Pub Date : 2023-01-31DOI: 10.5027/andgeov50n1-3459
R. Tomassini, C. I. Montalvo, E. Beilinson, D. Barasoain, G. I. Schmidt, E. Cerdeño, A. Zurita, Ricardo Bonini, Á. Miño‐Boilini, L. Rasia, G. Gasparini
Arenas Blancas is a poorly known fossiliferous site located in the lower reach of the Chasicó creek (Buenos Aires Province, Argentina), with great relevance from a biostratigraphic viewpoint. The Macrochorobates scalabrinii Biozone was defined in this site, proposed as the biostratigraphic basis of the early Huayquerian Stage/Age (Late Miocene); however, the geological context and faunal record of this site have never been studied in detail. In this work, we perform a multi-proxy analysis of the Arenas Blancas site, as well as a nearby site here called Curva de la Vaca, and provide new interpretations on their origin, age, and biostratigraphy. Sedimentological, stratigraphic, and geomorphological characteristics of both sites suggest that the sequences include fluvial/alluvial deposits that represent Quaternary terraces. The taxonomic analysis of the Arenas Blancas faunal assemblage evidences the presence of 14 mammal taxa, together with some fishes, reptiles, and birds; the assemblage is correlated with the assemblage from the Cerro Azul Formation assigned to the Chasicoan Stage/Age (Late Miocene), also recovered from the lower reach of the Chasicó creek. The use of the taxa proposed in previous works to characterize the Macrochorobates scalabrinii Biozone (including this species) is not supported due to their taxonomic status and/or temporal distribution. Taphonomic evaluation indicates that the assemblage is constituted by reworked specimens; in this frame, it is proposed that fluvio/alluvial events, occurred during the Late Pleistocene-Holocene, eroded the Upper Miocene substrate and reworked fossil remains. Based on the present evidence, the Macrochorobates scalabrinii Biozone is here rejected as a valid biostratigraphic unit.
Blancas竞技场是一个鲜为人知的含化石遗址，位于Chasicócreek下游（阿根廷布宜诺斯艾利斯省），从生物地层学的角度来看具有重要意义。在该遗址中确定了Macrochorobates scalabrinii生物带，该生物带被认为是华克期早期（中新世晚期）的生物地层学基础；然而，该遗址的地质背景和动物区系记录从未得到详细研究。在这项工作中，我们对Arenas Blancas遗址以及附近一个名为Curva de la Vaca的遗址进行了多代理分析，并对它们的起源、年龄和生物地层学提供了新的解释。两个地点的沉积学、地层和地貌特征表明，该序列包括代表第四纪阶地的河流/冲积沉积物。对Arenas Blancas动物群的分类学分析表明，存在14个哺乳动物分类群，以及一些鱼类、爬行动物和鸟类；该组合与属于Chasican阶/时代（中新世晚期）的Cerro Azul组的组合相关，该组合也从Chasicócreek下游回收。由于其分类地位和/或时间分布，不支持使用先前工作中提出的分类群来表征阶带大合唱藻生物区（包括该物种）。Taphonomic评估表明，该组合是由重新加工的标本组成的；在这一框架中，认为河流/冲积事件发生在更新世晚期全新世，侵蚀了上中新世基底和改造的化石遗迹。根据目前的证据，Macrochorobates scalabrinii生物带被认为是一个有效的生物地层单元。
{"title":"On the validity of the Macrochorobates scalabrinii Biozone (early Huayquerian Stage/Age, Late Miocene). Multi-proxy analysis of the enigmatic Arenas Blancas site (Buenos Aires Province, Argentina)","authors":"R. Tomassini, C. I. Montalvo, E. Beilinson, D. Barasoain, G. I. Schmidt, E. Cerdeño, A. Zurita, Ricardo Bonini, Á. Miño‐Boilini, L. Rasia, G. Gasparini","doi":"10.5027/andgeov50n1-3459","DOIUrl":"https://doi.org/10.5027/andgeov50n1-3459","url":null,"abstract":"Arenas Blancas is a poorly known fossiliferous site located in the lower reach of the Chasicó creek (Buenos Aires Province, Argentina), with great relevance from a biostratigraphic viewpoint. The Macrochorobates scalabrinii Biozone was defined in this site, proposed as the biostratigraphic basis of the early Huayquerian Stage/Age (Late Miocene); however, the geological context and faunal record of this site have never been studied in detail. In this work, we perform a multi-proxy analysis of the Arenas Blancas site, as well as a nearby site here called Curva de la Vaca, and provide new interpretations on their origin, age, and biostratigraphy. Sedimentological, stratigraphic, and geomorphological characteristics of both sites suggest that the sequences include fluvial/alluvial deposits that represent Quaternary terraces. The taxonomic analysis of the Arenas Blancas faunal assemblage evidences the presence of 14 mammal taxa, together with some fishes, reptiles, and birds; the assemblage is correlated with the assemblage from the Cerro Azul Formation assigned to the Chasicoan Stage/Age (Late Miocene), also recovered from the lower reach of the Chasicó creek. The use of the taxa proposed in previous works to characterize the Macrochorobates scalabrinii Biozone (including this species) is not supported due to their taxonomic status and/or temporal distribution. Taphonomic evaluation indicates that the assemblage is constituted by reworked specimens; in this frame, it is proposed that fluvio/alluvial events, occurred during the Late Pleistocene-Holocene, eroded the Upper Miocene substrate and reworked fossil remains. Based on the present evidence, the Macrochorobates scalabrinii Biozone is here rejected as a valid biostratigraphic unit.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":null,"pages":null},"PeriodicalIF":0.9,"publicationDate":"2023-01-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42098477","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 : 2022-09-30DOI: 10.5027/andgeov49n3-3402
Marcelo García, G. Aguilar, María Pía Rodríguez, J. Metcalf
