El Margen Continental Patagónico guarda gran parte de los registros de eventos tectónicos, sedimentarios, climáticos y oceanográficos que participaron en la evolución de la región Patagónica y del Atlántico Suroccidental. Su conocimiento es esencial para comprender cabalmente la geología y biodiversidad de Patagonia. Los rasgos geotectónicos y morfosedimentarios regionales se caracterizan por diferentes tipos de márgenes continentales (pasivo, transcurrente y transpresivo), en cada uno los cuales sus elementos constituyentes, la plataforma, el talud y la emersión, adquieren configuraciones morfológicas y sedimentarias particulares. Las características de las secuencias estratigráficas y sus discontinuidades documentan las distintas etapas de evolución del margen y los procesos mayores intervinientes. Se analizan y describen los eventos tectónicos, palaeoclimáticos y palaeoceanográficos de extensión regional que caracterizaron a la región desde la apertura de Gondwana hasta el Cuaternario y condicionaron sus aspectos morfosedimentarios. Se concluye que la región evolucionó en tres etapas mayores de acuerdo al predominio de diferentes factores: 1) etapa dominada por factores endógenos, ocurrida en tiempos Mesozoicos, cuando los procesos mayores fueron la tectónica de placas y la apertura oceánica; 2) etapa transicional, en el Terciario bajo, cuando el proto-Océano Atlántico ya comenzaba a evolucionar hacia un mar abierto y los factores condicionantes climático-oceanográficos se hicieron al menos tan importantes como los tectónicos; 3) etapa dominada por factores exógenos, ocurrida en tiempos post-Oligocenos, cuando el Océano Atlántico ya estaba definitivamente instalado y la circulación de las corrientes oceánicas influenció las características de los ambientes sedimentarios; esta etapa culminó en el Cuaternario cuando las fluctuaciones glacioeustáticas le dieron a la región su configuración actual.
Patagonia, the southernmost region of South America, is the only land mass in the world (except Antarctica) that extends south of ∼40°S. It is a narrow wedge of land penetrating into the Southern Ocean completely surrounded by sea, in a context of an ‘ocean-dominated’ Southern Hemisphere. Oceanographic and oceanic-induced climatic factors have therefore greatly influenced the evolution of the region, and the land–sea interrelationship that characterizes the evolution of any coastal region in the world plays a more significant role here. This influence was particularly significant in the Quaternary. During the glacioeustatic sea-level falls that accompanied the glacial (cold) periods, large extensions of the present Argentine continental shelf were exposed to subaerial conditions. Given the similarity between the extension of the Patagonian shelf south of the Colorado River mouth (39°50′S) and continental Patagonia (∼730 000 and 800 000 km2, respectively), the region experienced an alternating near duplication or reduction to half of the area during glacial/interglacial times. Undoubtedly, these changes are likely to have conditioned much of the geological, climatic, and biological evolution of Patagonia.
The Atlantic marine regions adjacent to Patagonia thus contain the stratigraphical, sedimentological, and biological records of the regional evolution, which are useful for interregional correlations and comparisons, and in turn can help solve some questions arising from ‘continental Patagonia’ geology. Essentially, in many places the records of the Quaternary transgressions and regressions are better preserved on the shelf than on the coasts. On the other hand, sedimentary sequences preserved on the slope contain nearly continuous records of palaeoceanographic and palaeoclimatic changes.
This contribution synthesizes our current knowledge of the Patagonian continental margin (PCM). Two main objectives are pursued: (1) to describe the morphosedimentary and stratigraphic aspects of the shelf, slope and rise; and (2) to present a new synthesis of the regional evolution, considering the different tectonic, climatic, oceanographic, and sedimentary conditioning factors involved.
THE ARGENTINE AND THE PATAGONIAN CONTINENTAL MARGINS
The Argentine continental margin (ACM) (Fig. 1) is one of the most extensive margins worldwide, and it is the largest in South America, covering an area of around 2 × 106 km2. The margin exhibits a regional orientation from NNE to SSW, and an extension of 2400 km from the Río de la Plata (35°S) to Cape Horn (55°S). Its width is variable: 550 km at the latitude of the Río de la Plata, 1000 km in front of San Jorge Gulf, 100 km south of Tierra del Fuego, and at the latitude of the Santa Cruz River in the direction of the eastern extreme of the Malvinas Plateau, it reaches a maximum width of ∼2000 km. The boundaries of the ACM are: to the west, the coastal regions of the Pampean and Patagonian plains, as well as the southernmost extreme of the Andes in Tierra del Fuego; to the east the Argentine Basin; to the north the Uruguayan continental margin; and to the south the Scotia Sea.
The southern boundary of the PCM at sea is the line of 200 nautical miles from the coastal baseline, south of Cape Horn. Therefore, the PCM extends between 43° and 58°S and includes the marine regions adjacent to continental Patagonia, Tierra del Fuego, Malvinas (Falkland) Islands and the Northern Scotia Ridge. From an oceanographic viewpoint, Matano, Palma & Piola (2010) defined the Patagonian shelf as the sector of the Argentine continental shelf extended from the Brazil–Malvinas Confluence (∼38°S) to the southern tip of South America (55°S).
