Pub Date : 2020-05-29DOI: 10.5027/ANDGEOV47N2-3214
I. Méndez-Bedia, G. Gallastegui, P. Busquets, S. N. Césari, C. Limarino, E. Prats, R. Cardó, F. Colombo
In the Argentinian Andes (Frontal Cordillera) the upper part of the late Carboniferous-early Permian San Ignacio Formation is made up of lacustrine-palustrine microbial carbonates and interbedded volcanic deposits. In this lacustrine-palustrine environment a natural monospecific forest was developed. The deposits of this sedimentary-volcanic succession were repeatedly subjected to subaerial exposure and modified by pedogenesis to varying degrees giving rise to paleosoils development. Diagenetic microfabrics were well preserved in the carbonates and volcanic rocks. The carbonate microfabrics comprise a wide spectrum of features consisting of root marks and stumps-related structures (rhizoliths, alveolar texture, tunnel-like structures and coprolites of arthropods), pisoids, coated grains and pseudomicrokarst, cracking, brecciated and nodular fabrics, and grainification also occur corresponding to different stages in the pedogenic evolution. Meteoric dissolution and cementation processes are observed; examples are well identified by scanning electron microscope showing silica-filled voids in partially dissolved carbonates and growths of inorganic carbonate microcrystals or of microbial origin in voids. Other different types of cements can be seen such as discontinuous carbonate crusts, ribbon spar, cavities with silt infillings and pendant cements. The whole set of these microfabrics are indicative of wetting, desiccation and meteoric conditions (vadose and phreatic). The abundance of plant roots and associated micro-organisms mainly of bacterial origin (micro-rods, short rod-shapes, nano-fibres, filaments and nano-spheres) played an important role in the pedogenic and subaerial diagenetic processes affecting these deposits. The immature character of the paleosoils and absence of calcretes point out to short intervals of subaerial exposure due to oscillating fluctuations in water level, intermittent volcanic supply, tectonic subsidence and oscillating climatic conditions. The whole of the macro and microfabrics reveals that the prevailing weather could correspond to an intermediate between semi-arid to sub-humid, however the alternating wetting and drying conditions in which the fossil forest developed and the abundance and diversity of micro-organisms, suggest a transition to sub-humid climate conditions.
{"title":"Pedogenic and subaerial exposure microfabrics in a late Carboniferous-early Permian carbonate-volcanic lacustrine-palustrine system (San Ignacio Formation, Frontal Cordillera, Argentina)","authors":"I. Méndez-Bedia, G. Gallastegui, P. Busquets, S. N. Césari, C. Limarino, E. Prats, R. Cardó, F. Colombo","doi":"10.5027/ANDGEOV47N2-3214","DOIUrl":"https://doi.org/10.5027/ANDGEOV47N2-3214","url":null,"abstract":"In the Argentinian Andes (Frontal Cordillera) the upper part of the late Carboniferous-early Permian San Ignacio Formation is made up of lacustrine-palustrine microbial carbonates and interbedded volcanic deposits. In this lacustrine-palustrine environment a natural monospecific forest was developed. The deposits of this sedimentary-volcanic succession were repeatedly subjected to subaerial exposure and modified by pedogenesis to varying degrees giving rise to paleosoils development. Diagenetic microfabrics were well preserved in the carbonates and volcanic rocks. The carbonate microfabrics comprise a wide spectrum of features consisting of root marks and stumps-related structures (rhizoliths, alveolar texture, tunnel-like structures and coprolites of arthropods), pisoids, coated grains and pseudomicrokarst, cracking, brecciated and nodular fabrics, and grainification also occur corresponding to different stages in the pedogenic evolution. Meteoric dissolution and cementation processes are observed; examples are well identified by scanning electron microscope showing silica-filled voids in partially dissolved carbonates and growths of inorganic carbonate microcrystals or of microbial origin in voids. Other different types of cements can be seen such as discontinuous carbonate crusts, ribbon spar, cavities with silt infillings and pendant cements. The whole set of these microfabrics are indicative of wetting, desiccation and meteoric conditions (vadose and phreatic). The abundance of plant roots and associated micro-organisms mainly of bacterial origin (micro-rods, short rod-shapes, nano-fibres, filaments and nano-spheres) played an important role in the pedogenic and subaerial diagenetic processes affecting these deposits. The immature character of the paleosoils and absence of calcretes point out to short intervals of subaerial exposure due to oscillating fluctuations in water level, intermittent volcanic supply, tectonic subsidence and oscillating climatic conditions. The whole of the macro and microfabrics reveals that the prevailing weather could correspond to an intermediate between semi-arid to sub-humid, however the alternating wetting and drying conditions in which the fossil forest developed and the abundance and diversity of micro-organisms, suggest a transition to sub-humid climate conditions.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":" ","pages":""},"PeriodicalIF":0.9,"publicationDate":"2020-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49588545","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-05-29DOI: 10.5027/andgeov47n2-3218
Ricardo M. Gómez, Maísa A. Tunik, S. Casadío
The Upper Cretaceous nonmarine deposits of the Neuquén Basin have an important regional exposure. These deposits are included in the Neuquén Group, a well-studied unit in both the south and central part of the basin. However, the northernmost exposed between the Laguna del Diamante and the Atuel River-assigned to the Diamante Formation-have not been studied in detail. In the studied area, the Diamante Formation corresponds to a braided fluvial system with moderate sinuosity evolving through time towards an anastomosing fluvial system. Petrographic analyses indicate that sandstones belong to feldspatic litharenites and litharenites, while the source area indicates provenance from both a recycled orogen and a transitional arc. The latter could be linked to the exhumation of the Andean orogen located to the west and to the input from the north-eastern border of the Neuquén Basin, represented by the rocks currently exposed in the San Rafael Block. On a macroscopic scale, the presence of calcareous lithic fragments in the mid and upper part of the surveyed stratigraphic section, strongly suggest an input from the west. This indicates an important change in the polarity of the basin and the presence of a deformation front located to the west of the study area in accordance to previous proposals in equivalent deposits to the south of study area.
