Gonzalo D. Veiga

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.

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.

Tectonostratigraphic evolution of a Mesozoic graben border system: the Chachil depocentre, southern Neuquén Basin, Argentina

The Chachil depocentre is one of a number of early Mesozoic extensional basins that form the early depocentres of the southern Neuquén Basin in Argentina. The synrift volcanic fill is composed of andesites, rhyolites and volcaniclastic deposits. Coarse-grained, non-marine facies dominate the sedimentary fill, mainly in the form of sediment gravity flow deposits. Stream flow deposits and minor non-marine carbonates are also locally present. The evolution of the graben border system was mainly controlled by subsidence along the main boundary fault (the Chihuido Bayo fault system) and recurrent volcanic activity. Marked changes in the thickness of the synrift megasequence indicate that episodic normal faulting in the hanging wall was also important. The integration of structural, magmatic and sedimentary data from the study area has led to the definition of three stages in the evolution of the synrift succession. The early rift stage is defined by the interplay between bimodal volcanism and gravity-driven sedimentation. The mid-rift stage is marked by the transition to acidic magmatism (rhyolitic and pyroclastic flows), also associated with coarse-grained non-marine deposition. The late-rift stage is dominated by fine-grained turbidites and pyroclastic falls related to the first marine sedimentation in the Neuquén Basin.

Anatomy of a mixed marine-non-marine lowstand wedge in a ramp setting. The Record of a Barremian-Aptian complex relative sea-level fall in the central Neuquen Basin, Argentina

Abstract During the Cretaceous, western Argentina was occupied by the Neuquén Basin, a back-arc-foreland basin that was open through the proto-Andes to the Pacific Ocean in the west. The Neuquén Basin contains a thick succession of sediments that include the offshore marine deposits of the Agrio Formation. These deposits represent a time when the arc was an island chain and the Neuquén Basin was freely connected to the Pacific. This offshore marine succession is punctuated by two intervals of arid continental deposits that represent major, second-order, relative falls in sea level. In both of these cases there is no evidence of tectonic uplift or angular truncation along a basal bounding unconformity. The upper of the two lowstand wedges is characterized by a complex arragement of shallow-marine and continental deposits. Shallow-marine deposits sharply overlying offshore shales and capped by a master sequence boundary are interpreted as falling-stage deposits recording a complex relative sea-level fall. On top of a regional erosion surface, a drying-upwards succession of fluvial-aeolian deposits is developed, recording a fully non-marine stage in the evolution of the basin. These deposits are overlain by a marginal marine evaporite succession. The absence of a return to fully open-marine conditions is attributed to uplift in the Andes and marks the transition of the Neuquén Basin from a back-arc to a foreland system. This succession has important implications for the basin’s evolution and in the timing of the uplift of the Andes, is a very spectacular example of a lowstand wedge and is also a major hydrocarbon reservoir.

Estratigrafía de 'sin-rift' (Triásico Superior-Jurásico Inferior) de la Cuenca Neuquina en la sierra de Chacaico, Neuquén, Argentina

La sierra de Chacaico (Neuquen, Argentina) constituye una de las localidades mas importantes para el analisis de la estratigrafia y relaciones espaciales del relleno inicial de la Cuenca Neuquina. Por medio de estudios estructurales y sedimentologicos de campo ha sido posible distinguir la naturaleza volcano-sedimentaria de la secuencia de sin-rift asi como su evolucion estratigrafica durante el periodo extensional Triasico Superior-Jurasico Inferior. La sucesion de sin-rift puede subdividirse en tres etapas evolutivas. La parte inicial esta integrada por volcanitas basicas a intermedias y depositos sedimentarios volcanoclasticos, mayormente gruesos. La seccion media se halla dominada por depositos piroclasticos primarios y reelaborados y finaliza con delgadas secuencias carbonaticas lacustres. En el techo de esa seccion aparecen localmente lavas basalticas. La seccion superior consiste de una asociacion de depositos clasticos gruesos (conglomerados y brechas) y piroclasticos primarios de caida y de flujo. Toda el area estudiada conformo un unico depocentro de sin-rift cuyos margenes estaban orientados en direccion E-W a ENE-WSW. El borde sur funciono como la estructura de fallamiento normal mas importante, hacia la cual los espesores de la secuencia fueron maximos. El depocentro fue invertido tectonicamente de manera compleja, con generacion de anticlinales asociados a fallas normales invertidas. Algunas de esas estructuras anticlinales reflejan la existencia de escalones estructurales internos dentro del depocentro, asociados a antiguas fallas normales oblicuas de orientacion noreste. El patron estratigrafico y estructural de la secuencia de sin-rift es comparable al de la dorsal de Huincul, uno de los rasgos morfoestructurales mas prominentes en la evolucion del sector sur de la Cuenca Neuquina.