In the Western Cordillera of northern Chile, the Proterozoic-Paleozoic Belén Metamorphic Complex is covered by late Oligocene-early Miocene (25-18 Ma) rocks, and both units are involved in west-vergent contractional deformation, which results in exhumation. A Miocene age (18 to 6 Ma) for deformation has been previously constrained by stratigraphy and cross-cutting relationships. To understand the youngest exhumation event and reverse faulting, we obtained 21 (U-Th)/He ages from two samples of the metamorphic rocks and the associate inverse thermal modeling. Five zircon (U-Th)/He ages from one sample are 113 to 226 Ma, very scattered, while five zircon ages from the other, are 20 to 49 Ma. The high dispersion of zircon (U-Th)/He data prevents the geological interpretation of results. Apatite grains from both samples yielded 11 (U-Th)/He ages between 10.4 and 18.7 Ma, with 9 values from 12.0 to 15.5 Ma. A slight positive correlation between apatite single-grain dates and effective uranium for 4 crystals of one sample suggests relatively slow cooling. The T-t model including these 4 apatite ages shows continuous cooling from 15 to 0 Ma with a relatively more marked cooling period at 11-7 Ma. The middle-late Miocene thermal signal agrees with the geologic evolution of the region and would permit to date the last activity of the Chapiquiña-Belén reverse fault, which uplifted and exhumed the metamorphic rocks. This signal is relatively similar to that the eastern Altiplano, but differs considerably from that the forearc.
{"title":"(U-Th)/He ages of Proterozoic-Paleozoic basement rocks from northern Chile (18-19° S) and implications on the Neogene uplift history of the Western Cordillera","authors":"Marcelo García, G. Aguilar, María Pía Rodríguez, J. Metcalf","doi":"10.5027/andgeov49n3-3402","DOIUrl":"https://doi.org/10.5027/andgeov49n3-3402","url":null,"abstract":"In the Western Cordillera of northern Chile, the Proterozoic-Paleozoic Belén Metamorphic Complex is covered by late Oligocene-early Miocene (25-18 Ma) rocks, and both units are involved in west-vergent contractional deformation, which results in exhumation. A Miocene age (18 to 6 Ma) for deformation has been previously constrained by stratigraphy and cross-cutting relationships. To understand the youngest exhumation event and reverse faulting, we obtained 21 (U-Th)/He ages from two samples of the metamorphic rocks and the associate inverse thermal modeling. Five zircon (U-Th)/He ages from one sample are 113 to 226 Ma, very scattered, while five zircon ages from the other, are 20 to 49 Ma. The high dispersion of zircon (U-Th)/He data prevents the geological interpretation of results. Apatite grains from both samples yielded 11 (U-Th)/He ages between 10.4 and 18.7 Ma, with 9 values from 12.0 to 15.5 Ma. A slight positive correlation between apatite single-grain dates and effective uranium for 4 crystals of one sample \u0000suggests relatively slow cooling. The T-t model including these 4 apatite ages shows continuous cooling from 15 to 0 Ma with a relatively more marked cooling period at 11-7 Ma. The middle-late Miocene thermal signal agrees with the geologic evolution of the region and would permit to date the last activity of the Chapiquiña-Belén reverse fault, which \u0000uplifted and exhumed the metamorphic rocks. This signal is relatively similar to that the eastern Altiplano, but differs considerably from that the forearc.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":null,"pages":null},"PeriodicalIF":0.9,"publicationDate":"2022-09-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44410171","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 : 2022-09-30DOI: 10.5027/andgeov49n3-3507
R. Charrier
{"title":"Profesor Alfredo Lahsen Azar (1939-2022) Pionero de la energía geotérmica en Chile","authors":"R. Charrier","doi":"10.5027/andgeov49n3-3507","DOIUrl":"https://doi.org/10.5027/andgeov49n3-3507","url":null,"abstract":"","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":null,"pages":null},"PeriodicalIF":0.9,"publicationDate":"2022-09-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"70695438","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 : 2022-06-07DOI: 10.5027/andgeov49n3-3425
S. Vizcaino, M. Bargo, M. E. Pérez, I. Aramendía, J. Cuitiño, Eduardo Sebastián Monsalvo, E. Vlachos, J. Noriega, R. F. Kay
The early–middle Miocene continental Cerro Boleadoras Formation (CBF) crops out in the area of Cerro Boleadoras and Cerro Plomo on the western slope of the Meseta del Lago Buenos Aires, northwestern Santa Cruz Province, Argentina. The lower levels of the CBF consist of laterally extensive medium to pebbly sandstone beds with trough cross-bedding, interpreted as fluvial channel deposits, interbedded with tabular fine-grained floodplain deposits. Recent fieldwork provided fossil vertebrates from these levels with an estimated age between ~16.5 Ma and 15.1 Ma (late Burdigalian-early Langhian). The studied section temporally overlaps with the middle or upper sections of the Santa Cruz Formation (SCF) in the Austral–Magallanes Basin of southern Patagonia, the Río Frias Formations in Chile, and the lower Collón-Curá Formation of northern Patagonia. We compile an integrated faunal list for this locality, including specimens from previous collections, and discuss its chronological and paleoenvironmental implications. The taxa list includes most of the groups recorded in the SCF: one anuran, three birds, and at least 33 mammals (metatherians, xenarthrans, litopterns, notoungulate typotheres and caviomorph rodents), indicating a Santacrucian age sensu lato. We also recorded a testudine, which constitutes the