Neuquén盆地的上白垩纪非海相矿床具有重要的区域暴露。这些矿床包含在Neuquén群中，这是盆地南部和中部一个经过充分研究的单元。然而，尚未详细研究迪亚曼特组的Laguna del Diamante和Atuel河之间暴露的最北端。在研究区域，Diamante组对应于一个具有中等弯度的辫状河流系统，随着时间的推移，该系统逐渐演变为网状河流系统。岩石学分析表明，砂岩属于长石岩屑砂岩和岩屑砂岩，而源区表明来源于再循环造山带和过渡弧。后者可能与位于西部的安第斯造山带的剥露和来自Neuquén盆地东北边界的输入有关，以目前暴露在圣拉斐尔区块的岩石为代表。在宏观尺度上，所测地层剖面的中上部存在钙质岩屑，强烈表明有来自西方的输入。这表明盆地极性发生了重要变化，并且根据研究区域南部等效矿床的先前建议，研究区域西部存在变形锋。
{"title":"Fluvial deposits of the Diamante Formation as a record of the Late Cretaceous tectonic activity in the Southern Central Andes, Mendoza, Argentina","authors":"Ricardo M. Gómez, Maísa A. Tunik, S. Casadío","doi":"10.5027/andgeov47n2-3218","DOIUrl":"https://doi.org/10.5027/andgeov47n2-3218","url":null,"abstract":"The Upper Cretaceous nonmarine deposits of the Neuquén Basin have an important regional exposure. These deposits are included in the Neuquén Group, a well-studied unit in both the south and central part of the basin. However, the northernmost exposed between the Laguna del Diamante and the Atuel River-assigned to the Diamante Formation-have not been studied in detail. In the studied area, the Diamante Formation corresponds to a braided fluvial system with moderate sinuosity evolving through time towards an anastomosing fluvial system. Petrographic analyses indicate that sandstones belong to feldspatic litharenites and litharenites, while the source area indicates provenance from both a recycled orogen and a transitional arc. The latter could be linked to the exhumation of the Andean orogen located to the west and to the input from the north-eastern border of the Neuquén Basin, represented by the rocks currently exposed in the San Rafael Block. On a macroscopic scale, the presence of calcareous lithic fragments in the mid and upper part of the surveyed stratigraphic section, strongly suggest an input from the west. This indicates an important change in the polarity of the basin and the presence of a deformation front located to the west of the study area in accordance to previous proposals in equivalent deposits to the south of study area.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":" ","pages":""},"PeriodicalIF":0.9,"publicationDate":"2020-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46876853","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-05-29DOI: 10.5027/andgeov47n2-3057
Jorge Quezada, E. Jaque, N. Catalán, A. Belmonte, A. Fernández, F. Isla
The Tirúa-Mocha Island area (38.2°-38.4° S) in southern Chile has been affected by two megaearthquakes in only 50 years: the 1960 Mw=9.5 Valdivia earthquake and 2010 Mw=8.8 Maule earthquake. We studied in the field the vertical ground movements occurred during the interseismic period between both earthquakes and the coseismic period of 2010 Maule earthquake and 2011 Mw=7.1 Araucanía earthquake. During the 1960 earthquake, vertical coseismic ground movements are typical of subduction related earthquakes with Mocha Island, located close to the trench, experienced bigger ground uplift (150 cm) than that occurred in Tirúa (-20 cm), place located in the continental margin at the latitude of Mocha Island. Then during the 1960-2010 interseismic period, the 1960 coseismic uplift remained at Mocha Island unlike the normal interseismic subsidence that occurred northward at Arauco Peninsula and Santa María Island. Also Tirúa experienced the biggest interseismic uplift (180 cm) in all the area affected later by 2010 Maule earthquake. Then during the 2010 Mw=8.8 Maule earthquake an anomalous vertical coseismic ground uplift occurred in the study area, opposite to that of 1960 since Mocha Island experienced lower (25 cm) ground uplift than Tirúa (90 cm). Subsequently, during the Araucanía 2011 earthquake a ground uplift in Mocha Island (50 cm) and subsidence at Tirúa (20 cm) occurred. These unexpected vertical ground movements can be explained by the existence of an upper plate splay fault located below the sea bottom between Tirúa and Mocha Island: the Tirúa-Mocha splay fault. Considering the last seismic cycle, the activity of this fault would have started after the 1960 Valdivia earthquake. During 2010 Maule earthquake, the main slip occurred at Tirúa Mocha splay fault. Finally during 2011 Araucanía earthquake, the slip occurred mainly at the updip of Wadati-Benioff plane with probable normal activity of Tirúa-Mocha splay fault. Simple elastic dislocation models considering the Wadati-Benioff plane and the Tirúa-Mocha splay fault activity, can account for all the vertical ground movements observed during 1960 earthquake, the 1960-2010 interseismic period, the 2010 Maule earthquake and the 2011 Araucanía earthquake.