TYPES OF MARGINS IN THE ACM
The ACM comprises three types of margins (Ramos, 1996; Hinz et al., 1999; Franke et al., 2007) resulting from the interaction among three major processes affecting different latitudes (Fig. 2): the westward motion of the South America Plate; translational movements relative to the Malvinas–Agulhas fracture zone; and the interaction with the Scotia Plate.
The volcanic-rifted continental margin corresponds to a typical lower-plate passive margin, with rift basins (Ramos, 1996) associated with sea-floor spreading. It is characterized by a young and thin crust with pre-rift associations and longitudinal rifts, and was strongly affected by volcanism. This margin extends from eastern Brazil to ∼48–49°S. The Argentine part of this margin extends between the Rio de la Plata and the northern PCM (north of central Santa Cruz province) (Fig. 1). The margin is structured (Franke et al., 2007) in four segments (I, II, III, and IV, from south to north), delimited at its southern boundaries by transference fracture zones (Malvinas, Colorado, Ventana, and Salado, respectively; Fig. 3). The Colorado transference zone, identified by magnetic (Ghidella et al., 1995) and seismic (Franke et al., 2007) data, is considered to be the northern limit of the ‘Patagonian shelf domain’ (Max et al., 1999; Ramos et al., 2004).
The transcurrent margin
The transcurrent margin is the part of the margin associated with the Malvinas–Agulhas fracture (Fig. 2), a complex transference fracture zone represented by a strike-displacement fault, along which the southern margin of the South America Plate was displaced to the west during continental separation. The major feature characterizing this margin is the Malvinas Escarpment, located at 48–49°S, which constitutes the northern boundary of the Malvinas Plateau (and includes the Malvinas/Falkland Islands and the Maurice Ewing Bank) (Fig. 3), a complex feature that resulted from the interaction between the South America and Scotia plates that shares morphostructural characteristics of the three types of margins.
According to Turic, Nevistic & Rebay (1996), north of 55°S (comprising both types of margins mentioned above) the shelf region was affected by down-warping processes conditioned by isostatic equilibrium and sediment overloading: thick sedimentary deposits were accumulated there, favoured by the increasing tilting of the continent towards the east as a result of the Andean uplifting in the Miocene.
The transpressive margin
The transpressive margin corresponds to the Scotia Arc, a complex tectonic element comprising ancient blocks of continental crust and volcanic arcs, displaced to the east from the southern extreme of the Andes Cordillera. This block was inserted between the South America and Antarctica plates, and constitutes an arc joining Tierra del Fuego with the Antarctic Peninsula. The northern part of the arc (which is part of the PCM) is the North Scotia Ridge that extends through Isla de los Estados, Burdwood Bank, and Georgia Islands to the east, and ends at the South Sandwich volcanic arc. The Malvinas Trough represents the boundary between the transcurrent and the transpressive margins, i.e. the boundary between the South America and Scotia plates (Fig. 2).
REGIONAL OCEANOGRAPHIC SETTING
The ACM is located in a key region of the world ocean because surface and deep Antarctic-sourced water masses penetrate deeply in mid latitudes and interact with North Atlantic-sourced water masses (Piola & Rivas, 1997; Wefer, Mulitza & Ratmeyer, 2004). The oceanographic factors that dominate are different in nearshore and offshore regions.
The nearshore regions comprise two sectors according to the dominant oceanographic shallow processes: waves and tides. The boundary between them is not precise because of the gradual change of the conditions prevailing north and south. In a general sense, regions north of ∼42°S are wave dominated and microtidal (< 2 m amplitude), whereas regions south of 42°S are tide dominated, although waves here are very high (up to 8 m), mainly because of wind activity. Most of the Patagonian coasts correspond to the second sector.
The deep regions located offshore, represented by the shelf, slope, and rise, are current dominated, where surface and subsurface currents are driven by geostrophic and thermohaline circulation. This circulation determines a vertical stratification of water masses (Piola & Matano, 2001; Matano et al., 2010; Piola et al., 2010; Fig. 4), which conditions most of the sedimentary processes and morphology of the margin.
The PCM is dominated in its surface water masses by northwards-flowing Antarctic-sourced cold and low-salinity currents corresponding to the Malvinas Current (MC). This water mass is a rapid and barotropic branch of the Antarctic Circumpolar Current (ACC), transporting between 40–70 Sverdrup (Sv; 1Sv = 106 m3 s−1 of water), and includes less salty Antarctic Intermediate Water (AAIW) at depths below 700–1000 m. MC meets the southwards-flowing Brazil Current (BC) in the Brazil–Malvinas confluence zone (BMCZ) centred around 38°S. The north and south shifting of the BMCZ as a result of climatic influences can affect regions located, even at 40°S. Matano et al. (2010) consider the Patagonian continental shelf (PCS) as the shelf sector located south of the BMCZ.
Below the MC flows the North Atlantic deep water (NADW), which flows polewards, and is characterized by high temperature and salinity.
The circumpolar deep water (CDW), which corresponds to the deeper fraction of the ACC, affects the ACM at depths below 800–1000 m. It flows northwards and is divided into two fractions: the upper (UCDW) and the lower (LCDW) circumpolar deep water. The UCDW flows northwards through the Drake Passage into the Argentine Basin along the 1000–1500-m isobath. The LCDW enters the Argentine Basin as a dense water mass over the Malvinas Plateau and east of the Ewing Bank, and follows the slope at a depth of 3000–3500 m.
