Basin Argentina Research Articles

Late Paleozoic coal beds and coaly mudstones in northwestern basins of Argentina: Paleoenvironmental context and paleoclimatic significance

Coals and coaly mudstones are found in the late Paleozoic Paganzo Basin, forming a key stratigraphic level deposited during the Bashkirian – Moscovian interval of relative tectonic quiescence. Coal beds vary from bituminous high volatility type C to sub-bituminous type A and, the coaly mudstones are kaolinitic-rich and presently used in the ceramic industry. Taking in account the stratigraphic position, facies associations, and the characteristics of the organic-rich levels, three domains were recognized within the basin: 1) The western domain (Precordillera) where paralic coals formed in transitional (mainly estuarine) environments, 2) The central domain (western Sierras Pampeanas) with limnic coals formed in floodplain areas, and 3) The eastern domain (eastern Sierras Pampeanas and Nevados del Famatina), in which coals are almost absent and are replaced by kaolinitic-rich coaly mudstones. Sea-level changes and climatic conditions related to the end of the Gondwanan glaciation played critical roles in the formation of coals and coaly mudstones. Regarding sea-level changes, three main transgressive events were identified in the Paganzo Basin. The oldest one corresponds to the “Namurian” postglacial transgression that produced no coal beds. The second event is the Pennsylvanian transgression 1, which favored the formation of coals and coaly mudstones in both coastal (i.e estuarine) and inland (i.e fluvial) environments. The third transgression, Pennsylvanian transgression 2, produced only very thin and scarce coal seams. During the transition from glacial (late Serpukhovian) to early postglacial climates (early Bashkirian), the vegetation was scarce and not suitable for the development of extensive mires. The post-glacial (late Bashkirian-early Moscovian) climatic amelioration, characterized by humid and temperate conditions, facilitated the proliferation of the NBG flora composed of hygrophilous elements as equisetalean, arborescent to sub-arborescent lycophytes and pteridosperms fronds, together with some gymnosperm that produced large amounts of organic detritus. On the contrary, the transition from subhumid to semiarid climate (late Moscovian-Kasimovian) led to the declination of the arborescent floras and the disappearance of coal beds. Although climate can be assumed to have simultaneously affected the whole basin, fluvial systems needed time to adjust to the sea-level changes and, therefore, at least two possibilities should be considered regarding the chronostratigraphic significance of the coal horizon. Firstly, that sea-level rise effectively controlled the equilibrium profile of channels well inland and coals and coaly mudstones across the Paganzo Basin form a time-transgressive horizon with a lag time not yet quantified based on the available paleontological and radiometric data. Secondly, that the growing of the riparian vegetation led by the ameliorating climatic conditions resulted in the almost synchronic formation of coals in the three domains.

Revised Timing of Cenozoic Atlantic Incursions and Changing Hinterland Sediment Sources during Southern Patagonian Orogenesis

Abstract New detrital zircon U-Pb geochronology data from the Cenozoic Magallanes-Austral Basin in Argentina and Chile ~51° S establish a revised chronostratigraphy of Paleocene-Miocene foreland synorogenic strata and document the rise and subsequent isolation of hinterland sources in the Patagonian Andes from the continental margin. The upsection loss of zircons derived from the hinterland Paleozoic and Late Jurassic sources between ca. 60 and 44 Ma documents a major shift in sediment routing due to Paleogene orogenesis in the greater Patagonian-Fuegian Andes. Changes in the proportion of grains from hinterland thrust sheets, comprised of Jurassic volcanics and Paleozoic metasedimentary rocks, provide a trackable signal of long-term shifts in orogenic drainage divide and topographic isolation due to widening of the retroarc fold-thrust belt. The youngest detrital zircon U-Pb ages confirm timing of Maastrichtian-Eocene strata but require substantial age revisions for part of the overlying Cenozoic basinfill during the late Eocene and Oligocene. The upper Río Turbio Formation, previously mapped as middle to late Eocene in the published literature, records a newly recognized latest Eocene-Oligocene (37-27 Ma) marine incursion along the basin margin. We suggest that these deposits could be genetically linked to the distally placed units along the Atlantic coast, including the El Huemul Formation and the younger San Julián Formation, via an eastward deepening within the foreland basin system that culminated in a basin-wide Oligocene marine incursion in the Southern Andes. The overlying Río Guillermo Formation records onset of tectonically generated coarse-grained detritus ca. 24.3 Ma and a transition to the first fully nonmarine conditions on the proximal Patagonian platform since Late Cretaceous time, perhaps signaling a Cordilleran-scale upper plate response to increased plate convergence and tectonic plate reorganization.

