene time the land-derived sediments filled the trap west of the ridge and prograded eastward over the ridge top and built the present continental rise atop the Mesozoic abyssal plain. Continuous seismic reflection profiles show that the rise is a huge prism of generally seaward-dipping, interbedded pelagic sediments and turbidites that contain many masses of sediment displaced from higher on the continental rise and from the continental slope. Such slides continue to occur, a large one having occurred in 1929. The volume of the Cenozoic continental rise in the study region is nearly 3 million km3, about half the volume of all sediments deposited on basement during Mesozoic time. The interbedding of sandy turbidites with organic-rich silts and clays displaced from the continental slope may constitute a thick sequence of oil reservoir and oil source beds, but no exploratory drilling into them has been done.

Continental Margin Off Western Africa: Angola to Sierra Leone

About 30,750 line-km of geophysical traverses (seismic reflection and refraction, magnetics, and gravity) were made in the Gulf of Guinea and vicinity aboard R/V Atlantis II during 1972 and 1973 as part of the International Decade of Ocean Exploration program. These traverses, supplemented by about 50,000 line-km of previous ones by other ships, provide a basis for mapping and understanding the geologic structure, history, and origin of the region. The deep indentation of the outline of western Africa is paralleled by a similar bend of the Mid-Atlantic Ridge and by the prominent bulge of northeastern Brazil. These sharp bends are the result of left-lateral offsets by many transform faults in a belt of equatorial fracture zones. Some of the fracture zones continue eastward and intersect the entire length of the east-west coast of the Gulf of Guinea and penetrate the continent at the Benue trough or graben. The valleys of the fracture zones have been sites of sediment deposition, whereas the ridges have served as dams that force the sediment to move westward. Where enormous quantities of sediment have been delivered to the ocean by the Niger-Benue Rivers, a large delta has deeply buried the irregular topography of the fracture zones. In this entire belt of fractured ocean floor the structure, physiography, and stratigraphy have been controlled by lateral movement, or translation, of the ocean floor with respect to the continent. In contrast, the regions southeast and northwest of the belt of equatorial fractures have fewer large fracture zones, smoother topography, and simpler sediment wedges. These two regions owe their origin to simple divergence during sea-floor spreading, when new oceanic basement added at the Mid-Atlantic Ridge increased the distances between the African continent, the Mid-Atlantic Ridge, and the American continents. Deposition of sediments along the margins of the originally narrow Atlantic Ocean was dominated early by coarse-grained and largely nonmarine sediments. South of the Gulf of Guinea these deposits were followed by evaporites as products of restricted water circulation in a long narrow arm of the ocean. There was little flow of water across the equator because of the sliding-v lve nature of the region of translation between the two regions of divergence. As spreading continued, the ocean widened, and thick prisms of marine sediments were deposited on the continental margins. Large deltas in western Africa first began at the south, with the now submerged deltas of the Orange and the Congo Rivers being chiefly Mesozoic in age and having no present coastal projection. The Niger delta farther north is mostly Cenozoic in age. Petroleum prospects appear to be far greater in the Niger delta and the region of divergence south of it than in the entire region west of the delta. The favorable continental margin contains thicker sediments, large ancient and modern deltas, and salt and mud diapirs. End_Page 2209------------------------------

