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Crystalline-rock aquifer system of the Llano Uplift, Central Texas, USA

Le système aquifère de roches cristallines du bombement de Llano, Texas Central, EUA

Sistema acuífero de roca Cristalina del Llano Uplift, Central Texas, EEUU

美国德克萨斯中部拉诺隆起的结晶岩含水层系统

Sistema aquífero cristalino do alto Llano, Texas Central, EUA

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Abstract

The crystalline-rock aquifer system of the Llano Uplift (Central Texas, USA) hosts an important local water resource that has been relatively little studied. The Uplift is a structural dome of Precambrian granitic and metamorphic rocks. Late Paleozoic normal faulting and fractures, decompressive fractures, weathering, lithology, and rock fabric control the aquifer properties. Data from driller reports (over 2,000 wells) show that wells in granites have higher median yields than wells in metamorphic rocks. There is a weak correlation of well yield with regolith thickness, and median regolith thickness is greater over granites than over metamorphic rocks. Fracture permeability, which is very heterogeneous, is the major control. Wells are shallow (generally <100 m depth), but more recent wells have been drilled more deeply. Permeability data imply decreased open-fracture density and aperture with depth, although sample bias is a consideration. Diamond drill cores show that many near-surface fractures with significant aperture are filled by rock fragments from weathered surrounding rocks and that fracture skins are thinner and contain iron oxides in a more reduced state with depth. Fracture skins can be porous, with porosity ranging to over 10%. There is a need to compare crystalline-rock aquifer systems to assess weathering, tectonics, fractures, and mineralogy/petrology to assess the characteristics of these systems, which are critical water resources in large areas of the world.

Resume

Le système aquifère de roches cristallines du bombement de Llano (Texas central, Etats-Unis d’Amérique) renferme une importante ressource en eau localement qui a été relativement peu étudiée. Le bombement est un dôme structural de roches granitiques et métamorphiques précambriennes. Les fractures et failles normales de la fin du Paléozoïque, les fractures de décompression, l’altération, la lithologie et la fabrique de la roche contrôlent les propriétés aquifères. Les données de rapports de forages (plus de 2,000 puits) montrent que les puits dans les granites ont des débits médians plus élevés que ceux dans les roches métamorphiques. Il y a une faible corrélation entre entre le débit des forages et l’épaisseur du régolithe, et l’épaisseur médiane du régolithe est plus forte pour les granites que pour les roches métamorphiques. La conductivité hydrauliqueest majoritairement contrôlée par la perméabilité de fractures, qui est très hétérogène. Les puits sont peu profonds (généralement <100 m), mais des puits plus récents ont été forés plus profondément. Les données de conductivité hydraulique impliquent une décroissance de la densité des fractures ouvertes et de l’ouverture avec la profondeur, bien qu’il faille considérer le biais lié à l’échantillonnage. Les carottes réalisées en forage diamanté montrent que de nombreuses fractures proches de la surface et d’ouverture significative sont remplies par des fragments de roche issus des roches altérées voisines et aussi que l’épaisseur des encroûtements aux épontes des fractures diminue avec la profondeur et qu’ils contiennent des oxydes de fer dans un état plus réduit. Les revêtements des fractures peuvent être poreux, avec des valeurs de porosité pouvant atteindre 10%. La comparaison des propriétés des systèmes aquifères de roches telles que l’altération, la tectonique, les fractures et la minéralogie/pétrographie est nécessaire pour évaluer les caractéristiques de ces systèmes, qui constituent des ressources en eau critiques dans de vastes régions du monde.

Resumen

El sistema acuífero de roca cristalina del Llano Uplift (Central Texas, EEUU.) alberga un importante recurso hídrico local que ha sido relativamente poco estudiado. El Uplift es una cúpula estructural de rocas graníticas y metamórficas precámbricas. Las fallas y fracturas normales tardías del Paleozoico, las fracturas descompresivas, la meteorización, la litología y la fábrica de la roca controlan las propiedades del acuífero. Los datos de los informes de los perforistas (más de 2,000 pozos) muestran que los pozos en granitos tienen rendimientos medios más altos que los pozos en rocas metamórficas. Existe una débil correlación entre el rendimiento de los pozos y el espesor del regolito, y la mediana del espesor del regolito es mayor en los granitos que en las rocas metamórficas. La permeabilidad de las fracturas, que es muy heterogénea, es el principal control. Los pozos son poco profundos (generalmente <100 m de profundidad), pero los pozos más recientes han sido perforados más profundamente. Los datos de permeabilidad implican una disminución de la densidad de las fracturas abiertas y de la apertura con la profundidad, aunque el sesgo de la muestra debe considerarse. Los testigos de perforaciones con diamantina muestran que muchas fracturas cercanas a la superficie con una apertura significativa son rellenadas por fragmentos meteorizadas de rocas circundantes erosionadas y que los rellenos de fracturas son más delgadas y contienen óxidos de hierro en un estado más reducido en profundidad. Los rellenos de las fracturas pueden ser porosos, con una porosidad que supera el 10%. Es necesario comparar los sistemas acuíferos de roca cristalina para evaluar la meteorología, la tectónica, las fracturas y la mineralogía/petrología, a fin de evaluar las características de estos sistemas, que son recursos hídricos críticos en grandes zonas del mundo.

