Argentine Continental Margin Research Articles

Variations in sediment transport at the central Argentine continental margin during the Cenozoic

The construction of the sedimentary cover at most passive continental margins includes gravitational downslope transport and along‐slope contourite deposition, which are controlled by tectonics, climate and oceanography. At the eastern continental margin of Argentina the history of deposition and erosion is intimately linked to the evolution of the South Atlantic and its water masses. Here we present a detailed seismic investigation of the mixed depositional system located between 41°S and 45°S. The study provides a northward complement to prior investigations from the southern Argentine margin and together with these may be used as background information for future ocean drilling in the region. Prominent features in our seismic cross sections are submarine canyons, mass wasting deposits, contourite channels, and sediment drifts. Four major seismic units above regional reflector PLe (∼65 Ma) are separated by distinct unconformities of regional extent. Using a dense grid of reflection seismic profiles, we mapped the depocenter geometries of the seismic units and derived a chronology of the depositional processes during the Cenozoic. While the Paleocene/Eocene (∼65–34 Ma) is characterized by hemipelagic sedimentation under relatively sluggish bottom water conditions, strong Antarctic bottom water (AABW) circulation led to widespread erosion on the slope and growth of a detached sediment drift during the Oligocene and early Miocene (∼34–17 Ma). After deposition of an aggradational seismic unit interpreted to represent the Mid‐Miocene climatic optimum (∼17–14 Ma), gravitational downslope sediment transport increased during the middle to late Miocene (∼14–6 Ma) possibly related to tectonic uplift in South America. The Pliocene to Holocene unit (<∼6 Ma) is very heterogeneous and formed by interactions of downslope and along‐slope sediment transport processes as indicated by the evolution of canyons, slope plastered drifts and channels.

How to Include Ignorance into Hydrocarbon-Resource Assessments? A Case Study Applied to the Presence of Source Rock at the Argentine Deep Water Margin

The underexplored deep Argentine continental margin may be a prospective area for hydrocarbon resources. This study quantifies the degree of certainty about a fundamental part in a petroleum system, the presence of source rocks. Probability theory and, in particular, Bayesian networks provide powerful assessment tools based on incomplete knowledge. However, they should not be applied uncritically in cases where necessary assumptions like independence are not fulfilled or prior probabilities are not known. We discuss the difference between a low probability and the ignorance of a hypothesis, and apply an alternative method to the assessment of source rock presence. The results of this probabilistic argumentation system coincide with the intuitive judgement, stating that there is a quantifiable evidence for a source rock, but no evidence against it.

Relict sea-floor ploughmarks record deep-keeled Antarctic icebergs to 45°S on the Argentine margin

This paper reports ploughmarks on the Argentine continental margin up to 44° 50′S, the most northerly morphological evidence of giant icebergs in the southern Atlantic. More than 2500 ploughmarks up to 32 km long, 685 m wide and more than 20 m deep have been observed at modern water depths up to 646 m. Taking into account these iceberg sizes, and ocean current directions which appear to have remained similar from the last full-glacial to the present interglacial, the icebergs that produced the ploughmarks are interpreted to be calved from fast-flowing ice streams draining huge basins of the West Antarctic Ice Sheet and the western Antarctic Peninsula during the Last Glacial Maximum (LGM) about 20,000 years ago. Ploughmarks at the deepest water depths correspond to icebergs more than 500 m thick and at least 2 km wide, given a sea-level rise of 120 m since the LGM. The icebergs, after having drifted northeast in the Antarctic Circumpolar Current, were transported northward by the Falklands/Malvinas Current; a total distance of 2000 to 4000 km. The waters north of the Falkland Islands in particular were probably several degrees colder than today to prevent rapid iceberg melting and deterioration.