The first upper Silurian land-derived palynological assemblage from South America: Depositional environment and stratigraphic significance

A thick succession of quartz-rich sandstones cropping out in the middle part of the Villacorta Creek, on the western slope of the Sierra del Famatina (Valle Hermoso, La Rioja Province, northwestern Argentina) was recently recognized as the Villacorta Formation. The sandstone package rests disconformably on greenish sandstones of the Ordovician Suri Formation and is overlain along a marked angular unconformity by rocks of the upper Carboniferous Agua Colorada Formation (Paganzo Group). The Villacorta Formation is divided into four intervals, the lower one is formed by quartz-rich sandstones showing large-scale cross-stratification; the second part is similar but includes some levels of intraformational conglomerates and shows a decrease in both grain-size and the scale of the cross-bedding sets. The third section comprises medium- and coarse-grained sandstone with intercalation of fine-grained sandstones. The upper interval corresponds to two thickening and coarsening upwards successions. This last section was deposited in a deltaic environment where the top of mouth bars and lagoon deposits preserved palynological assemblages and phytodebris. A low-diversity palynological assemblage recovered from these mudstone levels includes abundant nematophyte cuticular-like fragments, cryptospores (monads, dyads, tetrads) and trilete spores (e.g., Scylaspora scripta, Emphanisporites protophanus). The stratigraphic relationships of the succession and the palynological assemblage suggest a late Silurian age for the Villacorta Formation. The exclusive presence of land-derived components in the organic residue allows to capture a snapshot of the terrestrial vegetation and constitutes the first record of this type of palynological association and nematophyte remains in this southern region. This assemblage improves previous paleogeographic reconstruction enlarging the extent of Silurian basins in Argentina. The shallowing successions of the Famatina region pass to marine deposits in the Precordillera area, indicating a westward transition to offshore environments.

Dynamic of a lacustrine sedimentary system during late rifting at the Cretaceous–Palaeocene transition: Example of the Yacoraite Formation, Salta Basin, Argentina

AbstractThe architecture of lacustrine systems is the result of the complex interaction between tectonics, climate and environmental parameters, and constitute the main forcing parameters on the lake dynamics. Field analogue studies have been performed to better assess such interactions, and their impact on the facies distribution and the stratigraphic architecture of lacustrine systems. The Yacoraite Formation (Late Cretaceous/Early Palaeocene), deposited during the sag phase of the Salta rift basin in Argentina, is exposed in world‐class outcrops that allowed the dynamics of this lacustrine system to be studied through facies analysis and stratigraphic evolution. On the scale of the Alemania‐Metán‐El Rey Basin, the Yacoraite Formation is organized with a siliciclastic‐dominated margin to the west, and a carbonate‐dominated margin to the east. The Yacoraite can be subdivided into four main ‘mid‐term’ sequences and further subdivided into ‘short‐term’ sequences recording high frequency climate fluctuations. Furthermore, the depositional profiles and identified system tracts have been grouped into two end‐members at basin scale: (a) a balanced ‘perennial’ depositional system for the lower part of the Yacoraite Formation and (b) a highly alternating ‘ephemeral’ depositional system for the upper part of the Yacoraite Formation. The transition from a perennial system to an ephemeral system indicates a change in the sedimentary dynamics of the basin, which was probably linked with the Cretaceous/Tertiary boundary that induced a temporary shutdown of carbonate production and an increase in siliciclastic supply.

Phylogenetic position of Diplostomum spp. from New World herons based on complete mitogenomes, rDNA operons, and DNA barcodes, including a new species with partially elucidated life cycle.

