Eastern Cordillera Research Articles

Relationship between coal composition, coal facies, and desorbed methane gas content: Thermal histories and exhumation processes in the middle Magdalena Valley basin, Colombia

We analyzed a total of 25 coal samples from high volatile bituminous to medium volatile bituminous range from 2 wells drilled in the Landázuri area of the Middle Magdalena Valley Basin at the Eastern Colombia Cordillera. This study reveals the relationship between the measured gas concentration, petrographic characteristics, and coal facies. We identify that gas desorption in coal seams is related to maceral composition.Three coal seam groups were defined in Landázuri 1 based on petrographic and statistical evaluation. Two groups were identified in Landázuri 2 based on gas content.The evaluation of maceral content and preservation of reflectance vitrinite found that the highest desorbed gas values correlate with the abundance of inertinite and structured inertinite macerals, followed by the percentage of vitrinite. Structured inertinite lumens may serve as macro/mesopores which allow for higher gas desorption because they are free of mineral matter or other macerals.Using existing coal facies diagrams, two types of coal facies were differentiated: lower delta plain (fen, marsh) and back-barrier low moor developed in a humid environment, where reotrophic conditions prevail and herbaceous and aquatic/marginal vegetation predominate.The relationship of these transitional facies with gas content shows are not directly related to the higher gas values measured at Landázuri. Instead, the relationship between the high degree of destruction of organic matter in these facies (marine incursions and other flows) and the measured gas content shows the importance of quantifying the structured macerals as potential gas retainers.Using low-temperature thermochronometers apatite fission-track, and (U–Th)/He in zircon, combined with vitrinite reflectance data, we reconstructed time-Temperature histories for areas near the wells. Differences in gas content can be explained by an asynchronous behaviour of the burial-cooling processes in this part of the Colombian Eastern Cordillera. Our work shows it is possible to distinguish gas desorption in coal seams based on the maceral composition.

Characterization of the Soapaga Fault deformation in the central part of the Eastern Cordillera, Colombia

The eastern border of the Floresta Massif is characterized by the confluence of the Soapaga Fault and the Corrales Fault, two reverse faults that are linked to each other in a simple transfer zone and whose deformation influenced the formation of the Betéitiva Syncline. The Betéitiva Syncline, located on the Soapaga Fault footwall, shows geometric features, such as upright to inverted backlimbs and thickness changes related to the fault's deformation. However, none of the previously proposed kinematic models (fault-propagation, fault-bend, and buckling–folding models) clearly explain the evolution of the Betéitiva Syncline related to the Soapaga Fault or the differentiation between this fault and the Corrales Fault. In this work, we developed a structural analysis to differentiate the Soapaga Fault from the Corrales Fault through a detailed description of the fault zones, geomorphic evidence, geometric relationships, and deformation features related to each fault. Then, we developed a structural analysis of the evolution of the aforementioned structures using five balanced cross-sections that illustrate how they are related to one another. Three kinematic models or structural processes were used to explain the evolution of the study area: the trishear fault-propagation fold model, the fault-propagation fold model, and the gravitational fold and fault model. This work presents the structural, geological, and geomorphic evidence that allowed us to differentiate the Corrales Fault from the Soapaga Fault. Additionally, our results show an increase in the Soapaga Fault displacement (4.7 km–9.5 km), the Corrales Fault displacement (0.6 km–1.3 km), and the total shortening (1.9 km–3.4 km) from north to south. In addition, we propose a structural evolution model that better explains the geometric features of the Betéitiva Syncline regarding the deformation of the Soapaga Fault, which requires 10x less shortening than previous structural models. Finally, we propose that the deformation of the Soapaga Fault is a scale-dependent variable because of the relationship that this fault has with regional structures, such as the Floresta Anticlinorium, but also with local structures, such as the Betéitiva Syncline.