southernmost record of tortoises in South America and worldwide. Faunal differences between the vertebrate fossil content of the CBF and the mentioned sections of the Santa Cruz, Río Frías and Collón-Curá formations may reflect ecologic, climatic and geographic differences rather than temporal ones. The co-occurrence of arboreal or semiarboreal, browsing, frugivorous, and grazing mammals suggests the presence of both trees and open environments for the area occupied by the CBF rocks. However, it is not possible to discern whether these two environments coexisted or alternated, and whether one environment predominated over the other. Marker taxa, such as the chinchillid rodents Prolagostomus and Pliolagostomus, and the typothere Pachyrukhos indicate a trend to aridification during the Miocene in southern Patagonia, as previously reported for the upper part of the SCF along the Río Santa Cruz and south to the Río Coyle, along the Atlantic coast and the Río Gallegos.
早-中中新世大陆Cerro Boleadoras组(CBF)生长在阿根廷圣克鲁斯省西北部Meseta del Lago Buenos Aires西坡的Cerro Boleadoras和Cerro Plomo地区。CBF的下部由横向广泛的中卵石砂岩层组成，具有槽状交错层理，可解释为河道沉积，与板状细粒洪泛平原沉积互层。最近的野外调查提供了这些水平的脊椎动物化石，估计年龄在~16.5 Ma至15.1 Ma之间(burdigian晚期- Langhian早期)。研究剖面与南巴塔哥尼亚的Austral-Magallanes盆地的Santa Cruz组(SCF)中上剖面、智利的Río Frias组和北巴塔哥尼亚的Collón-Curá组下剖面在时间上重叠。我们编制了该地区的综合动物名单，包括以前收集的标本，并讨论了其年代和古环境意义。该分类群列表包括了在SCF中记录的大多数类群:一种无脊椎动物，三种鸟类，以及至少33种哺乳动物(异肢动物，异肢动物，穴居动物，无节肢动物和洞穴形啮齿动物)，表明了一个圣克鲁斯时代的感觉。我们还记录了一只龟，它构成了南美洲和世界上最南端的龟的记录。CBF与上述Santa Cruz、Río Frías和Collón-Curá地层的脊椎动物化石含量的区系差异可能反映了生态、气候和地理差异，而不是时间差异。树栖或半树栖、食食、果食和食草哺乳动物的共存表明，在CBF岩石占据的地区，树木和开放环境都存在。然而，无法辨别这两种环境是共存还是交替存在，以及一种环境是否优于另一种环境。标记类群，如栗鼠(chinchillid啮齿动物)Prolagostomus和Pliolagostomus，以及typothere Pachyrukhos，表明中新世在巴塔哥尼亚南部有干旱化的趋势，如先前报道的沿Río Santa Cruz和Río Coyle以南的SCF上部，沿大西洋海岸和Río Gallegos。
{"title":"Fossil vertebrates of the early-middle Miocene Cerro Boleadoras Formation, northwestern Santa Cruz Province, Patagonia, Argentina","authors":"S. Vizcaino, M. Bargo, M. E. Pérez, I. Aramendía, J. Cuitiño, Eduardo Sebastián Monsalvo, E. Vlachos, J. Noriega, R. F. Kay","doi":"10.5027/andgeov49n3-3425","DOIUrl":"https://doi.org/10.5027/andgeov49n3-3425","url":null,"abstract":"The early–middle Miocene continental Cerro Boleadoras Formation (CBF) crops out in the area of Cerro Boleadoras and Cerro Plomo on the western slope of the Meseta del Lago Buenos Aires, northwestern Santa Cruz Province, Argentina. The lower levels of the CBF consist of laterally extensive medium to pebbly sandstone beds with trough cross-bedding, interpreted as fluvial channel deposits, interbedded with tabular fine-grained floodplain deposits. Recent fieldwork provided fossil vertebrates from these levels with an estimated age between ~16.5 Ma and 15.1 Ma (late Burdigalian-early Langhian). The studied section temporally overlaps with the middle or upper sections of the Santa Cruz Formation (SCF) in the Austral–Magallanes Basin of southern Patagonia, the Río Frias Formations in Chile, and the lower Collón-Curá Formation of northern Patagonia. We compile an integrated faunal list for this locality, including specimens from previous collections, and discuss its chronological and paleoenvironmental implications. The taxa list includes most of the groups recorded in the SCF: one anuran, three birds, and at least 33 mammals (metatherians, xenarthrans, litopterns, notoungulate typotheres and caviomorph rodents), indicating a Santacrucian age sensu lato. We also recorded a testudine, which constitutes the southernmost record of tortoises in South America and worldwide. Faunal differences between the vertebrate fossil content of the CBF and the mentioned sections of the Santa Cruz, Río Frías and Collón-Curá formations may reflect ecologic, climatic and geographic differences rather than temporal ones. The co-occurrence of arboreal or semiarboreal, browsing, frugivorous, and grazing mammals suggests the presence of both trees and open environments for the area occupied by the CBF rocks. However, it is not possible to discern whether these two environments coexisted or alternated, and whether one environment predominated over the other. Marker taxa, such as the chinchillid rodents Prolagostomus and Pliolagostomus, and the typothere Pachyrukhos indicate a trend to aridification during the Miocene in southern Patagonia, as previously reported for the upper part of the SCF along the Río Santa Cruz and south to the Río Coyle, along the Atlantic coast and the Río Gallegos.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":null,"pages":null},"PeriodicalIF":0.9,"publicationDate":"2022-06-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47845383","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 : 2022-06-07DOI: 10.5027/andgeov49n3-3340
Marcos Podesta, Paola G. Orozco, P. Alvarado, F. Fuentes