Diplostomum ardeae Dubois, 1969 has seldom been reported since its description from the great blue heron (Ardea herodias L., 1758) in the USA. Sequences obtained in this study from the barcode region of cytochrome c oxidase 1 (CO1) in diplostomids from black-crowned night heron (Nycticorax nycticorax (L., 1758)) in Puerto Rico matched data from D. ardeae from A. herodias in the type region. We also obtained DNA barcodes from morphologically similar diplostomids from a rufescent tiger heron (Tigrisoma lineatum (Boddaert, 1783)) and from metacercariae from eye lenses of Trachelyopterus galeatus (Linnaeus, 1766) from the Paraná River basin in Argentina and Brazil, respectively. Barcodes matched (97-100% identity) in these South American adult and larval specimens as well as in recently published sequences from metacercariae from 11 other siluriform fishes from the same region. Barcodes from the South American species, which we describe as Diplostomum lunaschiae n. sp., differed from those of D. ardeae by 7.2-9.8%, and the new species differs from D. ardeae in its size, pharynx:oral sucker length ratio, egg:body length ratio, and distribution of vitellaria. As in prior phylogenetic analysis of CO1 sequences, both D. ardeae and D. lunaschiae n. sp. were not associated with Diplostomum. In more character-rich analyses of nuclear rDNA and of mitochondrial genomes, D. ardeae was an early divergent member of clades of species of Diplostomum. Consequently, we continue to consider D. ardeae and D. lunaschiae n. sp. members of Diplostomum, in contrast to recent suggestions that these species may belong to a different genus.

Sismicidad somera de intraplaca en la provincia de Buenos Aires (Argentina) y áreas adyacentes: ¿están relacionadas con la Fosa de Quilmes?

In the light of the November 30th, 2018 (N30) earthquake activity, some neighborhoods of the city of Buenos Aires were shaken by a 3.8 mb earthquake (4.53 km estimated depth). We examined the historical and recent seismic records in order to analyze possible mechanisms related to the distribution of tectonic stresses as responsible for such unusual earthquakes in a region where only very little seismic activity is reported. According to this, at list one historical event occurred on June 5th, 1888 and other small magnitude earthquakes are mentioned since 1848 interpreted as being associated with the Rio de la Plata faulting. But there is, still no consensus about the role of this structure compared to other structures with orientation SW-NE. The lack of evidence to support one over the other structures makes it difficult to analyze these earthquakes. The presence of the Quilmes Trough connecting the Santa Lucía Basin in Uruguay and the Salado Basin in Argentina was recently proposed to play a tectonic role by a system of ENE-WSW trending controlled by extensional faulting related to the beginning of the Gondwana breakup. This depocenter with a thickness of almost 2,000 m of Mesozoic and Tertiary sequences could be acting as a zone of weakness in the crust and therefore responsible for the mentioned earthquake activity. The orientation of this structure correlates well with the present convergence vector between the Nazca and the South American plates and could therefore be propitious for strain release triggering shallow intraplate seismicity. We propose that most of the epicenters from historical and recent earthquakes might be aligned sub-parallel to the principal axis of the Quilmes Trough. Nevertheless, more data is needed to produce a reliable earthquake monitoring system in order to elucidate the tectonic stress regime and the existence of such structures at depth

Facies heterogeneity and source potential of carbonate-mudstone-dominated distal ramp deposits, Agrio Formation, Neuquén Basin, Argentina