{"title":"Unexpected coseismic surface uplift at Tirúa-Mocha Island area of south Chile before and during the Mw 8.8 Maule 2010 earthquake: a possible upper plate splay fault","authors":"Jorge Quezada, E. Jaque, N. Catalán, A. Belmonte, A. Fernández, F. Isla","doi":"10.5027/andgeov47n2-3057","DOIUrl":"https://doi.org/10.5027/andgeov47n2-3057","url":null,"abstract":"The Tirúa-Mocha Island area (38.2°-38.4° S) in southern Chile has been affected by two megaearthquakes in only 50 years: the 1960 Mw=9.5 Valdivia earthquake and 2010 Mw=8.8 Maule earthquake. We studied in the field the vertical ground movements occurred during the interseismic period between both earthquakes and the coseismic period of 2010 Maule earthquake and 2011 Mw=7.1 Araucanía earthquake. During the 1960 earthquake, vertical coseismic ground movements are typical of subduction related earthquakes with Mocha Island, located close to the trench, experienced bigger ground uplift (150 cm) than that occurred in Tirúa (-20 cm), place located in the continental margin at the latitude of Mocha Island. Then during the 1960-2010 interseismic period, the 1960 coseismic uplift remained at Mocha Island unlike the normal interseismic subsidence that occurred northward at Arauco Peninsula and Santa María Island. Also Tirúa experienced the biggest interseismic uplift (180 cm) in all the area affected later by 2010 Maule earthquake. Then during the 2010 Mw=8.8 Maule earthquake an anomalous vertical coseismic ground uplift occurred in the study area, opposite to that of 1960 since Mocha Island experienced lower (25 cm) ground uplift than Tirúa (90 cm). Subsequently, during the Araucanía 2011 earthquake a ground uplift in Mocha Island (50 cm) and subsidence at Tirúa (20 cm) occurred. These unexpected vertical ground movements can be explained by the existence of an upper plate splay fault located below the sea bottom between Tirúa and Mocha Island: the Tirúa-Mocha splay fault. Considering the last seismic cycle, the activity of this fault would have started after the 1960 Valdivia earthquake. During 2010 Maule earthquake, the main slip occurred at Tirúa Mocha splay fault. Finally during 2011 Araucanía earthquake, the slip occurred mainly at the updip of Wadati-Benioff plane with probable normal activity of Tirúa-Mocha splay fault. Simple elastic dislocation models considering the Wadati-Benioff plane and the Tirúa-Mocha splay fault activity, can account for all the vertical ground movements observed during 1960 earthquake, the 1960-2010 interseismic period, the 2010 Maule earthquake and the 2011 Araucanía earthquake.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":" ","pages":""},"PeriodicalIF":0.9,"publicationDate":"2020-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45763092","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-05-29DOI: 10.5027/andgeov47n2-3131
Esteban Lannutti, M. G. Lenzano, J. Barón, M. Durand, L. Lenzano
The Puente del Inca, next to the homonymous village, is a natural bridge located on the Cuevas River, at about 2,700 m above sea level in the Central Andes of the province of Mendoza, Argentina. Declared Provincial Natural Monument in 2005, since the beginning of the 20th century it has registered a continuous weakening, mainly related to erosive processes that cause detachments of part of the material that makes up its structure. The objective of this study is to determine the factors involved in the structural regression and restitution of the natural bridge. To achieve this, visual inspection, characterization of thermal springs, measurement of accretion-erosion rate of travertine deposits, testing of materials and the development of a structural numerical model by the Finite Element Method, were carried out. Results indicate that the deterioration is linked, mainly, with changes in the geobiological system that regulates the travertine deposition and to the heterogeneous structure of the bridge. Changes of the geobiological system are related to the activity of the thermal waters present in the area, whose discontinuity and/or fluctuations in the flow rate are influenced by both natural and anthropogenic factors. The heterogeneity of the structure conditions the stability of the monument due to the presence of a less resistant material in the lower part of the bridge arch. From the numerical simulation, safety factors between 1.5 and 3 were determined for the bridge, thus, we conclude the structure is stable against its own weight, as long as the bridge maintains the current conditions.
毗邻同名村庄的Puente del Inca是一座天然桥梁，位于阿根廷门多萨省安第斯山脉中部海拔约2700米的Cuevas河上。2005年被宣布为省级自然古迹，自20世纪初以来，它一直在持续弱化，主要与侵蚀过程有关，导致构成其结构的部分材料脱落。本研究的目的是确定影响天然桥梁结构回归和恢复的因素。为了实现这一目标，进行了目视检查，表征温泉，测量石灰华沉积物的吸积-侵蚀速率，材料测试以及通过有限元方法开发结构数值模型。结果表明，这种恶化主要与调节石灰华沉积的地球生物系统的变化和桥梁的非均质结构有关。地质生物系统的变化与该地区存在的热水的活动有关，其流量的不连续性和/或波动受到自然和人为因素的影响。结构的非均质性决定了纪念碑的稳定性，因为在桥拱的下部有一种阻力较小的材料。从数值模拟中，确定了该桥的安全系数在1.5到3之间，因此，我们得出结论，只要桥梁保持当前状态，结构在自重下是稳定的。
{"title":"Análisis de los procesos de regresión y restitución del monumento natural Puente del Inca, Andes Centrales, Provincia de Mendoza, Argentina","authors":"Esteban Lannutti, M. G. Lenzano, J. Barón, M. Durand, L. Lenzano","doi":"10.5027/andgeov47n2-3131","DOIUrl":"https://doi.org/10.5027/andgeov47n2-3131","url":null,"abstract":"The Puente del Inca, next to the homonymous village, is a natural bridge located on the Cuevas River, at about 2,700 m above sea level in the Central Andes of the province of Mendoza, Argentina. Declared Provincial Natural Monument in 2005, since the beginning of the 20th century it has registered a continuous weakening, mainly related to erosive processes that cause detachments of part of the material that makes up its structure. The objective of this study is to determine the factors involved in the structural regression and restitution of the natural bridge. To achieve this, visual inspection, characterization of thermal springs, measurement of accretion-erosion rate of travertine deposits, testing of materials and the development of a structural numerical model by the Finite Element Method, were carried out. Results indicate that the deterioration is linked, mainly, with changes in the geobiological system that regulates the travertine deposition and to the heterogeneous structure of the bridge. Changes of the geobiological system are related to the activity of the thermal waters present in the area, whose discontinuity and/or fluctuations in the flow rate are influenced by both natural and anthropogenic factors. The heterogeneity of the structure conditions the stability of the monument due to the presence of a less resistant material in the lower part of the bridge arch. From the numerical simulation, safety factors between 1.5 and 3 were determined for the bridge, thus, we conclude the structure is stable against its own weight, as long as the bridge maintains the current conditions.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":" ","pages":""},"PeriodicalIF":0.9,"publicationDate":"2020-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46733153","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-31DOI: 10.5027/andgeov47n1-3200