Middle slope contourite deposits and associated sedimentary facies off NE Argentina

The Argentine continental margin is characterised by a large contourite depositional system driven by southern-sourced water masses flowing at different water depths. Interest in contourite deposits is increasing in geoscience and related fields, though knowledge of the Argentine contourite system is still limited. In particular, studies based on core data providing detailed descriptions of sedimentary facies are lacking, as are investigations of principal sediment source areas and of key factors controlling sedimentary processes. This study combines visual core description and downcore grain-size analyses as well as petrographic thin-section and magnetic susceptibility analyses of 14 cores from intermediate water depths of 616–1,208 m to characterise contourite deposits on the north-eastern Argentinean slope. Gravel-rich, sandy–silty and muddy contourites as well as hemipelagic facies were identified. The deposition of these contourites was presumably controlled by sea level, the depth range of the Antarctic water mass, climate conditions and windborne terrigenous supply. It is proposed that, during glacial lowstands, muddy contourites were deposited at depths <900 m, whereas sandy–silty sequences dominated at deeper depths. During the late Pleistocene–Holocene transition, sandy–silty contourites covered the entire middle slope. Hemipelagic facies draped limited sectors of the middle slope when the sea level reached the present-day position and gravel-rich contourites became restricted to contouritic channels and moats. Northern Patagonia and the southern Pampa are the most plausible sources for sediments deposited via along-slope processes, whereas the Tandilia Range is the best candidate for sediments deposited via down-slope processes.

Submarine canyon formation and evolution in the Argentine Continental Margin between 44°30'S and 48°S

In the framework of the Vulnerable Marine Ecosystems (VME) of the High Seas of the South West Atlantic, large areas of the Argentine Continental Margin (ACM) between 44°30'S and 48°S have been swath-mapped for the first time, obtaining full data coverage of the seafloor in this region between the outermost continental shelf and the middle slope down to 1600 m water depth. The slope is characterized by the presence of smooth terraces (Nagera, Perito Moreno and Piedra Buena) that widen towards the south, limited by morphological steps with evident signs of erosion in the form of scours. These terraces form part of the Argentine contourite depositional systems, generated by the interaction of the northwards flowing Antarctic water masses with the seafloor. Within the studied area, seven canyons and their multiple branches dissect the upper and middle continental slopes, from west to east, across the terraces and the steps. These canyons, which belong to the Patagonia submarine canyon system and are collected at a depth of ~ 3.5 km by a slope-parallel, SSW–NNE-oriented channel known as the Almirante Brown transverse canyon, display a large variety of morphologies. These include incisions from just a dozen of metres to 650 m, straight to highly meandering sections with sharp bends, well-developed levees and walls that reach 35° in slope gradient, hanging branches, conspicuous axial incisions and multiple knickpoints. Only the northernmost canyon indents in the continental shelf, whereas the others start at the limit between the upper and middle slopes, and are often fed by small, straight, leveed gullies. The action of both across-slope processes represented by submarine canyons and along-slope processes represented by terracing and scouring conform the ACM as a peculiar mixed margin, with the presence of both contour and gravity currents at the same place at the same time. We propose that at present, along-slope erosion and transport mainly occurs along the Perito Moreno terrace, whereas across-slope processes are much more dominant in the Nagera terrace. Erosive bedforms such as crescent scours, generated by contour currents, contribute to the progressive bottom-up erosion of the Nagera terrace and act as an initial collector of across-slope transported sediment, that later, due to flow focusing and recurrence, incise and interconnect creating definitive canyons that progress upslope by retrogressive erosion until their head indents the shelf break. Changes in the balance between across-slope and along-slope transport would imply a disequilibrium in the combination of processes leading to canyon formation, producing canyon abandonment, and partial or total filling. These changes could be produced by a variation in the depth of the main interfaces of Antarctic water masses leading to either an increase or a decrease in the erosion and transport capacity of contour currents, and/or by an enhancement of across-slope transport related to an increase of sediment availability.

Giant mounded drifts in the Argentine Continental Margin: Origins, and global implications for the history of thermohaline circulation