Subduction and Hydrogen Release: The Case of Bolivian Altiplano

Natural hydrogen is known to be generated in the crust by water/rock interactions, especially the oxidation of iron-rich rock or radiolysis. However, other sources, especially deeper ones, exist. In the context of subduction, the dehydration of the slab, the destabilization of the NH4, and the hydration of the mantle wedge above the subducting lithosphere may generate H2. We present here a compilation of the known gases in the central part of the Pacific subduction and the results of a first field acquisition dedicated to H2 measurements in Bolivia between La Paz and South Lipez. Various zones have been studied: the emerging thrust faults of the western borders of the Eastern Cordillera, the Sajama area that corresponds to the western volcanic zone near the Chile border northward from the Uyuni Salar, and finally, the Altiplano-Puna Volcanic Complex in South Lipez. Soil gas measurement within and around the Salar itself was not fully conclusive. North of the Uyuni Salar, the gases are very rich in CO2, enriched in N2 and poor in H2. On the opposite, southward, all the samples contain some H2; the major gas is nitrogen, which may overpass 90% after air correction, and the CO2 content is very limited. On the western border of the Cordillera, the δC13 isotope varies between −5 and −13‰, and it is not surprisingly compatible with volcanic gas, as well as with asthenospheric CO2. The methane content is close to 0, and only a few points reach 1%. The isotopes (−1‰) indicate an abiotic origin, and it is thus related to deep H2 presence. The high steam flow in the geothermal area of South Lipez combined with the H2 content in the water results in at least 1 ton of H2 currently released per day from each well and may deserve an evaluation of its economic value. The nitrogen content, as in other subduction or paleo-subduction areas, questions the slab alteration.

Sediment dispersal and basin evolution during contrasting tectonic regimes along the western Gondwanan margin in the central Andes

A >15-20 km-thick succession in southern Bolivia forms the most-complete stratigraphic record in western Gondwana. Upper Neoproterozoic–Carboniferous clastic rocks record ∼300 Myr of marine, nonmarine, and glacially influenced sedimentation in diverse basin systems generated by variable tectonic regimes along the western edge of Gondwana during active and passive-margin conditions. New provenance results help resolve key uncertainties regarding source regions and sediment dispersal patterns. The findings are integrated with spatial variations in stratigraphic thicknesses to evaluate regional patterns of basin subsidence, magmatism, and deformation during long-term evolution of the western Gondwanan margin in the central Andes.Detrital zircon U–Pb geochronological data for 17 sandstone samples reveal sedimentary input from Precambrian cratonic basement provinces and pre-Andean basement, magmatic arc, and fold-thrust belt source regions. The basement age signatures indicate derivation from the flanking Brasiliano (900-560 Ma) and Pampean (650-500 Ma) provinces to the south and east, and the distal Rio de la Plata craton (2400-2000 Ma) along the eastern South American margin ∼1000–1500 km to the southeast. Although the greater Amazonian craton was not a major contributor, subordinate Amazonian signatures from the Sunsás (1300-950 Ma) province to the east and northeast selectively fed the northern basin regions of the central Andes. Despite the lack of Paleozoic igneous rocks in Bolivia, detrital zircons of Ordovician age attest to the pre-Andean influence of the subduction-related Famatinian magmatic arc. Limited Devonian-Carboniferous igneous material was contributed locally from western pre-Andean highlands or regionally by axial northward transport from selected igneous sources in Argentina and Chile. Episodic recycling of Neoproterozoic–Paleozoic basin fill, including a sharp reappearance of Famatinian-age detritus, can be linked to periods of Paleozoic crustal shortening and foreland sedimentation ascribed to Famatinian, Ocloyic, Chañic, or Gondwanide phases of deformation.The spatial distribution of sediment sources along with temporal shifts in sediment routing highlight several stages in the paleogeographic evolution of the western Gondwanan margin preserved in the central Andes. Initial regional subsidence spanned a multiphase Neoproterozoic to early Paleozoic history of Rodinia breakup, Brasiliano-Pampean orogenesis, and post-orogenic back-arc extension prior to final late Paleozoic amalgamation of Gondwana. The early Paleozoic onset of subduction and Famatinian arc magmatism led to high-magnitude subsidence (>10–15 km) likely driven by Ordovician slab rollback in an extensional back-arc basin. Thereafter, intermittent Paleozoic contraction in a poorly understood pre-Andean system (best expressed in the Eastern Cordillera of Bolivia and neighboring segments of northern Argentina and southern Peru) generated transient topographic loads that produced superimposed flexural foreland and successor basin systems.