J. M. Cabrera, P. Temporetti, F. Pedrozo
Lake Caviahue, in Patagonia (Argentina), is a very acid water body (Patagonia, Argentina) due to the influx of volcanic fluids. Over the past 18 years, the lake has been progressively alkalinizing and pH is close to the ferric iron precipitation threshold (pH>3,0). Should iron precipitate, water and sediment composition will be altered. To set a baseline, trace metal partitioning (Cu, Cd, Cr, Pb, Zn and Mn) was studied for three sediment cores (0 to 16 cm depth) at three sampling stations using a sequential extraction procedure (SEP) and the acid volatile sulfide/simultaneously extracted metals (AVS/SEM) protocol. The total metals content of the sediments ranged between 0 to 408 μg g-1 of dry sediment, with Pb>Cr>Mn>Cu>Zn>Cd. No Cd was measured above the limit of detection. The sediment was richest in Mn and Cr, two non-toxic metals at the pH / Eh combination of the lake basin. Total Pb was at a moderate contamination level although it is associated with the most recalcitrant fraction and therefore has low mobility. The sediment fractions with higher metal content were the oxidizable and residual, the former commonly associated with labile organic matter and pyrite while the latter are related to recalcitrant organic matter and rock forming minerals. In addition, a high correlation was found between Cr, Pb and Mn,thus these metals may be subject to analogous precipitation processes and possibly to co-variation in the volcano effluents. Furthermore, no metals were detected in the exchangeable/carbonates fraction, which is the most labile of all the sediment fractions. The metal content in (SEM) was likewise below the toxicity thresholds of two international sediment quality guidelines and the ratio AVS/SEM was over one, indicative of non-toxicity. Both results indicate that metal mobility in the acidic sediments of lake Caviahue is very low.
{"title":"Trace metal partitioning and potential mobility in the naturally acidic sediment of Lake Caviahue, Neuquén, Argentina","authors":"J. M. Cabrera, P. Temporetti, F. Pedrozo","doi":"10.5027/andgeov47n1-3200","DOIUrl":"https://doi.org/10.5027/andgeov47n1-3200","url":null,"abstract":"Lake Caviahue, in Patagonia (Argentina), is a very acid water body (Patagonia, Argentina) due to the influx of volcanic fluids. Over the past 18 years, the lake has been progressively alkalinizing and pH is close to the ferric iron precipitation threshold (pH>3,0). Should iron precipitate, water and sediment composition will be altered. To set a baseline, trace metal partitioning (Cu, Cd, Cr, Pb, Zn and Mn) was studied for three sediment cores (0 to 16 cm depth) at three sampling stations using a sequential extraction procedure (SEP) and the acid volatile sulfide/simultaneously extracted metals (AVS/SEM) protocol. The total metals content of the sediments ranged between 0 to 408 μg g-1 of dry sediment, with Pb>Cr>Mn>Cu>Zn>Cd. No Cd was measured above the limit of detection. The sediment was richest in Mn and Cr, two non-toxic metals at the pH / Eh combination of the lake basin. Total Pb was at a moderate contamination level although it is associated with the most recalcitrant fraction and therefore has low mobility. The sediment fractions with higher metal content were the oxidizable and residual, the former commonly associated with labile organic matter and pyrite while the latter are related to recalcitrant organic matter and rock forming minerals. In addition, a high correlation was found between Cr, Pb and Mn,thus these metals may be subject to analogous precipitation processes and possibly to co-variation in the volcano effluents. Furthermore, no metals were detected in the exchangeable/carbonates fraction, which is the most labile of all the sediment fractions. The metal content in (SEM) was likewise below the toxicity thresholds of two international sediment quality guidelines and the ratio AVS/SEM was over one, indicative of non-toxicity. Both results indicate that metal mobility in the acidic sediments of lake Caviahue is very low.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":"47 1","pages":"46-60"},"PeriodicalIF":0.9,"publicationDate":"2020-01-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47761728","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-31DOI: 10.5027/andgeov47n1-3219
Alejandro Montes, Fernando Santiago, M. Salemme, R. López
Laguna Las Vueltas (LLV) area retains the morphology of a late Pleistocene watershed that was flooded during a mid-Holocene marine transgression. Sediments associated with a paleosol dated at 22,582 cal yr BP reflect subaerial exposure of the area prior to the submergence during the marine transgression. This transgression produced an extensive tidal flat near the mouth of the former LLV watershed by 7,477 cal yr BP. Subsequent decoupling of the Las Vueltas valley from the sea occurred through the growth of a baymouth barrier and a beach-ridge plain to the east. This decoupling turned the lagoon into a pan environment in which subsequent lake-level fluctuations were controlled by climate. A lunette dune developed at the pans in the former lagoon, providing a narrow corridor where humans trapped, killed and processed guanacos as early as 3,402 cal yr BP. Changes in aeolian sedimentation hint at increased aridity during the past 500 years.