ABSTRACTThe carbonate-mudstone-dominated Lower Cretaceous Agrio Formation is the youngest marine source rock of the hydrocarbon-prolific Neuquén Basin in Argentina, yet its facies variability and unconventional hydrocarbon potential remains relatively understudied. Detailed studies of mudstone facies variability in thick, carbonate mudstone successions deposited largely below storm wave base (i.e., chalk–marl, black shale, limestone), like the Agrio Formation, are rare and instead commonly focus on biostratigraphy or organic geochemistry alone. A continuous northern section of the Agrio Formation and a southern composite section of the lower Pilmatué and middle Avilé members, totaling ∼ 1,200 m of outcrop, were measured. From these measured sections, programmed pyrolysis (n = 339 samples), X-ray diffraction (XRD; n = 69), and thin sections (n = 69) were used to develop a high-resolution integrated macrofacies and microfacies scheme. The four most volumetrically abundant facies include detrital-quartz-silt-bearing fine mudstone (facies 1), radiolarian-bearing calcareous fine mudstone (facies 2), detrital-quartz-silt- and shell-bearing calcareous fine mudstone (facies 3), and calcareous wackestone (facies 4). All four facies are volumetrically dominated by carbonate mud matrix (i.e., micrite) that represents either 1) original pelagic coccolithophore deposition modified by diagenesis, 2) transported carbonate mud (i.e., bottom currents like contour currents or sediment gravity flows), or 3) a combination of both. Outcrop observations, XRD mineralogic trends, and petrographic variations in grain composition between detrital quartz silt, radiolarian and microfossil to macrofossil content (mainly benthic foraminifera and bivalves) distinguish the four mudstone facies. The facies scheme indicates distinctly more heterogeneous and current-influenced sedimentation in the downdip sub-storm wave base than previously described in the Agrio and in carbonate-dominated basinal settings in general. A depositional model is proposed for further testing that may prove valuable towards re-evaluating basinal carbonate mudstone successions worldwide. Utilizing TOC, S2, and HI value cutoffs, this study defines five discrete stratigraphic packages in the Agrio Formation that have the highest source potential, collectively totaling ∼ 140 m thick. The novel integration of macrofacies and microfacies analysis, stratigraphy, and a geochemical analysis allow both depositional insights and the assessment of a potential source rock. The study adds to a growing body of literature on 1) carbonate ramp (or slope) to basinal processes and 2) facies models for organic-rich, carbonate-dominated mudstone successions that are unconventional hydrocarbon systems.

The Huncal Member of the Vaca Muerta Formation, Neuquén Basin of Argentina: Insight into biostratigraphy, structure, U-Pb detrital zircon ages and provenance

In the western sector of the Neuquén Basin, the organic-rich shales of the Vaca Muerta Formation are intercalated with turbidite sandstone intervals and slump structures integrated in the Huncal Member. The age of the Huncal sandstones based on the integration of the ammonite faunas, calcareous nannofossils and U-Pb analysis is late Berriasian in their type locality. The origin of this deposit is probably related with two lobes of turbiditic sandstones linked with a progradational regressive phase. Sandstones are lithic arkoses and feldspathic litharenites derived from recycled orogenic and dissected arc sources. The U-Pb ages confirm a mixed sedimentary provenance from the south and southeastern margins of the basin, specifically from Paleozoic and Triassic–Jurassic rocks of the North Patagonian Massif and the Huincul High. The complex fold and fault system described in the sandstone levels was interpreted as the result of slump processes with a main transport direction to the SW and W. Sandstone deposits with slump structures assignable to the Huncal Member are present throughout the entire basin from the Tithonian to the Valanginian in the Vaca Muerta Formation. Therefore, the Huncal Member is a diachronic lithostratigraphic unit and its deposition probably depended on different factors such as relative sea level changes, the position in the ramp system or in the platform and slope configuration and internal morphostructural features of the Neuquén Basin.

Climatic signals in lacustrine deposits of the Upper Yacoraite Formation, Western Argentina: Evidence from clay minerals, analcime, dolomite and fibrous calcite

AbstractAuthigenic clays are an important control on reservoir quality in lacustrine carbonates but remain challenging to predict. Lacustrine depositional systems respond to climatic variations in rainfall, surface runoff and groundwater input, and evaporation, and result in rapid and frequent changes in lake volume; this is expressed through changing water depth and shoreline position. In the upper portion of the Early Palaeocene Yacoraite Formation of the Salta Basin in Argentina, extensive lacustrine deposits were deposited during the sag phase of rifting. Prior high‐resolution stratigraphic studies have suggested that climatic factors control microbial carbonate sequences within a ‘balanced fill’ lake, with variation in the lake level having a major influence on facies association changes. This study characterizes the evolution of facies and mineralogy within the Yacoraite Formation, focusing on the distribution of clay minerals, making a link between the high, medium and low‐frequency sequence stratigraphic cycles. The low‐frequency transgressive hemicycle of the upper portion of the Yacoraite Formation is comprised of abundant siliciclastic facies, suggesting a wetter period. Microbialites occurring in this interval are coarse‐grained and agglutinated. Detrital clay minerals such as illite and chlorite and associated siliciclastic sediments were input to the lake during high‐frequency transgressive periods. During high‐frequency regressive hemicycles, sedimentation was dominated by carbonate facies with Ca‐rich dolomite and the authigenic clays are comprised of chlorite/smectite mixed‐layers. By contrast, the low frequency regressive hemicycle records fine‐grained agglutinated microbialite with horizons of fibrous calcite, more stoichiometric dolomite, barite and authigenic magnesian smectite. This indicates elevated ion concentrations in the lake under intense evaporation during an arid period. Understanding the conditions that are favourable for formation and preservation of authigenic clays within the lacustrine environment can improve understanding of reservoir quality in comparable economically important deposits.