Continental-Oceanic Crustal Transition Off Southwest Africa

More than 1,426 mi (2,300 km) of 48-trace, 12-fold seismic reflection profiles were used to examine the nature of the continent-ocean boundary off southwest Africa. South of the Orange River, faulted blocks, which we interpret as rifted continental crust, can be traced seaward to the 3 km isobath. The transition to oceanic basement occurs in a zone 19 to 25 mi (30 to 40 km) wide beneath the continental rise. Although structural details are obscured by a thick, seaward-dipping wedge of pre-AII (pre-late Early Cretaceous) overburden, oceanic crust at the presumed contact is not older than magnetic anomaly M9 (126 to 121 m.y.B.P.). North of the Orange River, a pronounced hinge in continental crust correlates with magnetic anomaly G of Rabinowitz (1976). Seaward of the hinge, a complex fault-block terrane is evident. Oceanic basement can not be traced with any certainty landward of anomaly M4. Intracontinental stretching and associated volcanism appear to have been important in the early history of the Cape Basin. Rifting and local dike intrusion may explain the presence of some lineated magnetic anomalies previously attributed to sea-floor spreading. Related extrusives form at least part of the pre-AII wedge just seaward of the hinge zone. Our interpretation of the seismic data suggests that the initiation of normal spreading in the Cape Basin postdates by 4 to 9 m.y. the Valanginian age derived from prevailing plate tectonic reconstructions of the South Atlantic.

Structure and Sedimentary History of Southeastern Mediterranean Sea-Nile Cone Area

A detailed seismic, magnetic, and sonobuoy study of the southeastern Mediterranean and Nile cone area shows the details of recent sedimentation and tectonic activity, and especially the effects of salt deformation due to movement of a late Miocene evaporite sequence. The continental rise in this area can be divided into two major subprovinces: the Nile cone and the Levant platform. The name Levant platform is applied to the rough topography extending northward from the Sinai that separates the essentially smooth Nile cone on the west from the Cyprus basin on the east. The previously reported suggestion of two offshore fans (the Rosetta and Damietta) off the present mouths of the Nile is not confirmed; rather, one major feature is present--the Nile cone. Both the Levant pl tform and Nile cone have considerable thicknesses of Nile-derived sediment, but the topographic irregularities of the Levant platform result from greater vertical and horizontal flow of evaporites than on the Nile cone. The movements of evaporites have resulted in large numbers of collapse structures and a 100 km-long salt ridge at the northern edge of the Levant platform. The offshore Miocene evaporites are acoustically detectable (reflector M) and have been mapped. They are perhaps correlative with a nearshore reflector (P) that underlies much of the Nile cone area. Reflector P is either a middle to late Miocene carbonate sequence that prograded eastward during the Messinian regression or an erosional surface cut into pre-Messinian strata. This reflector marks the top of a broad anticlinal structure off the Nile delta and Sinai Peninsula. The volume of post-Messinian sediment (essentially all is Nile derived) is about 387,000 cu km (assuming an average sediment velocity of 2 km/sec), or an average sediment thickness of 1.89 km for the area and an average sedimentation rate of 37 cm/1,000 years.

Drainage of late Wisconsin glacial lakes and the morphology and late quaternary stratigraphy of the New Jersey–southern New England continental shelf and slope

Abstract We propose that late Wisconsin deposition and erosion (Hudson Shelf and Block Island valleys) on the shelf and slope from New Jersey to southern New England were a consequence of the catastrophic drainage of glacial lakes behind terminal moraine systems and the huge volume of water stored beneath the Laurentian ice sheet and subsequent erosion of the lake sediments by flash floods. The morphology imparted by glaciation regulated the discharge associated with the ablation of the glaciers. Associated with the deposits west of Hudson Shelf Valley are the remains of mammoth and mastodon which were transported from their living habitats along the lake shores to their present burial sites on the shelf. The floods also triggered gravity flows on the upper continental slope which made possible the transportation of coarse debris over hundreds of km into the deep-sea. That these catastrophic flood morphologies can still be recognized on the middle to outer shelf suggest that much of its surface was little modified during the late Pleistocene/Holocene transgression. Thus the late Pleistocene/Holocene transgression may have been characterized by short periods when sea level rose rapidly allowing for the preservation of relict features.