Laguna Las Vueltas（LLV）地区保留了更新世晚期分水岭的形态，该分水岭在全新世中期海侵期间被淹没。与22582 cal yr BP的古土壤相关的沉积物反映了海侵期间淹没之前该地区的陆上暴露。这一海侵在7477 cal yr BP的前LLV分水岭河口附近形成了一片广阔的滩涂。随后，Las Vueltas山谷与大海脱钩，原因是海湾河口屏障和东部海滩山脊平原的生长。这种脱钩将泻湖变成了一个泛环境，随后的湖面波动受到气候的控制。早在3402年英国石油公司（BP），前泻湖的平底锅上就形成了一个新月形沙丘，为人类捕获、杀死和加工鸟粪提供了一条狭窄的走廊。风成沉积的变化表明，在过去500年中，干旱加剧。
{"title":"Late Pleistocene and Holocene geomorphologic evolution of Laguna Las Vueltas area, Tierra del Fuego (Argentina)","authors":"Alejandro Montes, Fernando Santiago, M. Salemme, R. López","doi":"10.5027/andgeov47n1-3219","DOIUrl":"https://doi.org/10.5027/andgeov47n1-3219","url":null,"abstract":"Laguna Las Vueltas (LLV) area retains the morphology of a late Pleistocene watershed that was flooded during a mid-Holocene marine transgression. Sediments associated with a paleosol dated at 22,582 cal yr BP reflect subaerial exposure of the area prior to the submergence during the marine transgression. This transgression produced an extensive tidal flat near the mouth of the former LLV watershed by 7,477 cal yr BP. Subsequent decoupling of the Las Vueltas valley from the sea occurred through the growth of a baymouth barrier and a beach-ridge plain to the east. This decoupling turned the lagoon into a pan environment in which subsequent lake-level fluctuations were controlled by climate. A lunette dune developed at the pans in the former lagoon, providing a narrow corridor where humans trapped, killed and processed guanacos as early as 3,402 cal yr BP. Changes in aeolian sedimentation hint at increased aridity during the past 500 years.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":"47 1","pages":"61-76"},"PeriodicalIF":0.9,"publicationDate":"2020-01-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46045470","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-31DOI: 10.5027/ANDGEOV47N1-3177
Gabriel A. Casal, Patricia Vallati, L. Ibiricu, Andrea de Sosa Tomas, Nicolás Foix, José Óscar Alllard, R. Martínez
The presence of stromatolites from Cretaceous outcrops at the headwaters of the río Chico locality in the Golfo San Jorge Basin is reported for the first time. They are present in the uppermost part of the Lago Colhué Huapi Formation (Coniacian-Maastrichtian) of the Chubut Group. The presence of, up to now, four structures interpreted as stromatolites in this locality are not only important because it is the first record in the basin, but because it contributes to the knowledge of these bioconstructions in continental environments. The stromatolite called E1, which is characterized and discussed in detail, is associated with an abundant and diverse fossil record represented by palynomorphs, fragments of silicified trunks, dinosaur remains and eggshells. The study, integrated mainly with the recorded pollen and spores, is important for paleoenvironmental interpretations of the site and contributes to the paleoecological and paleoclimatic understanding of the most modern stratigraphic interval of the Chubut Group.
首次报道了Golfo San Jorge盆地río Chico地区源头白垩纪露头的叠层石的存在。它们存在于Chubut群的Lago-ColhuéHuapi组（Coniacian-Maastrichtian）的最上部。到目前为止，该地区存在四个被解释为叠层石的结构，这不仅很重要，因为这是该盆地的第一个记录，还因为它有助于了解大陆环境中的这些生物结构。被称为E1的叠层石，经过详细的表征和讨论，与丰富多样的化石记录有关，这些化石记录以坡缕石、硅化树干碎片、恐龙遗骸和蛋壳为代表。这项研究主要与记录的花粉和孢子相结合，对该遗址的古环境解释很重要，并有助于对楚布特群最现代地层间隔的古生态和古气候理解。
{"title":"Primer registro de estromatolitos en el Maastrichtiano tardío del Grupo Chubut, Cuenca del Golfo San Jorge, Patagonia central, Argentina","authors":"Gabriel A. Casal, Patricia Vallati, L. Ibiricu, Andrea de Sosa Tomas, Nicolás Foix, José Óscar Alllard, R. Martínez","doi":"10.5027/ANDGEOV47N1-3177","DOIUrl":"https://doi.org/10.5027/ANDGEOV47N1-3177","url":null,"abstract":"The presence of stromatolites from Cretaceous outcrops at the headwaters of the río Chico locality in the Golfo San Jorge Basin is reported for the first time. They are present in the uppermost part of the Lago Colhué Huapi Formation (Coniacian-Maastrichtian) of the Chubut Group. The presence of, up to now, four structures interpreted as stromatolites in this locality are not only important because it is the first record in the basin, but because it contributes to the knowledge of these bioconstructions in continental environments. The stromatolite called E1, which is characterized and discussed in detail, is associated with an abundant and diverse fossil record represented by palynomorphs, fragments of silicified trunks, dinosaur remains and eggshells. The study, integrated mainly with the recorded pollen and spores, is important for paleoenvironmental interpretations of the site and contributes to the paleoecological and paleoclimatic understanding of the most modern stratigraphic interval of the Chubut Group.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":" ","pages":""},"PeriodicalIF":0.9,"publicationDate":"2020-01-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48546609","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-31DOI: 10.5027/andgeov47n1-3112
María José J. Espeche, R. Lira, Nicolás Antonio Viñas
