Geocronología

Here we report a detailed archaeomagnetic and rock-magnetic study of a pottery kiln from Burgos (Spain) to reconstruct its burning conditions and date its last use and abandonment age. During the course of a rescue archaeological excavation carried out in 2015 in the center of Burgos city, a medieval pottery workshop was discovered. Two well-preserved kilns appeared and archaeomagnetic analyses were performed on one of them. In addition to a large amount of pottery remains, some numismatic and documental evidences provided a general chronological estimation, but the abandonment age of the workshop remains unknown. On the basis of the existing archaeological information we carried out an archaeomagnetic study in order to date its last use. 69 archaeomagnetic samples were collected from the combustion chamber and the kiln's fire tunnel. Stepwise alternating field and thermal demagnetization of the natural remanent magnetization (NRM) were carried out to retrieve the mean direction. Additional experiments consisted in the acquisition of isothermal remanence (IRM), low-field magnetic susceptibility and its anisotropy as well as thermomagnetic curves. Despite the high temperatures expected in the combustion chamber, it appeared that the bricks´ samples from the fire tunnel exhibit the most successful directional results. Magnetite and variable contributions of hematite are the main ferromagnetic minerals observed in the thermomagnetic curves. The type of lithology studied, its previous magnetic history and their location in the kiln strongly condition the observed directional and rock-magnetic results. Additionally, AMS data revealed the manufacturing fabric of the fire tunnel's bricks, showing moderately high anisotropy degrees, but not enough to casts doubts on the directional NRM record. In order to test the reproducibility of the dating results, archaeomagnetic dating was carried out using different geomagnetic field models and the Iberian secular variation curve. The small differences observed in the dating results are mainly due to the density and type of input data of these records. The combination of the archaeomagnetic analyses with the archaeological and documental suggests that last kiln's usage took place during the first half of the XVIth century CE. Overall, this paper illustrates how the combination of archaeological data and archaeomagnetic analyses may improve our understanding about the manufacturing processes, use and age of abandonment of archaeological combustion structures.

Abstract. The Eastern Sierras Pampeanas system spreads over a vast centerwest area of Argentina forming the central Andean foreland; it presents plain surfaces, paleosurfaces of erosive origin their mountain summits, called “Pampas de Altura”. The Cenozoic stratigraphic studies in these areas are punctual, perhaps, product of the lack of continuity of stratigraphy and the difficult access to areas of study, being relegated to traditional research inside the Pampas of Argentina. This work aims to perform a comprehensive analysis of stratigraphic registry of this “Pampas de Altura”, with an integrated vision, in order to analyze their paleoclimatic, paleoenvironmental and geochronological significance and provide a framework for future studies in the region. The stratigraphic and sedimentary analysis identified five facies within Cordoba's Pampas de Altura. This is to say: 1) sandy-silt with calcium carbonate megatabiques, 2) red silty-sand with clasts, 3) laminated and
cemented silty-sands, 4) yellowish massive sandy-silt with clasts and 5) dark massive sandy silt. These facies were grouped into associated fluvial/Aeolian facies. These facies were included in a lithostratigraphic unit named Vaca Corral Formation, and has an age between 37,095 ± 2,020 (OSL) and 7,206 cal. (AMS) yr. BP. The evidence allowed hypothesizing a paleoenvironment similar to the current savannah and/or grassland for the Pampa de Altura's, discussed in this work, during the Late Pleistocene period and most of the Holocene period as well. Vegetal communities would have been dominated by grasslands, smallsize bushes and gallery forests. Great rivers tracks were not registered and probably seasonal floods were frequent at that time. From the middle Holocene period climate change to more wet and cold conditions is registered, with a replacement in the dominance of grasses by trees and shrubs. These conditions would have remained up to 1,500 year BP. The evidence discussed here, lithological, pedogenetic and temporary, permits us to hypothesize a possible resilience of
the mountain system; which became less susceptible to drastic climate changes.