The Iglesia basin, in the San Juan Province, is the northern part of a Cenozoic piggy-back basin (the Iglesia-Calingasta-Uspallata basin), located above of the active flat-slab subduction zone of western Argentina. This basin is located between two mountain ranges, the Cordillera Frontal to the west and the Precordillera to the east, affected by thin and thick skinned tectonics, respectively. It is elongated in a north-south direction (70 km) and has a maximum width of 35 km. We have analyzed 17 seismic reflection lines corresponding to more than 500 km of interpreted sections. A strong reflection at 0.5-2 s is interpreted as indicative of shales and sandstones in the upper part of the Agua Negra Formation (Carboniferous); this represents a seismic basement for the Iglesia basin clearly separating stratified (shallower) from chaotic (deeper) reflectors. Several high angle faults have been recognized in the interpreted seismic lines affecting the Neogene fill and even the basement; some of them correlate with structures observed at the surface such us El Tigre Fault System. A 3D model shows the seismic top of basement smoothly deepening down to 3,650 m beneath the Pismanta area. The basement continues upward to the east with a steeper slope clearly defining an asymmetrical shape for the basin. Moreover, there are three depocenters of similar maximum depths around the geothermal hot spring of the Pismanta center. Some of the interpreted faults may be helping in the outflow process of meteoric water heated by a normal geothermal gradient. Considering earthquake data framework, our observations correlate with transpressive deformation likely associated with El Tigre Fault System, which affects Neogene and Pleistocene strata of the Iglesia Group.
{"title":"The Iglesia basin (San Juan, Argentina), seismic interpretation, geometry basin, and implications for geothermal systems","authors":"Marcos Podesta, Paola G. Orozco, P. Alvarado, F. Fuentes","doi":"10.5027/andgeov49n3-3340","DOIUrl":"https://doi.org/10.5027/andgeov49n3-3340","url":null,"abstract":"The Iglesia basin, in the San Juan Province, is the northern part of a Cenozoic piggy-back basin (the Iglesia-Calingasta-Uspallata basin), located above of the active flat-slab subduction zone of western Argentina. This basin is located between two mountain ranges, the Cordillera Frontal to the west and the Precordillera to the east, affected by thin and thick skinned tectonics, respectively. It is elongated in a north-south direction (70 km) and has a maximum width of 35 km. We have analyzed 17 seismic reflection lines corresponding to more than 500 km of interpreted sections. A strong reflection at 0.5-2 s is interpreted as indicative of shales and sandstones in the upper part of the Agua Negra Formation (Carboniferous); this represents a seismic basement for the Iglesia basin clearly separating stratified (shallower) from chaotic (deeper) reflectors. Several high angle faults have been recognized in the interpreted seismic lines affecting the Neogene fill and even the basement; some of them correlate with structures observed at the surface such us El Tigre Fault System. A 3D model shows the seismic top of basement smoothly deepening down to 3,650 m beneath the Pismanta area. The basement continues upward to the east with a steeper slope clearly defining an asymmetrical shape for the basin. Moreover, there are three depocenters of similar maximum depths around the geothermal hot spring of the Pismanta center. Some of the interpreted faults may be helping in the outflow process of meteoric water heated by a normal geothermal gradient. Considering earthquake data framework, our observations correlate with transpressive deformation likely associated with El Tigre Fault System, which affects Neogene and Pleistocene strata of the Iglesia Group.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":null,"pages":null},"PeriodicalIF":0.9,"publicationDate":"2022-06-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48859803","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 : 2022-06-07DOI: 10.5027/andgeov49n3-3399
Ana Mestre, M. Gallardo, M. J. Salas, S. Heredia
The microfossil hosted in the strata of the upper part of the San Juan Formation has been widely studied in several sections to the northward of the Argentinian Central Precordillera. In contrast, the coeval strata at the Los Baños de Talacasto section, in the southern part of the Central Precordillera, have scarce biostratigraphic and sedimentological data. In this work, a conodont association together with single ostracod species are documented for the first time in this section. The record of the Lenodus crassus and L. pseudoplanus zones confirms the Darriwilian age for these beds and accurately correlates them with equivalent strata of the San Juan Formation studied in several sections of the Central and Eastern Precordillera. The microfacies analysis verifies the presence of Nuia síbirica Maslov, 1954, peloids, intraclasts, cyanobacteria, calcareous algae, and a possible microbialite indicating a shallow warm-water subtidal environment, in equatorial to subequatorial climate. This suggests a low latitudes position for Precordillera during the early-middle Darriwilian. The conodont genus Aurilobodus Xiang and Zhang is recognized for the first time from the Precordillera, and the Aurilobodus leptosomatus An specimens are described and illustrated. This genus shows affinities to the warm water in shallow marine environments of North China, Central Asia, South Tibet, western Thailand, Australia, and Newfoundland, suggesting probable ties between Precordillera and these regions. The record of the ostracod Pilla nodospinosa Salas in the study section would agree with the correlation of the top of San Juan Formation with the lower levels of the Las Aguaditas Formation in the Central Precordillera, and also suggests paleobiogeographic links with Eastern Gondwana and Australia regions during the Darriwilian times.
