Amazon Cone Research Articles

Mineralogy of Quaternary Sediments from the Valley of Vema Fracture Zone (Central Atlantic)

The content of sediment forming minerals in two cores from the eastern (ANS45-37) and western (ANS45-48) parts of the Vema transform fault valley is studied using the semi-quantitative XRD analysis of bulk powder sediment samples. The mineral composition of deep-sea sediments from the Amazone cone is also analyzed for comparison. It appeared that the average composition of the terrigenous component of both cores (according to prevailing quartz, secondary mica, plagioclase and potassium feldspar, as well as smectite, chlorite, kaolinite, illite) is quite similar and approximately corresponds to the composition of sediments from the Amazon cone. The ratio of four clay minerals suggests the supply of terrigenous material to the Amazon and Orinoco due to the erosion of the Andes and humid tropical weathering in the lower course of the rivers with further transportation of the suspended load to the ocean. This material was transported to the Vema transform valley due to the interplay between the gravity flows from the South American continental slope and the current of the Antarctic Bottom Water. Data on biogenic calcite (planktic foraminiferal tests, nannofossils) and opal A (radiolarians, sponge spicules) are obtained in addition. In the study area, several authigenic (diagenetic) minerals are identified. In particular, siderite and greigite are first found in the sediments from the Vema valley and Amazon cone, respectively.

Exploratory plays of the Foz do Amazonas Basin, NW portion, in deep and ultra-deep waters, Brazilian Equatorial Margin

A petroleum geology study was carried out at Foz do Amazonas Basin northwest portion, a new exploratory frontier basin on the Brazilian Equatorial Margin. Seismic and well data were interpreted to search similar play of some oil discoveries in correlated basins. The study area is in deep/ultra-deep waters, where the influence of the Amazon Cone is less expressive and 90 km east of Zaedyus oil discovery on Guiana-Suriname Basin. There were interpreted four seismic horizons, four chronostratigraphic intervals, two hypothetical petroleum systems, two exploratory Cretaceous plays, and an estimative of the source rock maturity. The petroleum systems are: Cassiporé/Codó–Limoeiro (late Aptian-Late Cretaceous) and Limoeiro–Limoeiro (Cenomanian/Turonian-Late Cretaceous). The top seismic horizons interpreted are Basement, Albian, Cretaceous, and Middle Miocene, determining four chronostratigraphic intervals: Early Cretaceous, Late Cretaceous, Paleocene to Middle Miocene, and Late Miocene to Recent. There were interpreted two Cretaceous plays: Slope to Plain and Deep/Ultra-Deep Waters. The main source rocks are the shales of the Codó Formation (late Aptian) and Limoeiro Formation (Cenomanian-Turonian). The Codó Formation reaches 10% TOC, has Type I kerogen, excellent potential for oil generation, and thermal maturity. The Limoeiro Formation may have 4,4% TOC, Type II kerogen, and very good petroleum potential. The reservoir rocks are Late Cretaceous Limoeiro Formation turbiditic sandstones. The seal rocks are marine shales from Late Cretaceous Limoeiro Formation. The traps are mainly stratigraphic (pinch-out). The migration routes are mainly lateral or, exceptionally, vertical, where the faults are present. The estimated source rock maturity indicates that at least one of them is below the oil window top. It was concluded that there is a great hydrocarbon exploratory potential to this part of the basin, similar to the discoveries that occurred in the correlated basins of the equatorial margins of Africa and South America.

