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Joints, Tensile Strength and Preferred Fracture Orientation in Sandstones

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Joints, Tensile Strength and Preferred Fracture Orientation in Sandstones Document generated on 09/25/2021 1:34 a.m. Atlantic Geology Joints, tensile strength and preferred fracture orientation in sandstones. New Brunswick and Prince Edward Island, Canada E.Z. Lajtai and P. Stringer Volume 17, Number 2, Summer 1981 Article abstract The orientations of tensile fractures in Upper Devonian, Carboniferous and URI: https://id.erudit.org/iderudit/ageo17_2art01 Lower Permian sandstones of southern New Brunswick and Prince Edward Island show correlation at the submicroscopic scale of microcrack alignments, See table of contents the macroscopic scale of joints, and the megascopic scale of airphoto lineaments. Planes of minimum tensile strength and planes of preferred fracture, induced in visibly unfractured sandstone test-samples by line-loading Publisher(s) and point-loading tests, are identified as microcrack alignments. The joints and airphoto lineaments have microcrack alignments parallel to them, but not all Maritime Sediments Editorial Board microcrack alignments are represented at the macroscopic and megascopic scales. ISSN Deformed Carboniferous sandstones east of Saint John have two prominent 0843-5561 (print) orthogonal Joint systems with microcrack alignments parallel to the four joint 1718-7885 (digital) sets. The airphoto lineaments are parallel to only three joint sets. Southeast and northeast striking joints in one orthogonal system are tentatively interpreted as planes of extension and release respectively, related to the direction of Explore this journal compression during the Variscan-Appalachian orogeny. The other orthogonal system, with joint sets striking close to 010° and 100°, is interpreted as post-Triassic in age, possibly related to the presently acting crustal stresses in Cite this article eastern North America. Lajtai, E. & Stringer, P. (1981). Joints, tensile strength and preferred fracture orientation in sandstones. New Brunswick and Prince Edward Island, Canada. Atlantic Geology, 17(2), 70–87. All rights reserved © Maritime Sediments, 1981 This document is protected by copyright law. Use of the services of Érudit (including reproduction) is subject to its terms and conditions, which can be viewed online. https://apropos.erudit.org/en/users/policy-on-use/ This article is disseminated and preserved by Érudit. Érudit is a non-profit inter-university consortium of the Université de Montréal, Université Laval, and the Université du Québec à Montréal. Its mission is to promote and disseminate research. https://www.erudit.org/en/ Maritime Sediments and Atlantic Geology VOL. 17 AUGUST 1981 NUMBER 2 Joints, tensile strength and preferred fracture orientation in sandstones, New Brunswick and Prince Edward Island, Canada E. Z. Lajtai, department of Geological Engineering University of Manitoba, Winnipeg, Manitoba, Canada RST 2N2 and P. Stringer, Department of Geology, University of New Brunswick P.O. Box 4400, Fredericton, New Brunswick, Canada ESB 5A3 The orientations of tensile fractures in Upper Devonian, Carboniferous and Lower Permian sand- stones of southern New Brunswick and Prince Edward Island show correlation at the submicroscopic scale of microcrack alignments, the macroscopic scale of joints, and the megascopic scale of air- photo lineaments. Planes of minimum tensile strength and planes of preferred fracture, induced in visibly unfractured sandstone test-samples by line-loading and point-loading tests, are identified as microcrack alignments. The joints and airphoto lineaments have microcrack alignments parallel to them, but not all microcrack alignments are represented at the macroscopic and megascopic scales. Deformed Carboniferous sandstones east of Saint John have two prominent orthogonal joint systems with microcrack alignments parallel to the four joint sets. The airphoto lineaments are parallel to only three joint sets. Southeast and northeast striking joints in one orthogonal system are tentatively interpreted as planes of extension and release respectively, related to the direction of compression during the Variscan-Appalachian orogeny. The other orthogonal system, with joint sets striking close to 010° and 100°, is interpreted as post-Triassic in age, possibly related to the presently acting crustal stresses in eastern North America. II yacorrespondance dans les gres du Devonien superieur, du Carbonifere et du Permien inferieur du sud du Nouveau-Brunswick et de l'ile du Prince-Edouard entre 1'orientation des fractures de tension a diverses echelles: microscopique — alignement des microfissures; macroscopique — dia- clases; et megascopique — lineaments observes a partir de photographies aeriennes. On reconnait comme alignements de microfissures les plans de force