The deepest water masses are represented by the very cold and dense Antarctic bottom water (AABW), which originates in the Weddell Sea. It is introduced in the Argentine Basin at different depths (> 3500 m) and is deflected to the west against the Malvinas Scarp, producing both a circulation parallel with the South Atlantic Ridge and an anticyclonic gyre on the Argentine Basin, where it remains trapped as a result of the complex sea-floor morphology.
The MC is the main component of the oceanic circulation in the PCS (Matano et al., 2010; Piola et al., 2010), which is influenced by strong westerly winds, large tidal amplitudes, and low-salinity water discharges. The MC conditions most of the cross-shelf processes and shelf-break dynamics. A jet derived from the MC in the inner shelf south of 49°S originates the Patagonian Current (Matano et al., 2010).
The circulation of the water masses on the PCS has a significant importance at a global scale as it influences the global carbon budget, as large volumes of CO2 are absorbed there from the atmosphere (Bianchi et al., 2005). Biological implications are significant because deep-water masses move onshore and generate coastal upwelling off the shore of southern Patagonia (Matano et al., 2010), which transports nutrient-rich waters to the surface and produces a persistent zone of high chlorophyll content at the shelf–slope transition. It impacts coastal ecosystems and defines the region as one with the highest primary productivity (Lopez Gappa, 2000; Matano et al., 2010).
Morphosedimentary features are landforms on the earth surface (including both continents and oceans) with particular morphologies shaped by erosive and depositional sedimentary processes. Major features described herein (of different orders depending on the regional development and scale of processes involved) are continental margins, shelf, slope, rise, terraces, scarps, ridges, submarine canyons, channels, and plateaus. In most of the cases they are different depending upon which type of margin they develop on, except the continental shelf that shows relatively homogeneous and continuous characteristics overlapping the different margins.
Morphosedimentary features in the volcanic-rifted continental margin
The continental shelf is described as a whole independently of the types of margins upon which it develops. Parker et al. (1996, 1997 and 2008). described most of the morphological and sedimentological aspects of the shelf, which were later synthesized by Cavallotto (2008). The shelf extends from the shoreline to the shelf break in transition to the slope, where the sea-floor slope drastically increases. The regional shelf average gradient is around 1 : 1000. The shelf break is at depths between 110 and 165 m, and is located at distances from the coast of 350 km in front of Puerto Deseado, 850 km in front of Bahía Grande – including the Malvinas (Falkland) Islands shelf – and only 10 km south of Isla de los Estados. The sedimentary cover is terrigenous and siliciclastic, mostly (> 65%) composed of relictic and palimpsestic sands reworked during transgression after the last glacial maximum (LGM). They have a mineralogical composition showing a pampean–patagonian affinity (volcanic–pyroclastic association). Subordinate fractions are shelly deposits representing former, presently submerged shorelines. Gravels are particularly important in the PCS where they constitute ∼25% of the surface sediments. They are associated with glacial–fluvial activity during Quaternary low-stand periods, particularly at LGM times.
Parker et al. (1996) described different physiographic features defined by morphology and sedimentology: deltaic front of the Colorado and Negro rivers, Patagonian gulfs, Patagonian inner shelf, Patagonian outer shelf, Tierra del Fuego shelf, and Malvinas (Falkland) Islands shelf. On the other hand, Parker et al. (1997) described four levels of terraces, the top surfaces of which are at 25–35, 85–95, 110–120, and 130–150 m, with all of them separated from each other by terrace scarps or ‘steps’ of steeper gradient. The origin of these terraces has been related to wave action during short interruptions in the velocity of the post-LGM sea-level rise (Perillo & Kostadinoff, 2005; Violante, 2005; Ponce et al., 2011); however, a combination of different factors such as isostasy, tectonism, and sea-level fluctuations could have influenced the modelling of the terraces (Ponce et al., 2011).
The characteristics and distribution of shelf sediments are mainly a function of sedimentary processes that took place during the Quaternary sea-level fluctuations (particularly the postglacial transgression), as well as the dynamics and sediment supply of coastal waters. During the last sea-level low-stand, around 18 thousand years ago, the shelf was a continental region probably very similar to Patagonia. As sea levels rose during the post-glacial transgression, erosive coastal retreat provoked sediment transference offshore. Isla & Cortizo (2005) estimated that a total volume of 243.8 × 106 tons per year of sediment is presently eroded from the Patagonian cliffs into the sea. If this volume is extrapolated to every stage of the transgression, a broad estimation can be obtained about the enormous volume of sediments produced at the coastline and deposited on the shelf surface during the last 18 thousand years. Ponce et al. (2011) modelled several evolutive stages of the PCM during the last transgression, and concluded that the coastline recession occurred at a mean rate of 22–38 m per year, depending on the region considered.
