Central Precordillera Research Articles

Relationships between landform properties and vegetation patterns in the Cerro Zonda Mt., Central Precordillera of San Juan. Argentina

Arid environments are dynamic systems due, in part, to the action of hydrological processes. However, studies on the relationship between the abiotic factors and the properties of arid vegetation communities of Argentina are scarce. In this paper, the connection between these two parameters is researched using DEM analysis and exhaustive field work in order to model vegetation changes and landform evolution. The aim of this paper is to determine whether geomorphological properties influence vegetation patterns along the Cerro Zonda Mt. This mountain range is located in an arid central sector of Monte Desert, a phytogeographic region of Argentina. The classification of geomorphological units was made by combining interpretation of images provided by Google Earth and the analysis of Alos Palsar DEM using SAGA GIS. The geomorphic units were sorted into categories based on slope, elevation, hillslope aspect, terrain roughness and topographic wetness. In addition, surface characteristics of units were evaluated considering the properties of coverage of rock fragments, fine sediment and mulch. Afterwards, community patterns were explored using PCA analysis and linear correlations. Our results showed five geomorphological units: a) Pediment of mountain upland; b) Active landforms; c) Inactive landforms; d) Inactive landforms raised by neotectonics and e) Lacustrine landforms. The vascular flora is represented by 35 species distributed in 15 families. The landforms' surface property of rock fragments is correlated both directly with species richness and inversely with vegetation coverage. Furthermore, roughness explains the higher vegetation diversity while slope promotes an increase in the vegetation cover, but this does not suggest more species richness. In addition, our results indicate that topographic wetness, as well as hillslope aspect and elevation, are not determining variables of vegetation patterns across the landforms. The vegetation patterns of Cerro Zonda Mt. are associated with topographic properties as slope and roughness. In turn, rock fragment surface coverage also affects these vegetation patterns. The landforms' properties not only expose spatial changes in the development of the plant communities but also show the direct implications of geomorphological changes over such communities. In particular, there is a need to integrate a geomorphological view for the analysis of vegetation in arid zones.

Conodont biostratigraphy and depositional history of the Darriwilian (Middle Ordovician) Pygodus serra Zone, western Argentina

The stratigraphic record of the Pygodus serra conodont Zone in the Cuyania terrane of western Argentina is discussed in this contribution. Three well-known sections were sampled in Precordillera and the San Rafael Block. The studied successions are composed mainly by clastic rocks with variable amount of carbonate. The lower part of Ponón Trehué Formation of the San Rafael Block and the La Cantera Formation of the Eastern Precordillera are composed of conglomerate and represent the input of coarse clastic deposits to the Cuyania basin. The Los Azules Formation, in Los Amarillitos section of the Central Precordillera, has a massive sandstone bed with carbonate nodules in a section largely of black shale. Key conodonts recovered from these sections indicate a correlation to the E. robustus and E. lindstroemi subzones of the Pygodus serra Zone of the upper Darriwillian Stage (Middle Ordovician Series). The species in the Ponón Trehué Formation are almost all the same as those in the Precordillera sections. Correlation of the clastic sedimentary successions between the three sections indicates that vertical facies changes were not controlled by eustasy. More likely, they were controlled by differential tectonic subsidence.

Stratigraphy and facies analysis of the La Chilca Formation, Central Precordillera: Insights on the postglacial Ordovician–Silurian boundary and Early Silurian deposits from Argentina

The La Chilca Formation is exposed at the Poblete Norte and Ancha sections, Talacasto, Central Precordillera. Detailed facies analysis on field outcrops suggests a tide‐dominated tidal plain model on the Silurian siliciclastic ramp, integrated by supratidal to subtidal plain deposits. The lower part of the La Chilca Formation (upper supratidal plain) is herein attributed to reworked glaciomarine deposits that occurred during interglacial Hirnantian age, under conditions of sea rise and the action of tidal amplification. The intertidal–supratidal depositional environments are represented by tidal channels, scour and fill, tidal dunes, and hummocky cross‐stratified sandstones. The estimated tidal conditions for the tidal plain were mesotidal to macrotidal, under action of combined flows, attributed to the interaction of tidal currents and orbital low‐velocity waves. The identification of microbially induced sedimentary structures (MISS) in these deposits, such as petee polygonal structures, pitted texture, imploded sand volcanoes, Astropolithon sp., Manchuriophycus sp., domal build‐ups, biofilms, elephant skin, and trace fossils such as Chondrites sp., Zoophycos sp., Undichnia sp., Planolites sp., Monomorphichnus sp., and gastropod traces, may have played a protection and stabilization role on the clastic sediment surfaces against erosion of the La Chilca Formation deposits. The prolific growth of microbial mat on the intertidal to supratidal zone occurs explicitly at the turning points of transgression–regression cycles in shallow‐water environments under stable base‐level conditions. The middle‐upper section of the La Chilca Formation shows non‐cyclic rhythmites that are replaced by the cyclic rhythmites, suggesting an open tidal plain subject to an orbital/climatic rather than a tectonic control of cyclicity (Milankovitch cycles). The aggradation in this section indicates a shallowing towards the top of the formation. The La Chilca Formation records two third‐order transgressive events and a shallowing event. The transgressive events are found in the base and top of the formation and are marked by the presence of Fe‐oolites, while the shallowing event is determined by the presence of MISS.