Resumen. El sistema de Sierras Pampeanas Orientales se extiende en un amplio sector del centro-oeste de la Argentina conformando el antepaís andino central y posee superficies de aplanamiento de origen erosivo en las áreas cumbrales, denominadas “pampas de altura”. Los estudios estratigráficos cenozoicos en estas áreas son puntuales, producto quizás de la poca continuidad de la estratigrafía y el difícil acceso a las áreas
de estudio, siendo relegadas a investigaciones tradicionales dentro de la llanura pampeana argentina. Se pretende con este trabajo, realizar un análisis comprensivo del registro estratigráfico de estas “pampas de altura”, con una visión integradora, que permita analizar su significado paleoclimático, paleoambiental y geocronológico y aportar un marco para futuros estudios en la región. El análisis estratigráfico y sedimentario permitió identificar 5 facies en las pampas de altura de las Sierras Pampeanas de Córdoba: 1- Limo arenoso con megatabiques de carbonato de calcio; 2- Arena limosa pardo rojiza con clastos; 3- Arena limosa laminada y cementada; 4- Limo arenoso amarillento; y 5-Arena limosa oscura maciza. Estas facies fueron agrupadas en asociación de
facies fluviales/eólicas. Se incluyeron a estas facies en nueva formación denominada Vaca Corral y tiene una edad entre los 37,095 ± 2,020 (OSL) y 7,206 cal. (AMS) años AP. La evidencia permitió hipotetizar un paleoambiente análogo a las sabanas y/o praderas actuales para las pampas de altura tratadas en este trabajo durante el Pleistoceno Tardío y gran parte del Holoceno. Las comunidades vegetales habrían estado dominadas por pasturas, arbustos de pequeño porte y bosques en galería. No se registraron grandes cauces de ríos y probablemente fueron frecuentes los anegamientos estacionales poco profundos. A partir del Holoceno medio se registra un cambio climático a condiciones más húmedas y frías, con un remplazo en la dominancia de las gramíneas
por árboles y arbustos. Estas condiciones se habrían mantenido hasta 1,500 años AP. La evidencia discutida aquí, litológica, pedogenética y temporal, permiten hipotetizar una posible resiliencia del sistema serrano, que lo hizo menos propenso a cambios climáticos muy drásticos.

El estudio de los glaciares tropicales es una temática de gran importancia para las áreas de paleo-clima, gestión de riesgos naturales, manejo del recurso agua, generación de energía, etc. El presente trabajo muestra algunos adelantos en el estudio de la dinámica glaciar cuaternaria en Los Andes Centrales de Mérida. La reconstrucción de los glaciares deducida de la geocronología con el 10 Be in-situ, contribuyen con el conocimiento climático regional y global desde el Último Máximo Glaciar. En el valle de Mucubají, el avance máximo del glaciar ocurrió hace más de18000 años y el retiro definitivo hace menos de 9000 años. El proceso transcurrió en dos grandes etapas. El glaciar retrocedió durante el Pleistoceno de ~3500 m a ~3850 m. La parte superior del glaciar estuvo confinado y desapareció rápidamente durante el Holoceno. En Los Zerpa el avance máximo ocurrió hace aproximadamente 13000 años.

Este trabajo tiene como finalidad presentar la concepción de la magnitud "tiempo" en las ciencias geológicas, lo cual es fundamental para comprender la evolución de la Tierra, ya que los procesos y eventos geológicos terrestres, han ocurrido desde hace cientos y miles de millones de años. Se refieren los intentos científicos para calcular la edad de la Tierra, los cuales están basados en: las velocidades de depositación de las secuencias sedimentarias, la salinidad de los mares, el método de flujo y en la radiactividad, siendo este último empleado en la actualidad para realizar las estimaciones absolutas de las edades de las rocas y de la Tierra. Así mismo, se presentan los métodos de cronología absoluta y relativa para establecer la secuencia de eventos geológicos para poder reconstruir el pasado de una región determinada de la corteza terrestre, utilizándose ambos métodos de manera complementaria con la finalidad de que la historia geológica propuesta sea aceptable y confiable científicamente.