Partially buried giant drifts are located in the southern-most sector of the Argentine continental margin, generating a bathymetric jump at the base of the slope. They are characterised as giant, asymmetrical elongated, mounded contourite drifts, and are described in detail here for the first time. This description is mainly based on the bathymetric and multichannel seismic reflection profiles data. Their origin and evolution in the Argentine Basin are decoded and their implications in an area crucial for geologic and palaeoceanographic reconstruction between the Atlantic and Antarctica are discussed. The buried giant drifts are divided into two zones by a large seamount. The major giant-drift (50 km wide, 300 km long, and having a sedimentary thickness of nearly 1 km) is located to the south of this seamount and oriented to the north, where its summit outcrops at present seafloor. Here, its asymmetric external shape is characterised by a steep, western side and a gently dipping, smooth eastern side, with internal reflections prograding eastward. Based on its position, morphology and internal characteristics, it has been deduced that this giant-drift was generated in the Argentine Basin by the southward branch of a confined large loop of the Antarctic Bottom Deep Water, from the Eocene-Oligocene boundary (coevally with the Drake Passage opening) until the middle Miocene. However, after a major palaeoceanographic change in the mid-to-late Miocene a new oceanographic scenario was established that fossilised these giant drifts. These changes produced a new depositional style in the margin, resulting in initiation of the present contourite depositional system and morphologic features over the slope. Extension of North Atlantic Deep Water circulation into the Southern Hemisphere and deepening of Antarctic Bottom Water circulation in the Argentine Basin are herein postulated as the factors that controlled said changes. These giant drifts show strong seismic evidence of hydrates and free gas that are ripe for energy resources exploration, thus demonstrating the economic potential of contourite deposits in deep marine environments.

The structure of the lower crust at the Argentine continental margin, South Atlantic at 44°S

It is well established that the Argentine passive margin is of the rifted volcanic margin type. This classification is based primarily on the presence of a buried volcanic wedge beneath the continental slope, manifested by seismic data as a seaward dipping reflector sequence (SDRS). Here, we investigate the deep structure of the Argentine volcanic margin at 44°S over 200 km from the shelf to the deep oceanic Argentine Basin. We use wide-angle reflection/refraction seismic data to perform a joint travel time inversion for refracted and reflected travel times. The resulting P-wave velocity-depth model confirms the typical volcanic margin structure. An underplated body is resolved as distinctive high seismic velocity ( v p up to 7.5 km/s) feature in the lower crust in the prolongation of a seaward dipping reflector sequence. A remarkable result is that a second, isolated body of high seismic velocity ( v p up to 7.3 km/s) exists landward of the first high-velocity feature. The centres of both bodies are 60 km apart. The high-velocity lower-crustal bodies likely were emplaced during transient magmatic–volcanic events accompanying the late rifting and initial drifting stages. The lateral variability of the lower crust may be an expression of a multiple rifting process in the sense that the South Atlantic rift evolved by instantaneous breakup of longer continental margin segments. These segments are confined by transfer zones that acted as rift propagation barriers. A lower-crustal reflector was detected at 3 to 5 km above the modern Moho and probably represents the lower boundary of stretched continental crust. With this finding we suggest that the continent–ocean boundary is situated 70 km more seaward than in previous interpretations.

ABSTRACT: Seafloor Stability of the Argentine Continental Margin - Considerations for Offshore Hydrocarbon Exploration and Production

ABSTRACT: Seafloor Stability of the Argentine Continental Margin - Considerations for Offshore Hydrocarbon Exploration and Production

The Argentine continental margin north of 48°S: sedimentary successions, volcanic activity during breakup

New seismic data and geophysical results from the outer Argentine continental margin and from seismic flow-line transects across the oceanic Argentine/Brazil and Cape/Angola Basins are presented. The results document that the Early Cretaceous South Atlantic continental breakup and initial sea-floor spreading were accompanied by large-scale, transient volcanism emplacing voluminous extrusive constructions on the conjugate outer continental margins of the South Atlantic. On the Argentine margin we interpret three major tectono-volcanic crustal units beneath a thick and tectonically undisturbed sedimentary succession of Cretaceous and Tertiary age: (1) Syn-rift basins and even pre-rift features are present on the outer shelf; (2) a deeply buried, 60–120 km wide and several thousand metres thick volcanic wedge occupies most of the slope. The wedge is characterized by seaward-dipping reflectors and is associated with the distinct magnetic Anomaly G; (3) the seaward adjacent oceanic crust of Cretaceous age shows isochronous changes of the seismic images along our flow-line transects, suggesting some form of episodicity of rich magma and poor to moderate magma supply at the pre-existing mid-ocean ridge of the South Atlantic during Cretaceous time.