Pierce & Siegel (1979) and Gaiero et al. (2003) estimated a total volume of 70 × 106 tons per year of terrigenous sediments transferred to the sea bypassing the Patagonian coasts: 56% (39 × 106 tons per year) corresponds to coastal erosion; 41% (29 × 106 tons per year) corresponds to atmospheric processes (dust transport); and 3% (2 × 106 tons per year) corresponds to fluvial activity (Cavallotto, 2008). The Patagonian fluvial network introduces relatively reduced sedimentary volumes, as the smaller rivers carry low volumes of sediments, whereas the bigger rivers usually have estuarine environments that retain most of the sedimentary load. However, streams were more significant in pre-Holocene times, as evidenced by today's oversized fluvial valleys with respect to the present fluvial dynamic, as well as by the large volumes of gravel on the southern shelf surface that cannot be transported by present streams. Kokot (2004) estimated that the Santa Cruz river presently has a discharge of one-tenth of the discharge in the Pleistocene. Iantanos, Estrada & Isla (2002) stressed the significant diminishing in the discharge of the Deseado River since the last glaciation. Isla & Cortizo (2005) considered that Chubut and Chico rivers experienced a reduction of 21–24% in the area of their watersheds, and 32–34% in their annual discharges, with respect to upper Pleistocene times.
Based on the processes intervening in the sedimentary regime of the ACS, Violante (2004) classified the shelf as passive and autochthonous.
The slope in the volcanic-rifted continental margin has an average gradient of 1 : 50 (> 1.5°), its width varies from 140 to 270 km between 35 and 49°S, reaching less than 50 km wide further south, and its foot reaches average depths of around 3200 m. The slope is dominated by gravitational (down-slope) and along-slope processes that give rise to a varied morphosedimentary configuration, characterized by depositional and erosive features. The main features are represented by the impressive countouritic depositional system (Hernández-Molina et al., 2009), one of the largest contouritic systems worldwide, which is genetically associated with the circulation of the Antarctic-sourced currents along the slope and its interaction with the sea floor. Countouritic deposition associated with erosive processes at the interphases between the different water masses gave rise to several terraces: north of 43°S extends the Ewing terrace, averaging a water depth of 1000 m, whereas between 43 and 48°S extend the terraces Nágera (at a water depth of 500 m), Perito Moreno (at a water depth of 1000 m), Piedrabuena (at a water depth of 2100–2500 m), and Valentin Feilberg (at a water depth of 3500–4000 m). Fully erosive features are represented by submarine canyon systems, the most important named Ameghino (43°–44°30′S) and Almirante Brown (44°30′–46°S), which are deflected to the north in their down-slope portions through the influence of the contouritic processes.
The continental rise extends offshore from the foot of the slope north of 44°S, and its outer boundary approaches the abyssal plains at depths deeper than 5000 m. The average gradient is around 1 : 100. Depositional features are principally coalescent submarine fans at the base of submarine canyons. South of 44°S the rise disappears and the lower terrace slope connects directly with the abyssal plain.
Although it does not belong to the ACM, as it corresponds to the abyssal plains, it is important to mention that very large sedimentary drifts develop in this region. These drifts are formed by huge mud-wave fields (Zapiola, Argyro, and Ewing drifts; Flood & Shor, 1988; von Lom-Keil, Spieβ & Hopfauf, 2002) containing most of the finest terrigeous sediments carried towards the deepest ocean by the Antarctic-sourced water masses that recirculate into the Argentine Basin after being detached from the continental margin.
Morphosedimentary features in the transcurrent margin
This margin is represented by the continental mass that extends eastwards of the Malvinas/Falkland Islands, constituting the Malvinas Plateau; as stated above it contains geomorphic features that share different characteristics of the three types of margins. Its nearly flat surface reaches a water depth of 2000 m on average, and is delimited by the 3000-m contour line. Further east there is another flat, shallower surface that constitutes the M. Ewing Bank. The entire surface including the Malvinas Plateau and the M. Ewing Bank is about 1800 km long and 300 km wide. The northern boundary of the plateau is the Malvinas Escarpment, which represents the eastern extension of the lower slope in the southern part of the passive margin, north of 48°S. The escarpment is very steep between water depths of 2200 and 5100 m, with a gradient of 1 : 5 and even 1 : 1 (slope of 45°). The escarpment extends towards the east outside the plateau in what is known as the Malvinas fracture zone. The Malvinas Channel, extended at the foot of the Malvinas fracture zone, reaching depths of 5800 m, marks the transition to the abyssal plain. On the other hand, the southern part of the Malvinas Plateau shows a gentle gradient reaching depths of 3500 m in the Malvinas Trough, with depths grading from 300 to 3500 m from west to east. It is a long submarine valley extended for more than 1500 km from the west part of Malvinas/Falkland Islands to the Abyssal Plain north of Georgia Islands. The Malvinas Trough separates the Malvinas Plateau from the North Scotia Ridge. As a result of the topographical characteristics, represented by a relatively shallow, transverse-to-the-continent feature, the Malvinas Plateau constitutes a topographic barrier to the deep Antarctic-sourced water masses, so interfering in the oceanic circulation, and conditioning most of the oceanographic and sedimentary characteristics of the passive continental margin located to the north (Arhan, Heywood & King, 1999).
Morphosedimentary features in the transpressive margin
This margin comprises different morphostructural features associated with the Scotia Arc. However, as only the northern part of this arc (Northern Scotia Ridge) is considered as a part of the PCM, the present description is restricted to this area. It represents the submarine extension of the Southern Andes, which reaches its more significant features in Isla de los Estados, Burdwood Bank, Cormorán, and Negra Rocks, and the insular shelf around Georgia Islands. All these islands are composed of rocks similar to those of the Andes cordillera (Dalziel & Elliot, 1971; Ramos, 1996). In particular, the Burdwood Bank is an isolated piece of continental shelf, 360 km long and 115 km wide, at depths averaging 180 m.