{"title":"Biostratigraphic and paleobiogeographic significance of the Darriwilian microfossils from the top of San Juan Formation in the Los Baños de Talacasto section, Central Precordillera (Argentina)","authors":"Ana Mestre, M. Gallardo, M. J. Salas, S. Heredia","doi":"10.5027/andgeov49n3-3399","DOIUrl":"https://doi.org/10.5027/andgeov49n3-3399","url":null,"abstract":"The microfossil hosted in the strata of the upper part of the San Juan Formation has been widely studied in several sections to the northward of the Argentinian Central Precordillera. In contrast, the coeval strata at the Los Baños de Talacasto section, in the southern part of the Central Precordillera, have scarce biostratigraphic and sedimentological data. In this work, a conodont association together with single ostracod species are documented for the first time in this section. The record of the Lenodus crassus and L. pseudoplanus zones confirms the Darriwilian age for these beds and accurately correlates them with equivalent strata of the San Juan Formation studied in several sections of the Central and Eastern Precordillera. The microfacies analysis verifies the presence of Nuia síbirica Maslov, 1954, peloids, intraclasts, cyanobacteria, calcareous algae, and a possible microbialite indicating a shallow warm-water subtidal environment, in equatorial to subequatorial climate. This suggests a low latitudes position for Precordillera during the early-middle Darriwilian. The conodont genus Aurilobodus Xiang and Zhang is recognized for the first time from the Precordillera, and the Aurilobodus leptosomatus An specimens are described and illustrated. This genus shows affinities to the warm water in shallow marine environments of North China, Central Asia, South Tibet, western Thailand, Australia, and Newfoundland, suggesting probable ties between Precordillera and these regions. The record of the ostracod Pilla nodospinosa Salas in the study section would agree with the correlation of the top of San Juan Formation with the lower levels of the Las Aguaditas Formation in the Central Precordillera, and also suggests paleobiogeographic links with Eastern Gondwana and Australia regions during the Darriwilian times.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":null,"pages":null},"PeriodicalIF":0.9,"publicationDate":"2022-06-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48958567","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 : 2022-06-07DOI: 10.5027/andgeov49n3-3389
L. Martín, D. Marchal, S. Barredo, C. Naides, Silvia Blanco
The fissility is the ability of some rocks to split along relatively smooth surfaces parallel to the bedding. This property observed mostly in fine-grained rocks is particularly expressed in outcrops, where rocks are subjected to weathering processes. Most authors associate the fissility to the abundance of clay minerals and their orientation parallel to the bedding. The horizontal fabric can be promoted by depositional conditions such as sediment composition, quantity of total organic carbon content (TOC) and depositional mechanisms, giving rise to a primary fissility. Alternatively, the alignment of platy minerals can be linked to the burial history of the rock, by processes such as mechanical compaction or secondary mineral growth, resulting in a secondary fissility. The present study aims to identify the main controls of fissility development at the micro- and macroscopic scale in rocks of the Vaca Muerta Formation exposed in the Cerro Mulichinco area and in a 121-meter-long core extracted from a well within the Neuquén Basin. In outcrops, fissility is related to fine-grained laminated facies with low carbonate content, revealing the strong control exerted by lithology. The TOC measurements allow establishing a positive correlation between organic matter content and fissility intensity. Moreover, the analysis of the transgressive-regressive cycles shows that fissility is higher around the maximum flooding surfaces. Regarding their mechanical characteristics, the different interfaces observed in core are classified into first and second-order, the last one including fissility planes. Some of these interfaces evolve from potential (partially open) to effective (totally open) discontinuities in response to changes of stress conditions during the core extraction and due to the stress relaxation through time: weeks (T1), months (T2) and years (T3) after extraction. The time evolution of the effective core discontinuities points out rock intervals that are variably broken and core segments that remain intact. The Drying Alcohol Discontinuities (DAD) methodology reveals potential discontinuities within apparently intact core segments. By using this technique, a 4-class index is established as a proxy for fissility degree. When integrated with geological, petrophysical and geomechanical data, this index enables characterizing the main mechanisms controlling rock fissility that express through discontinuities promoting the loss of competence of a rock. Consequently, this mechanical property is considered to influence the efficiency of hydraulic fracture in shale reservoir completion.