Conodont and graptolite biostratigraphy of the lower–middle Darriwilian (Middle Ordovician), Cerro Viejo of Huaco, Argentine Precordillera

AbstractThe distribution of conodonts and graptolites from the upper 13 m of the San Juan Formation and the first metre of the lower member of the Los Azules Formation, Los Gatos and Amarilla creeks, Cerro Viejo of Huaco, Central Precordillera from San Juan, Argentina, is studied. The conodonts from the top stratum of the San Juan Formation at the Los Gatos creek section correspond to the Lenodus variabilis Zone, whereas at the Amarilla creek section, they represent the Yangtzeplacognathus crassus Zone, verifying the diachronous top surface of the San Juan Formation. In the lower member of the Los Azules Formation, 81 conodont elements are recorded from bedding plane surfaces, where the presence of Paroistodus horridus horridus (Barnes & Poplawski) allows to recognize the Y. crassus Zone. On the basis of the graptolite and conodont association, we estimate that the top of the San Juan Formation at Los Gatos creek corresponds to the Levisograptus dentatus Zone, L. dentatus Subzone, whereas at the Amarilla creek, the Arienigraptus angulatus Subzone of the same zone is represented. The presence of the graptolite A. angulatus (Mu) in the lower member of the Los Azules Formation in both sections indicates the L. dentatus Zone (A. angulatus Subzone). The records of graptolites in limestones of the San Juan Formation and of abundant conodonts on bedding plane surfaces of the lower member of the Los Azules Formation are important, taking into account the type of facies where they occur. The conodont records in the referred type of preservation are scarce in the Ordovician, which allows for a more precise biostratigraphic correlation between conodont and graptolite biostratigraphy.

Graptolite and conodont faunas of Upper Ordovician (Sandbian) successions of the Argentine Precordillera: Biostratigraphic implications

Graptolites and conodonts are frequent fossils in Sandbian (Late Ordovician) outcrops of the Argentine Precordillera, where they present continuous and abundant records that enable detailed biostratigraphic studies. This work deals with the integration of previously available information on early Sandbian graptolites and conodonts, together with new paleontological data from the Las Aguaditas Creek (middle and upper members of the Las Aguaditas Formation), Las Chacritas River (Las Aguaditas Formation), and Cerro La Chilca (Los Azules Formation) sections from the Central Precordillera of the San Juan Province. The presence of the key graptolite Nemagraptus gracilis and the associated graptolite taxa allow for the documentation of the N. gracilis Zone. Specimens of N. gracilis and key conodont species, such as Pygodus anserinus coexist, along with a rich conodont fauna including Baltoniodus variabilis, whose range straddles the boundary between the Pygodus anserinus and the Amorphognathus tvaerensis zones. Detailed biostratigraphic ranges of the rich graptolite and conodont associations documented in the three study areas are provided. These records and an accurate taxonomic review allow a thorough description of the recognized zones. The use of conodont and graptolite zones from different areas of the Precordillera enables the verification of a diachronous contact, representing a hiatus that spans the middle to upper Darriwilian. Moreover, a brief taxonomic description of the identified graptolite species is provided.