Demerara Plateau - the structure and evolution of a transform passive margin

SUMMARY The Amazon Cone Experiment acquired two transects (Profile sD&A )across the Demerara Plateau, part of the French Guiana-Northeast Brazil continental margin, to better understand rift and transform-style margin evolution. Profile A images an intermediate-type margin formed as a result of trans-tensional extension. In this paper we describe the modelling of wide-angle and multichannel seismic and gravity data from Profile D, to reveal whole crustal structure and features exhibiting transform characteristics. Combining these results with other studies in the region and comparing our results with ‘young’ rift analogues, we develop a model of along-margin segmentation that explains the evolution of the west equatorial Atlantic. Interpretation of the velocity-depth model for Profile D shows a 35‐37 km thick continental crust which thins to 10‐11 km over a distance of 320 km. This thinning is accommodated in two regions. The narrowest region, associated with the ocean‐continent transition, is interpreted to have formed by dextral shearing of the margin along major transform zones that accommodated the initial break-up geometry of the Central Atlantic. Given the orientation of the margin relative to local fracture zone traces it is likely that the second region of thinning, located 162 km landward of the ocean‐continent transition, is a result of rifting suborthogonal to the profile orientation. There is no evidence of rotated faulted blocks, half graben structures or riftrelated magmatism, manifest as either seaward-dipping reflectors or as a high-velocity region within the lower crust. The Demerara Plateau is, therefore, interpreted as a margin segment comprising thinned continental crust bound to the north and south by transform-type zones in which trans-tensional extension is accommodated. In contrast to Profile A, modelling suggests that the eastern margin exhibits a relatively broad region of crustal thinning associated with extension consistent with a rift-type setting. Offshore, unusually thin oceanic crust of 3.3‐5.7 km thickness is identified which is consistent with similar observations further south. In the absence of identifiable magnetic anomalies, best estimates of the initial half-spreading rate of ∼20 mm yr −1 suggest that the thin crust throughout the region is unlikely to be a result of ultra-slow spreading but, instead, it is most likely due to a reduced magma supply at numerous, long-lived transform faults reflected by those presently offsetting the Mid-Atlantic Ridge in this equatorial setting.

Crustal structure of the French Guiana margin, West Equatorial Atlantic

SUMMARY Geophysical data from the Amazon Cone Experiment are used to determine the structure and evolution of the French Guiana and Northeast Brazil continental margin, and to better understand the origin and development of along-margin segmentation. A 427-km-long combined multichannel reflection and wide-angle refraction seismic profile acquired across the southern French Guiana margin is interpreted, where plate reconstructions suggest a rift-type setting. The resulting model shows a crustal structure in which 35‐37-km-thick pre-rift continental crust is thinned by a factor of 6.4 over a distance of ∼70 km associated with continental breakup and the initiation and establishment of seafloor spreading. The ocean‐continent boundary is a transition zone up to 45 km in width, in which the two-layered oceanic-type crustal structure develops. Although relatively thin at 3.5‐5.0 km, such thin oceanic crust appears characteristic of the margin as a whole. There is no evidence of rift-related magmatism, either as seaward-dipping sequences in the reflection data or as a high velocity region in the lower crust in the P-wave velocity model, and as a such the margin is identified as non-volcanic in type. However, there is also no evidence of the rotated fault block and graben structures characteristic of rifted margins. Consequently, the thin oceanic crust, the rapidity of continental crustal thinning and the absence of characteristic riftrelated structures leads to the conclusion that the southern French Guiana margin has instead developed in an oblique rift setting, in which transform motion also played a significant role in the evolution of the resulting crustal structure and along-margin segmentation in structural style.

Proposição de um modelo crustal para a região do Cone do Amazonas baseado em dados gravimétricos

O Cone do Amazonas, situado na Bacia da Foz do Amazonas, Margem Equatorial Brasileira, e caracterizado por um espesso pacote sedimentar terrigeno depositado a partir do Mioceno Medio, como consequencia da reversao da drenagem do rio Amazonas decorrente da orogenia Andina. Este trabalho investigou, por meio de uma analise integrada de dados gravimetricos, a arquitetura crustal da regiao do Cone do Amazonas. Modelagens gravimetricas 2D, baseadas em modelos de densidades construidos a partir de informacoes sismicas, mostraram a espessura sedimentar, a espessura crustal, a morfologia e a profundidade da interface crosta-manto. Aplicou-se um algoritmo de inversao nao-linear, adotando-se um modelo de densidades variando exponencialmente com a profundidade para a crosta. Assim, os valores de profundidade da interface crosta-manto, bem como de espessura crustal, foram ajustados, tornando-se compativeis com valores obtidos por sismica de refracao. Os resultados alcancados neste estudo permitiram a caracterizacao da crosta na regiao do Cone do Amazonas, possibilitando a observacao das principais caracteristicas de sua estruturacao geologica.

