Neogene Micropaleontology and Stratigraphy of Argentina (introduction) (original) (raw)
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The Patagonian transgression, early Miocene (Burdigalian age), in Aysén and Magallanes
During the early Miocene an extended eustatic cycle referred to as the Patagonian transgression flooded a vast part of southern South America. In Chile, the Guadal Formation in Aysén and the recently identified Estancia 25 de Mayo Formation in Magallanes represent this event. The main facies are thick, massive, fine to middle sandstones interbedded with subordinated thin mudstone levels, with characteristically greyish and bluish colors. Fossil content includes more than 80 species of marine invertebrates, infrequent vertebrates, and traces. Biogeographic similitudes of the fossil assemblage are with correlative units in western Argentina, and with the Monte León Formation along the Atlantic coast of Santa Cruz province. Stratigraphy, facies analysis and the fossils suggests a direct Atlantic connection, and the isolation of the basin from the western Pacific depocenters by the existence of an orogenic belt already uplifted.
Problems and concepts of paleopedology in Argentina
Quaternary International, 1998
Pedogenesis has played a major role in the evolution of the Pampean landscape which resulted in the common occurrence of pedogenic features in the stratigraphic successions, but has not been recognized in previous studies. The main problems facing paleopedological research are the identification of paleosol profiles, paleosol horizons and their interpretation. Paleosols have been traditionally interpreted as truncated profiles because A and E horizons were not recognized. This may be apparent or real. Erosion generally leaves supporting evidence such as a lag deposit, but superposed cycles of pedogenesis can obliterate original morphological properties, which alters the appearance of buried A and E horizons. Abrupt boundaries have always been interpreted as erosional surfaces because of an overgeneralization that all land surfaces are eroded. When paleosols are exposed in roadcuts and quarries, they acquire new morphological features such as induration and tension cracks, which have been misinterpreted as original properties. In the current view, the basic criteria for paleosol recognition in exposures are based on the vertical and lateral arrangements of soil morphological features within a detailed stratigraphic framework provided by sedimentary facies analysis. The framework can be verified by micromorphological studies of polygenetic horizons to determine the sequence of episodes of pedogenesis and to evaluate the effects of diagenesis following burial.
Micromorphology of paleosols at the continental border of the Buenos Aires province, Argentina
Revista Mexicana De Ciencias Geologicas, 2004
The sediments with loessial characteristics of the Pampean plain are called "loess y limos pampeanos" (Pampean loess and silts), to distinguish those of purely eolian origin (loess) from those reworked by water (silts). In the continental border of La Plata River, mainly loess-like sediments of the late and middle Pleistocene outcrop, approximately at 20 km from the shoreline. Between that border and the shoreline they underlie at 2-5 m depth the Holocene sediments deposited by the regressive events in a successive lowering of sea level during the late 6,000 years. The objectives of this work are: a) to integrate previous and new information on several sedimentary successions, b) to study the micromorphological features of the paleosols and c) to carry out a regional correlation of paleosols between the localities of San Pedro and La Plata based on field and micromorphological features. The profiles are between 18.5 and 12.5 m a.s.l. with macroscopic pedological features which show partially continuous development. The buried paleosols are superposed and welded, with different degrees of pedogenesis indicated by macro-and micromorphological features of illuviation and hydromorphism. In some cases, these features would have formed simultaneously with the deposition of the eolian dust trapped by grass. The degree of pedogenesis would depend on the ratio between the intensity of the accretion and reworking processes, and pedogenesis. The sedimentary units were affected partially or totally by pedogenesis. Two zones rich in volcanic glass were detected at San Pedro profile; new information reveals the same situation near La Plata city. The uppermost zone, with 20-30% volcanic glass, constitutes in some cases the parent material of the present soil; the deeper zone, contains 50-70% volcanic glass. Micromorphological observations confirm the presence of scarce to abundant clasts of old illuvial horizons and loess embedded in the matrix of most of the paleosols. Pedological processes are most evident in the deeper part of the profiles, indicated by a strong grade of structure and abundant laminated and juxtaposed textural and amorphous features. The lower paleosols can be considered pedostratigraphic units useful for correlation in the continental border in northeastern Buenos Aires Province, Argentina.