The transgression of an erg — sedimentation and reworking/soft-sediment deformation of aeolian facies: the Cretaceous Troncoso Member, Neuquén Basin, Argentina

Abstract The Cretaceous Troncoso Inferior Member of the Huitrín Formation comprises fluvial and aeolian facies that form a drying-upwards succession within the Neuquén Basin. The basal fluvial sandstones were deposited as braided river deposits and lie unconformably on top of either the deep-marine Agrio Formation or, locally, the shallow-marine Chorreado Member (Huitrín Formation.). In places, the fluvial sandstones are interbedded with remnants of aeolian deposits recording an arid environment and ephemeral flows. In the study area the upper section is predominantly aeolian and was controlled by northerly winds with both linear and transverse dune types being deposited. The depositional system was rapidly flooded and dune topography (relief ranging between 2 and 35 m) was preserved on its top surface. In addition to dune topography, the Troncoso dunes also show evidence of reworking and in situ soft-sediment deformation related to the flooding. The principal aim of this paper is to document the soft-sediment deformation and preservation of topography associated with the flooding of the dune field. Within the soft sediment deformed and reworked sediments at the top of the Troncoso Inferior Member spatial and temporal relationships indicate that they formed in a specific sequence. Initially, water-escape processes created convolutedly folded and dish structures that were concentrated in areas of slightly higher preserved dune topography. Secondly, the convolutedly folded and dish facies were eroded and reworked by wave undercutting and migrating three-dimensional dunes in a shallow-marine environment. This subaqueous reworking resulted in an interbedded massive and cross-stratified unit. With further deepening of the water, the topography became stabilized and the uppermost part of the interval (0.1–0.3 m) was reworked by waves across most of the basin. In the topographic lows between dunes, liquefaction-induced sediment gravity flows deposited massive-flatlaminated facies. The reworked and soft-sediment deformed aeolian dune topography is overlain by the evaporites of the Troncoso Superior Member. The distribution of flood-related facies and the amount of preserved dune topography (2–35 m) indicates that the transgression must have been rapid but of low energy.

Facies characterization and sequential evolution of an ancient offshore dunefield in a semi-enclosed sea: Neuquén Basin, Argentina

This study analyses a 30-m-thick, sand-dominated succession intercalated between offshore mudstones in the Lower Cretaceous record of the Neuquén Basin, Argentina, defining facies associated with unidirectional currents as sand dunes (simple and compound), rippled sand sheets and heterolithic sheets. These facies associations are related to the development of an offshore, forward-accreting dunefield developed as a response to the onset of a tidal-transport system. The reported stratigraphic record results from the combination of the gradual downcurrent decrease of the current speed together with the long-term climbing of the entire system. Maximum amplification of the tidal effect associated with incoming oceanic tides to this epicontinental sea would develop at the time of more efficient connection between the basin and the open ocean. Thus, the onset of the offshore tidal system approximately corresponds to the time of maximum flooding conditions (or immediately after). The short-term evolution of the tidal-transport system is more complex and characterized by the vertical stacking of small-scale cycles defined by the alternation of episodes of construction and destruction of the dunefield. The development of these cycles could be the response to changes in tidal current speed and transport capacity.

Linking mid-scale distributive fluvial systems to drainage basin area: geomorphological and sedimentological evidence from the endorheic Gastre Basin, Argentina

Abstract The analysis of mid-scale distributive fluvial systems (DFS) provides an opportunity to study fluvial depositional patterns in sedimentary basins in a manner similar to megafans, but under the more homogeneous external conditions that control small alluvial fans from the source area to the depositional area. Four mid-scale Quaternary DFS and the areas of their drainage basins were analysed with the aim of understanding and predicting the mechanism and magnitude of fluvial fan deposition. These DFS were located in the Gastre Basin, Argentina, an endorheic basin that has evolved during the Quaternary under relatively uniform climatic and tectonic conditions. Using a geomorphological and sedimentological approach, the measured areas of the DFS were compared with the calculated areas using different regression equations for variables such as the drainage basin area and the drainage basin slope. The results suggest a moderate to high percentage of deviation in all cases, indicating that at an intermediate scale the size of the DFS is not solely a function of the size of the drainage basin. Adjacent DFS with similar depositional areas and catchment bedrock lithologies were linked with very different drainage basin areas and vice versa. This study suggests that the horizontal accommodation space, determined by the incision and distribution of previous deposits, and the drainage basin slope may also influence the size of mid-scale DFS.