Synthesizing the stratigraphy of the PCM is complex given the existence of several sedimentary basins in the subsurface of the margin, each with its own geological and sedimentological characteristics. It can, however, be broadly described as comprising two major units.
The pre-Cretaceous basement corresponds to the deep geological substratum of the basins and interbasinal regions. Its characteristics were defined by lithological aspects recorded in offshore drillings as well as in seismic records, in which the differential seismic velocity (the speed at which a seismic wave propagates into the rocks depending on lithology, consolidation, hardness, and other physical properties) enable the identification of different rock layers. According to Zambrano & Urien (1974) and Ramos (1999), in the northern part of the PCM (between 39 and 43°S) there is an igneous basement covered by metamorphized palaeozoic continental–marine sedimentites, with seismic velocities between 5 and 5.5 km s−1. South of 44°S, above a precambrian metamorphic basement, there are upper palaeozoic–lower mesozoic metamorphic and acid intrusive rocks, with seismic velocities of up to 6 km s−1, followed by silurian–jurasic pyroclastic and acidic mesosilicic extrusive rocks interbedded with continental sediments, with seismic velocities between 4.2 and 5.1 km s−1.
Post-Cretaceous sedimentary filling of the basins
Post-cretaceous sequences are composed of continental–marine shales, filites, lutites, limestones, sandstones and conglomerates with thicknesses of 6–8 km. The upper part of the sequences represents the Neogene, which is not restricted to the basins but overpass their limits and extend homogeneously onto most of the margin. Two sequences are recognized: the lower sequence (Miocene–lower Pliocene) is integrated by marine deposits formed during the Miocene transgression, which covered most of South America (Aceñolaza, 2000; Hernández et al., 2005), as well as by regionally extended lower Pliocene continental (mostly fluvial) deposits; the upper sequence (middle Pliocene–Quaternary) south of 43°S is mainly represented by marine (interglacial)–continental (glacial) sequences, which became progressively fully marine offshore of the outer shelf. The discontinuous distribution of Quaternary deposits on the PCM can be attributed to post-glacial isostatic rebound (which was more significant in that region, closer to the glaciated areas of Patagonia) and the consequent increment in erosive processes. Malumián (1999) correlated the stratigraphy of several Patagonian basins and related them to the global eustatic curve. Malumián & Nañez (1996) and Nañez & Malumián (2008) studied and correlated geological units in the shelf on the basis of the micro- and nanofossil content.
SEISMIC STRATIGRAPHY OF THE PCM
Seismic stratigraphy is an invaluable tool for synthesizing submarine sedimentary successions as it enables us to record them at a regional scale. The seismic method consists of the release of a shock wave from a seismic source on the surface, which penetrates the sea floor and is reflected and/or refracted at the boundary between structurally or texturally contrasting geological units. The boundary is visually manifested in the seismic records as a ‘seismic horizon’. In the marine environment, most of the seismic horizons constitute regional stratigraphic unconformities of strong erosive character, resulting from the occurrence of significant and often abrupt climatic, oceanographic, and tectonic events. The geological units separated by seismic reflectors are seismic sequences (interpreted as ‘depositional sequences’) representing particular environments developed during the successive evolutionary stages of the continental margin.
Table 1. Seismic horizons in the Argentine continental margin
base of the Quaternary
The seismic reflector AR3 (Hinz et al., 1999) is found at water depths of 1700–2500 m, deepening to the east, and separates units with seismic velocities higher than 3 km s–1 (below) and lower (above). According to Hinz et al. (1999), AR3 is of Maastrichtian (lower Campanian) age (∼81 Ma), and constitutes the erosional surface of a complex drift sequence developed offshore of the outer shelf. This horizon is correlated with reflector ARII (Urien & Zambrano, 1996), which these authors (based on correlations with offshore oil drillings) assigned to the Maastrichtian marine transgression.
The seismic reflector AR4 (Hinz et al., 1999) is relatively subhorizontal at water depths of 1000–1300 m, separating seismic units with seismic velocities higher than 2.4 km s−1 (below) and lower (above); therefore, it separates consolidated from semi-consolidated beds, following the terminology used by Ewing & Lonardi (1971). The reflector is considered to be equivalent to reflector ARI of Urien & Zambrano (1996), which represents the Eocene–Oligocene boundary. Hinz et al. (1999) associate AR4 with erosion produced by a strong expansion of Antarctic ice masses in the late Eocene, which was responsible of the thermohaline circulation that highly influenced the depositional regimes. Sedimentary sequences below and above this horizon are represented by complex giant sedimentary drifts. Sequences above correspond to the lower unit of the contourite depositional system defined by Hernández-Molina et al. (2009, 2010).