Journal of South American Earth Sciences, 2002
A combined macromorphological and micromorphological approach was used to identify and differentiate paleosols at a representative exposure of the Pleistocene sedimentary succession in the northern Pampas of Buenos Aires (La Plata area). Five pedological units (Pu), which apparently represent four discrete paleosols plus the surface soil, were initially differentiated on the basis of field-scale morphological properties. The succession was divided into B and C horizon with an A horizon only clearly identified in the surface soil and a weak A horizon at depth. Micromorphology suggests a complex pedosedimentary history of welding, with some degree of water reworking indicated by fragments of sorted layers and the significant grain-size heterogeneity of the parent material. The micromorphological data do not support the field differentiation of Pu4, Pu3, and Pu2 into discrete paleosols. Pedological features (i.e. excrements, secondary carbonate coatings, illuvial clay coatings) occur throughout without any obvious breaks or patterns. Pu4, Pu3, and Pu2 are therefore interpreted as an accretionary and/or welded pedocomplex. The Gorina section does not conform to the simple classical model proposed by other authors in the region of alternating loess and paleosol units associated with arid (loess deposition) and wet (soil formation) intervals, respectively. q
Andean Geology, 2015
Early Miocene shallow marine deposits in the region of Lago Posadas-Meseta Belgrano (Argentina) represent part of the "Patagoniense" transgression, an Atlantic marine incursion that flooded large part of Patagonia, including the Austral (foreland) Basin (southern Patagonia). These deposits, referred as El Chacay (Argentina) or Guadal (Chile) formations, and the transition to the overlying Santa Cruz Formation were divided into six facies: subtidal sandbars, shallow marine sandy deposits, muddy shelf deposits, estuarine complex deposits, fluvial channels and fluvial floodplains. These are arranged in a general transgressive-regressive cycle, subdivided into two stratigraphic sequences, separated by a major erosional surface. 87 Sr/ 86 Sr ages from shell carbonate in eight oysters yielded an age range of 20.3 to 18.1 Ma for these "Patagoniense" deposits. Correlation with other dated "Patagoniense" sections in southern Patagonia, like those at Lago Argentino or Comodoro Rivadavia, indicates that they belong to a single transgression that flooded several Patagonian basins approximately at the same time. Eustasy, flexural subsidence created by tectonic loading in the adjacent fold-and-thrust belt, and basin floor paleo-topography controlled the duration of the depositional event and the sedimentation style of these shallow marine deposits.
Advances in Geomorphology and Quaternary Studies in Argentina, 2019
The present contribution involves deposits of continental Neogene age that outcrop in the Bahía Blanca region. The studies that were carried out in order to contribute to the paleoenvironmental and chronostratigraphical knowledge involve sedimentological, paleomagnetic and magnetic susceptibility analyses. One of the outcrops studied (Cueva de Los Leones) is mainly composed of facies of fine to thick sandstones, largely pedogenized and with important amounts of calcrete, whereas the other (Loma Sarmiento) has a predominance of loess-like silt facies, reddish brown in colour, an abundance of rhizoconcretions and evidence of pedogenic processes. Both outcrops culminate in an important layer of calcrete of regional scope, with particular morphologies at each of the sites. The paleomagnetic analysis leads to the conclusion that the sedimentation of the deposits underlying the calcareous crust at the two sites was not synchronous. Reverse polarity was recorded in the Cueva de Los Leones in contrast to Loma Sarmiento, where normal polarity is predominant. The paleomagnetic samples of calcareous crust on the top presented anomalous polarity directions in both sections, with very high values of magnetic intensity. The reverse directions in the Cueva de Los Leones can be assigned to the Gilbert Cron (6.0-3.6 Ma), without discarding younger ages in the Pliocene. In contrast, Loma Sarmiento seems to be older (Late Miocene). The susceptibility records also indicated differences between the two sites that would be linked to pedogenic development; they were higher in Loma Sarmiento and judging by the F% factor, in Cueva de los Leones the superparamagnetic particles contribution is higher. Petrographic, X-Ray difractometry, paleontological and soil micromorphology analyses, in some places at the studied sites, also contributed to the paleoenvironmental/paleoclimatic reconstruction carried out.