Andean icefields to Amazonian jungles: a radical new view of the late pleistocene of South America

Late Pleistocene glaciation in the Altlplano of the Central Andes was far more extensive than previously recognized, with all of the northern Altiplano being covered by a glacial icecap. Large paleolakes existed in the Altiplano, but their relationship with glacial episodes is unclear. One Altiplano paleolake filled the basin to the 4,100±m contour drained catastrophically through the Achocalla Valley of Bolivia approximately 45,000-40,000 years B.P. The catastrophic floodwaters flowed north and south upon reaching the lowlands of eastern Bolivia, scouring much of lowland Amazônia and depositing the Belterra Clay. Asymmetric ripplemarks on the Amazon Cone record the passing of the floodwaters into the Atlantic Ocean. The Belterra Clay dammed the eastern outlet of the Amazon drainage to an elevation of 260±m, resulting in the formation of a great freshwater lake, Lake Amazonas Lake Amazonas accumulated vast quantities of fine-grained sediments through deposition in basin-wide "birds's foot" deltas before it began draining eastward about 13,500 years B.P. Except for minor drainage system refinements, the Amazon Basin had assumed its current physical parameters by 6,000 years B.P. Paleolakes may have covered the Altiplano up to the 3,950±m contour in the early Holocene, desiccating to form the modern lakes and salares only in the late Holocene. The geological, biological and climatological ramifications of the Altiplano glaciation and paleolakes, the catastrophic flood and the formation, filling and draining of Lake Amazonas are enormous.

Amazon cone isotopic stratigraphy: Evidence for the source of the tropical freshwater spike

The isotopic stratigraphy of piston cores from the upper fan of the Amazon Cone reveals a negative planktic foraminiferal δ 18O deviation from the global oxygen isotope signal that extends back into the last deglacial period. The benthic foraminiferal δ 18O signal agrees with the global oxygen isotope signal and supports the lithological interpretation that the glacial/interglacial boundary occurs at the iron-rich crust on the Amazon Cone. The negative planktic foraminifera δ 18O deviation from the global δ 18O signal is interpreted as evidence of increased deglacial Amazon freshwater discharge to the equatorial Atlantic Ocean. 14C dates suggest that the Amazon tropical freshwater discharge initiated at 13.5 ky B.P., peaked at 9.6 ky B.P., and terminated at approximately 6 ky B.P. The timing of the tropical meltwater spike (Showers and Margolis, 1985) which peaked in the southern Venezuelan Basin at 9.6 ky B.P. can be linked to the flooding of the Amazon shelf and the reinitiation of the Brazilian Coastal Current. This change in equatorial circulation and advection of Amazon River freshwater to the North Atlantic via the Caribbean Surface Current, the Gulf of Mexico-Loop Current, the western Atlantic-Gulf Stream and the North Atlantic Drift may be the cause for the rapid termination of the Younger Dryas cool period.

A Study of Channel Patterns and Morphology of Amazon Deep-Sea Fan Using Long-Range Side-Scan Sonar (GLORIA): ABSTRACT

The Amazon Cone is a large deep-sea fan off northeast Brazil. Previous studies utilizing 3.5-kHz echograms, seismic profiles, and piston cores show that the modern fan is characterized by a complex of leveed distributary channels which are bounded to the east and west by large slump/debris-flow deposits. Seismic data show that older channel-levee complexes extend beneath these debris flows. We recently conducted a detailed study of the modern distributary-channel system of the fan using the Institute of Oceanographic Sciences (England) long-range side-scan sonar instrument (GLORIA). We were able to continuously insonify a 20 to 30-km wide swath of sea floor along each ship track; thus we were able to continuously map many of the distributary channels of the upper and middle fan for distances of a few tens to 150 km. These sonographs suggest that the central leveed channel on the uppermost fan arises within the floor of the lower Amazon Canyon near the 1,500 m isobath. This central channel leads into at least four leveed channels between 1,500 and 2,500 m. On the middle fan between 2,500 and 4,300 m, these channels divide into additional distributaries. Differences in acoustic reflectivity between channels as well as discontinuity of some channels suggest that certain channels have become inactive and abandoned, and were subsequently partly buried by overbank deposits. This is confirmed by seismic and 3.5 kHz data. The apparent branching of some channels observed on the sonographs may thus be due in part to channel abandonment and avulsion. Most channels on the middle fan are characterized by continuous, highly developed meanders, much like those characteristic of late-maturity rivers on land. The number, tightness, and intricacy of the meanders generally increase from upper to middle fan. Meander migration scars, abandoned meanders, ch tes, neck cutoffs, and breaching of channel levees to form additional distributaries are observed. Portions of the large slump/debris-flow deposits which flank the distributary-channel complex are also visible on the side-scan sonographs. At some locations these debris flows appear to have crossed distributary channels and buried, obliterated, or diverted them. End_of_Article - Last_Page 561------------