Forage biomass yield and arbuscular mycorrhizal symbiosis in a legume and C3 and C4 grasses under increasing soil phosphorus availability

Phosphorus (P) is the main limiting factor for forage production in grasslands. It is important to determine levels of available P in soil that optimise production with minimum impact on arbuscular mycorrhizal (AM) symbiosis. We investigated the effects of increasing P availability on biomass production, root morphology, AM symbiosis and P acquisition of a forage legume (Lotus tenuis), a C3 grass (Schedonorus arundinaceus) and a C4 grass (Panicum coloratum) growing on a P-deficient soil in pots with P applied at rates of 0–160 mg kg–1 dry soil. The three forage species responded strongly to addition of P, with 90% of maximum shoot growth reached at available P levels of 24.3 mg kg–1 for L. tenuis, 14.4 mg kg–1 for P. coloratum and 11.2 mg kg–1 for S. arundinaceus. Lotus tenuis and P. coloratum produced higher yields of shoot biomass than S. arundinaceus. Root dry weight was higher in the legume than in the grasses, with the root-mass fraction being lowest in P. coloratum. AM colonisation was higher in L. tenuis roots than in grass roots, and decreased with increased soil P availability, especially in grasses. Low to moderate additions of P did not affect, and could even improve, AM colonisation in L. tenuis roots. For L. tenuis, it is possible to increase forage yield while maintaining high values of AM colonisation at 10–20 mg kg–1 of available P, but for grasses, especially S. arundinaceus, it is difficult to achieve both objectives. The presence of L. tenuis in grasslands or pastures may contribute to maintaining the native AM inoculum under a wide range of soil P availability in regions such as the Salado River basin of Argentina.

A new species of Imparfinis Eigenmann & Norris, 1900 (Siluriformes: Heptapteridae) from the Tapajós basin, Brazil.

Imparfinis comprises 20 valid species in the Heptapteridae, being the most diverse taxonomic group of catfishes of the Nemuroglanis subclade. The genus has one of the widest geographical distributions in the neotropical region, found on both sides of the Andes, from Costa Rica to the Paraná and Uruguay river basins in Argentina. A new species of Imparfinis is described from streams of the upper Rio Tapajós and its tributary Teles Pires in northern Brazil. The new species is diagnosed from all congeners by the presence of a vertical dark brown band W-shaped at the base of the caudal-fin rays, a thick dark brown lateral stripe from the snout to the end of the caudal peduncle, dark brown head, long maxillary barbel surpassing the distal margin of the pectoral fin, and presence of 39 or 40 total vertebrae.

Comparative structural analysis of inverted structures in the San Bernardo fold belt (Golfo San Jorge basin, Argentina): Inversion controls and tecto-sedimentary context of the Chubut Group

The Golfo San Jorge Basin is one of the most important hydrocarbon-bearing basins of Argentina. Its position in the distal Patagonian Broken Foreland confers a particular structural architecture. Specifically, the western margin was affected by positive tectonic inversion, with higher degrees of inversion at the San Bernardo Fold Belt (SBFB). This paper synthesizes the comparative structural analysis of outcropped and buried inverted structures in the SBFB. The study areas are regionally distributed, and the geometric characterizations integrate field structural data, topographic trends, cutting records, borehole logs, and 2D-3D seismic data. The geometries of the studied inversion folds are poorly vergent and can be classified as asymmetric anticlines or pop-up growth folds. The architecture of the inverted fault system shows: along-strike variability in reverse displacement, inherited extensional growth-strata, selective inversion, oblique inversion, and heterogeneous deformation. Pre-inversion major controls include: 1) an extensional network linked to the fabric of the pre-rift basement, 2) a non-coaxial extensional phase during the early Upper Cretaceous times, and 3) a basin-scale paleohigh of crystalline rocks located to the east of the SBFB. The kinematic analysis evaluates the timing of the tectonic inversion phase, which is actually under discussion. Our results are consistent with a tecto-sedimentary context dominated by normal fault reactivations during the deposition of the basal units of the Chubut Group (Barremian-Albian), and a reverse-reactivation during the record of the Laguna Palacios Formation (late Upper Cretaceous). The geometric-kinematic characterization of the inverted structures at the SBFB contradicts many of the evidence that supports recently proposed geodynamic models for the Cretaceous evolution of Southern Patagonia.

