Luis A. Spalletti

The Neuquén Basin: an overview

Abstract The Neuquén Basin of Argentina and central Chile contains a near-continuous Late Triassic-Early Cenozoic succession deposited on the eastern side of the evolving Andean mountain chain. It is a polyphase basin characterized by three main stages of evolution: initial rift stage; subduction-related thermal sag; and foreland stage. The fill of the basin records the tectonic evolution of the central Andes with dramatic evidence for base-level changes that occurred both within the basin and along its margins. The record of these changes within the mixed siliclastic-carbonate succession makes the basin an excellent field laboratory for sequence stratigraphy and basin evolution. The 4000 m-thick fill of the basin also contains one of the most complete Jurassic-Early Cretaceous marine fossil records, with spectacular finds of both marine and continental vertebrates. The basin is also the most important hydrocarbon-producing province in southern South America, with 280.4 × 106 m3 of oil produced and an estimated 161.9 × 106 m3 remaining. The principal components of the hydrocarbon system (source and reservoir) crop out at the surface close to the fields. The deposits of the basin also serve as excellent analogues to reservoir intervals worldwide.

Mesozoic break-up of SW Gondwana: implications for regional hydrocarbon potential of the southern South Atlantic

Abstract This work provides new palinspastic palaeofacies reconstructions of SW Gondwana incorporating rotation of a Falkland/Malvinas microplate. We discuss the implications of this for the tectonic evolution of the southern South Atlantic and hence for the regional hydrocarbon potential. Existing Gondwana reconstructions display good fits of major continents but poorly constrained fits of microcontinents. In most continental reconstructions, the Falkland/Malvinas Plateau was assumed to be a rigid fragment of pre-Permian South American crust. However, it has been suggested, on the basis of palaeomagnetic data, that the Falkland/Malvinas Islands were rotated by ∼180° after 190 Ma. This rotation hypothesis has been successfully tested on the basis of Devonian stratigraphy and palaeontology, Permian stratigraphy and sedimentology and Late Palaeozoic and Early Mesozoic structure, making it unlikely that the plateau behaved as a rigid structure during breakup. We have explored the consequences of accepting this hypothesis for the tectonic evolution of SW Gondwana by compiling new palaeogeographic maps for the Permian–Cretaceous of the southern Atlantic area. To achieve a realistic close fit, we have devised a pre-rift proxy for the ocean–continent boundary for the South Atlantic. In order to produce the best fit, it is necessary to subdivide South America into four plates. The consequences of this are far-reaching. Our work suggests that although sedimentary basins were initiated at different times, three major tectonic phases can be recognised; in regional terms these can be thought of as pre-, syn- and post-rift. During the pre-rift time (until the Late Triassic), the area was dominated by compressional tectonism and formed part of the Gondwana foreland. The Falkland/Malvinas Islands lay east of Africa, the Falkland/Malvinas Plateau was ∼33% shorter and Patagonia was displaced east with respect to the rest of South America, in part along the line of the Gastre Fault System. Potential source facies are dominantly post-glacial black shales of Late Permian age deposited in lacustrine or hyposaline marine environments; these rocks would also be an effective regional seal. Sandstones deposited in the Late Permian would be dominantly volcaniclastic with poor reservoir qualities; Triassic sandstones tend to be more mature. There was significant extension from about 210 Ma (end-Triassic) until the South Atlantic opened at about 130 Ma (Early Cretaceous). In the early syn-rift phase, extension was accompanied by strike-slip faulting and block rotation; later extension was accompanied by extrusion of large volumes of lava. Early opening of the South Atlantic was oblique, which created basins at high angle to the trend of the ocean on the Argentine margin, and resulted in microplate rotation in NE Brazil. Intermittent physical barriers controlled deposition of Upper Jurassic–Cretaceous anoxic sediments during breakup; some of these mudrock units are effective seals with likely regional extent. During crustal reorganisation, clastic sediments changed from a uniform volcaniclastic provenance to local derivation, with variable reservoir quality. In the late rift and early post-rift phase, continental extension changed from oblique to normal and basins developed parallel to the continental margins of the South Atlantic. This change coincides with the main rifting in the Equatorial basins of Brazil and the early impact of the Santa Helena Plume. It resulted in widespread development of unconformities, the abandonment of the Reconcavo–Tucano–Jatoba rift and the end of NE Brazil plate rotation, which remained attached to South America. There was extensive deposition of evaporites, concentrated in (but not restricted to) the area north of the Rio Grande Rise/Walvis Ridge. Widespread deposits can be used to define potential regional elements of hydrocarbon systems and to provide a framework for relating more local elements. Our main conclusion is that the regional hydrocarbon potential of the southern South Atlantic has been constrained by the tectonic evolution.