Late Pleistocene-Holocene History of Chaco-Pampa Sediments in Argentina and Paraguay
E&G Quaternary Science Journal
A substantially improved reconstruction of the Late Pleistocene-Holocene morphological and geological history of the Paraguayan Chaco and the Argentine Pampa Plain (Chaco-Pampa Plain) is presented. Due to the vast extension of the area, satellite images are the clue to synthesize previous and new multidisciplinary geoscientific results to set up a more reliable regional picture. For this synoptic view the interpretation of remote sensing data was complemented by ground investigation and physical age determinations. Luminescence ages (75 IRSL and 12 TL) of loess, loessoids and sands were determined to reconstruct the climatic history of the study area in the Marine Isotopic Stages (MIS) 3 to 1. Loess deposition dominated the period prior to MIS 2. For MIS 2 and MIS 1 the numerical ages of lacustrine and alluvial sediments intercalating the loess provide evidence for alternating humid and dry periods in the Chaco-Pampa Plain. Prevailing dry climate lasted from 8.5 to 3.5 ka (middle MIS 1) when sandy sediments deposited as dunes or filled palaeoriver beds. Temporarily limited phases of palaeoriver activity were related to sporadic torrential rainfall in the Andes Mountains. Source regions of loess, loessoid and sandy deposits were localized in the southwestern Pampa and the neighbouring Andean slopes and the Altiplano. These sediment were transported towards east and later northeast as reconstructed by the morphological features and remnants of past aeolian activity periods. (Spätpleistozän-holozäne Geschichte der Chaco-Pampasedimente in Argentinien und Paraguay) Kurzfassung: Eine erheblich verbesserte Rekonstruktion der spätpleistozänen/holozänen morphologischen und geologischen Geschichte des paraguayischen Chaco und der argentinischen Pampa Ebene wird vorgestellt. Wegen der großen Ausdehnung des Gebietes waren Satellitenbilder der Schlüssel, um frühere und neue interdisziplinäre geowissenschaftliche Ergebnisse zu einem verlässlicheren Bild zusammenzuführen. Für diesen synoptischen Überblick wurde die Interpretation von Fernerkundungsdaten durch Geländeuntersuchungen und physikalische Altersbestimmungen ergänzt. Viele Lumineszenz alter (75 IRSL und 12 TL) von Löß, lößähnlichen Sedimenten und Sanden wurden bestimmt, die bei der Rekonstruktion der Klimageschichte des Untersuchungsgebiets im Marinen Isotopen Stadium 3 bis 1 (MIS 3-1) beitragen. Lößablagerungen überwiegen in der Zeit vor MIS 2. Die numerische Alter von lakustrinen und alluvialen Sedimenten, im Löß zwischengelagert, belegen wechselnde feuchte und trockene Perioden in der Chaco/Pampa-Ebene im MIS 2 und MIS 1. Überwiegend trockene Bedingungen herrschten von 8.5 bis 3.5 ka BP (mittleres MIS 1), als Sande in Form von Dünen oder in Paläoflussbetten abgelagert wurden. Zeitlich begrenzte Phasen extrem verstärkter Paläoflussaktivität im Mittelholozän wurden auf sporadische Starkregenereignisse in den Anden zurückgeführt. Die Ursprungsgebiete des Löß, der lößartigen Sedimente und der sandigen Ablagerungen wurden in der südwestlichen Pampa, den benachbarten Andenabhängen und im Altiplano lokalisiert. Diese Sedimente wurden von dort nach Osten und später nach Nordosten transportiert, wie sich aus den morphologischen Mustern als Zeugnisse früherer äolischer Aktivität rekonstruieren ließ.
