K. O. Emery

Sea cliffs: Their processes, profiles, and classification

General concavity or convexity of sea-cliff profiles is controlled by relative rates of erosion by marine and subaerial processes, as well as by positions of more resistant strata in the cliffs. Profiles supplemented by on-site examination can establish the activity and dominance of erosional processes and indicate changes in regimen. A sharp angle at the sea-cliff base generally indicates active marine erosion, whereas a smooth curve at the base means that subaerial erosion may dominate. Talus shows absence of marine erosion. Studies of profiles can be useful for estimating stability for residences, railroads, and highways at the top, face, and base of sea cliffs. Generally increased erosion and retreat of sea cliffs are in prospect because of projected regionally wetter and stormier climate, rising sea level, and increased human activities.

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 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.

Coastal neo-tectonics of the Mediterranean from tide-gauge records

Abstract Records from tide gauges in Israel and Egypt supplement the many geological and archeological investigations that have contributed information about relative sea-level changes in the Mediterranean region. Seven such records reveal changes during the past few decades that accord with prior inferences about land movements in this region (emergence along the coast of Israel and at Alexandria and subsidence at the Nile Delta and the head of the Gulf of Suez). Twenty-four other tide-gauge records for the rest of the Mediterranean region indicate more uniformity (submergence of land or rise of sea level) in the west, but with greater movements of the land attributed to probable plate underthrusting in Turkey and Greece, to volcanism near Mount Etna, to deltaic compaction at Izmir, and to deltaic compaction coupled with water pumping at the Po Delta.

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.

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.

Changing coastal levels of South America and the Caribbean region from tide-gauge records

Abstract Tide-gauge records from southern Mexico, the Caribbean Islands, and Central and South America that span the interval 1940–1970 reveal long-term changes of relative sea level according to regression analysis and eigenanalysis. The results indicate such large variations in both direction and rate of secular movement as to rule out changes in volume of ocean water as being more than a subordinate factor. The only satisfactory explanation is that the land level beneath the tide gauges is rising in some places and sinking in others. Complex spatial patterns of relative seal-level change in southern Mexico and the Caribbean mirror the tectonic regime of these regions, exhibiting both submergence and emergence of the land. Central American tide-gauge records similarly show considerable complexity, responding to relative movement along plate boundaries. The Pacific coast of South America appears to correlate with the depth of the Benioff zone; subduction of aseismic ridges produces local highs in the Benioff zone, flanked by troughs at either side. Near the Benioff highs, relative land level is rising; between these ridges relative land level is falling. Sea-level trends in southern and Atlantic coasts of South America are closely linked with continental crustal rifting and subsidence. Data do not allow unambiguous separation of changes in ocean level from changes in land level, and no simple eustatic ocean level change can be estimated accurately from these data.

Structure and Origin of Southeastern Bahamas

The structural framework of the southeastern Bahamas has been reconstructed from seismic profiler, magnetic, and gravity data. The Bahama Escarpment that marks the boundary between the southeastern Bahamas and the deep-sea floor may follow an ancient fracture zone. The geophysical data suggest that the crust beneath the southeastern Bahamas has a thickness (20 km) intermediate between those of the crusts of continents and ocean basins, and that this basement is partly volcanic in origin. The Bahamas may have formed by subsidence of a continental crust and carbonate accretion. If the southeastern Bahamas were formed in this manner, the crustal foundation must be very thin because the carbonate apron may be as thick as 10 km. Another explanation is that the Bahamas are unde lain by oceanic crust, in which case the Bahamas could have been formed in two ways. The northwestern Bahamas may be located at the site of a trough formed before or at the time the Atlantic Ocean was open. After this trough was filled nearly to sea level with terrigenous sediment, carbonate deposition was initiated. The southeastern Bahamas, on the other hand, may be located along a fracture zone that was formed during the opening of the Atlantic. The sedimentary section may be entirely carbonate. As the continents separated, the sediment-filled trough and the fracture zone subsided with carbonate accretion keeping pace with subsidence. The interpretation of the southeastern Bahamas being built on oceanic crust eliminates the problem of its overlap onto Africa in continental drift recon tructions.

Continental Margins--Classification and Petroleum Prospects

Continental margins can be classified according to their stages of development as judged from published continuous seismic reflection profiles. Mapping of the stages of initial, youth, maturity, and old age (= destruction) shows that their distributions are related to plate movements and to sediment supply. The main information gaps are in the Arctic Sea, off Antarctica, in the open Indian Ocean, and along parts of eastern Asia. Even in these regions the stages of development can be inferred from related regions having available profiles and from general knowledge of the topography and structure. The distribution for the entire earth is about 6% initial, 48% youth, 25% maturity, and 21% old age. Best petroleum prospects are believed to be mature margins and thick basin fi ls within youthful margins.