Basin formation, magmatism, and exhumation document southward migrating flat-slab subduction in the central Andes

The Central Andean Plateau is the largest plateau formed in a non-collisional setting and is taller, wider, has thicker crust, and has greater retroarc shortening than any other region of the modern Andes. It also hosts some of the thickest Cenozoic strata in the Andes, which were deposited synchronously with punctuated widening and narrowing of the magmatic arc. Multiple hypotheses have been advanced to explain these unique characteristics, yet the detailed, plateau-scale Cenozoic basin history needed to test these hypotheses remains unavailable and therefore has not been integrated into the established records of deformation and magmatism. Here we synthesize new detrital zircon maximum depositional ages (MDAs) and sediment provenance analysis of non-marine strata from ∼15°–16.5°S with existing records of sediment accumulation, deformation, and magmatism from ∼14°–24.5°S to develop a model of the Eocene–early Miocene evolution of the Altiplano-Puna plateau region. Stratigraphic correlations based on the new MDAs show a Paleocene–Eocene unconformity/condensed section followed by resumption of rapid sediment accumulation. Rapid sediment accumulation resumed at 46–43 Ma at 15–16°S, at 36 Ma at 18°S, and as young 19 Ma at ∼23°S. Provenance data presented herein indicate that Eastern Cordilleran detritus appeared in Altiplano strata at progressively younger ages to the south. The southward progression of this basin reorganization proceeded in lockstep with initiation of exhumation in the Eastern Cordillera, and also a lull in the magmatic arc followed by widespread volcanic flare-up. We present a model in which the observed stratigraphic hiatus, along with the magmatic lull and flare-up, orogenic widening, and high-magnitude shortening, are upper plate responses to shallowing and resteepening of an late Paleocene–early Miocene flat slab beneath the Altiplano-Puna plateau. In the proposed model initial slab flattening was driven by late Paleocene–early Eocene subduction of buoyant oceanic crust: potentially a Manihiki Plateau Conjugate. Subsequently, southward shallowing and resteepening of a late Eocene–early Miocene flat slab beneath the Altiplano-Puna Plateau was driven by subduction of an asperity on the Nazca Plate which we tentatively identify with the Juan Fernandez Ridge. The geographic coincidence of all of these features suggests that the flat slab hydrated and weakened the upper plate lithosphere, thereby facilitating later development of the suite of unique features which characterize the Central Andean Plateau.

Cenozoic Exhumation and Deformation of the Intermontane Pastos Chicos Basin in the Southern Central Andes: Implications for the Tectonic Evolution of the Andean Plateau (Puna) and the Eastern Cordillera Between 23° and 24°S, NW Argentina

AbstractThe Andean Plateau of north‐western Argentina (Puna) at a mean elevation of ca. 4.2 km constitutes the southern continuation of the Altiplano; it is a compressional basin‐and‐range province comprising fault‐bounded, high‐elevation mountain ranges and largely internally drained basins with often thick sedimentary and volcaniclastic fill. Growing sedimentological and structural evidence supports the notion that the north‐western Argentine Andes between 22° and 26°S developed from an initial extensive broken‐foreland system that extended across the present‐day eastern Andean flank during the early to middle Eocene. However, compelling evidence of the tectonic history of this region is still missing. Here, we present new apatite fission track and zircon (U–Th)/He thermochronological data and U–Pb zircon ages from intercalated volcanic ash deposits from the Pastos Chicos Basin (23.5°S, 66.5°W) to constrain basin formation and the timing of major crustal deformation in the northern Puna. Inverse thermal modeling of the thermochronological data provides further temporal constraints on the late Cenozoic cooling history of the crust in this region and, by inference, on the timing of upper‐crustal shortening, range uplift, and basin formation in the northern sector of the present‐day Puna Plateau. Specifically, we argue for plateau‐wide distributed deformation in the Eocene between 23° and 24°S, followed by spatially disparate and diachronous deformation (Oligocene to Pliocene).

