Jointing and fracture interactions in middle eocene chalks near beer sheva, Israel

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This study presents some observations on jointing and fracture interaction in synclinal Middle Eocene chalks (Horsha formation) near Beer Sheva, Israel. Three joint sets occur in an outcrop; set 344° (N16°W) developed first, and was followed by sets 028° and 062°. Each set reflects a different fracture mechanism. Joint spacing and fracture length were investigated in set 028° but there did not appear to be any correlation between the two parameters. The mean spacings in two layers 30-cm- and 60-cm-thick range closely between 23 ± 16 cm and 26 ± 19 cm, suggesting independence of joint spacing from bed thickness in set 028°. Log-probability plots of joint lengths from these two layers suggest an approximate lognormal distribution. The mean joint lengths are 82.2 cm ± 56.8 cm and 64.1 cm ± 44.0 cm in the thicker and thinner beds, respectively. Statistically, the length populations in the two beds are considered to be significantly distinguishable, implying that the mean fracture lengths show some dependence on layer thickness. Joint sets that reflect different fracture mechanisms show differences in intensity and in style of fracture termination. Terminations between parallel joints of the same set are differentiated from interactions between joints of different sets at acute angles. In joints of set 028°, various crack terminations are classified into seven styles. There is on the average a visible interaction every 120 cm along joints of set 028°. Interactions between sets 0.28° and 344° induce secondary fractures, which are identifiable as three style groups. The most frequently induced secondary fractures are either subparallel or subperpendicular to the two sets. The sum length of the secondary cracks in an interaction zone does not seem to be akin to the lengths of the parent joints. Especially puzzling interactions between sets 028° and 344° are triple junctions and complex triple junctions defined by these fractures. Interactions between sets 028° and 0.62° result in rhomboid structures at acute angles ranging between 18° and 51°. Evidently, these structures are the product of extension, and no shear offsets are visible along the fractures. Characteristically, some of these rhomboids are being subdivided into smaller structures by the same sets.