E&P Notes (October 2019)

E&P Notes (October 2019)

Stratigraphic revision of the El Imperial Formation (Pennsylvanian-Cisuralian), in its type locality, San Rafael Basin(Mendoza), Argentina

The El Imperial Formation (San Rafael Basin) shows a complete record of the late Carboniferous - early Permian stratigraphy in western basins of Argentina. The sedimentological and stratigraphic analysis in the type locality allowed recognizing eight facies associations. Facies association A includes diamictites and conglomerates related to the Late Paleozoic glacial event, facies B composed of shales and fine-grained sandstones, was formed during the postglacial transgression while facies C represents sandy littoral bars, that pass upward to shales and mudstones of the facies association D. The conglomerates and sandstones of the facies association E entirely correspond to fluvial sedimentation, followed by a new transgressive event represented in fine-grained sandstones and shales of the facies F. The last transgression is recorded in shales, mudstones and very fine-grained sandstones belonging to the facies G, which are covered by sandstones and conglomerates corresponding to multi-channel fluvial systems.From these facies associations, 33 palynofloras divided into three palynological associations were obtained. The palynological assemblages that characterized the basal section contain abundant trilete spores and monosaccate pollen grains, with a progressive increase of taeniate pollen towards the top of the unit. The presence of scolecodonts characterizes palynofloras from transitional to marine environments. The composition of the palynofloras allows correlations with the current biostratigraphic scheme of the center-west of Argentina. The presence of the Biozone Raistrickia densa- Convolutispora muriornata in the lower and middle part of the El Imperial Formation, together with Pakhapites fusus-Vittatina subsaccata Biozone in the upper part, allows us to refer the palynofloras to the late Serpukhovian/early Cisuralian.

Characterisation of the natural attenuation of chromium contamination in the presence of nitrate using isotopic methods. A case study from the Matanza-Riachuelo River basin, Argentina

The groundwater contamination by hexavalent chromium (Cr(VI)) in a site of the Matanza-Riachuelo River basin (MRB), Argentina, has been evaluated by determining the processes that control the natural mobility and attenuation of Cr(VI) in the presence of high nitrate (NO3−) contents. The groundwater Cr(VI) concentrations ranged between 1.9E-5 mM and 0.04 mM, while the NO3− concentrations ranged between 0.5 mM and 3.9 mM.In order to evaluate the natural attenuation of Cr(VI) and NO3− in the MRB groundwater, Cr and N isotopes were measured in these contaminants. In addition, laboratory batch experiments were performed to determine the isotope fractionation (ε) during the reduction of Cr(VI) under denitrifying conditions. While the Cr(VI) reduction rate is not affected by the presence of NO3−, the NO3− attenuation is slower in the presence of Cr(VI). Nevertheless, no significant differences on ε values were observed when testing the absence or presence of each contaminant. The ε53Cr determined in the batch experiments describe a two- stage trend, in which Stage I is characterized by ε53Cr ~−1.8‰ and Stage II by ε53Cr ~−0.9‰. The respective ε15NNO3 obtained is −23.9‰ whereas ε18ONO3 amount to −25.7‰. Using these ε values and a Rayleigh fractionation model we estimate that an average of 60% of the original Cr(VI) is removed from the groundwater of the contaminated site. Moreover, the average degree of NO3− attenuation by denitrification is found to be about 20%. This study provides valuable information about the dynamics of a complex system that can serve as a basis for efficient management of contaminated groundwater in the most populated and industrialized basin of Argentina.

Two new species of Gyrodactylus von Nordmann, 1832 (Monogenoidea: Gyrodactylidae) parasitizing introduced poeciliids in Patagonia.