Geology of the Argentine continental shelf and margin from aeromagnetic survey

New aeromagnetics show a complex of magnetic patterns and source depths of the north and central Argentine continental and adjacent oceanic crust. The 40 km wide, positive magnetic G-anomaly reflects an end-rift associated volcanic edifice at the continental margin. M4-MO and Cretaceous quiet zone oceanic crust are identified. The Colorado magnetic discontinuity forms the southern boundary of the Precambrian La Plata craton. To the south the continental crust of the Patagonia Platform is composed of amalgamated late Precambrian through end-Paleozoic weakly magnetic sedimentary prisms. The SW Patagonia Platform crust shows a strong high-frequency magnetic pattern associated with Mesozoic volcanics, while zones of mid-crustal level intrusives and some volcanics on the outer shelf are expressed as a broad, higher amplitude magnetic pattern.

The Basins on the Argentine Continental Margin: ABSTRACT

The Basins on the Argentine Continental Margin: ABSTRACT

BASIN EVOLUTION AND PROSPECTIVITY OF THE ARGENTINE CONTINENTAL MARGIN

Nine Mesozoic‐Tertiary basins of different orientations lie along the Argentine continental margin, over a distance exceeding 2,000 km: these are the Salado, Colorado, Valdes, Rawson, San Jorge offshore, North Malvinas (San Julian), West Malvinas and Magallanes (Austral) Basins on the Continental Shelf, together with the Continental Slope. These basins formed following the latest Jurassic — Early Cretaceous extension that accompanied the onset of South Atlantic rifting. Strain was modified by earlier basement fabrics, with consequent transtension. The Cape Fold Belt (Permo‐Triassic) provided a NW‐SE grain to pre‐Mesozoic cover off NE Argentina. In the central‐southern sector, several phases of oblique NEwards Pacific subduction and terrane accretion during the Permo‐Triassic formed back‐arc basins and volcanic belts, producing a more variable fabric orientated close to NNW‐SSE. Atlantic basin fill, of Lower Cretaceous‐Tertiary clastics, was deposited as a result of rift‐shoulder erosion and Atlantic flooding, following eastward tilting. Basin fill thickness is typically 2–4 km, but locally exceeds 6km. The hydrocarbon potential of these basins hinges as much upon the preservation of source rocks within the pre‐rift succession as it does on that of those within the base‐rift succession, and subsequent Atlantic anoxic events.

Sediment dispersal in the Argentine Basin: evidence from visible light spectra

Abstract Visible light reflectance spectra were determined for each of 236 core top samples from the Argentine Basin and surrounding areas. The spectra were analyzed by taking the first derivative of the raw spectral curves and factor analyzing the first derivative values using wavelengths as variables. We used a four-factor solution that explains about 90% of the cumulative variance. Factors were evaluated by comparing factor loadings to first derivative curves from known minerals and mineral combinations, by the mapped distributions of the factor scores, and by comparison to factors from other areas. The four factors represent (in order of decreasing variance) goethite, terrigenous vs pelagic sedimentation, organic matter, and chlorite. The goethite and chlorite factors are useful for interpreting sediment influx and dispersal. Goethitecomes from the Falkland Plateau and the Argentine continental margin. Once introduce to the deep ocean, goethite appears to be transported primarily by isobath-parallel currents. Chlorite, on the other hand, is transported into the Argentine Basin by Antarctic Bottom Water (AABW) and from the Falkland Plateau. However, along the Falkland Plateau chlorite is transported normal to the slope, probably by gravitational processes.

Deep-sea sedimentary processes in the Argentine Basin revealed by high-resolution seismic records (3.5 kHz echograms)

The regional distribution of nine 3.5 kHz seismic echo types reveals the areal extent of deep-sea sedimentary processes in the Argentine Basin. Correlation between four echo types (IB, IIA, IIB, IIIF) and the relative abundance of sand and silt layers in piston cores shows that the majority of coarse sediment bypasses the continental slope and rise. This coarse sediment is deposited on the proximal abyssal plain with progressively lower amounts reaching basinward across the distal plain. Scarps, irregular and blocky hyperbolic zones, and transparent layers along the Argentine continental margin indicate the widespread occurrence of mass-flow deposits. One large area of mass-flow deposits covers at least 5 × 10 4 km 2 of the continental rise and adjacent abyssal plain east of the Rio de la Plata. A field of large migrating mud waves, approximately 1.0 × 10 6 km 2 in extent, occurs on sediment drifts in the central basin. The mud waves range in amplitude up to 137 m and average 26 m; wavelengths are about 3–7 km. The migrating mud waves are current-controlled features which form in areas of relatively weak Antarctic Bottom Water (AABW) benthic circulation. A variety of hyperbolic echo types on the abyssal plain adjacent to the Argentine continental margin, south of the Rio Grande Rise, and in the Georgia Basin, are also interpreted as AABW current-produced erosional/depositional bedforms, although downslope sedimentary processes may also produce similar hyperbolic echoes. The distribution of bedforms inferred from 3.5 kHz echo character suggests that sediments is supplied to the basin principally by gravity-controlled mass flows. Sediment transported by AABW from higher latitudes is a secondary source. Sediments from both sources is winnowed by strong AABW flow along the Argentine continental rise. The fine-grained component is transported to the central basin where it is deposited as migrating mud waves in regions of relatively weaker AABW flow.