The seismic reflector AR5 (Hinz et al., 1999) is a strong reflector separating units with seismic velocities higher (below) and lower (above) than 2.15 km s−1. This horizon lies very close to seismic reflector H3 defined by Ewing & Lonardi (1971), at water depths around 1500–1700 m, which was originally considered as being of Eocene age, although the new seismic stratigraphic information undoubtedly relates it to younger – Miocene – units. Therefore, both horizons can be considered as equivalent at least in the middle slope, although in deeper regions they progressively separate from each other. According to Hinz et al. (1999), AR5 represents a renewed cooling with another extensive expansion of Antarctic ice masses that took place in the mid-Miocene, approximately 15 Ma. This period has a global significance as it was also recorded in other parts of the Southern Hemisphere, such as in Antarctica (Hernández-Molina et al., 2004, 2006), South Africa (Wildeboer Schut, Uenzelmann-Neben & Gersonde, 2002; Wigley & Compton, 2006), India, and in the South Pacific Ocean (Hernández-Molina, Maldonado & Stow, 2008). Above AR5 the intermediate and the upper units of the contourite depositional system develop (Hernández-Molina et al., 2009, 2010).
The seismic reflector H2 (Ewing & Lonardi, 1971) was well defined in the upper slope at water depths around 1200 m, separating the underlying semiconsolidated from the overlying nonconsolidated layers (in the sense of Ewing & Lonardi, 1971), with seismic velocities higher and lower (respectively) than 2 km s−1. As they did with seismic reflector H3 (equivalent to AR5), these authors originally considered it as being Eocene in age, but following reinterpretation it is now considered as representing the Miocene–Pliocene boundary. In regions offshore of the province of Buenos Aires, a seismic reflector also separating layers of seismic velocities higher and lower than 2 km s−1– and hence correlated with H2 – was recognized in the outer shelf (Violante et al., 2010). H2 marks the top of the upper unit of the contourite depositional system defined by Hernández-Molina et al. (2009, 2010).
H1 was described for the upper slope by Ewing & Lonardi (1971) at water depths averaging 800–1000 m: it is continuous along most of the margin, gently deepening in the seaward direction. In the bonaerensian outer shelf, Parker et al. (2005) described reflector L at around water depths of 200 m, and assigned it to the lower–mid Pliocene boundary. This reflector L was correlated with H1 on the basis of stratigraphic position and seismic-facies analysis.
N was described by Parker et al. (2005) for the bonarensian shelf as representing the beginning of the transgressive–regressive events associated with the alternating interglacial and glacial periods, hence it corresponds to the base of the Quaternary (in the sense of Gibbard et al., 2009, i.e. 2.6 Ma). It was correlated (Parker et al., 2008) with horizon ‘b’ defined by Ewing & Lonardi (1971), who considered that this reflector is continuous along the outer shelf at least up to the latitude of San Jorge Gulf (46°S), therefore appearing in the northern PCM but probably not in southern PCM. Although more research is still needed on the Quaternary deposits in the Patagonian shelf, the possibility of the occurrence of transgressive sedimentary sequences associated with the high-stand sequences preserved on the coast cannot be disregarded. The well-known records of sea-level transgressions in the coasts of Patagonia correspond to OIS 9 (240–342 kyr), OIS 7 (158–239 kyr), and OIS 5 (104–143 kyr) (Rostami, Peltier & Manzini, 2000) (OIS: Oxygen Isotopic Stage). The absence – or at least discontinuous distribution – of the Quaternary transgressive sequences in the southern PCM could have resulted from erosion associated with the isostatic uplift that affected Patagonia at an average speed of 0.9 m per kyr (Rostami et al., 2000).
GEOTECTONIC AND PALAEOCEANOGRAPHIC EVOLUTIVE CONTEXT OF THE PCM
The ACM, as a part of the South America Plate, is genetically associated with the cortical extension and sea-floor spreading caused by the break-up of Gondwana. However, the margin was also indirectly affected by processes taking place on the western part of the plate, like the intense tectonism and complex subduction regimes that were important during Mesozoic and Cenozoic times (Uliana & Biddle, 1988). Ortiz-Jaureguizar & Cladera (2006) considered that this tectonic development, together with other major external forcing factors such as sea-level fluctuations, changes in sea temperatures, and glaciations, were responsible for modifications in the palaeogeography of the southern part of South America, and hence in the biomes. Figure 2 shows the major geotectonic features and lineaments that resulted from the development of the margin, whereas Figure 5 synthesizes the tectonic, palaeoclimatic, and palaeoceanographic events participating in the regional evolution.
According to Urien & Zambrano (1996), Ramos (1996, 1999), and Turic et al. (1996), rift and wrench processes characterized the initial tectonic stages of the sea-floor spreading in middle Jurassic times. After that, voluminous volcanic effusions took place in late Jurassic–early Cretaceous times (Hinz et al., 1999; Franke et al., 2007, 2010). The separation between South America and Africa was completed with the reactivation of the Malvinas–Agulhas fracture in the Aptian (∼115 Ma), when the sea invaded former euxinic environments (with very restricted water circulation and stagnant conditions), and a proto-Atlantic ocean (with more open-water circulation and increasing oxygenation) was installed (Hinz et al., 1999). In the early Campanian (∼81 Ma) deep waters played an important role in shaping the sea floor, as evidenced by complex sedimentary drifts in the south-western Atlantic (Hinz et al., 1999). The Maastrichtian (70–65 Ma) was characterized by a global marine transgression. This event covered very large areas of Patagonia from San Jorge Gulf and the Austral Basin to Neuquén Basin, thus representing the first Atlantic transgression reaching the Andean basins (Malumián, 1999; Nañez & Malumián, 2008). According to the same authors, this transgression gave rise to the ‘first’ Argentine continental shelf, characterized by a very extensive shallow sea with widely distributed marginal environments. Sea levels remained high in the Palaeocene (65.5–55 Ma), and the resulting Atlantic transgression is known as Salamanca Sea. A regressive marine event took place at the end of this period, at the time that the Laramica tectonic phase occurred as a significant stage of cortical deformation associated with the beginning of the Andes uplift.