Gyrodactylus superbus (Szidat, 1973) Popazoglo Boeger, 2000 was described from Corydoras paleatus (Jenyns) (Callichthyidae) and represents the only known viviparous gyrodactylid reported from the Parano-Platense basin of Argentina. We describe two new species of viviparous neotropical gyrodactylids parasitizing the introduced poeciliid, Cnesterodon decemmaculatus (Jenyns, 1842) (Poeciliidae), from southern Argentina: Gyrodactylus decemmaculati n. sp. and Gyrodactylus breviradix n. sp. The new species differ from other gyrodactylids parasitizing poeciliids in the morphology of superficial bars and hooklets. Gyrodactylus decemmaculati n. sp. has a superficial bar with two robust and rounded anterolateral projections (each with a ventral lobe), and a subtriangular shield, and has a slender hooklet with a delicate recurved point, a straight, elongate shaft, and a depressed, acute toe. Gyrodactylus breviradix n. sp. has a superficial bar with two robust, elongate anterolateral projections, folded inward, and a trapezoidal shield, and has a hooklet with a short point, angled at 90º, ending before level of toe tip, a straight, short shaft, a round, prominent heel, and a pointed, depressed toe. These identifications were supported by DNA analyses based on sequences of the ITS2 region and a barcoding gap analysis. Sequences of the Cytochrome oxidase II gene and fragments of the Internal Transcribed Spacers 1 for both species are also provided.

Criteria for recognizing shelf‐slope clinoforms in outcrop; Jurassic Lajas and Los Molles formations, S. Neuquén Basin, Argentina

AbstractSeismic‐reflection data show that most deepwater (>200 m water depth) basins are filled by sand and mud dispersed across clinoformal geometries characterized by gently dipping topsets, steeper foresets and gently dipping bottomsets. However, the entire geometry of these ubiquitous clinoforms is not always recognized in outcrops. Sometimes the infill is erroneously interpreted as “layer cake” or “ramp” stratigraphy because the topset‐foreset‐bottomset clinoforms are not well exposed. Regional 2‐D seismic lines show clinoforms in the Lower to Middle Jurassic Challaco, Lajas, and Los Molles formations in S. Neuquén Basin in Argentina. Time equivalent shelf, slope and basin‐floor segments of clinoforms are exposed, and can be walked out in hundreds of metres thick and kilometres‐wide outcrops. The studied margin‐scale clinoforms are not representing a continental‐margin but a deepwater shelf margin that built out in a back‐arc basin. Lajas‐Los Molles clinoforms have been outcrop‐mapped by tracing mudstones interpreted as flooding surfaces on the shelf and abandonment surfaces (low sedimentation rate) in the deepwater basin. The downslope and lateral facies variability in the outcrops is also consistent with a clinoform interpretation. The Lajas topset (shelf) is dominated by fluvial and tidal deposits. The shelf‐edge rollover zone is occasionally occupied by a 40–50‐m‐thick coarse‐grained shelf‐edge delta, sometimes incising into the underlying slope mudstones, producing oblique clinoforms expressing toplap erosion on seismic. A muddy transgressive phase capping the shelf‐edge deltas contains tidal sandbodies. Shelf‐edge deltas transition downslope into turbidite‐ and debris flow‐filled channels that penetrate down the mud‐prone Los Molles slope. At the base‐of‐slope, some 300m below the shelf edge, there are basin‐floor fan deposits (>200 m thick) composed of sandy submarine‐fan lobes separated by muddy abandonment intervals. The large‐scale outcrop correlation between topset–foreset–bottomset allows facies and depositional interpretation and sets outcrop criteria recognition for each clinoform segment.

A new palynological assemblage (Carboniferous) from the Chacoparaná Basin, Western Gondwana, and revision of the biostratigraphic scheme