Temporal evolution and spatial variation of early tertiary volcanism in the Patagonian Andes (40°S–42°30′S)

Early to mid-Tertiary igneous activity in the Cordilleran Series (CS) of the Patagonian Andes between 40°S–42°30′S shows spatial variation and temporal trends that can be correlated with crustal thickness and slab depth. Volcanism in this region is concentrated in two sub-parallel arcs, the Pilcaniyeu Belt to the east and the El Maiten Belt to the west. Compilation of available K-Ar data and paleogeographic constructions suggests three major periods of volcanic activity. The first is a Paleocene-Eocene (60-42 Ma) event which developed primarily in the Pilcaniyeu belt; the second (Oligocene, 33-23 Ma) event is now exposed primarily along the El Maiten Belt. Miocene volcanics (16-11 Ma) have also been found in the northern sector of the El Maiten Belt. After an overall decline in the magnitude of volcanism, the third period of volcanic activity reached its maximum during the Pliocene-Pleistocene as large stratovolcanoes were built along the North Patagonian Cordillera. Temporal variations in the lower Tertiary CS, determined by superposition in selected cross-sections, indicate that the major episodes begin with silicic associations (ignimbritic, plinian, and obsidian rhyolitic facies) and end with intermediate and basic lava flows (stratovolcanoes and monogenetic cones). K2O and total alkalis decrease southward for a given silica content in the Pilcaniyeu and El Maiten Belts. The calc-alkaline andesites of the northern sector (39°30′S–41°30′S) are similar to many other Central Andes series, but towards the south (Cholila) there are also sequences with mild tholeiitic affinities. Spatial variations in major element composition seem to be related to the southward decrease in thickness of the underlying sialic crust. Variations in subduction geometry during the Cenozoic possible correlate with the compositional recurrence observed in the Cordilleran Series.

Tectonic and paleoenvironmental evolution of Mesozoic sedimentary basins along the Andean foothills of Argentina (32°–54°S)

Abstract Chronoenvironmental and tectonic charts are presented for Mesozoic basins located along the Andean foothills of the South American plate. On the basis of the main tectonic events, pre-Andean basins, break-up-related basins, extensional back-arc basins, and Andean foreland basins are recognized. The pre-Andean basins were formed by continental extension and strike-slip movement before the development of the Mesozoic–Cenozoic Andean magmatic arc. Upper Permian to Middle Triassic extension along Palaeozoic terrane sutures resulted in rifting, bimodal magmatism (Choiyoi group), and continental deposition (Cuyo basin). From the Late Triassic to the Early Jurassic, continental extension related to the collapse of the Gondwana orogen initiated a series of long, narrow half-grabens that filled with continental volcaniclastic deposits. These depocenters were later integrated into the Neuquen basin. Coeval development of the shallow marine Pampa de Agnia basin (42–44°S) is related to short-lived extension, probably driven by dextral displacement along major strike-slip faults (e.g. the Gastre fault system). Widespread extension related to the Gondwana breakup (180–165 Ma) and the opening of the Weddell Sea reached the western margin of the South American plate. As a result, wide areas of Patagonia were affected by intraplate volcanism (Chon Aike province), and early rifting occurred in the Magallanes basin. The Andean magmatic arc was almost fully developed by Late Jurassic times. A transgressive stage with starvation and anoxia characterized the Neuquen basin. In western Patagonia, back-arc and intra-arc extension produced the opening of several grabens associated with explosive volcanism and lava flows (e.g. Rio Mayo, El Quemado). To the south, a deep marginal basin floored by oceanic crust (Rocas Verdes) developed along the back-arc axis. In mid-to late Cretaceous times, Andean compressional tectonics related to South Atlantic spreading caused the inversion of previous extensional structures and the beginning of a retro-arc foreland phase in the Neuquen and Austral basins.