Integrated Characterization of Mudstones in the Andes of Colombia: Understanding Its Complexities for Risk Mitigation

This article presents a comprehensive vision of particularities and constraints of the Paja Formation in the Northern Andes of Colombia, supported by personal, institutional, and academic experiences, including a doctoral thesis in completion (geomorphology and risks research line). Such fine-grained marine rocks cause severe damage in diverse zones, with little spread, and are very unfavorable, especially within the Eastern Cordillera (departments of Santander and Cundinamarca), whose socio-environmental problems motivated a popular legal action in the municipality of Vélez due to the cracking and collapse of houses, damage to roads and landslides in the urban area, as well as flows, subsidence, and high hydrogeochemical dynamism or rare earths, although they also presented spontaneous ignition at the rural area. Understanding how these problems originate and interrelate is the main objective of the work. At the beginning, we include some brief definitions, terms, and key approaches to understand the consolidated geomaterials, location, and background of the problem; then, the results of meso–macro–micro studies, obtained by combining the field techniques and conventional instrumental laboratory analyses (tests on the chemistry of water and soil, description of samples with magnifying glasses, petrography with a polarized light microscope, micromorphology of regoliths–colluvions) of nanoscientists (emphasizing RXD-RXF, SEM, IR-Raman spectroscopy, TOC-TS) are presented. These characterizations and new knowledge must be socially and institutionally appropriated and applied in land use planning and risk management for the sustainability of challenging environments with the stratiforms of Lower Cretaceous rocks and associated Quaternary deposits in populated mountainous areas and contrasting intertropical hydroclimatological regimes, geologically active, so unstable and insecure.

Diversity of culturable bacteria of freshwater environments from an altitudinal gradient in the eastern Cordillera of Colombia.

Bacteria are an essential biotic component in freshwater environments. A group of 262 bacterial strains of freshwater environments from an altitudinal gradient in the Eastern Cordillera of Colombia was identified using the 16S rRNA gene sequence analysis. Hill numbers and related diversity indices were calculated to know the bacteria diversity in this collection and environments. In addition, the Bray-Curtis index was also calculated to know the differences in genera composition between sampled localities and their relationship with altitudinal gradient. The identified bacterial strains were grouped into 7 major phylogenetic groups (Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Flavobacteriia, Actinomycetes, Clostridia, and Bacilli), 38 genera, and 84 distinctive species. Diversity analysis based on Hill numbers showed that the diversity concerning bacteria inhabiting freshwater environments was consistently high. Dominant genera were Klebsiella, Serratia, and Pseudomonas, although other genera such as Bacillus, Lelliottia, and Obesumbacterium were well represented per locality. The highest bacterial diversity came from localities Cimitarra and El Carmen del Chucurí, while those originating from Santa Bárbara and Páramo del Almorzadero were relatively lower diverse. Differences in diversity were found to be mainly due to the spatial replacement of one genus by another and, to a lesser extent, to the loss or gain of taxa.

Late Miocene Exhumation of the Western Cordillera, Ecuador, Driven by Increased Coupling Between the Subducting Carnegie Ridge and the South American Continent

AbstractThe subduction of bathymetric highs, such as aseismic ridges, leads to far‐reaching changes in the dynamics of subduction zones with increased plate coupling and deformation in the upper plate. Subduction of the submarine Carnegie Ridge on the Nazca Plate has fundamentally impacted late Cenozoic magmatism and tectonic activity in the northern Andes. However, the timing of onset of Carnegie Ridge subduction has been a matter of debate. Time‐temperature inverse modeling of new thermochronological data from the Western Cordillera of Ecuador reveals two phases of cooling separated by isothermal conditions. The first cooling phase postdates early and middle Miocene magmatism in the Western Cordillera and is attributed to post‐magmatic thermal relaxation. The second cooling phase started after 6 Ma. Inferred to record the onset of tectonically controlled rock uplift and exhumation in the Western Cordillera, this phase is coeval with the last cooling phase recorded in the Eastern Cordillera. Based on these findings, we suggest that the onset of subduction of the Carnegie Ridge at ∼6–5 Ma increased plate coupling at the subduction interface, promoting shortening, regional rock uplift, and exhumation in the northern Andes. Overall, our results highlight the essential role of bathymetric highs in driving regional upper‐plate deformation at non‐collisional convergent plate margins.