Journal of South American Earth Sciences, 2005
The stratigraphy and sedimentology of the Oligocene Río Leona Formation (Austral/Magallanes Basin) is described and interpreted on the basis of a study of two sections located in the area close to Viedma and Argentino Lakes in southern Argentina. This nonmarine unit unconformably rests on top of the marine Eocene Man Aike Formation and is transitionally covered by the shallow marine, latest Oligocene–Early Miocene Centinela Formation. The Río Leona Formation is composed of varying percentages of conglomerates, sandstones, and mudstones, as well as thin but conspicuous levels of carbonaceous shales and volcaniclastic beds. The rocks represent sedimentation in different fluvial systems. Sedimentological data indicate a progressive change in the fluvial styles from high-energy braided systems to low-energy meandering and anastomosed systems. The landscape is envisaged to have evolved during the Oligocene from a high gradient piedmont area to a low-lying coastal plain. The Patagonian Sea flooded the area by the end of the Oligocene (22–25 My). Within the framework of the nonmarine sequence stratigraphic scheme for foreland basins, the Río Leona Formation deposits represent good examples of degradational, transitional, and aggradational systems tracks. The development of a basal unconformity and the erosion of the Eocene rocks suggest an initial uplift of the source area to the west. Changes in the fluvial systems indicate an increase of the accommodation space. The Oligocene evolution of this part of the Austral Basin probably reflects a combination of tectonic processes and eustatic sea-level changes. The former may be related to an episode of uplift in the southern Andes, followed by a period of tectonic quiescence and active volcanism with strong subsidence typical of foreland basin evolution. The latter may be linked to late Paleogene glaciation–deglaciation events in Antarctica.Sobre la base del estudio de dos afloramientos ubicados en el área de los lagos Viedma y Argentino (Provincia de Santa Cruz, Argentina), se describe y analiza la estratigrafía y sedimentología de la Formación Río Leona, Oligoceno de la Cuenca Austral o de Magallanes. Esta unidad sedimentaria de origen no marino, se apoya en discordancia sobre las sedimentitas marinas eocenas de la Formación Man Aike y es cubierta en forma transicional por sedimentitas marinas litorales de la Formación Centinela asignadas al intervalo Oligoceno tardío-Mioceno temprano. La Formación Río Leona está compuesta por conglomerados, areniscas y fangolitas en variadas proporciones y delgados pero conspicuos niveles carbonosos y volcaniclásticos. Estas rocas representan sedimentación en distintos tipos de sistemas fluviales que evolucionaron desde entrelazados de alta energía hasta meandrosos y anastomosados de baja energía. La sucesión temporal de los mismos permite proponer que la paleogeografía de esta porción de la cuenca evolucionó durante el Oligoceno desde un área pedemontana con altos gradientes hacia una planicie costera baja que finalmente es inundada por el mar ‘Patagoniano’ hace unos 22 a 25 Ma. Dentro del esquema de la estratigrafía secuencial para secuencias no-marinas en cuencas de antepaís, los depósitos de la Formación Río Leona representan un buen ejemplo de los cortejos depositacionales degradacional, transicional y agradacional. El desarrollo de la discordancia basal y la erosión de las rocas eocenas infrayacentes sugieren un levantameinto inicial del área de aporte que se encontrada ubicada hacia el Oeste. Los cambios registrados en los sistemas fluviales indican un progresivo incremento el espacio de acomodación dentro de la cuenca. La evolución de esta parte de la Cuenca Austral durante el Oligoceno refleja una combinación de factores que involucran eventos tectónicos y cambios eustáticos en el nivel del mar. Los primeros pueden relacionarse con un evento de estructuración y ascenso en los Andes Patagónicos australes seguido por un período de calma tectónica, volcanismo activo y fuerte subsidencia en la cuenca de antepaís. Los segundos, en cambio, podrían estar relacionados con los episodios de glaciación y deglaciación ocurridos durante el Paleógeno tardío en Antártida.