Sequence stratigraphy of a tidally dominated carbonate–siliciclastic ramp; the Tithonian–Early Berriasian of the Southern Neuquén Basin, Argentina

The Tithonian–Berriasian Vaca Muerta, Carrín Curá and Picún Leufú formations in the southern Neuquén Basin were deposited on a tidally dominated, mixed carbonate–siliciclastic ramp. Basinal, outer, middle, shallow and back ramp facies associations are recognized and a sequence stratigraphic analysis reveals that the ramp record consists of three shallowing‐upwards sequences (Ti1, Ti2 and Ti3) set within a lower‐order progradational cycle. A higher order of cyclicity is superimposed on to the middle (Ti2) sequence. The majority of the ramp facies belong to the transgressive and highstand systems tracts; however, at the base of Ti2, a lowstand systems tract is identified, characterized by a basal unconformity and an abrupt basinward shift of the shallow marine lithofacies. Transgressive systems tracts were characterized by slow sedimentation rates and rapid sea‐level rises that affected carbonate productivity. Highstand systems tracts show the greatest carbonate productivity and an increased progradation rate on account of a reduction in accommodation space generation. Palaeogeography played a major role in the development of the depositional systems. Partial isolation from the Pacific Ocean reflecting the growth of the Andean magmatic arc and geographic restriction due to tectonic inversion in the central part of the basin resulted in a meso‐macrotidal regime that produced a tidally dominated sedimentary record in the shallow and back ramp environments. Coeval anoxic conditions in the central part of the Neuquén Basin favoured distal ramp and basinal black shale deposition during episodes of relative sea‐level rise.

Dating the Triassic continental rift in the southern Andes: the Potrerillos Formation, Cuyo Basin, Argentina

The Triassic successions of western Argentina commonly show thin pyroclastic levels intercalated within thick fluvial and lacustrine terrigenous deposits. The Potrerillos Formation is the thickest Triassic unit in the Cuyo Basin. It is composed of alternating cycles of gravelly- sandy- and muddy-dominated intervals, in which several laterally-continuous tuff horizons occur. U-Pb SHRIMP ages were determined on zircon grains from three tuff levels located between the lowermost and the middle sections of the Potrerillos Formation. The ages for the time of deposition of the tuffs are 239.2 ± 4.5 Ma, 239.7 ± 2.2 Ma and 230.3 ± 2.3 Ma (Middle Triassic). Chemical data indicate that these acid to intermediate volcaniclastic rocks are derived from coeval basic magmas displaying tholeiitic to slightly alkaline signatures. They are associated with the rift stage that followed the extensive post-orogenic volcanism of the Choiyoi Group, that in turn has been ascribed to slab break-off in neighbouring areas. Two of the studied samples also record a subpopulation of inherited zircon grains with crystallisation ages of 260-270 Ma. The latter are considered to be an indirect measurement for the age of the Choiyoi Group in the Cuyo basin. The rift-related Triassic event represents the culmination of the Gondwanian magmatic cycle, and is interpreted as the result of subduction cessation and anomalous heating of the upper mantle previous to the western Gondwana break-up.

Sedimentologic and sequence stratigraphic model of a Neocomian marine carbonate–siliciclastic ramp: Neuquén Basin, Argentina

Abstract Facies analysis of the upper member of the Agrio Formation (Hauterivian–Barremian) in Loma La Torre, central Neuquen Basin, allowed the definition of an open marine ramp mainly characterised by the accumulation of fine-grained sediments of the basinal to outer ramp settings, with subordinated mid to inner ramp silts, carbonate sands, and carbonate buildups. Accumulation conditions for the fine-grained facies match with a low energy environment, below storm-wave base level, poor to null oxygenation of the substrate, and alternating conditions of siliciclastic input and micritic carbonate production. Carbonate-rich facies (wackestones, packstones, grainstones, and biolitites) as well as siltstones and heterolithic intervals may have accumulated under more oxygenated and moderate energy conditions, with some evidence of distal orbital flows. An oxygenation curve was traced from the combined analysis of ichnofacies and sedimentary facies. This curve shows that diversity of ichnofossils is controlled not only by the oxygenation of the substrate, but also by its consistency or even the energy of the environment. Vertical association of facies and stratal geometry allowed the definition of five stratigraphic sections, which were grouped into three depositional sequences related to third order cycles of sea-level change. Each sequence is characterised by a basal transgressive interval (TST) followed by a highstand systems tract (HST). The TSTs are relatively thick and show an aggradational stacking of outer ramp to basinal deposits. The HSTs are characterised by shallower deposits that may show either a strong progradational arrangement, evidenced by low angle clinoforms, or a subhorizontal stratal pattern with a shallowing upward trend. The progradational arrangement of HSTs indicates low rates of accommodation/siliciclastic input or accommodation/carbonate production. This contrasts with transgressive intervals, in which the increase of these rates cause a starved stage, especially in the deepest portions of the basin. The relatively shallower position of the HST deposits allowed the development of high-frequency cycles that seem to be related to high-order eustatic oscillations. In theses cycles, transgressive periods, characterised by anoxic conditions and saturated substrates, pass vertically into highstands represented by more oxygenated and looser substrates in which benthic organisms spread, supplying significant volumes of carbonate sand.