Lower–Middle Ordovician brachiopods from the Eastern Cordillera of Peru: biostratigraphical and palaeobiogeographical significance

Lower–Middle Ordovician brachiopods from the Eastern Cordillera of Peru: biostratigraphical and palaeobiogeographical significance

Lithospheric and Slab Configurations From Receiver Function Imaging in Northwestern South America, Colombia

AbstractThe northwestern corner of South America presents a highly complex geodynamical setting including the convergence between South America, Nazca, and Caribbean plates; which also entails heterogeneity and complexity in the lithospheric structure. Here, we present depth‐migrated common‐conversion point (CCP) teleseismic receiver function profiles carried out using seismological data between 2011 and 2020 from the Colombian Geological Service, combined with some stations of observatories and international networks. These profiles provide better constraints on the lateral interpretation of the main lithospheric discontinuities in this region. We provide an updated crustal thickness map displaying some differences from previous studies, especially in the northernmost part of the cordilleran system. Our results also show some intracrustal features such as the regional detachment surface beneath the Eastern Cordillera and the presence of deep melt reservoirs under the Central Cordillera and the Paipa‐Iza complex in the Eastern Cordillera. We also discuss the lithospheric thickness and the potential existence of a highly hydrated and serpentinized mantle wedge under the forearc over the slab segment south of the Caldas tear. Furthermore, the CCP profiles further support the subduction system segmentation under northwestern South America, with at least five slab segments, two of them overlapping between ∼5.5 and ∼8°N, three belonging to the Nazca plate, and two having a Caribbean origin. The Bucaramanga nest appears to be linked to the interaction between the two northmost slab segments, while the Cauca cluster might be related to a slab dehydration process of a highly fractured zone.

Mixed Signals From the Stable Isotope Composition of Precipitation and Plant Waxes in the Northern Tropical Andes

AbstractWe evaluate the efficacy of the stable isotope composition of precipitation and plant waxes as proxies for paleoaltimetry and paleohydrology in the northern tropical Andes. We report monthly hydrogen (δ2Hp) and oxygen (δ18Op) isotope values of precipitation for an annual cycle, and hydrogen isotope values of plant waxes (δ2Hwax) obtained from modern soils along the eastern and western flanks of the Eastern Cordillera of Colombia. δ2Hp, δ18Op, as well as the unweighted mean δ2Hwax values of n‐C29, n‐C31, and n‐C33 n‐alkanes in the eastern flank show a dependence on elevation (R2 = 0.90, 0.82, and 0.65, respectively). In stark contrast, the stable isotope compositions of neither precipitation nor plant waxes from the western flank correlate with elevation (R2 < 0.23), on top of a negligible (p‐value >0.05) correlation between δ2Hwax and δ2Hp. In general, δ2Hwax values along the eastern flank of the Eastern Cordillera seem to follow the trend of a simple Rayleigh distillation process that is consistent with studies elsewhere on the eastern side of the Andes in South America. Neither δ2Hp nor δ18Op, and therefore δ2Hwax, offer reliable estimates of past elevations in the western flank, due perhaps to water vapor source mixing, evaporation overprint, contrasting plant communities, and/or differences in evapotranspiration. Thus, δ2Hwax is only reliable for paleohydrology and paleoaltimetry reconstructions on the eastern flank of the Andes, whereas interpretations based on δ2Hp and/or δ18Op west of the highest point of the Eastern Cordillera need to consider mixing of moisture sources in addition to precipitation amount.

Polygenic soils in the southern central Ecuadorian highlands as the result of long-lasting pedogenesis, geodynamic processes and climate change