Disponível on-line no endereço www.igc.usp.br/geologiausp-103-Resumo O Cinturão Salvador-Esplanada-Boquim (CSEB) é provavelmente um ramo nordeste do Orógeno Itabuna-Salvador-Cu-raçá. Neste trabalho, inédito para a região, é apresentada a caracterização petrográfica, petroquímica e geocronológica preliminar das rochas granulíticas da porção central-norte desse cinturão, onde foram separadas quatro unidades granulíti-cas ácidas e intermediárias, além de bandas de granulitos máficos, granulitos aluminosos e quartzitos. As quatro unidades ácidas e intermediárias são ortogranulitos foliados onde os granulitos máficos encontram-se encaixados sob a forma de lentes. Essas rochas foram atravessadas por diques orosirianos, dacíticos-riolíticos (Diques de Arauá). O estudo litogeoquí-mico dos granulitos ácidos e intermediários revelou quatro séries cálcio-alcalinas de baixo a alto K, assim denominadas: enderbítica (Ed1), enderbítica (Ed2); charnockítica de composição monzogranítica (MCh) e charnoenderbítica (Ch-Ed). Diagramas multielementares indicam que as quatro unidades possuem anomalias negativas de Nb, Ti e P e baixos teores de high field strenght elements (HFSE), sugerindo haver associação genética ligada a zonas de subducção e arcos vulcânicos. Os granulitos máficos, por sua vez, apontam para uma filiação toleítica com características tipo island-arc tholeiite (IAT). Os dados geocronológicos indicam idades que sugerem a presença de eventos tectônicos ocorridos na área, a saber: forma-ção do protólito do granulito (Ed2) em 2,90 Ga (idade-modelo T DM-Nd); cristalização do granulito (Ed2) em 2582 ± 11 Ma e do MCh em 2473 ± 13 Ma; metamorfismo regional com idades entre 2087 e 2073 Ma; e preenchimento de falhas em forma de diques em torno de 2015 ± 12 Ma. Os dados obtidos, sobretudo aqueles da litogeoquímica, estão permitindo interpretar que essas rochas metamórficas de alto grau se adequam àquelas do tipo arco vulcânico. Essas, ao colidirem no Paleoprote-rozoico com o Bloco Serrinha, situado a oeste da área estudada, formaram uma cadeia de montanhas, a qual, ao ser erodida, expôs suas raízes granulíticas. Palavras-chave: Granulito; Geoquímica; Geocronologia; Cinturão Salvador-Esplanada-Boquim. Abstract The Salvador-Esplanada-Boquim Belt is probably a northeastern branch of the Itabuna-Salvador-Curaçá Orogen. This study, which is unprecedented in the region, presents the petrographic, petrochemical and geochronological characterization of granulite rocks from the central-northern portion of this belt, where four acidic and intermediate granulite units were separated , in addition to mafic granulite bands, aluminous granulites and quartzites. These four acidic and intermediate units are foliate orthogranulites, where mafic granulites are embedded in the form of lenses. These rocks were crossed by orosirian, dacitics-rhyolite dykes, (Arauá Dykes). The lithogeochemical study of acidics and intermediate granulites has revealed four calcium-alkaline series from low to high K, named: (i) enderbitic (Ed1), (ii) enderbitic (Ed2); (iii) charnockitic of monzogranitic composition (MCh) and (iv) charnoenderbitic (Ch-Ed). Multielement diagrams indicate that the four units have negative Nb, Ti and P anomalies and low levels of HFSE, suggesting that there is a genetic association attached to subduction zones and volcanic arcs. On the other hand, the mafic granulites indicated a tholeiitic affiliation with charac-As raízes granulíticas do cinturão Salvador-Esplanada-Boquim,