The Brazilian continental margin

The Brazilian continental margin, with its interesting morphology, structure and sediments, has become better known only during the last two decades. Six physiographical provinces can be recognized at the continental margin and the adjacent coast: (1) Cabo Orange-Parnaiba delta; (2) Parnaiba delta-Cabo Sa˜o Roque; (3) Cabo Sa˜o Roque-Belmonte; (4) Belmonte-Cabo Frio; (5) Cabo Frio-Cabo Santa Marta; and (6) Cabo Santa Marta-Chui. The shelf is rather wide near the Amazon Mouth, becoming narrower eastwards, continuing very narrow along the northeastern and eastern coast, and becoming wider again in the south towards the Plate River. Prominent morphological features along the margin are the Amazon cone, the marginal plateaus off northeastern Brazil, the Sa˜o Francisco cone and canyon, the Abrolhos Bank, and the deep-sea plateaus of Pernambuco and Sa˜o Paulo. On the shelf proper a number of relief elements exist, such as sand waves east of the Amazon, submarine terraces at various places, and irregularities of structural origin. The shelf break is rather smooth in the far north and south, more abrupt in the remainder. Surface sediments of the Brazilian shelf show five distinct facies types: littoral quartz sands, mud, transition sand-mud, coralline algae, and biodetrital. The terrigenous elastic fractions dominate off the Amazon and in southern Brazil; between these areas they occupy a very narrow strip near the coast. The carbonate facies, predominantly composed of calcareous algae, is abundant between the Parnaiba delta and Cabo Frio; to the south this facies is more biodetrital and restricted to the outer shelf. Economically important on the Brazilian continental margin besides oil, are sands and gravels, carbonate deposits, evaporites and some subsurface coal. Other possible mineral resources could be phosphate, heavy minerals and clays for ceramics.

Mass-Transport Processes on Amazon Cone: Western Equatorial Atlantic

Three large mass-transport deposits occur on the upper to middle Amazon Cone. These slump/debris-flow complexes have been mapped from 3.5-kHz echograms and cover approximately 75,000 sq km or about 10% of the cone. The debris flows traveled down slopes with gradients as low as 1:90 to 1:165 (0.3 to 0.6°) for distances up to 300 km. These deposits average 10 to 50 m in thickness and the total amount of sediments displaced may exceed 3,800 cu km. The upslope parts of these complexes generally consist of hummocky slump or slide deposits which are commonly bounded by scarps of varying relief. Downslope from this zone of removal the deposits consist of thin ( The occurrence of such widespread mass movements on the Amazon Cone indicates that mass transport is probably an important sedimentation process on deep-sea fans. However, most studies and models of modern deep-sea fans have minimized or ignored the potential importance of mass-transport processes in fan development. We suggest that, in the future, both ancient and modern deep-sea fans be more closely examined for mass-transport deposits so that the importance of this process to overall fan development can be more accurately assessed.