Influence of bottom currents on sediment texture and sea-floor morphology in the Argentine Basin

Summary The relative bottom-current speed below 4000 m in the Argentine Basin was inferred from the regional pattern of mean particle size of the non-biogenic silt fraction of about 300 sea-floor samples. The pattern reveals a strong deep western boundary current (DWBC) entering the Argentine Basin from the Georgia Basin through a gap in the Falkland Fracture Zone. The DWBC turns W and flows as a contour current along the Falkland Escarpment and Argentine continental margin. At the southern end of the Rio Grande Rise, the DWBC is deflected to the E and SE where it flows along the lower flank of the Mid-Atlantic Ridge as a weak basin-wide return flow. Several smaller return-flow gyres in the southern and western basin are delineated in the regional pattern. The DWBC is strongest along the western margin of the Argentine Basin where 3.5 kHz echograms reveal very prolonged echoes with no sub-bottom reflectors. That reflector pattern is indicative of coarse turbidites, some of which have been winnowed to produce coarse-grained lag deposits under the axis of flow. In the interior of the basin, where bottom-current flow is weakest, sediment consists of clay and silt which is deposited in large migrating mud waves with wavelengths of 3–10 km and heights of up to 137 m (average 26 m). The mud waves consist of material swept into the basin by the DWBC and delivered to the basin margin by down-slope processes. Thus material is winnowed by the DWBC and deposited as a fine-grained chaff in abyssal antidunes which migrate to the centre of the basin and form thick drift deposits.

Structure of Argentine Continental Margin

Data from 100 seismic-refraction profiles recorded on the extra-Andean part of the Argentine continental margin are presented in ten schematic structure sections and two generalized structure maps. The results are correlated with previously reported measurements in the offshore Buenos Aires province and the Argentine basin, and with seismic-refraction measurements, bore holes, and surface geology on land. Between Buenos Aires and the southern end of Tierra del Fuego four of the major subsurface elements of Argentina extend eastward under the continental shelf; from north to south they are: (1) the Rio Salado basin, (2) the Bahia Blanca basin, (3) the San Jorge basin, and (4) the Magellan basin. Another sediment-filled depression, the Malvinas basin, was discovered lying between the Falkland Islands and Tierra del Fuego. The Rio Salado, Bahia Blanca, and Malvinas basins extend out to the edge of the continent and have been filled without barriers to sediment transport at the seaward ends. The overall effect of bending appears to be well demonstrated by the arcuate trends of the main structural elements and the tectonic lines of extra-Andean Patagonia, for the most part NNW-SSE with a complementary W-E direction. The data are consistent with the supposition of a mass drifting westward with some clockwise rotation.

Structure of the Argentine Continental Margin: ABSTRACT

Long lines of end-to-end seismic refraction profiles, shot parallel with the coast line, have defined several major sediment-filled depressions on the Argentine continental shelf. Two large elongate depressions which trend perpendicular to the coast and extend out to the edge of the continental shelf in the Province of Buenos Aires, are the only ones presently known. Extensions of land basins exhibiting a similar seismic sequence of layers were found to close on the continental shelf in the Golfo San Jorge, Magellan Straits, and Tierra del Fuego regions. The section of shelf lying between the Province of Buenos Aires and Tierra del Fuego exhibits a marked lithologic change from that to the north and south. Undoubtedly the northern Patagonian Mesozoics extend well out onto the continental shelf in this region and may be part of the seaward extension of the central craton of Argentina. The basins, therefore, are associated with the evolution of a central craton, a dominant continental mass of enormous proportions which has influenced the geologic history of Argentina. End_of_Article - Last_Page 362------------