In the high altitude (>3000 m asl) grasslands of northern Ecuador, the Páramo ecosystem, soils generally formed on recent volcanic ashes being <10,000 years old in the northern and central part of the cordilleras. With time these soils evolved in to non-allophanic Andisols with hydric properties such as evidenced in profiles from the Cajas massif located in the western central Cordillera. However soils at the Fierro Urcu massif and at the Oña plateau at the eastern Cordillera formed on a non pyroclastic indurated parent material. Compared to non-allophanic Andisols from the Cajas massif, they present similar morphological properties with a dark 50 cm thick carbon-rich (80–120 g kg−1) topsoil with a very low bulk density (<0.65 g cm−3) and high water retention. These top soils appear to have originated from fall-out, derived from close and still active volcanoes of Sangay and/or Tungurahua, both located in a distance of 200–250 km NNE of the Oña Massif, respectively. Geochemical determinations of trace and rare earth elements, however, confirm the lack of volcanic ash contribution in the Saraguro and Oña samples based on Eu/Eu* and Ce/Ce* anomalies relative to the lack of such anomalies in the Cajas samples. The Oña and Saraguro soils appear to be the result of a long-lasting double pedogenic process. The basement of the Saraguro soil is a Ferralsol with gibbsite and kaolinite and the Oña soil is a 5 m thick kaolinitic Ferralsol. The upper part of these profiles are the result of a major climatic change with increasing colder conditions, generating an increase in organic matter content and a very acidic environment leading to the instability of kaolinite and gibbsite, thereby liberating large amounts of free aluminium and iron oxides. The evolution of these very highly evolved Ferralsols into soils with Andic properties is proposed to be the result of rapid tectonic uplift calculated at a rate exceeding 0.6 mm/year in the region leading to a general decrease in temperature limiting carbon mineralization and favouring the formation of organo-metallic complexes. The soils studied here reveal compelling new evidence and information about the uplift of a part of the Ecuadorean Andes.

Water-Vapour Monitoring from Ground-Based GNSS Observations in Northwestern Argentina

The Central Andes in northwestern Argentina are characterized by steep topographic and climatic gradients. The humid foreland areas at 1 km asl elevation rapidly rise to over 5 km in the eastern Cordillera, and they form an orographic rainfall barrier on the eastern windward side. This topographic setting combined with seasonal moisture transport through the South American monsoon system leads to intense rainstorms with cascading effects such as landsliding and flooding. In order to better quantify the dynamics of water vapour transport, we use high-temporal-resolution global navigation satellite system (GNSS) remote sensing techniques. We are particularly interested in better understanding the dynamics of high-magnitude storms with high water vapour amounts that have destructive effects on human infrastructure. We used an existing GNSS station network with 12 years of time series data, and we installed two new ground stations along the climatic gradient and collected GNSS time series data for three years. For several stations we calculated the GNSS signal delay gradient to determine water vapour transport direction. Our statistical analysis combines in situ rainfall measurements and ERA5 reanalysis data to reveal the water vapour transport mechanism for the study area. The results show a strong relationship between altitude and the water vapour content, as well as between the transportation pathways and the topography.

Decoding of groundwater recharge in deep aquifers of foreland Basins using stable isotopes (δ18O and δD) and anion-cation analysis: A case study in the southern Llanos Basin, Colombia

The topographic and structural configuration of the deformation front in foreland basins has implications in the process of hydrologic recharge of deep aquifer systems. Deep aquifer systems are favored when tectonic deformation has exhumed aquifer units to the surface, creating a connection between the surface and subsurface. The indirect water recharge processes established by this connection allow meteoric water to infiltrate and travel long distances through a basin. Therefore, establishing recharge zones and determining the influence of meteoric waters in deep aquifers represents a challenge to understand the dynamics and hydraulic potential of this type of system. This study presents new oxygen and hydrogen isotopic data from 162 samples of surface water and 109 samples from groundwater. Groundwater samples were taken from deep wells (700 m–4000 m deep) that penetrated the aquifer units that make up the Basal Flow Unit (BFU) present in the southern Llanos basin in Colombia. In addition, anion (−) and cation (+) analysis (HCO3−, CO3−2, SO4−2, Cl−, Na+, K+, Ca+2, Mg+2) was performed on 67 surface water samples and 93 groundwater samples from wells to evaluate their composition and existing relationship with the recharge areas. We used these isotope compositions (δ18O - δD) and hydrochemical data to identify the provenance and degree of influence of meteoric waters in the Basal Flow Unit. Thus, we evaluated the isotopic behavior of the Andean Foreland in Colombia. These data allowed us to propose the altitude ranges of the recharge zones for a deep regional aquifer (BFU) that extends along the southern Llanos basin in Colombia. The δ18O and δD values from surface water sources showed consistency with respect to the isotopic behavior reported in global datasets (GMWL), the Local Meteoric Water Line (LMWL) established for Colombia, and data from IAEA-GNIP stations examined in this study. The new δ18O and δD analysis, together with datasets from previous studies carried out in the area, made it possible to establish the Local Meteoric Water Line for the Llanos basin following the equation δD = 7.8 (±0.10) x δ18O + 11.66 (±0.75). δ18O and δD values of groundwater samples taken in the deep wells follow the proposed Llanos Local Meteoric Water Line. It supports a high influence of meteoric waters in the Basal Flow Unit, from areas close to the recharge zones up to more than 300 km away to the east of the basin and suggesting its origin from approximate altitude ranges that vary between 580 m and 2060 m, related to the La Macarena Range and the eastern flank of the Eastern Cordillera. The physicochemical data was used as an additional proxy and showed the lowest ion concentrations in proximity to the recharge zones, recognizing in general groundwater with low concentrations of Cl− (4.1–1278 mg/L) and Na (4.7–1749 mg/L) along the BFU present in the southern Llanos basin.