Eolian permian deposits in west and northwest Argentina

Abstract The sedimentary and stratigraphic characteristics of eolian Permian deposits exposed in Mendoza, La Rioja and San Juan Provinces (west and northwest Argentina) are described in this paper. The eolianites are fine and medium sandstones with large-scale cross-bedding, multiple parallel truncation planes and some asymmetrical ripples. Three genetic types of deposits have been identified: dune facies, eolian sand-sheet facies and mixed fluvial-eolian facies. Fine and medium sandstones with large-scale cross-bedding and multiple parallel truncation planes are here interpreted as dune deposits (mainly crescentic dunes), and unstratified or flat-bedded sandstones as eolian sand-sheet deposits. Mixed fluvial and eolian sequences, composed of sandstones, mudstones and some matrix-supported conglomerates, represent a transitional facies between those formed in eolian and fluvial environments. This considerable deposition of eolian sediments was probably brought about by the existence of an extensive, medium to low-latitude continent and the withdrawal of marine environments. The environments with highest aridity occurred towards the south and west of the region. The eolian circulation pattern was controlled by a long ensialic volcanic arc emerging towards the west of the basin.

The Cambrian-Ordovician siliciclastic platform of the Balcarce Formation (Tandilia System, Argentina): Facies, trace fossils, palaeoenvironments and sequence stratigraphy

The Lower Palaeozoic sedimentary cover of the Tandilia (Balcarce Formation) is made up of thick quartz arenite beds together with kaolinitic claystones and thin fine-grained conglomerates. The Balcarce Formation was formed in the nearshore and inner shelf environments of a tide-dominated and storm influenced open platform. It shows many features suggesting tidal sedimentation. Coarse-grained facies were formed by sand bar migration and accretion. Heterolithic packages are interpreted as interbar (swale) deposits. Subordinated HCS sandstones indicate storm events. The recognition of thick progradational clinoforms allows to confirm that the Balcarce sea was open to the south, as suggested years ago through palaeocurrent interpretation. The great abundance and variety of trace fossils is among the most outstanding characteristics of this unit. The ichnotaxa that have been recognised so far are: Ancorichnus ancorichnus, Arthrophycus alleghaniensis, Arthrophycus isp., Bergaueria isp., Cochlichnus isp., Conostichus isp., Cruziana furcifera, Cruziana isp., Daedalus labeckei, Didymaulichnus lyelli, Didymaulichnus isp., Diplichnites isp., Diplocraterion isp., Herradurichnus scagliai, ?Monocraterion isp., Monomorphichnus isp., Palaeophycus alternatus, Palaeophycus tubularis, Palaeophycus isp., Phycodes aff. pedum, Phycodes isp., Plagiogmus isp., Planolites isp., Rusophycus isp., Scolicia isp. and Teichichnus isp. Trace fossils have traditionally been used to assign the Balcarce Formation to the Lower Ordovician, due to the presence of Cruziana furcifera. However, Plagiogmus is typical of Cambrian successions world-wide.

Geological Factors and Evolution of Southwestern Gondwana Triassic Plants

Abstract A synergistic model based on reciprocal influences between biotic and abiotic factors is developed for the Triassic of southwestern Gondwana. Changes in physical environment exerted a strong influence on the characteristics and evolution of plant assemblages. The Permian-Triassic extinction, and the change from palaeophytic to mesophytic floras, is one of the most striking examples of direct influence of physical environment upon plant communities. Pangea coalescence, the distribution of land masses and seas, the spreading of continental climates (megamonsoonal conditions) and the waning polar glaciation determined the expansion of xeromorphic morphotypes that became dominant during the whole Mesozoic. In southwestern Gondwana, the introduction or invasion of immigrant lineages suggests a strong asymmetrical interchange from the Euroamerican realm to the Gondwana realm. In addition, generalised extensional volcanism, development of intracratonic rifts and the palaeolatitudinal location of climatic zones during the early-Middle Triassic favoured extinction of the Glossopteris flora and explosive diversification of endemic groups. From the chronological viewpoint, the Barrealian, Cortaderitian and Florian stages are recognised in the Triassic of southwestern Gondwana. These stages are respectively characterised by: (a) appearance of mesophytic elements, and coexistence of Palaeozoic and Mesozoic groups, (b) maximum diversification of the Dicroidium flora, and (c) Dicroidium flora decline and replacement by morphotypes with strong Jurassic affinity. These palaeofloristic changes seem to be strongly influenced by tectonic evolution of sedimentary basins, temporal and regional distribution of sedimentary environments, and intra-Triassic palaeoclimatic change.