Upslope flow of turbidity currents on the northwest flank of the Cearà Rise: western Equatorial Atlantic*

ABSTRACTA piston core (RC16‐57) raised from the northwestern flank of the Ceará Rise contained several turbidites up to 62 cm thick with grain sizes ranging from clay to coarse sand. These turbidites were similar in composition to terrigenous turbidites found throughout the Amazon Cone, continental rise and abyssal plains of the western Equatorial Atlantic. The core site (RC16‐57) on the Ceará Rise, however, was 156 m above the level of the adjacent Amazon Cone (the source of the turbidites). Thus the turbidity currents which deposited these beds apparently had to flow upslope for 17 km to reach the core site. Sub‐bottom reflectors observed on a 3.5 kHz echogram that extended from the Amazon Cone upslope past the core site suggested that these and deeper turbidites extended from the cone up the rise flank to distances of up to 40 km from the cone/rise boundary and to elevations up to 400 m above the level of the cone at the base of the rise. An equally plausible explanation could be that the turbidity currents that deposited these sediments were in excess of 400 m in thickness and thus would not require uphill flow to reach their observed location on the rise flank. The absence of terrigenous turbidites from the bases of topographic knolls on the continental rise and abyssal plains throughout the western Equatorial Atlantic indicated, however, that turbidity currents were normally less than 100 m thick and hence would seem to rule out this explanation. The average gradient of the rise flank in this region was about 1 : 1000 (\sim 0.5°).

Morphology and Structure of Amazon Upper Continental Margin

Integration of extensive geophysical data (seismic refraction, reflection, gravity, and magnetic) with geologic information derived from exploratory drilling and land-based studies allows the delineation of four major phases of development of the Amazon continental margin. Early rifting during the Cretaceous separated a Precambrian craton in the Amazon region from its counterpart in Liberia; Eburnian or Trans-Amazonian (1,800 to 2,000 m.y.) magnetic anomalies, however, are still evident on both margins. During rifting, or perhaps soon after, the Amazon graben was formed, possibly augmented by intersection of the St. Paul's fracture zone onto the Amazon margin. During the Cretaceous and Paleogene, terrigenous sediments were derived primarily from the erosion of nearby shield rocks. Depositional environments ranged from continental to shallow marine, the depocenter being the Marajo basin. The presence of a thick sequence of shelf-edge carbonates suggests that terrigenous influx was low, and/or that the shelf-edge platform restricted terrigenous sedimentation to the inner and middle shelf. Uplift of the Andes Mountains during the Miocene initiated eastward drainage of the Amazon River. Erosion of the mountains provided a 60-fold increase in terrigenous influx to the Amazon margin, which overwhelmed the shelf-edge barriers and began formation of the Amazon Cone. Accumulation since that time has accounted for as much as 12 km of sediment. Quaternary fluctuations in sea level have caused two distinct phases of margin sedimentation. During glacially lowered sea level, Amazon sediment emptied directly into the adjacent outer margin, resulting in high accumulation rates. During high sea-level stands, such as at present, sediment is transported northwest along the inner shelf, with little material escaping to the outer shelf or beyond.

Sediment Distribution in Western Atlantic Off Northern Brazil--Structural Controls and Evolution

A sediment-isopach map of the western equatorial Atlantic off northern Brazil reveals major structural trends and significant sediment accumulations. The main features of the region beyond the continental slope are: the Amazon Cone, the aseismic Ceara Rise, the Demerara and Ceara Abyssal Plains, the North Brazilian Ridge, and the Fernando de Noronha and Para-Maranhao basins which are semi-isolated basins located on the continental margin seaward of the coastal basins of the Brazilian shelf. The maximum thickness of sediments in the area (> 10 km) exists under the shallowest (upper) part of the Amazon Cone. Sediment thickness in the center of Fernando de Noronha and Para-Maranhao basins exceeds 3 km. Beyond the Amazon Cone and these basins, the sediments are generally about 1 km thick in the abyssal plains and on the higher parts of Ceara Rise. Starting approximately 100 m.y. ago, sediments on the outer margin were derived from segments of the shelf between 37 and 39°W long. and between 43 and 45°W long. The northern Brazilian shelf between 39 and 43°W, and west of 45°W, off the Amazon River, contained shelf-edge structural highs which did not allow sediments to be carried to the outer margin until Cenozoic time. The shelf-edge highs are believed to be landward extensions of the Romanche and St. Paul's fracture zones. Beginning in Miocene time, a major phase of terrigenous transport to the outer margin resulted in the Amazon Cone. At least 2 km of sediment was deposited on the rest of the outer margin during the same interval. Sediments have been carried to the abyssal plains through gaps in the North Bra ilian Ridge only since middle to late Miocene. The Ceara Rise is covered primarily by pelagic sediments. However, its western part has been buried under approximately 3 km of Amazon Cone sediments during the last 6 m.y.