At the beginning of the Eocene (55–50 Ma), when the proto-Atlantic ocean was well developed, sea levels were globally high in a climatic context of high temperatures; surface and bottom ocean temperatures at low latitudes were typical of subtropical seas. Marine waters reached some sectors of the present Patagonian lands according to microfaunistic evidence (Malumián & Nañez, 2009). As this ocean progressively evolved into a fully open ocean, thermohaline circulation developed, which in the first stages was mainly controlled by salinity rather than by temperature (Oberhansli & Hsü, 1986).
At the end of the Eocene (40–35 Ma) a new cooling of the deep-water masses was accompanied by a marine regression and the displacement of the zones of formation of deep waters from low to high latitudes. At 34 Ma, the first evidence of formation of ice masses appeared in East Antarctica (Einsele, 2000), which occurred together with a global decrease in temperature (stage Oi-1; Zachos et al., 2001). Global tectonic deformations were documented at this time, which had their largest effects on western South America where the Inca tectonic phase – a new reactivation of the Andean uplift – took place.
In the early Oligocene (around 32–30 Ma) the opening of the Drake Passage took place (Lawver & Gahagan, 2003; Livermore et al., 2004). This event, together with the opening of the Tasmania Passage (which started earlier, around 38 Ma) produced the initiation of the circulation of the ACC, with the consequent isolation of Antarctica (Einsele, 2000; Zachos et al., 2001). The resulting interruption in heat transference from low to high latitudes produced the definitive cooling of Antarctica, where large ice masses accumulated in its eastern regions and a regressive event took place. These processes progressively gave rise to the present model of thermohaline circulation in the world ocean, which in the PCM was responsible for the progressive shaping of the margin towards its present configuration, including the formation of the Contourite depositional system. The Pehuenche tectonic phase, a major process involved in the Andean uplift, developed simultaneously with the final stages of the opening of the Drake Passage (∼29 Ma).
Nearly simultaneously with these events, in the southern tip of South America the occurrence of the first stages of the north-west–south-east sea-floor spreading and transtensional motions associated with the Scotia plate allowed it to penetrate beneath the Drake Passage, with the consequent formation of the Scotia Arc. This event lasted for a long time, between 30 and 6 Ma (Dalziel, 1982; Livermore et al., 2004). In the intermediate times, in the late Oligocene (25 Ma), a global warming with sea-level rise and associated marine transgressions occurred: some parts of Patagonia were affected by this event, which is recorded by very shallow and limited extended seas (Malumián & Nañez, 2009). Soon after, in the lower Miocene (23 Ma), a new global cooling with low sea levels led to a new glaciation (Mi-1; Zachos et al., 2001).
The Miocene continued, between 17 and 14.5 Ma, with another increase in temperature, a reduction in the Antarctic ice masses, and the occurrence of a new sea-level rise on a global scale (Haq, Hardenbol & Vail, 1987). Zachos et al. (2001) describe a regional warm phase prevailing at those times, characterized by global low ice volumes and slightly high bottom water temperatures, with the exception of several brief periods of glaciations. These processes, along with a significant regional subsidence (van Andel et al., 1977; Kennett, 1982), gave rise to Atlantic marine transgressions, which in the Pampean regions correspond to the Paranense Sea, and in Patagonia to the marine facies described by Malumián & Nañez (2009). According to Berggren et al. (in Malumián & Nañez, 1996), the climatic optimum of the Neogene occurred in the middle Miocene, between 15.6 and 13.6 Ma. Hernández et al. (2005) dated the Paranense transgression to between 15 and 13 Ma.
The expansion of Antarctic ice masses and consequent regressive events represented by seismic horizon AR5 – considered by Hinz et al. (1999) as having occurred at ∼15Ma – is included in the time span of the warmer and transgressive mid-Miocene. This apparent inconsistency deserves more attention and further research. However, it can be preliminarily considered that some of the ‘several brief periods of glaciations’ mentioned by Zachos et al. (2001) as having taken place in a warmer period, may have been highly significant – at least around Antarctica – and are evidenced by horizon AR5. In this case, AR5 should be slightly older (around 16 Ma) than it is presently considered to be.
Following the regional warm periods of the mid-Miocene, a gradual cooling and re-establishment of a major ice sheet on Antarctica occurred between 14 and 10 Ma (Zachos et al., 2001). As a result, the whole Antarctic continent was affected by the cold climate, a new marine regression occurred, and the circulation of the AABW began to be very active (Einsele, 2000). At the same time, the orogenic Quechua phase contributed to the uplifting of the Cordilleras Patagónica and Principal, which nearly reached their present configuration (Yrigoyen, 1979, 1999). This new geomorphological scenario led to changes in the climatic conditions as a result of the ‘barrier effect’ of the recently raised mountains that interfered with the wind pattern and the circulation of moist air masses, resulting in an increase in aridity in Patagonia. Besides, a substantial increase in sediment supply from west to east was favoured as a result of the increasing slope gradients between the new high-relief mountains and the sea. At the same time, as a consequence of the combination of all the aforementioned tectonic, geomorphological, and climatic changes, the south-western Atlantic Ocean experienced the circulation of the NADW and the AAIW. The interaction between the two currents gave rise to most of the morphosedimentary changes that allowed the PCM to evolve towards its present configuration.