Quaternary tectonic activity of Maradona Fault System, Central Precordillera, Argentina

Several parallel east verging thrusts with Quaternary tectonic activity have been identified in the eastern piedmont of Cordón de Las Osamentas, Central Precordillera of Argentina. We grouped these structures into the Maradona Fault System (MFS). The main morphostructural units, called mountainous and transitional, were analyzed. The form and evolution of these units are closely related to the geological structure of the area. With the morphotectonic analysis, we estimated the length of MFS and the affected alluvial levels. Besides, the morphology of the escarpments was determined through detailed transversal topographic profiles made in a sector where the alluvial levels are affected by the faults. This analysis allowed the recognition of several fault segments and the selection of the points that would present the best natural exposures of the fault- sedimentary deposits relationships. Topographic profiles performed with differential GPS let to identify centimetric fault scarps through slope changes observed in each transect orthogonal to the direction of the structures. Five natural exposures were identified allowing determination of the kinematics of the different fault sections and the deposits affected by these structures. In all the analyzed exposures, faults are reverse with low angle (thrust), dipping to the west and affecting Neogene deposits and alluvial units assigned to the late Pleistocene-Holocene. The Maradona fault system affects younger alluvial levels to the east, showing the gradual migration of the Andean deformation eastward. The progression of the deformation in this sector of Central Precordillera would have evolved according to a pattern of thrusts with angles in surface near 45°, with sequences of internal deformation and reactivation during the Pleistocene-Holocene. Finally, the SFM is regarded as a potential seismogenic source capable to produce earthquakes with magnitude >Mw 6 and hit the main populated centers of the province of San Juan (>700,000 inhabitants), and several dams built along the San Juan River.

Bioestratigrafía y provincialismo de conodontes del tramo medio-superior de la Formación San Juan en el cerro Viejo de Huaco, Precordillera, Argentina

The present study deals with the conodont biostratigraphy from the middle and upper parts of the San Juan Formation (Lower-Middle Ordovician) exposed at the Los Gatos creek section, west of the cerro Viejo de Huaco, Central Precordillera of San Juan Province. The numerous conodonts recovered, corresponding to 55 species, allow to recognize a series of biozones in the studied section. The biostratigraphic analysis carried out herein allows determining associations of conodonts assignable to the Oepikodus evae, Oepikodus intermedius, Baltoniodus triangularis-Tripodus laevis and Lenodus variabilis zones that correspond to the middle Floian to lower Darriwilian. The Baltoniodus navis and Microzarkodina parva zones were not identified due to insufficient information provided by this stratigraphic section. In general, the levels corresponding to the Baltoniodus triangularis-Tripodus laevis Zone contain more conodonts, than the records of other localities from Precordillera. These specimens are well preserved with a color alteration index (CAI) of 2-2.5, indicating overburden paleotemperatures ranging from 60° and 155° C. The large presence of fragmented conodonts, with recrystallized surfaces and crystal overgrowth, could indicate the effect of distinctive diagenetic fossil processes on the bearer limestone. The analysis of the diversity and abundance of genera and species of conodonts by chronostratigraphic intervals presents a great percentage of cosmopolitan taxa, 18% and 38% in common, at species level, only with the North-American Midcontinent Province (NAMP), and with a small percentage with the North-Atlantic Province (NAP) and the Precordillera, which allows to approximate a greater paleobiogeographic affinity of the conodonts recovered with those of the NAMP than those of the NAP for the entire interval studied. On the other hand, the Precordillera is verified as a province with its own characteristics as identified by several authors.

Lower middle Darriwilian (Ordovician) graptolites and index conodonts from the Central Precordillera of San Juan Province, Argentina

This study includes 3 localities from the Jáchal area in the Central Precordillera of San Juan Province, from north to south: the Oculta creek, Las Aguaditas creek, and Cerro La Chilca sections. We deal with the graptolite faunas and conodont index species recorded from units that overlie the carbonate San Juan Formation, spanning the lower part of the Los Azules, Las Aguaditas, and Gualcamayo formations in their respective areas. The index graptolites and associated species are reported, which enable the recognition of the Levisograptus dentatus Zone in the Central Precordillera. The presence of graptolites in the limestones from the top of the San Juan Formation at the Cerro La Chilca section is documented for the first time. The record of representatives of the Lenodus variabilis, Yangtzeplacognathus crassus, Eoplacognathus pseudoplanus, Histiodella, and Periodon lineages recognized in these units, linked with respective graptolite zones, provide precise information for global correlation purposes. The use of conodont and graptolite zones from different areas of the Central Precordillera enables the verification of the diachronous contact between the San Juan Formation and overlying units, which spans the lower to middle Darriwilian, in our investigated sections and classical localities previously documented.