{"title":"Relationship between fissility, composition, rock fabric and reservoir properties in Vaca Muerta Formation (Neuquén Basin, Argentina): from outcrop to subsurface core data","authors":"L. Martín, D. Marchal, S. Barredo, C. Naides, Silvia Blanco","doi":"10.5027/andgeov49n3-3389","DOIUrl":"https://doi.org/10.5027/andgeov49n3-3389","url":null,"abstract":"The fissility is the ability of some rocks to split along relatively smooth surfaces parallel to the bedding. This property observed mostly in fine-grained rocks is particularly expressed in outcrops, where rocks are subjected to weathering processes. Most authors associate the fissility to the abundance of clay minerals and their orientation parallel to the bedding. The horizontal fabric can be promoted by depositional conditions such as sediment composition, quantity of total organic carbon content (TOC) and depositional mechanisms, giving rise to a primary fissility. Alternatively, the alignment of platy minerals can be linked to the burial history of the rock, by processes such as mechanical compaction or secondary mineral growth, resulting in a secondary fissility. The present study aims to identify the main controls of fissility development at the micro- and macroscopic scale in rocks of the Vaca Muerta Formation exposed in the Cerro Mulichinco area and in a 121-meter-long core extracted from a well within the Neuquén Basin. In outcrops, fissility is related to fine-grained laminated facies with low carbonate content, revealing the strong control exerted by lithology. The TOC measurements allow establishing a positive correlation between organic matter content and fissility intensity. Moreover, the analysis of the transgressive-regressive cycles shows that fissility is higher around the maximum flooding surfaces. Regarding their mechanical characteristics, the different interfaces observed in core are classified into first and second-order, the last one including fissility planes. Some of these interfaces evolve from potential (partially open) to effective (totally open) discontinuities in response to changes of stress conditions during the core extraction and due to the stress relaxation through time: weeks (T1), months (T2) and years (T3) after extraction. The time evolution of the effective core discontinuities points out rock intervals that are variably broken and core segments that remain intact. The Drying Alcohol Discontinuities (DAD) methodology reveals potential discontinuities within apparently intact core segments. By using this technique, a 4-class index is established as a proxy for fissility degree. When integrated with geological, petrophysical and geomechanical data, this index enables characterizing the main mechanisms controlling rock fissility that express through discontinuities promoting the loss of competence of a rock. Consequently, this mechanical property is considered to influence the efficiency of hydraulic fracture in shale reservoir completion.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":null,"pages":null},"PeriodicalIF":0.9,"publicationDate":"2022-06-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44108721","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 : 2022-06-07DOI: 10.5027/andgeov49n3-3478
Aldo Manzuetti, W. Jones, M. Ubilla, Daniel Perea
Felidae entered South America from North America during the Ensenadan Stage/Age (Early to Middle Pleistocene). For Uruguay their fossil record is scarce but informative, although mostly correspond to large felids (Smilodon, Panthera onca). In this work describes fossil remain of a skull and a mandible assigned, based on morphology and statistical analysis, to Puma concolor. These materials were unearthed from Late Pleistocene sediments (Sopas Formation) from northern Uruguay. Based on these remains some paleobiological and paleoecological aspects are discussed. This specimen had a body mass of about 45 kg, with the potential capacity to hunt animals over 200 kg, which is in agreement with previous reports for other fossil puma remains. During Late Pleistocene, in the current territory of Uruguay, Puma concolor would have shared ecological niche with other large carnivore such as sabre-tooth cats (Smilodon populator, S. fatalis), the jaguar Panthera onca, the canid Protocyon troglodytes, and short-faced bears Arctotherium, feeding upon medium-sized animals in diverse environment.