Continental Margin Off Western Africa: Senegal to Portugal

About 22,000 km of continuous seismic-reflection, magnetic, and gravity profiles, 118 radiosonobuoy recordings, 98,000 km of geophysical profiles from previous investigations, 15 deep-sea-drilling logs, and many dredge samples served to reconstruct the history of the continental margin and adjacent deep-ocean floor between Senegal and Portugal. Initial structures of the margin south of Morocco formed by divergence when Africa and North America separated 180 m.y. ago. The margin off western Portugal had a similar origin when the Iberian Peninsula and North America separated about 80 m.y. ago. Between these two divergent segments the area of the Strait of Gibraltar formed by a combination of translation from 180 to 72 m.y. ago and plate convergence from 63 m.y. ago to the p esent, with convergence becoming more intense during the past 10 m.y. Oldest sedimentary rocks atop basement include an evaporite of Late Triassic to Early Jurassic age. When the apron deposited by upbuilding and outbuilding became thick enough, mobility of the evaporites deformed the overlying sediments especially north of the Canary Islands. Except off Morocco and the Strait of Gibraltar and possibly off southern Senegal the sedimentary blanket is dominantly calcareous, reflecting the general lack of fluvial influx. Included is a middle-Late Jurassic algal reef that constitutes the lower continental slope off Morocco. During Aptian-Cenomanian time the deep ocean off much of northwestern Africa had only sluggish bottom circulation, recorded by organic-rich sediments. A major hiatus in deep-ocean sedimentary rocks and in three prominent sedimentary ridges (off Madeira, near Agadir canyon, and north of Conception bank) probably was caused by temporarily intensified circulation. Tertiary tectonics modified the divergent margin south of Morocco by folding of shelf strata off the western High Atlas, emplacement of the Canary Island Ridge, folding of slope strata off Spanish Sahara, and uplift of the Cape Verde plateau. These orogenies also may have uplifted oceanic basement beneath the upper rise and formed the volcanic seamounts along this ridge. Maximum modification by Tertiary diastrophism occurred on the margin of translation-convergence near the Strait of Gibraltar. There, the convergence phase caused uplift of Gorringe bank. Plate convergence also deformed sediments atop oceanic basement, aided by the mobility of Triassic-Jurassic evaporites. More recently, probably because of uplift of the Iberian Peninsula during the Pliocene, well-stratified Miocene and younger deposits atop the deformed lower unit slid oceanward away from the peninsula, with the megaslide coming to rest against the Moroccan continental slope. Associated folding also involved the lower deformed sequence.

Gas venting and late Quaternary sedimentation in the Persian (Arabian) Gulf

Abstract High resolution 3.5 kHz echo sounding profiles and piston cores were used to reconstruct the microtopography and late Quaternary depositional history of the Persian Gulf. Perversive throughout the seafloor of the Gulf is an extensive network of pockmarks formed by seepages of thermogenic gas. These gas seeps and bottom water exiting the Gulf via the Strait of Hormuz are the most significant processes controlling present-day sedimentation in the region. Erosion by these seeps has been so intense in the Baiban Shelf in the Strait of Hormuz as to create a “hoodoo” like terrain on the outer shelf. The surfical geology of the Gulf documents a short lived transgression 29,400 to 22,800 years ago during the Wisconsin regression which began 125,000 years ago, the Wisconsin regressive maxima when sea level dropped to −120/−130 m about 21,000/20,000 years ago and the climate was dry and eolian and paralic sedimentation characterized the Gulf, the Holocene transgression 18,000 to 12,000 years ago when the climate was more humid than during the climax of the Wisconsin regression, a dry phase 12,000 to 9000 years ago when the Persian Gulf was a site of eolian and carbonate deposition, and the present sediment cycle during the last 9000 years under a more humid regime. It was during the present cycle that southeast trending marl lobes were deposited off Iran, Arabia acquired its hyper-arid climate about 3000 years ago and the Gulf attained its present configuration about 1000 years ago as a result of the construction of the Tigris Euphrates Delta at its head and tectonism and aggradation along its Arabian and Iranian flanks.