Multiple geophysical methods examining neotectonic blind structures in the Maradona valley, Central Precordillera (Argentina)

A high-resolution superficial geophysical study was carried out in an area of the retroarc region of the Andes mountains, located in the southwest of San Juan Province (31°45′ S, 68°50′ W), Central Precordillera of Argentina. The main objectives of this study were to confirm the presence of blind neotectonic structures and characterize them by observing variations in magnetic susceptibility, density and p-wave velocities. Geological evidence demonstrates the existence of a neotectonic fault scarps affecting Quaternary alluvial deposits in eastern piedmont of de Las Osamentas range, in addition to direct observation of the cinematic of this feature in several natural exposures. The Maradona valley is characterized by the imbricated eastern-vergence Maradona Fault System that uplifts Neogene sedimentary rocks (Albarracín Formation) over Quaternary (Late Pleistocene-Holocene) alluvial deposits. The combined application of different geophysical methods has allowed the characterization of a blind fault geometry also identified on a natural exposure. The magnetic data added to the gravimetric model, and its integration with a seismic profile clearly shows the existence of an anomalous zone, interpreted as uplifted blocks of Miocene sedimentary rocks of Formation Albarracín displaced over Quaternary deposits. The application and development of different geophysical methods, together with geological studies allow to significantly improving the knowledge of an area affected by Quaternary tectonic activity. Finally, this multidisciplinary study, applied in active blind structures is very relevant for future seismic hazard analysis on areas located very close to populated centers.

Carbonate microfacies of the San Juan Formation (Ordovician: Oepikodus evae and Oepikodus intermedius conodont zones), Niquivil, Central Precordillera, Province of San Juan (Argentina)

This contribution presents the description and interpretation of carbonate microfacies of the San Juan Formation (Ordovician) at the Niquivil section, considering the stratigraphical interval between the Oepikodus evae and Oepikodus intermedius conodont zones. The distribution of the microfacies and the conodonts assemblages allow us to identify different sub-environments within the late Floian carbonate ramp of the Central Precordillera. Five microfacies were recognized from the base to the top: M1 Bioclastic mudstone-wackestone; M2 Bioclastic-peloidal wackestone; M3 Intra-bioclastic wackestone; M4 Intra-bioclastic packstone; M5 Peloidal grainstone. The vertical distribution of these microfacies indicates a shallowing trend of the carbonate ramp in the Niquivil section for this temporal interval, which suggests a middle ramp environment with low energy, without wave action, and that evolved towards the middle-inner ramp environment with more energy by wave action and development of tempestites.

Darriwilian (Middle Ordovician) conodont biofacies from the Central Precordillera of Argentina

Abstract Conodont biofacies structure was recognized throughout detailed stratigraphic sampling at three localities from the Central Precordillera of Argentina; from north to south: Las Aguaditas Creek, Las Chacritas River and Cerro La Chilca sections. A total of 50 carbonate samples yielded over 9900 identified specimens, including a new conodont species, Fahraeusodus jachalensis, which is described due to its paleoecological connotations. The Yangtzeplacognathus crassus – Eoplacognathus pseudoplanus and Periodon macrodentatus zones are dominated by the ubiquitous species Periodon macrodentatus and Paroistodus horridus at the three study areas. This enables the recognition of the Periodon–Paroistodus biofacies in the Central Precordillera. Numerical cluster methodology allowed the identification of conodont sub-biofacies: the Protopanderodus–Semiacontiodus sub-biofacies in the Las Aguaditas Creek, the Protopanderodus–Parapaltodus sub-biofacies in the Las Chacritas River, the Parapaltodus sub-biofacies at the Cerro La Chilca and the Fahraeusodus–Ansella sub-biofacies in all of the study sections. These sub-biofacies present distinctive attributes, which are correlative with changes in the depositional environment. The diverse Protopanderodus–Parapaltodus sub-biofacies developed with a shallowing of the basin while the Fahraeusodus–Ansella sub-biofacies is characteristic of deep environments. Shallower water settings resulted in higher species richness, diversity, and evenness of conodont communities, the eustatic change that led to the drowning of the carbonate platform produced a shift to deeper, anoxic benthic conditions that in turn induced a change in the taxonomic composition of the fauna, and affected the diversity of the conodont communities.