On-going mining operations in a marble quarry (Cantera Centro) from Malagueno, Sierra Chica de Cordoba, Argentina, have unearthed veins, veinlets and lenses of sulfides (pyrrhotite>pyrite≅ chalcopyrite>>sphalerite). These veins and lenses are up to 0.3 m thick and 2-3 m long, although intermittently can extend about a hundred meters. They are associated with skarns. The metasedimentary host sequence, largely composed of gneisses, amphibolites and marbles, was intruded by amphibolic metagabbro and metadiorite dykes, metatonalite plutons and alkali-feldspar metagranites; the whole complex was metamorphosed into medium to high amphibolite facies and strongly deformed as a result of the a regional event M2-D2/D3 that affected the Neoproterozoic-Cambrian basement during the Pampean orogeny. Except for gneisses, all the other metamorphic lithologies register evidence of differential alteration into skarn, although the process was preferentially developed on marbles, amphibolites and metagranite, and to a lesser exten on mafic and mesosilicic dykes. The metasomatic rocks are characterized by a garnet>>pyroxene skarn (Grs27-48Adr22-34Alm15-27Sps9-21), formed after the replacement of the metagranite; a pyroxene-rich skarn (Hd42-63Di32-50Jo3-5) developed after para-amphibolite, and a garnet (Adr54-71Grs22-40Alm4-7Sps1-2) (±wollastonite) skarn that replaced a calcic marble. The dykes show poor metasomatic replacement and lack sulfides of metasomatichydrothermal origin. Retrograde mineral associations include hastingsite, ferroactinolite, epidote, clinozoisite, sericite, plagioclase (An18), chamosite and calcite. The sulfide mineralization is paragenetically associated with late-stage, infilling skarn-hydrothermal minerals that were sequentially deposited as: calcite→clinochlore→quartz→pyrite→pyrrhotite→chalcopyrite+sphalerite; these phases occur as veins and veinlets within the garnetpyroxene skarn, and as massive pyrrhotite lenses in the piroxene-rich skarn. Microthermometric data from fluid inclusions in sulfide related calcite, together with the geothermometric data from the Fe/Mg ratios in clinochlore and the phase equilibria data from intergrown high and low T oC pyrrhotite phases, all constrain the infilling gangue phases and sulfide crystallization temperature within the ~360 °C to 250 °C range; the gradual termal decrease is in agreement with the temporal depositional sequence of the infilling phases. Fluid inclusion petrographic data and salinity estimations suggest that sulfide precipitation was triggered by boiling, from a fluid of moderate to high salinity (~14.5 to 33.5 wt% eq. NaCl). Evidence that the fluid evolved under dominantly reducing conditions are the high Fe+2/(Fe+2+Fe+3) ratios and molar proportions of subcalcic garnet (Alm+Sps) in garnet-pyroxene skarn, the presence of fluid inclusion sulfide daughter crystals in calcite, the high H2S/SO42- ratios in the sulfide-bearing fluid and the presence of pyrrhotite among the sulfide phases.
{"title":"Aportes a la metalogénesis del basamento proterozoico superior-cámbrico de las Sierras Pampeanas Orientales: mineralogía, termometría e isótopos de azufre del skarn Malagueño (Cu- Fe±Zn), Sierra Chica, Córdoba, Argentina","authors":"María José J. Espeche, R. Lira, Nicolás Antonio Viñas","doi":"10.5027/andgeov47n1-3112","DOIUrl":"https://doi.org/10.5027/andgeov47n1-3112","url":null,"abstract":"On-going mining operations in a marble quarry (Cantera Centro) from Malagueno, Sierra Chica de Cordoba, Argentina, have unearthed veins, veinlets and lenses of sulfides (pyrrhotite>pyrite≅ \u0000chalcopyrite>>sphalerite). These veins and lenses are up to 0.3 m thick and 2-3 m long, although intermittently can extend about a hundred meters. They are associated with skarns. The metasedimentary host sequence, largely composed of gneisses, amphibolites and marbles, was intruded by amphibolic metagabbro and metadiorite dykes, metatonalite plutons and alkali-feldspar metagranites; the whole complex was metamorphosed into medium to high amphibolite facies and strongly deformed as a result of the a regional event M2-D2/D3 that affected the Neoproterozoic-Cambrian basement during the Pampean orogeny. Except for gneisses, all the other metamorphic lithologies register evidence of differential alteration into skarn, although the process was preferentially developed on marbles, amphibolites and metagranite, and to a lesser exten on mafic and mesosilicic dykes. The metasomatic rocks are characterized by a garnet>>pyroxene skarn (Grs27-48Adr22-34Alm15-27Sps9-21), formed after the replacement of the metagranite; a pyroxene-rich skarn (Hd42-63Di32-50Jo3-5) developed after para-amphibolite, and a garnet (Adr54-71Grs22-40Alm4-7Sps1-2) (±wollastonite) skarn that replaced a calcic marble. The dykes show poor metasomatic replacement and lack sulfides of metasomatichydrothermal origin. Retrograde mineral associations include hastingsite, ferroactinolite, epidote, clinozoisite, sericite, plagioclase (An18), chamosite and calcite. The sulfide mineralization is paragenetically associated with late-stage, infilling skarn-hydrothermal minerals that were sequentially deposited as: calcite→clinochlore→quartz→pyrite→pyrrhotite→chalcopyrite+sphalerite; these phases occur as veins and veinlets within the garnetpyroxene skarn, and as massive pyrrhotite lenses in the piroxene-rich skarn. Microthermometric data from fluid inclusions in sulfide related calcite, together with the geothermometric data from the Fe/Mg ratios in clinochlore and the phase equilibria data from intergrown high and low T oC pyrrhotite phases, all constrain the infilling gangue phases and sulfide crystallization temperature within the ~360 °C to 250 °C range; the gradual termal decrease is in agreement with the temporal depositional sequence of the infilling phases. Fluid inclusion petrographic data and salinity estimations suggest that sulfide precipitation was triggered by boiling, from a fluid of moderate to high salinity (~14.5 to 33.5 wt% eq. NaCl). Evidence that the fluid evolved under dominantly reducing conditions are the high Fe+2/(Fe+2+Fe+3) ratios and molar proportions of subcalcic garnet (Alm+Sps) in garnet-pyroxene skarn, the presence of fluid inclusion sulfide daughter crystals in calcite, the high H2S/SO42- ratios in the sulfide-bearing fluid and the presence of pyrrhotite among the sulfide phases.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":"47 1","pages":"14-45"},"PeriodicalIF":0.9,"publicationDate":"2020-01-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49493481","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-31DOI: 10.5027/andgeov47n1-3195