Distribution and morphology of large submarine sediment slides and slumps on Atlantic continental margins

Abstract Numerous large sediment slides and slumps have been discovered and surveyed on the continental margins of Northwest Africa, Southwest Africa, Brazil (Amazon Cone), the Mediterranean, the Gulf of Mexico, and North America over the past 10 years. The mass movements are of two primary types: (1) translational slides, and (2) rotational slumps. Translational slides are characterized by a slide scar and a downslope zone of debris flows, after traveling in some areas for several hundreds of kilometers on slopes of less than 0.5°. Rotational slumps are bounded by steep scarps, but they do not involve large‐scale translation of sediments, although seismic records indicate disturbance in the down‐dropped block. Many of the slides and slumps have occurred in water depths greater than 2000 m on initial slopes of less than 1.5°. The largest slide so far discovered is off Spanish Sahara; in this case, the slide scar is 18,000 km2 in area, at least 600 km3 in volume of translated sediments. No apparent consist...

Evolution and origin of Ceará Rise: An aseismic rise in the western equatorial Atlantic

The Ceara Rise is an aseismic rise located in the western equatorial Atlantic between latitudes 3° and 8 °N, and longitudes 38 ° and 48 °W. An analogous feature, the Sierra Leone Rise, is located in the eastern equatorial Atlantic. These two rises on the two sides of the Atlantic are bounded by the same fracture zones and are approximately equidistant from the ridge axis. The northern boundary of the two rises is formed by the 8 °N fracture zone, whereas the southern boundary is formed by the 4 °N fracture zone. Seismic-profiler records show that the acoustic basement under the Ceara Rise is smooth in areas where it is also the shallowest. Moreover, areas of smooth and shallow acoustic basement coincide with the presence of a seismic layer with a velocity of 3.5 km/sec overlying the oceanic layer 2. These observations indicate that volcanic extrusives probably account for the basement relief of the Ceara Rise. On the basis of the drilling data and on the basis of its distance from the Mid-Atlantic Ridge, we suggest that the Ceara Rise originated approximately 80 m.y. ago. At that time, the region of the Mid-Atlantic Ridge located between the 4° and 8 °N fracture zones underwent a period of extensive volcanic extrusion. This volcanic pile reached a thickness of 2 km. As the spreading continued at the ridge, the volcanic pile was split into two segments. The western segment formed the Ceara Rise; the eastern segment formed the Sierra Leone Rise. The Ceara Rise has continuously subsided since its formation and has been covered by calcareous and siliceous pelagic sediments. However, a large influx of terrigenous sediments from the Brazilian margin, which coincided with the growth of the Amazon Cone since early Miocene time, has buried the western part of the rise under several kilometres of terrigenous sediments. The onlapping of terrigenous sediments over the flank of the rise has continued until at least the end of Pleistocene time.

Structure of the northern Brazilian continental margin

Results from two-ship seismic refraction profiles and supplementary data from several sonobuoys show that typical oceanic crust underlies both the landward and seaward sides of the North Brazilian Ridge. The Ceara Rise is also an oceanic structure, probably formed by local tectonic activity. Sediments of the upper Amazon Cone are more than 11 km thick, but they thin both landward and seaward. Owing to the great sedimentary thickness, refraction arrival times from oceanic basement could not be identified, but basement is assumed to extend under the cone.

Sound-velocity characteristics of sediment from the eastern South American margin

The eastern margin of South America has been divided into 14 zones, each with a characteristic sound-velocity distribution relative to depth below the sea floor, as determined from sonobuoy data. Regressions of sound-velocity data determined from one-way travel time for each region can now be used to compute thickness from vertical reflection data. The velocity characteristics of the Pelotas Basin and Pelotas Rise are similar to those of the Amazon Cone and some other large deltaic deposits, which suggests that the discharge of the Rio de la Plata may have formerly been diverted along the shelf to the north. The velocity characteristics of the Rio Colorado Basin show that it is distinct from and cuts across the Argentine shelf and Argentine Rise. The Falkland Plateau velocity data are identical to the Argentine Rise contourites, suggesting a common provenance. Velocity characteristics in the adjacent Pelotas and Santos Basins are quite different, suggesting that the Torres Arch has been an important barrier for some time.