Evolución del relieve de un frente montañoso y respuesta del sistema erosivo fluvial ante el forzamiento causado por la actividad tectónica (Andes centrales, provincia de San Juan, Argentina)

Fil: Ocana, Raul Emmanuel. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Fisicas y Naturales. Instituto de Geologia Dr. Emiliano Aparicio; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Fisicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; Argentina

Palynology and palynofacies analysis of a Silurian (Llandovery–Wenlock) marine succession from the Precordillera of western Argentina: Palaeobiogeographical and palaeoenvironmental significance

Abstract A comprehensive survey of the marine and terrestrial palynomorphs from the La Chilca Formation, exposed in the Central Precordillera of San Juan, Argentina, is presented. Twenty-four samples from the Quebrada Ancha section were analysed. Marine phytoplankton predominated in the entire stratigraphic unit while the contribution of terrestrial palynomorphs was much less. Acritarchs and chlorophytes represent a highly diversified assemblage, including stratigraphically relevant taxa such as Domasia trispinosa , Tylotopalla caelamenicutis , Tylotopalla digitifera , Crassiangulina variacornuta , Stellinium rabians and Percultisphaera cf. stiphrospinata . Terrestrial palynomorphs are exclusively represented by six cryptospore species; namely Gneudnaspora divellomedia , Pseudodyadospora laevigata , Rugosphaera cerebra , Tetrahedraletes medinensis , cf. Imperfectotriletes vavdovae and Velatitetras retimembrana . Comparisons with coeval phytoplankton assemblages from Gondwana and other palaeoplates such as Laurentia, Baltica and Avalonia show strong similarities, suggesting a cosmopolitan distribution pattern during the Llandovery and Wenlock. The first palynofacies analysis for the early Silurian of Argentina was performed. Depositional environments based on palynofacies match with those previously suggested based on sedimentological data. Two main palynofacies associations are recognised. Association B occurs in the lower part of the La Chilca Formation and is mostly composed of amorphous organic matter (AOM) representative of low energy inner shelf environments. Association A is recognised in the upper part of the formation, where sandstone beds become more frequent, and is characterised by a greater amount of possible palynomorph remains (PPR), indicating a proximal platform to shoreface environment. Three excursions of Total Organic Carbon (TOC) were identified in the section, probably as a result of sea level rise episodes that produced anoxic conditions and preservation of organic matter.

Conodont biostratigraphy and global correlation of the middle Darriwilian-lower Sandbian (Ordovician) Las Aguaditas Formation, Precordillera of San Juan, Argentina

Middle Darriwilian to lower Sandbian conodonts were recorded from the Las Aguaditas Formation at its type section in the Argentine Precordillera. A total of 9,974 conodont specimens were recovered from 46 carbonate samples, which represent 68 species of 38 genera. A biostratigraphic study verified a middle Darriwilian age for the interval spanning the contact between the San Juan and the Las Aguaditas formations. The following zones are determined in the study section: the Lenodus variabilis Zone, with the Periodon gladysae and Paroistodus horridus subzones following the Precordilleran scheme; the L. variabilis, Yangtzeplacognathus crassus, and the Eoplacognathus pseudoplanus zones with the Microzarkodina hagetiana and M. ozarkodella subzones, and the Pygodus anserinus Zone, according to the Scandinavian scheme; the Periodon macrodentatus Zone, with the Histiodella sinuosa, H. holodentata and H. cf. holodentata subzones, and the P. zgierzensis Zone with the H. kristinae Subzone that correlates the North American scheme. A stratigraphic gap was recognized between the lower and middle members of the Las Aguaditas Formation. It comprises the Eoplacognathus suecicus and Pygodus serra zones, and the lower subzone of the Pygodus anserinus Zone. The variation of conodont diversity through the study section conforms to shallowing and deepening patterns, which accompanies the changes of the provenance lithology. Three conodont assemblages were quantitatively recognized: a) Diverse conodont association, b) Low diversity conodont association and c) Recovery phase association. We propose to use the North American biozonal scheme of conodonts for the Central Precordillera because of the affinity of documented index taxa, which provides a more accurate intercontinental correlation for the global Middle Ordovician Series.