Condorodus n. gen., a new Ordovician conodont genus from Argentina: origin, evolution and dispersal through the western margin of Gondwana

Ordovician conodont collections from several Argentinian basins including the Eastern Cordillera, Famatina and Precordillera allow recognition of a group of conodonts that comprise a new genus here named Condorodus n. gen. Species of this genus have an apparatus composed of six elements recovered so far: Pa, Pb, Sb1, Sb2, Sc and Sd. The differences mainly between the P elements support recognizing three species, from the older to younger: C. diablensis n. gen., n. sp., C. gracielae n. gen., n. sp. and C. chilcaensis n. gen., n. sp., that appeared in the upper Floian (Lower Ordovician) and vanished in middle Darriwilian time (Middle Ordovician). The Eastern Cordillera is here assumed as the place of origin of the Condorodus n. gen. lineage during the late Floian, and then this genus dispersed through the western margin of Gondwana, reaching the Precordillera in the early Darriwilian, from there it could have dispersed to different regions of Gondwana, Perigondwana and Laurentia during the late Darriwilian, and probably give rise to conodont apparatuses of similar morphology in the Late Ordovician.

Evidence of Jurassic extension in NW Argentina: Characterization of fault-related strata at the Salta Group base using sandstone provenance and zircon U–Pb geochronology

The present-day structure of the Eastern Cordillera in NW Argentina is governed by structural and lithological heterogeneities inherited from preceding deformation phases, which influence the localization of newly-formed faults and the inversion of pre-existing structures. The Salta Rift Basin formed during a Late Jurassic-Cretaceous extensional phase and created a dominant structural and stratigraphic imprint in NW Argentina that is particularly evident within the Eastern Cordillera, where uplift and exhumation have exposed the Salta Group syn-rift succession. Although in general, the Salta Group rests upon Paleozoic rocks, locally the Tacurú Group forms an intermediate succession, consisting of interfingering eolian sandstones and proximal fault-related conglomerates with a Jurassic maximum depositional age. This succession might be the key to unraveling the Mesozoic history of NW Argentina, prior to the deposition of the Salta Group. The conglomerates represent the earliest deposits related to extension in the western Lomas de Olmedo sub-basin, which is also documented in predominantly Jurassic zircon (U–Th-Sm)/He cooling ages of the rift shoulders. The detrital zircon U–Pb age signature and sandstone provenance of the Tacurú Group conglomerates differs strongly from the Salta Group syn-rift strata, which show a more regional signal. These variations and the angularity of the unconformity may be connected to a rotation of the extension direction in the western Lomas de Olmedo sub-basin.

First-Order Stratigraphic Boundaries of the Late Cretaceous–Paleogene Retroarc Foreland Basin in Colombia