Continental Margin Off Western Africa: Cape St. Francis (South Africa) to Walvis Ridge (South-West Africa)

Approximately 17,000 km of continuous gravity, magnetic, and seismic-reflection profiles were recorded to determine the structure of the continental margin from Cape St. Francis to Walvis Ridge, and of the adjacent Agulhas and Cape deep-ocean basins. These and previous sea-floor and land data suggest that basement structures are the result of the breakup of Gondwanaland and the dispersion of the fragments to their present positions. This breakup may have been initiated as early as the Carboniferous Period, but most of the dispersion has taken place since Middle Jurassic. Igneous activity during the early phase may have led to the emplacement of ridges along the continental margin. Later, block faulting and volcanism along the fracture zones that delineate the flow lines of the drifting continents produced Walvis Ridge, Cape Rise, and the Agulhas Plateau. One of these fracture zones, the Agulhas fracture zone, dominates the structural grain of the continental margin and deep-ocean floor off the African southern coast. Sediments as thick as 7 km buried the fragmented continental basement and adjacent oceanic basement off the west coast and formed a broad continental rise and abyssal plain within Cape basin. The source of much of this clastic debris is believed to be the Orange River. In contrast, sedimentation off the southern coast since the breakup of Gondwanaland has been very limited, mostly being restricted to the narrow St. Francis basin atop the shelf. The adjacent continental slope and Agulhas basin have only a thin sediment cover. Much of the sediment that was present on the slope has slumped into the narrow Agulhas fracture zone at the base of the slope. Numerous swells on the southwestern end of the Walvis Ridge, undulating topography of the ocean floor in the western part of the Cape basin, swells on the upper continental rise, and the rough topography of the Agulhas Plateau were formed by the movement of the South Atlantic Bottom Water that enters Cape basin on the west side, flows along the southern flank of Walvis Ridge, and then is deflected southward by the continental slope. Pleistocene eustatic changes in sea level considerably modified the shelf, upper slope, and the eastern end of Walvis Ridge by wave action, turbidity currents, and the Benguela Current.

Catastrophic meltwater discharge down the Hudson Valley: A potential trigger for the Intra-Allerød cold period

Glacial freshwater discharge to the Atlantic Ocean during de- glaciation may have inhibited oceanic thermohaline circulation, and is often postulated to have driven climatic fluctuations. Yet attributing meltwater-discharge events to particular climate oscil- lations is problematic, because the location, timing, and amount of meltwater discharge are often poorly constrained. We present ev- idence from the Hudson Valley and the northeastern U.S. conti- nental margin that establishes the timing of the catastrophic drain- ing of Glacial Lake Iroquois, which breached the moraine dam at the Narrows in New York City, eroded glacial lake sediments in the Hudson Valley, and deposited large sediment lobes on the New York and New Jersey continental shelf ca. 13,350 yr B.P. Excess 14 C in Cariaco Basin sediments indicates a slowing in thermohaline circulation and heat transport to the North Atlantic at that time, and both marine and terrestrial paleoclimate proxy records around the North Atlantic show a short-lived (,400 yr) cold event (Intra- Allerod cold period) that began ca. 13,350 yr B.P. The meltwater discharge out the Hudson Valley may have played an important role in triggering the Intra-Allerod cold period by diminishing thermohaline circulation.

Structure of Continental Margin off Atlantic Coast of United States

Seismic profiler recordings in 44 profiles between Nova Scotia and the Florida Keys indicate that the continental margin was formed by upbuilding on the shelf and prograding on the slope. Upbuilding on the shelf during the Tertiary and Quaternary Periods ranged from 200 to 1,000 meters, and seaward prograding on the slope during the same span of time was from 5 to more than 35 kilometers. The greatest progradation occurred where the slope is flanked by the Blake Plateau rather than by the deep sea. The beds are truncated along some sections of the slope as though the slope had been steepened by submarine erosion. Off Nova Scotia the beds of the slope continue into the continental rise; off New England the rise consists of sedimentary layers that have buried the base of th continental slope; and off southeastern United States the beds of the Florida-Hatteras Slope have prograded atop the older surface of the Blake Plateau.