Andrea F. Sterren, G. A. Cisterna, O. López-Gamundí, M. M. Vergel
The Agua del Jaguel Formation crops out in the southernmost part of the Calingasta-Uspallata basin, in central western Argentina. The lower part of the unit is characterized by a glacigenic sequence with diamictites and mudstones with dropstones. In the latter, elements of the Aseptella-Tuberculatella/Rhipidomella-Micraphelia Fauna (AT/RM Fauna), such as the brachiopods Rhipidomella discreta Cisterna, Micraphelia indianae Simanauskas and Cisterna and Orbiculoidea? sp., the gastropods Murchisonia? sp., and Glabrocingulum (Stenozone)? sp., the bivalve Nuculanidae indet., rugose corals, and indeterminate fragments of nautiloids and hyolithids, have been identified. The importance of this fauna mainly resides in its paleoenvironmental and biostratigraphic implications. AT/RM Fauna is characteristic of restricted environments with relatively low concentrations of oxygen and nutrients in the seafloor, which is consistent with the glaciomarine sequences in fjord-type coasts suggested for the Agua del Jaguel Formation. The relatively low diversity of the fauna in this unit compared to that defined in the El Paso Formation, located further north in the basin, might suggest more restricted sectors for benthic colonization, related to the paleovalle’s isolation from oceanic waters. The postglacial mudstones with marine invertebrate faunas of late Serpukhovian-Bashkirian age would have been deposited in relatively restricted (palaeofjord) part of the Uspallata-Calingasta basin as well as in open shelf environments. The marine flooding over drastically different coast configurations and the availability of nutrient and oxygen in the water column would have propitiated the development of faunas with important differences in the taxonomic composition and the paleoecological structure (AT/RM and Levipustula Faunas) occurring at the same time interval. Radiometric data in Agua del Jaguel Formation and paleontological records in the glacial-postglacial sedimentary succession in the basin (marine invertebrate faunas, palynomorphs and plants) are the most important tools to adjust the timing of the postglacial transgression. This information herein presented complements the scheme proposed for the Carboniferous sequences throughout the central western of Gondwana but is not sufficient to assign a more precise age of the fauna studied within the late Serpukhovian-Bashkirian interval.
Agua del Jaguel组产于阿根廷中西部Calingasta Uspallata盆地的最南端。该单元的下部以含杂岩的冰川序列和含落石的泥岩为特征。在后者中，结核足类/小缝足类动物群（AT/RM动物群）的元素，如腕足类Rhipideomella discita Cisterna、小缝足属Simanauskas和Cisterna和Orbiculoidea？sp.，腹足类Murchisonia？sp.和Glabrocingulum（Stenozone）？sp.，双壳类裸藻科。，褶皱珊瑚，以及鹦鹉螺和舌骨类的不确定碎片已经被确认。该动物群的重要性主要在于其古环境和生物地层学意义。AT/RM动物群的特征是海底氧气和营养物质浓度相对较低的受限环境，这与Agua del Jaguel组所建议的峡湾型海岸的冰川海洋序列一致。与位于盆地北部的埃尔帕索组相比，该单元的动物群多样性相对较低，这可能表明与古河谷与海水隔离有关的底栖生物定殖区更受限制。Serpukhovian Bashkirian晚期的冰川后泥岩和海洋无脊椎动物群可能沉积在Uspallata Calingasta盆地相对受限的（古峡湾）部分以及开放陆架环境中。在截然不同的海岸结构和水柱中营养物质和氧气的可用性上发生的海洋洪水会促进动物群的发展，在同一时间间隔内，动物群在分类学组成和古生态结构（AT/RM和Levipustula动物群）上存在重要差异。Agua del Jaguel组的辐射数据和盆地冰后沉积序列的古生物记录（海洋无脊椎动物、坡缕菌和植物）是调整冰后海侵时间的最重要工具。本文提供的这些信息补充了针对整个冈瓦纳大陆中西部石炭系序列提出的方案，但不足以确定晚谢尔普霍夫阶-巴什基尔阶区间内研究的动物群的更精确年龄。
{"title":"El registro más austral de la Fauna Aseptella-Tuberculatella / Rhipidomella-Micraphelia (Serpukhoviano tardío-Bashkiriano) en la cuenca Calingasta-Uspallata, Argentina","authors":"Andrea F. Sterren, G. A. Cisterna, O. López-Gamundí, M. M. Vergel","doi":"10.5027/andgeov47n1-3195","DOIUrl":"https://doi.org/10.5027/andgeov47n1-3195","url":null,"abstract":"The Agua del Jaguel Formation crops out in the southernmost part of the Calingasta-Uspallata basin, in central western Argentina. The lower part of the unit is characterized by a glacigenic sequence with diamictites and mudstones with dropstones. In the latter, elements of the Aseptella-Tuberculatella/Rhipidomella-Micraphelia Fauna (AT/RM Fauna), such as the brachiopods Rhipidomella discreta Cisterna, Micraphelia indianae Simanauskas and Cisterna and Orbiculoidea? sp., the gastropods Murchisonia? sp., and Glabrocingulum (Stenozone)? sp., the bivalve Nuculanidae indet., rugose corals, and indeterminate fragments of nautiloids and hyolithids, have been identified. The importance of this fauna mainly resides in its paleoenvironmental and biostratigraphic implications. AT/RM Fauna is characteristic of restricted environments with relatively low concentrations of oxygen and nutrients in the seafloor, which is consistent with the glaciomarine sequences in fjord-type coasts suggested for the Agua del Jaguel Formation. The relatively low diversity of the fauna in this unit compared to that defined in the El Paso Formation, located further north in the basin, might suggest more restricted sectors for benthic colonization, related to the paleovalle’s isolation from oceanic waters. The postglacial mudstones with marine invertebrate faunas of late Serpukhovian-Bashkirian age would have been deposited in relatively restricted (palaeofjord) part of the Uspallata-Calingasta basin as well as in open shelf environments. The marine flooding over drastically different coast configurations and the availability of nutrient and oxygen in the water column would have propitiated the development of faunas with important differences in the taxonomic composition and the paleoecological structure (AT/RM and Levipustula Faunas) occurring at the same time interval. Radiometric data in Agua del Jaguel Formation and paleontological records in the glacial-postglacial sedimentary succession in the basin (marine invertebrate faunas, palynomorphs and plants) are the most important tools to adjust the timing of the postglacial transgression. This information herein presented complements the scheme proposed for the Carboniferous sequences throughout the central western of Gondwana but is not sufficient to assign a more precise age of the fauna studied within the late Serpukhovian-Bashkirian interval.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":"47 1","pages":"110-124"},"PeriodicalIF":0.9,"publicationDate":"2020-01-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42956339","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-31DOI: 10.5027/andgeov47n1-3138