The present-day Magdalena Valley, Eastern Cordillera, and Llanos Basin were part of a regional multiphase basin that started as an extensional basin in the Jurassic. Then, it was transformed into a retroarc foreland basin in the Late Cretaceous and subsequently separated into a hinterland and a foreland basin during the late Eocene. We incorporated new data from the Llanos Basin and Eastern Cordillera and chronostratigraphically correlated it with the adjacent basins. Their correlation contributed to establishing the stratigraphic boundaries formed during the first-order changes that marked the beginning and the end of the lifespan of the retroarc foreland basin. In the initial stage, the lateral extent along the dip of the significant depocenter that extended from the Magdalena Valley to the westernmost Llanos Basin was approximately 300–470 km. At the same time, the western flank of the Eastern Cordillera and most of the Llanos Basin were uplifting. The contact between the retrograding marine facies of the Upper Lidita, Buscavidas, Umir, Guadalupe, and Palmichal units marks a late Campanian maximum regressive surface formed during the initial exhumation of the orogen, the Central Cordillera. Therefore, it represents a first-order stratigraphic surface. This surface does not extend across the whole basin. In the uplifting areas, subaerial unconformities truncate the middle Campanian rocks. Their combination represents the lower first-order boundary of the sequence, and their diachroneity is the product of the northward migration of the orogen. The Teruel, Hoyon, Bogota, Cacho, Socha, Barco, and Cuervos formations are truncated by a regional subaerial unconformity. It was formed during orogenic unloading that marks the termination of the foreland basin in the Eocene. As it marks another first-order change, it is the upper boundary of the first-order sequence.

Constraining Andean Propagation of Exhumation at the Limit of the Eastern Cordillera, NW Argentina, Using Low‐Temperature Thermochronology in a Structural Context

AbstractWithin the Central Andes of NW Argentina, the spatiotemporal distribution and style of deformation is strongly influenced by pre‐Cenozoic heterogeneities, mostly related to the Salta rift extension in the Cretaceous. At the enigmatic junction of the thin‐skinned Subandean belt and the thick‐skinned Santa Barbara System, the Tilcara Range and adjacent San Lucas block, located within the Eastern Cordillera, show thermochronological and field evidence of multiple exhumation events. Mesozoic (140‐115 Ma), pre‐Andean exhumation of basement highs is constrained by unconformities between basement and syn‐rift strata, as well as zircon (U‐Th‐Sm)/He cooling ages. Cenozoic Andean exhumation is quantified by apatite (U‐Th‐Sm)/He and fission track cooling ages, which were reset between the Late Cretaceous and Miocene. These data show that the westernmost Tilcara Range began exhuming in the late Oligocene‐early Miocene (26‐16 Ma), after which exhumation propagated to the border of the Eastern Cordillera in the middle Miocene (22‐10 Ma). The onset of rapid exhumation in the San Lucas block, which is located east of the Tilcara Range, occurred in the late Miocene (10‐8 Ma) in its western part, and in the late Miocene‐early Pliocene (6‐4 Ma) in its eastern part. Internal deformation of the San Lucas block, disturbing zircon (U‐Th‐Sm)/He and apatite fission track age patterns, predates propagation of rapid exhumation. The here presented low‐temperature thermochronology data set thus quantifies the multi‐phase exhumation history of the Eastern Cordillera of NW Argentina and constrains the timing of Andean propagation of exhumation within the Eastern Cordillera and the adjacent structural transition zone.

Catálogo polínico de plantas usadas por tres abejorros del género Bombus (Hymenoptera: Apidae) en la Cordillera Oriental de los Andes colombianos

More than 20 species of bumblebees are reported in South America, nine of them in the Eastern Cordillera of the Colombian Andes. They are known to be important pollinators, but their distribution has been decreasing due to increasing land use change; this has caused an impoverishment of Andean ecosystems. Identifying and protecting the floral resources used by wild bees is key in the management of wild areas to mitigate the impact of human activities in highly intervened areas such as cities or crops. Pollen analysis is an important tool to preserve floral resources used by bees, but it depends on having pollen catalogs that allow adequate taxonomic identifications. The objective of this study was to develop a pollen catalog of plants used by three of the most common Andean bumblebees, Bombus rubicundus, B. hortulanus and B. pauloensis. Sampling was conducted in six localities of the Bogotá savanna, covering an altitudinal gradient between 2600 and 3500 m a. s. l. Floral samples from all localities were prepared by acetolysis, and then analyzed. A reference catalog was digitized from the pollen extracted from the body of workers of each bumblebee species. Pollen grains were described according to their size and morphology. The similarity of the resources used by the different bumblebee species was compared using Jaccard’s similarity and distance index. Pollen grains from 99 plants were isolated, identified and described, with Taraxacum officinale, Vallea stipularis, Senna multiglandulosa, Trifolium pratense and Trifolium repens being the pollen sources shared by all species. The most generalist species and the one that used the most exotic plants was B. pauloensis. An illustrated pollen catalog is provided with all the resources used by the bees.