{"title":"Nuevo registro de Puma concolor Linnaeus, 1771 (Carnivora, Felidae) para el Pleistoceno tardío de Uruguay y su importancia paleoecológica","authors":"Aldo Manzuetti, W. Jones, M. Ubilla, Daniel Perea","doi":"10.5027/andgeov49n3-3478","DOIUrl":"https://doi.org/10.5027/andgeov49n3-3478","url":null,"abstract":"Felidae entered South America from North America during the Ensenadan Stage/Age (Early to Middle Pleistocene). For Uruguay their fossil record is scarce but informative, although mostly correspond to large felids (Smilodon, Panthera onca). In this work describes fossil remain of a skull and a mandible assigned, based on morphology and statistical analysis, to Puma concolor. These materials were unearthed from Late Pleistocene sediments (Sopas Formation) from northern Uruguay. Based on these remains some paleobiological and \u0000paleoecological aspects are discussed. This specimen had a body mass of about 45 kg, with the potential capacity to hunt animals over 200 kg, which is in agreement with previous reports for other fossil puma remains. During Late Pleistocene, in the current territory of Uruguay, Puma concolor would have shared ecological niche with other large carnivore such as sabre-tooth cats (Smilodon populator, S. fatalis), the jaguar Panthera onca, the canid Protocyon troglodytes, and short-faced bears Arctotherium, feeding upon medium-sized animals in diverse environment.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":null,"pages":null},"PeriodicalIF":0.9,"publicationDate":"2022-06-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44120336","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 : 2022-03-07DOI: 10.5027/andgeov49n2-3377
B. Coira, C. Galli, Suzanne Mahlburg Kay, D. Stockli, P. Flores, Emilio Eveling
Ash-fall tuffs of the Pliocene-Pleistocene deposits of Humahuaca and Casa Grande intermountain basins, northwestern Argentina, have been differentiated into two groups based on new geochemical and geochronological data which correspond to the tuffs of the Pliocene- Lower Pleistocene alluvial fan deposits dominated by debris flow, deep sandy gravel braided, and shallow ephemeral lake deposits (Uquia and Mal Paso formations),and those recorded in Pleistocene alluvial fans sheet flood deposits. The two clusters of ages recognized: 4.3 to 2.6 Ma, and 2.2–pre 0.8 Ma, corresponding to those groups, are in agreement with pulses of ignimbrite eruptions in the Altiplano Puna Volcanic Complex (APVC), and Southerm Puna calderas located west of the study region. The ash-fall tuffs of both groups are mainly vitreous to phenocryst-poor of rhyodacite-dacite composition with minor andesites to trachyandesites, characterized by 58-69% SiO2 contents, A/CNK 1-1.4, FeO/MgO (0.8-2.8), which plot in the calc-alkaline range. They can be differentiated based on its immobile trace elements ratios as indicated by a slight enrichment in LREE, higher arc affiliation and somewhat higher Sm/Yb ratios in the case of the younger group, although in both Sm/Yb ratios are indicative of sequestration of HREE in residual hornblende. The new geochemical and geochronological data of those ash fall tuffs point to these as chrono-stratigraphic tracers of the Humahuaca and Casa Grande intermountain basins stratigraphy, during the Pliocene-Pleistocene, also as the key to identify their volcanic sources.
{"title":"Pliocene-Pleistocene ash-fall tuff deposits in the intermountain Humahuaca and Casa Grande basins, northwestern Argentina: tracers in chronostratigraphic reconstructions and key to identify their volcanic sources","authors":"B. Coira, C. Galli, Suzanne Mahlburg Kay, D. Stockli, P. Flores, Emilio Eveling","doi":"10.5027/andgeov49n2-3377","DOIUrl":"https://doi.org/10.5027/andgeov49n2-3377","url":null,"abstract":"Ash-fall tuffs of the Pliocene-Pleistocene deposits of Humahuaca and Casa Grande intermountain basins, northwestern Argentina, have been differentiated into two groups based on new geochemical and geochronological data which correspond to the tuffs of the Pliocene- Lower Pleistocene alluvial fan deposits dominated by debris flow, deep sandy gravel braided, and shallow ephemeral lake deposits (Uquia and Mal Paso formations),and those recorded in Pleistocene alluvial fans sheet flood deposits. The two clusters of ages recognized: 4.3 to 2.6 Ma, and 2.2–pre 0.8 Ma, corresponding to those groups, are in agreement with pulses of ignimbrite eruptions in the Altiplano Puna Volcanic Complex (APVC), and Southerm Puna calderas located west of the study region. The ash-fall tuffs of both groups are mainly vitreous to phenocryst-poor of rhyodacite-dacite composition with minor andesites to trachyandesites, characterized by 58-69% SiO2 contents, A/CNK 1-1.4, FeO/MgO (0.8-2.8), which plot in the calc-alkaline range. They can be differentiated based on its immobile trace elements ratios as indicated by a slight enrichment in LREE, higher arc affiliation and somewhat higher Sm/Yb ratios in the case of the younger group, although in both Sm/Yb ratios are indicative of sequestration of HREE in residual hornblende. The new geochemical and geochronological data of those ash fall tuffs point to these as chrono-stratigraphic tracers of the Humahuaca and Casa Grande intermountain basins stratigraphy, during the Pliocene-Pleistocene, also as the key to identify their volcanic sources.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":null,"pages":null},"PeriodicalIF":0.9,"publicationDate":"2022-03-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47555038","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 : 2022-03-07DOI: 10.5027/andgeov49n2-3400
Jorge E. Morales Leal, A. Menzies, H. Wilke, J. Zuluaga
The El Peñón low-sulfidation epithermal deposit, located in the Paleocene metallogenic belt in the Antofagasta region, northern Chile, consists of Au-Ag bearing veins and hydrothermal breccias hosted in volcanic rocks and surrounded by alteration haloes consisting of a series of minerals, such as adularia, K-micas, and clay minerals. They are the result of metasomatism generated by the passage of reduced, and near neutral pH, hydrothermal fluids, that transported gold through structures and its interaction with the host rock. we investigated the spatial distribution of these minerals (mainly adularia) in the wall rocks around the Aleste vein, in the northern part of the deposit, to establish their relationship with the Au-Ag ore and, thus, support the identification of new exploration targets. An X-ray diffraction technique to discriminate adularia from other K-feldspars in whole rock samples was developed by the detail study of the diffractogram patterns of an adularia standard. The study of an X-ray diffraction standard pattern allowed the recognition of this mineral by 4 secondary peaks. Our results indicated that adularia occurs in association with felsic volcanic rocks, and it is restricted in intermediate composition units. The spatial distribution of this mineral is correlated with the mineralized zones, being adularia semi-quantitative abundance in the range of 25 to 40% a good indicator of gold mineralization. Sericite and illite occur mainly where adularia is scarce or absent. The spatial distribution of these minerals showed the structural control in the evolution and flow path of the hydrothermal fluids toward the surface. Kaolinite is related to argillic alteration caused by steam-heated fluids, and the advanced argillic alteration associated with later supergene alteration. Chlorite usually is far from the mineralized areas; therefore, it could be a reliable indicator of the margins of the system. The characterization and spatial distribution pattern of the alteration minerals identified by the X-ray diffraction method in the veins of El Peñón deposit show the capability of this type of analyses in determining possible prospection targets.