M. J. Andreazzini, S. Degiovanni, A. Prieto, Alfonsina Tripaldi, M. Luque
Los mallines son humedales de alto valor ambiental y son frecuentes en diversas regiones de la Argentina, como la Patagonia, la Cordillera de los Andes y las Sierras Pampeanas. En particular en esta ultima region, los estudios sobre su funcionamiento y dinamica son aun insuficientes y parciales. En este trabajo, se analizan los factores geologico-geomorfologicos que influyeron en el origen, distribucion, morfometria y dinamica de mallines del sector sur de la Sierra de Comechingones (Sierras Pampeanas de Cordoba, Argentina) y se realiza una reconstruccion paleoambiental a partir del registro sedimentologico, polinico y diatomologico de uno de estos sistemas durante el Holoceno. El area de estudio forma parte de una paleosuperficie de erosion desarrollada sobre rocas de basamento igneo-metamorfico con anterioridad a la orogenia andina, la cual se preserva en el faldeo oriental de esta sierra y presenta distinto grado de incision fluvial. El mayor desarrollo de los mallines se asocia a ambientes geomorfologicos con baja actividad morfodinamica, preferentemente sobre rocas graniticas, y depositos loessicos/loessoides. La fuente principal de los materiales que forman los depositos de mallines es la removilizacion de las secuencias loessicas/loessoides cuaternarias situadas en el sector serrano. Los registros sedimentologico, polinico y diatomologico sugieren la persistencia de estos ambientes en los sectores altos de las Sierras de Comechingones durante gran parte del Holoceno, con contracciones y expansiones como respuesta a la variabilidad ambiental. Las condiciones climaticas fueron variables, con un periodo mas seco que el actual, al menos temporariamente, entre ca. 7.000 y 4.700 cal. anos AP (6.400 y 4.100 14C anos AP) y, a partir de los 4.700 cal. anos AP (4.326 14C anos AP) se habrian establecido condiciones climaticas semejantes a las actuales. Los procesos de incision e integracion de las redes de drenaje que ocurrieron en las ultimas centurias iniciaron la desactivacion y perdida de estos humedales, en especial a partir de la decada de 1970, por el incremento en las precipitaciones y, subordinadamente, los frecuentes incendios forestales.
{"title":"Mallines en la Sierra de Comechingones, Sierras Pampeanas, Argentina. Caracterización geológico-geomorfológica y reconstrucción paleoambiental durante el Holoceno","authors":"M. J. Andreazzini, S. Degiovanni, A. Prieto, Alfonsina Tripaldi, M. Luque","doi":"10.5027/andgeov47n1-3138","DOIUrl":"https://doi.org/10.5027/andgeov47n1-3138","url":null,"abstract":"Los mallines son humedales de alto valor ambiental y son frecuentes en diversas regiones de la Argentina, como la Patagonia, la Cordillera de los Andes y las Sierras Pampeanas. En particular en esta ultima region, los estudios sobre su funcionamiento y dinamica son aun insuficientes y parciales. En este trabajo, se analizan los factores geologico-geomorfologicos que influyeron en el origen, distribucion, morfometria y dinamica de mallines del sector sur de la Sierra de Comechingones (Sierras Pampeanas de Cordoba, Argentina) y se realiza una reconstruccion paleoambiental a partir del registro sedimentologico, polinico y diatomologico de uno de estos sistemas durante el Holoceno. El area de estudio forma parte de una paleosuperficie de erosion desarrollada sobre rocas de basamento igneo-metamorfico con anterioridad a la orogenia andina, la cual se preserva en el faldeo oriental de esta sierra y presenta distinto grado de incision fluvial. El mayor desarrollo de los mallines se asocia a ambientes geomorfologicos con baja actividad morfodinamica, preferentemente sobre rocas graniticas, y depositos loessicos/loessoides. La fuente principal de los materiales que forman los depositos de mallines es la removilizacion de las secuencias loessicas/loessoides cuaternarias situadas en el sector serrano. Los registros sedimentologico, polinico y diatomologico sugieren la persistencia de estos ambientes en los sectores altos de las Sierras de Comechingones durante gran parte del Holoceno, con contracciones y expansiones como respuesta a la variabilidad ambiental. Las condiciones climaticas fueron variables, con un periodo mas seco que el actual, al menos temporariamente, entre ca. 7.000 y 4.700 cal. anos AP (6.400 y 4.100 14C anos AP) y, a partir de los 4.700 cal. anos AP (4.326 14C anos AP) se habrian establecido condiciones climaticas semejantes a las actuales. Los procesos de incision e integracion de las redes de drenaje que ocurrieron en las ultimas centurias iniciaron la desactivacion y perdida de estos humedales, en especial a partir de la decada de 1970, por el incremento en las precipitaciones y, subordinadamente, los frecuentes incendios forestales.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":"47 1","pages":"77-109"},"PeriodicalIF":0.9,"publicationDate":"2020-01-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46815318","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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