Faulting and erosion in the Argentine Precordillera during changes in subduction regime: Reconciling bedrock cooling and detrital records

The Argentine Precordillera is an archetypal retroarc fold-and-thrust belt that records tectonics associated with changing subduction regimes. The interactions between exhumation and faulting in the Precordillera were investigated using apatite and zircon (U–Th–Sm)/He and apatite fission track thermochronometry from the Precordillera and adjacent geologic domains. Inverse modeling of thermal histories constrains eastward in-sequence rock cooling associated with deformation and erosion from 18 to 2 Ma across the Central Precordillera tracking thrusting during this time. The youngest AHe ages (5–2 Ma) and highest erosion rates are located in the eastern and western extremities of the Precordillera and indicate that recent denudation is concentrated at its structural boundaries. Moreover, synchronous rapid Pliocene cooling of the Frontal Cordillera, Eastern Precordillera, and Sierra del Valle Fértil was coeval with initiation of basement-involved faulting in the foreland. Detrital zircon U–Pb geochronology from the ca. 16–8.1 Ma Bermejo foreland basin strata suggests fluvial connectivity westward beyond the Frontal Cordillera to the Main Cordillera and Coast Range followed by an important shift in sediment provenance at ca. 10 Ma. At this time, we suggest that a substantial decrease in Permo-Triassic igneous sources in the Frontal Cordillera and concurrent increase in recycled zircons signatures of Paleozoic strata are best explained by uplift and erosion of the Precordillera during widening of the thrust-belt. Bedrock thermochronology and modeling indicate a 2–6 Myr lag time between faulting-related cooling in the hinterland and the detrital record of deformation in the foreland basin, suggesting that for tectonically active semi-arid settings, bedrock cooling may be more sensitive to onset of faulting. We suggest that high erosion rates in the Frontal Cordillera and Eastern Precordillera are associated with increased interplate coupling during shallowing of the subducting Nazca plate that may concentrate stress along weak structural boundaries of the Precordillera.

Late Quaternary evolution of the La Cantera Fault System (Central Precordillera, Argentina): A morphotectonic and paleoseismic analysis

The La Cantera Fault System (LCFS) is the most active Quaternary structure in the Central Precordillera of San Juan, in central-western Argentina; the system extends for 47km along the intermountain valley that separates the Sierra de La Cantera and La Invernada, north of the San Juan River. The average fault trend is 20°; it dips at angles varying between 15° and 30° W in the northern section, to approximately 40° W in the central section, and up to 60° W in the southern section. The fault affects Holocene to recent alluvium deposits in the western piedmont of the Sierra de La Cantera and is defined by a series of landforms found in compressive tectonic environments, including simple and compound counterslope fault scarps, staircased alluvial terraces, sag ponds, flexural scarps, aligned springs, broom-shaped drainage patterns, river diversions, beheaded channels, changes in incision depths, sinuosity and a river gradient along channels. Trench investigations indicated that at least three events occurred in the past 1.1–10.1ky. The topographic profiles of the selected channels and interfluves cutting across the northern and central trace of the fault were analyzed using a Stonex Vector GPS differential system to establish the relationship between the topography and slope of the rivers. This morphometric analysis of scarps indicates that active tectonics have played an essential role in controlling the drainage pattern in the piedmont, leading the rivers to adjust to these slope variations. Based on the analyzed geomorphologic, stratigraphic and structural characteristics, the LCFS is considered to be a relevant seismogenic source in the intraplate portion of southern South America, with a recurrence interval of at least 2000±500years for moderate magnitude earthquakes during the last 11,000years.

New Biostratigraphic Proposal for the Lower Palaeozoic Tucunuco Group (San Juan Precordillera, Argentina) Based on Marine and Terrestrial Palynomorphs

Abstract. This is an exhaustive analysis of the palynomorph assemblages, organic-walled marine phytoplankton and miospores from the Tucunuco Group. This group includes the La Chilca and Los Espejos formations and is exposed in the Central Precordillera of San Juan. Four localities were included in this study, i.e., Rio Jachal, Rio de las Chacritas, Cerro La Chilca and Quebrada Ancha. Previous data concerning several fossil groups, such as graptolites, conodonts and brachiopods, suggest a mainly Llandovery—Wenlock? age for the La Chilca Formation and a Wenlock to locally Lochkovian age for the Los Espejos Formation. Marine palynomorphs are predominant in the two formations in all the studied sections. Although terrestrial palynomorphs are the minor component, trilete spores are abundant, diverse and biostatigraphically relevant in the Los Espejos Formation. Correlations using marine palynomorph assemblages and miospore biozones recognized worldwide allow constraining the age of the two stratigraphic units....