de traction minimale et les plans de frac- ture preferentielle occasionnes par des essais de charges lineaires et ponct-uelles sur des echan- tillons types de gres prealablement exempts de fractures apparentes. Bien que parallSlement aux diaclases et aux lineaments on retrouve des alignements de microfissures, ce ne sont pas tous les alignements de microfissures qui s'expriment aux echelles macroscopique et megascopique. A l'est de Saint-Jean, les gres deformes du Carbonifere possedent deux systemes distincts de diaclases orthogonales accompagnes d'alignements de microfissures paralleles aux quatre ensembles de diaclases. Les lineaments observes sur photographie aerienne ne sont paralleles qu'a trois ensembles de diaclases. On interprete provisoirement les diaclases de direction sud-est et nord- est dans l'un des systemes orthogonaux comme, respectivement, des plans d'extension et de relache- ment relies a la direction de compression durant l'orogenese varisque-appalachienne. Des orien- tations se rapprochant de 010° et 100° caracterisent 1'autre systeme orthogonal qui serait d'age post-triasique et peut etre relie aux forces de tension contemporaines agissant sur l'ecorce terrestre dans l'est de l'Amerique du Nord. [Traduit par le journal] INTRODUCTION between joints and microfabric Although the rock occurring be- has been described in sandstone by tween joints is often assumed to Reik and Currie (19 74), and in be unaffected by fracture, there quartz diorite by Swolfs et al. have been a number of studies (1974). Microcrack alignments may demonstrating that fractures, at a exist either parallel or perpen- much smaller scale than joints, do dicular to the maximum principal exist in the seemingly unfractured stress direction(s) of the geo- rock. For example, correlation logical past (Lajtai and Alison 1979). Preliminary comparison of MARITIME SEDIMENTS AND ATLANTIC GEOLOGY microcrack alignments and jointing 17, 70-87 (1981) 71 LAJTAI & STRINGER. (Lajtai and Alison 1979) nas indi- of five predominantly terrestrial cated that joints have microcracks formations in southern New Bruns- running parallel to them, but not wick and Prince Edward Island necessarily all microcrack align- (Fig. 1), ranging from Upper De- ments become joints. vonian to Lower Permian in age. Microcrack alignments in oriented, The main purpose of the present visibly unfractured sandstone test- investigation has been to estab- samples have been compared with lish a correlation between tensile the strikes of subvertical (75- fractures occurring at the sub- 90° dipping) joints in the flat- microscopic (microcrack align- lying Pictou Formation and the ments) , macroscopic (joints), and gently dipping Perry Formation megascopic (airphoto lineaments) (mainly undeformed rocks of Wil- scales. Joint orientations have liams 1978, see Fig. 1), and in the been measured in the sandstones folded, thrust-faulted and locally LEGEND UPPER DEVONIAN CARBONIFEROUS and LOWER PERMIAN O mainly undeformed terrestrial deposits 100 km j | deformed predomitpredominantly terrestrial deposits 47°- Fig. 1 - Location map of sandstone formations investigated. Locality 1 - Perry Formation; 2 - Balls Lake Formation; 3 - McCoy Head Formation (Emerson Creek); 4 - McCoy Head Formation (Gardner Creek) ; 5 - Boss Point Formation; 6 - Pictou Formation. Areas of mainly undeformed and deformed Upper Devonian, Carboniferous and Lower Permian rocks are based on Williams (1978). MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 72 cleaved Pennsylvanian Boss Point sponds to the plane of minimum ten­ and McCoy Head Formations (deformed sile strength, and the second de­ rocks of Williams 1978). Airphoto termines the tensile strength for lineaments have previously been any plane (McWilliams 1966, Lajtai established (Naing 1976 , Naing and and Alison 1979, Lajtai 1980); Lajtai 1977) east of Saint John in minima on the tensile strength dis­ southern New Brunswick. tribution curve identify the azi­ muths of the microcrack alignments. Two methods for identifying pref­ erentially oriented microcracks Fracture tests were made on sand­ in rocks are the point-loading and stone samples from flat-lying or the line-loading (Brazilian) tests. gently dipping (0-15° dip) beds in The first produces a preferred four of the formations investi­ fracture orientation which corre- gated. Discs of sandstone, pre­ pared by slicing core drilled per­ pendicular to bedding in the sam­ ples, were loaded along the axis of the disc in the point-loading tests and along diametral planes in the line-loading tests. The discs were 22.9 mm in diameter and 7.9 mm thick. Thin sections of the sandstone test-samples show no preferred orientation of con­ stituent grains or grain boun­ daries , and the fractures produced during the tests are assumed to represent microcrack alignments. The fracture planes produced in both tests were
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