A new palynozone for the upper Serpukhovian–Bashkirian (uppermost Mississippian–lower Pennsylvanian) strata of the Chacoparaná Basin (Argentina) is formally proposed based on palynological assemblages from the Santiago Temple YCF.CO1, YCF.CO2 and YCF.CO3 boreholes, drilled in Córdoba province. Thirty samples were analyzed and 222 palynomorph species identified. The Anapiculatisporites concinnus (Ac) Assemblage Zone is present only in the Santiago Temple boreholes and is divided into two subzones (lower and upper). The Ac Assemblage Zone has been identified in the lower part of the Ordóñez Formation, which is characterized by the presence of diamictites that would have been deposited in glaciolacustrine and glaciomarine environments. The Ac Assemblage Zone is correlated with subzones A and B of the Raistrickia densa–Convolutispora muriornata Zone defined in the western basins of Argentina. The known zones in the Chacoparaná Basin succession are redefined based on a review of borehole information, and the new zone is incorporated. Palynostratigraphically, the basin is divided into the Anapiculatisporites concinnus Assemblage Zone (upper Serpukhovian–lower Bashkirian), Potonieisporites–Lundbladispora Interval Zone (late Bashkirian–Gzhelian), Cristatisporites Interval Zone (late Gzhelian–early Kungurian) and Striatites (Kungurian–early Roadian) Assemblage Zone.

Lower Ordovician (late Tremadocian?- Floian) palynomorphs from the Llanos Basin, Colombia: Biostratigraphic and paleogeographic significance

Four core and fifteen cutting samples of well RH-2 from the Ordovician of the Llanos Basin, Colombia, were palynologically analyzed. They yielded mostly diverse and well-preserved marine palynomorph assemblages, dominated by acritarchs. Forty-six acritarch taxa and two chitinozoan species were identified. Palynological assemblages indicate an Early Ordovician, maximum Floian age, for the upper core samples and a late Tremadocian?- Floian? age for the lower cutting samples. Based on a thorough revision of the updated stratigraphic ranges of the taxa, and especially on the First Appearance Datums (FADs) of selected acritarch taxa such as Arbusculidium filamentosum, Arkonia, Barakella, Aureotesta clathrata simplex, Coryphidium bohemicum and Frankea, the age of the four upper core samples could be constrained to the middle to late Floian and the 15 cutting samples most probably to the late Tremadocian or early to middle Floian. The recognized chitinozoan taxa, Eremochitina brevis and Velatachitina veligera, support the interpreted ages. The presence of taxa such as Arbusculidium, Arkonia, Aureotesta clathrata simplex, Coryphidium, Dactylofusa velifera var. brevis, Frankea and Striatotheca confirm affinities with the Perigondwana Province. Eisenackidium orientalis which first appears outside the Early to Middle Ordovician basins of Argentina, could become a useful species for correlations among Eastern Gondwana Ordovician basins. The genus Frankea, considered as a biostratigraphic and paleogeographic component of the Perigondwana Province, is recorded for the first time in Eastern Gondwana, and its occurrence could represent one of the oldest worldwide records of the genus. The palynological results constitute a step towards the establishment of a biostratigraphic framework for the Ordovician of Colombia.

Gravitational field calculation in spherical coordinates using variable densities in depth

SUMMARY We present a new methodology to compute the gravitational fields generated by tesseroids (spherical prisms) whose density varies with depth according to an arbitrary continuous function. It approximates the gravitational fields through the Gauss–Legendre Quadrature along with two discretization algorithms that automatically control its accuracy by adaptively dividing the tesseroid into smaller ones. The first one is a preexisting 2-D adaptive discretization algorithm that reduces the errors due to the distance between the tesseroid and the computation point. The second is a new density-based discretization algorithm that decreases the errors introduced by the variation of the density function with depth. The amount of divisions made by each algorithm is indirectly controlled by two parameters: the distance-size ratio and the delta ratio. We have obtained analytical solutions for a spherical shell with radially variable density and compared them to the results of the numerical model for linear, exponential, and sinusoidal density functions. The heavily oscillating density functions are intended only to test the algorithm to its limits and not to emulate a real world case. These comparisons allowed us to obtain optimal values for the distance-size and delta ratios that yield an accuracy of 0.1 per cent of the analytical solutions. The resulting optimal values of distance-size ratio for the gravitational potential and its gradient are 1 and 2.5, respectively. The density-based discretization algorithm produces no discretizations in the linear density case, but a delta ratio of 0.1 is needed for the exponential and most sinusoidal density functions. These values can be extrapolated to cover most common use cases, which are simpler than oscillating density profiles. However, the distance-size and delta ratios can be configured by the user to increase the accuracy of the results at the expense of computational speed. Finally, we apply this new methodology to model the Neuquén Basin, a foreland basin in Argentina with a maximum depth of over 5000 m, using an exponential density function.