The Argentine Precordillera: A foreland thrust belt proximal to the subducted plate

The Precordillera thrust belt of western Argentina is anomalously close, both horizontally and vertically, to the coeval subduction zone of the Nazca plate. The thin-skinned part of the belt has an unusually deep decollement that is well defined by industry seismic reflection and recent broadband experiments. New area and line-length balanced cross sections show that the central Precordillera has accrued ∼90 ± 21 km of shortening since 13 Ma; much of that shortening occurred between 12 and 9 Ma. Fault-slip data generally show shortening approximately west-northwest–east-southeast, orthogonal to the traces of the thrust and folds in the Precordillera and oblique to the mean vector of local global positioning system (GPS) data. The GPS strain rate is –63 ± 9 × 10 –9 /yr, whereas strain rate in the thrust belt, averaged over 13 m.y., is –56 ± 4 × 10 –9 /yr. Although the decollement of the Precordillera cannot cut into Paleozoic Cuyania(?) terrane basement east of the crest of the high Andes, broadband receiver function data show that significant crustal thickening must occur beneath and even east of the thrust belt. We suggest that top-to-the-west shear and thickening of the lower crust due to flat subduction explains the distribution of crustal thickening.

Application of logistic regression and frequency ratio in the spatial distribution of debris-rockslides: Precordillera of San Juan, Argentina

This study employs statistical modeling techniques and geomorphological mapping to analyze the distribution of debris-rockslides in relation to altitude, aspect, slope angle, lithology and distance to lineaments using optical remote sensing techniques with GIS. The study area includes a portion of the Central Precordillera of San Juan around 32°S, 69°W, where few geomorphological studies have been conducted. A debris-rockslides inventory map was prepared using debris-rockslides locations identified from satellite imageries, a total of 35 debris-rockslides were identified in an area of 288.13 km2 (with a total slide area of 6.36 km2). All of the debris-rockslides were rotational slides, affecting Upper Ordovician and Upper Devonian deposits. The volumes of debris-rockslides were estimated using the sloping local base level technique that allows computing the possible pre-rockslide topography and the bedrock–deposits interface. The relationship between the variables and the slide distribution was analyzed using the frequency ratio method and logistic regression models. The analytical results show that lithology and slope exert major influences. Lithology is found to be the most important aspect in the study area.

New glacial evidences at the Talacasto paleofjord (Paganzo basin, W-Argentina) and its implications for the paleogeography of the Gondwana margin

The Talacasto paleovalley is situated in the Central Precordillera of San Juan, Argentina, where upper Carboniferous-Permian rocks (Paganzo Group) rest on Devonian sandstones of the Punta Negra Formation. This outcrop is an excellent example of a glacial valley-fill sequence that records at least two high-frequency cycles of the advance and retreat of a glacier into the valley. The paleocurrent analysis shows transport predominantly to the south, indicating that at this site the ice flow differs from the other nearby paleovalleys. Evidence of the glacial origin of this valley can be seen in the glacial striae on the valley's sides, as well as the U-shape of the valley, indicated by very steep locally overhanging valley walls. Deglaciation is indicated by a set of retransported conglomerates deposited in a shallow-water environment followed by a transgressive succession, which suggests eustatic rise due to meltwater input to the paleofjord. The complete sedimentary succession records distinct stages in the evolution of the valley-fill, represented by seven stratigraphical units. These units are identified based on facies associations and their interpreted depositional setting. Units 1 to 5 show one cycle of deglaciation and unit 6 marks the beginning of a new cycle of glacier advance which is characterized by different types of glacial deposits. All units show evidence of glacial influence such as dropstones and striated clasts, which indicates that the glaciers were always present in the valley or in adjacent areas during sedimentation. The Talacasto paleofjord provides good evidence of the Late Paleozoic Gondwana glaciation in western Argentina and examples of sedimentary successions which have been interpreted as being deposited by a confined wet-based glacier in advance and retreat cycles, with eventual release of icebergs into the basin. The outcrop is also a key for reconstructing the local glacial paleogeography, and it suggests a new interpretation that is not in agreement with previous studies. Finally, the importance of the Talacasto paleovalley for the Paganzo basin lies in its orientation, because it allows the reconstruction of the ice paleoflow and indication, for the first time, that marine ingressions into this area were not taking place along the Jachal trough, as expected, but along a different connection to the sea, which for this work we will call the San Juan Paleotrough.