During the time span between approximately 16 and 5 Ma (seismic horizons AR5 and H2, respectively), very significant morphosedimentary changes took place in the slope. The contourite depositional system, which developed from the early Oligocene through different stages represented by the lower, intermediate, and upper units, reached its most significant expression, and finally – coincident with the deposition of the upper unit – began to evolve towards its present configuration (Hernández-Molina et al., 2009, 2010).
The new oceanographic and climatic conditions imposed at the end of the Miocene by the interaction between NADW and AAIW resulted in a decrease in oceanic temperatures and the permanent installation of the Antarctic ice masses. The climate in the Patagonian–Pampean region became cold and glaciers made their first appearance, with evidence of glacier advances recorded at 7 Ma (Rabassa et al., 2005). Global cooling was interrupted at around 5–4 Ma, and temperatures increased again at 4–3 Ma. During the late Pliocene a new Andean diastrophic phase (Diaguita) took place. This phase was responsible for the final uplift of the central Andes of Argentina and Chile, the Puna, the Pampean Mountain Range, and the Mesopotamia region. Soon after that another event of global consequence occurred, at around 3–2.6 Ma, when the definitive closing of the Panamá Isthmus led to an increase in activity of the Gulf Stream that resulted in a transport of warm and saline waters to the North Atlantic, inducing the intensification of the NADW, the formation of the ice masses in the Northern Hemisphere, and the beginning of the Northern Hemisphere glaciations. Consequently, the increase in temperature gradients influenced the circulation of the NADW towards the south-western Atlantic (Einsele, 2000). By now, glacial conditions were definitively settled in Antarctica and Patagonia. The AABW was definitely reactivated at the beginning of the Quaternary, and the deep-water circulation reached its present configuration with an increase of the AABW during glacial periods and of the NADW during interglacial periods (Duplessy et al., 1988; Sarnthein et al., 1994). The final stages of the evolution of the ACM were dominated by the Quaternary sea-level fluctuations associated with the alternating glacial–interglacial periods, particularly with the post-LGM transgression (Guilderson et al., 2000; Violante & Parker, 2004; Parker et al., 2008). Rignot, Rivera & Casassa (2003) consider that the melting of Patagonian glaciers – essentially resulting from climate change – significantly contributes to sea-level rise in a proportion 1.5 times greater than the melting of Alaskan glaciers. If this relation is considered for past glacial times, it can be concluded that Patagonia had a strong impact in Quaternary eustatic fluctuations.
EVOLUTION OF THE PCM
Three major stages of evolution can be synthesized.
1A stage dominated by internal (endogene) factors, corresponding to pre-Maastrichtian times, when tectonic conditioning factors prevailed over oceanographic and climatic factors. Major processes were related to plate tectonics, sea-floor spreading, and the separation of South America and Africa, with predominant continental-dominated sedimentation.
2A transitional stage, corresponding to early Tertiary times (pre-Eocene–Oligocene), when the first shallow marine environments settled in the region (initial stages of the Atlantic Ocean evolution), and oceanographic and climatic conditioning factors became at least as important as tectonic factors in the shaping of the margin.
3A stage dominated by external (exogene) factors, corresponding to post-Oligocene times, when the Atlantic Ocean was definitively installed and the climatic and oceanographic conditions evolved towards its present configuration. The stratification of the oceanic water masses ensued, thus inducing an ocean circulation pattern that became a major forcing factor in the morphosedimentary evolution of the margin. Climatic and oceanographic conditions were thus more important in the short term than tectonism (mainly characterized by long-term factors such as the Andean uplift and subsidence in the marine basins). The Quaternary glacio-eustatic sea-level fluctuations, and the resulting effects experienced by the coast and offshore (particularly the shelf) regions, gave the PCM its final (present) configuration.
The PCM offers enormous possibilities for a better understanding of the evolution of the adjacent continent. The records of the regional palaeoclimatic, palaeooceanographic, and geotectonic processes are preserved there, and are thus available for an integrated and multidisciplinary geological correlation between sea and land. Different types of margins are present, each showing particular features that are the result of a complex interaction among tectonic, sedimentological, dynamic, morphological, climatic, oceanographic, and evolutive conditioning factors.
In this contribution attempts were made to synthesize our current knowledge of the different processes participating in the development of the Patagonian continental shelf (PCS). Many questions are still unsolved and more research is needed. In doing so it is shown that an integration of marine and continental geology can provide insights that probably neither discipline can address satisfactorily in isolation.
This contribution is based on our presentation at the symposium entitled ‘Paleogeography and Paleoclimatology of Patagonia: Effects on Biodiversity’, held at the La Plata Museum in Argentina in May 2009 and organized by Jorge Rabassa, Eduardo Tonni, Alfredo Carlini, and Daniel Ruzzante. The authors thank Daniel Ruzzante and Jorge Rabassa for reviews and comments on the article.