摘要

拉诺隆起(美国德克萨斯中部)的结晶岩含水层系统是当地重要的水资源,该资源相关研究较少。隆起是前寒武纪花岗岩和变质岩的构造圆丘。晚古生代正断层和裂缝,减压裂缝,风化,岩性和岩石控制着含水层的特性。钻探(超过2,000口井)数据表明,花岗岩区的井中间出水量高于变质岩区。井出水量与风化层厚度的相关性较小,花岗岩的中间风积层厚度大于变质岩。裂缝渗透率高度不均匀是出水量的主要控制因素。井很浅(通常 < 100米深),但最近的井已经钻得更深。尽管井采样数据存在偏差,但渗透率数据表明开口裂缝密度和开度随深度减小。金刚石岩芯显示许多具有明显开度的近地表裂缝被周围风化岩石碎片填充,并且裂缝表层薄,而且氧化铁含量随深度减少。裂缝表层是多孔的,孔隙率可超过10%。有必要通过比较结晶岩含水层系统来评估其风化、构造和裂缝特征,通过比较矿物学/岩石学来评估其特征,这对于世界上大部分地区的水资源研究具有重要作用。

Resumo

O sistema aquífero cristalino do Alto Llano (Texas Central, EUA) abriga um importante recurso hídrico local que tem sido relativamente pouco estudado. O levantamento é um domo estrutural de rochas graníticas e metamórficas Précambrianas. Falhas normais e fraturas originadas no final do período Paleozóico, fraturas de descompressão, intemperismo, litologia e a matriz da rocha controlam as propriedades hidráulicas deste aquífero. Dados obtidos a partir de relatórios de perfuração (mais de 2,000 poços) mostram que poços em granitos têm vazões médias superiores aos poços em rochas metamórficas. A correlação entre a produtividade dos poços e a espessura do regolito é fraca, sendo a espessura mediana do regolito maior em rochas graníticas do que em metamórficas. A permeabilidade das fraturas, que são muito heterogêneas, apresentam maior controle sobre o fluxo de água. Os poços são rasos (geralmente <100 m de profundidade), embora os poços mais recentes sejam perfurados com profundidade maiores. Os dados de permeabilidade sugerem uma diminuição na abertura e densidade de fraturas com o aumento da profundidade, embora seja necessário levar em consideração o viés de amostragem do método. Testemunhos coletados com brocas diamantadas mostram que muitas fraturas próximas da superfície com abertura significativa são preenchidas por fragmentos de rocha oriundas do intemperismo, e que as películas que cobrem as fraturas são mais finas e contêm óxidos de ferro em um estado mais reduzido com o aumento da profundidade. As superfícies das fraturas podem ser porosas, podendo chegar a valores superiores a 10%. Há a necessidade de se comparar diferentes sistemas aquíferos cristalinos para avaliar o grau de intemperismo, tectônica, fraturas e mineralogia/petrologia para entender as características desses sistemas, que são recursos hídricos importantes em grande parte do mundo.

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Acknowledgements

We appreciate the support for this research from The University of Texas through the David P. Carlton Professorship in geology. We thank Glen Baum (Department of Geosystems and Petroleum Engineering) for providing the Hg-porosimetry data. The comments of Antonio Chambel, Uwe Troeger, and especially Rob Reed on earlier versions of this manuscript were very useful. Two anonymous reviewers gave us extensive comments and constructive criticisms, which we also greatly appreciate.

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Correspondence to John M. Sharp Jr.

Appendix: Figs. 10 and 11

Appendix: Figs. 10 and 11

Fig. 10
figure10

a Road cut in Llano County with granite bedrock overlain by weathered granitic regolith (grus); b Faulted and fractured (NNE, ~20°) granitic outcrop in Mason County; c Fault within a felsic gneiss (Packsaddle Schist) in the Llano River, Mason County. This is within a 240-m-wide zone trending to the NW (~200°). Two relatively high-yielding domestic supply wells are located about 60 and 210 m from the river’s edge. Regolith and alluvium were not present during well drilling of well that indicates yield is from fractures

Fig. 11
figure11

a Regolith showing the development of the saprolite/fissured layer developed on granite in Mason County. Where saturated, this is the zone of high permeability; b Close up showing incipient fissuring/layering

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Setlur, N., Sharp, J.M. & Hunt, B.B. Crystalline-rock aquifer system of the Llano Uplift, Central Texas, USA. Hydrogeol J 27, 2431–2446 (2019) doi:10.1007/s10040-019-02000-4

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Keywords

  • Crystalline rocks
  • Fractured rocks
  • Aquifer
  • Permeability
  • USA