New evidence for occurrence of debris flow deposits in the deep sea

Clearly defined debris flow deposits generated by a large sediment slide or slides off the Spanish Sahara have been identified on the lower continental rise west of the Canary Islands. The debris flow traveled over a slope as low as 0.1° and a distance of several hundred kilometres. The deposits cover an area of sea floor of about 30,000 km 2 . Recognition of the debris flow deposits is based on a characteristic geometry, a distinctive acoustic character, a pebbly mudstone fabric and sharp angular contacts in piston cores, and an undulating surface morphology revealed by bottom photographs. Similar deposits are observed on the Amazon cone and the North American continental rise and in the Gulf of Mexico and the Mediterranean Sea. Debris flows are probably a much more important depositional process on the sea floor than was previously suspected.

Echo character of the western equatorial Atlantic floor and its relationship to the dispersal and distribution of terrigenous sediments

The floor of the western equatorial Atlantic Ocean can be divided into several distinct provinces based on detailed characteristics of the bottom echos recorded with short-ping (< msec.) 3.5 and 12 kHz sound sources. Two major types of echos are recorded: (I) distinct echos; and (II) indistinct echos. Indistinct echos can be further sub-divided into (A) continuous prolonged echos; and (B) hyperbolic echos. Each class of echos contains two or more unique echo types. The regional distributions of the various echo types recorded from the continental rise, Amazon Cone, and abyssal plains reveal much information about sedimentary processes. In the western equatorial Atlantic, hyperbolic echos are recorded only from small, isolated portions of the continental rise. This contrasts with the continental rise of the western North Atlantic where previous investigators have shown that hyperbolic echos parallel bathymetric contours along the entire rise and thus reflect shaping of the rise by geostrophic contour currents (Heezen et al., 1966; Hollister, 1967). The fact that regions of hyperbolic echos show little or no relationship to bathymetric contours of the continental rise of the western equatorial Atlantic suggests that contour currents have been unimportant in shaping the rise in this region. The three most widespread echo types recorded from the continental rise, Amazon Cone, and abyssal plains reveal much information about terrigenous sediment dispersal and deposition in the western equatorial Atlantic. Comparison of the thicknesses and frequencies of coarse (silt- to gravel-size), bedded, terrigenous sediment in piston cores with the echo type recorded at each coring site shows a correlation between echo type and the relative amount of coarse, bedded sediment within the upper few meters of the sea floor. The regional distributions of these three echo types indicate that dispersal of coarse terrigenous sediment has been downslope across the continental rise and Amazon Cone to the abyssal plains via gravity-controlled sediment flows. The Amazon River is the major sediment source and most coarse sediment is deposited on the lower Amazon Cone and proximal portions of the Demerara abyssal plain.

Amazon Cone: Morphology, Sediments, Age, and Growth Pattern

The morphology, sediment distribution, and growth pattern of the Amazon cone are similar to those of other deep-sea fans; its sediment, at least during the late Quaternary Period, was deposited in response to glacial-interglacial cycles, and its age of formation is estimated to be middle to late Miocene. Sedimentation on the Amazon cone, at least during Quaternary time, has been climatically controlled. During high sea-level stands, terrigenous sediment is trapped on the inner continental shelf, and only pelagic sediment is deposited on the cone. During low sea-level stands, the Amazon River discharges terrigenous sediment into the Amazon Submarine Canyon, from where it is easily transported to the cone by gravity-controlled sediment flows. Wisconsin sedimentation rates on the cone were in excess of 30 cm/10 3 yr. Average sedimentation rates for the Pleistocene Epoch, based on the extrapolated age (2.2 m.y.) of a prominent acoustic reflector within the cone, range from 50 to 115 cm/10 3 yr. The Amazon cone began to form about 8 to 15 m.y. B.P. and is thus about one-tenth the age of the Equatorial Atlantic.