El Peñón低硫化浅成低温热液矿床位于智利北部Antofagasta地区古新世成矿带，由含金银脉体和热液角砾岩组成，寄存于火山岩中，周围由一系列矿物组成的蚀变晕，如阿黄、k云母、黏土矿物等。它们是由还原的、pH值接近中性的热液流体通过构造及其与宿主岩石的相互作用将金输送到金矿所产生的交代作用的结果。通过对矿床北部Aleste脉周围围岩中这些矿物(主要为adularia)的空间分布特征的研究，建立了它们与Au-Ag的关系，从而支持新的找矿目标的确定。通过对一种阿杜石标准样品的衍射图进行详细研究，建立了一种区分阿杜石与其他钾长石的x射线衍射技术。通过x射线衍射标准模式的研究，可以通过4个次级峰来识别这种矿物。结果表明，阿杜里亚与长英质火山岩伴生，局限于中间组成单元。该矿物的空间分布与成矿带具有相关性，其半定量丰度在25 ~ 40%之间，是金矿化的良好指示物。绢云母和伊利石主要出现在少或无成虫的地方。这些矿物的空间分布显示了热液流体向地表演化和流动路径的构造控制。高岭石与蒸汽加热流体引起的泥质蚀变有关，晚期泥质蚀变与后期表生蚀变有关。绿泥石通常远离矿化区;因此，它可以作为系统边际的可靠指标。利用x射线衍射方法在El Peñón矿床矿脉中识别的蚀变矿物特征和空间分布规律表明，这种分析方法能够确定可能的找矿靶区。
{"title":"Characterization and distribution of adularia and other alteration minerals by X-ray diffraction analysis at the El Peñón Au-Ag epithermal deposit, northern Chile","authors":"Jorge E. Morales Leal, A. Menzies, H. Wilke, J. Zuluaga","doi":"10.5027/andgeov49n2-3400","DOIUrl":"https://doi.org/10.5027/andgeov49n2-3400","url":null,"abstract":"The El Peñón low-sulfidation epithermal deposit, located in the Paleocene metallogenic belt in the Antofagasta region, northern Chile, consists of Au-Ag bearing veins and hydrothermal breccias hosted in volcanic rocks and surrounded by alteration haloes consisting of a series of minerals, such as adularia, K-micas, and clay minerals. They are the result of metasomatism generated by the passage of reduced, and near neutral pH, hydrothermal fluids, that transported gold through structures and its interaction with the host rock. we investigated the spatial distribution of these minerals (mainly adularia) in the wall rocks around the Aleste vein, in the northern part of the deposit, to establish their relationship with the Au-Ag ore and, thus, support the identification of new exploration targets. An X-ray diffraction technique to discriminate adularia from other K-feldspars in whole rock samples was developed by the detail study of the diffractogram patterns of an adularia standard. The study of an X-ray diffraction standard pattern allowed the recognition of this mineral by 4 secondary peaks. Our results indicated that adularia occurs in association with felsic volcanic rocks, and it is restricted in intermediate composition units. The spatial distribution of this mineral is correlated with the mineralized zones, being adularia semi-quantitative abundance in the range of 25 to 40% a good indicator of gold mineralization. Sericite and illite occur mainly where adularia is scarce or absent. The spatial distribution of these minerals showed the structural control in the evolution and flow path of the hydrothermal fluids toward the surface. Kaolinite is related to argillic alteration caused by steam-heated fluids, and the advanced argillic alteration associated with later supergene alteration. Chlorite usually is far from the mineralized areas; therefore, it could be a reliable indicator of the margins of the system. The characterization and spatial distribution pattern of the alteration minerals identified by the X-ray diffraction method in the veins of El Peñón deposit show the capability of this type of analyses in determining possible prospection targets.","PeriodicalId":49108,"journal":{"name":"Andean Geology","volume":null,"pages":null},"PeriodicalIF":0.9,"publicationDate":"2022-03-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47734336","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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