The relationship between joint aperture, spacing distribution, vertical dimension and carbonate stratification: An example from the Kimmeridgian limestones of Pointe-du-Chay (France)

Joint aperture and joint development have been studied in the Kimmeridgian limestones of the Pointe-du-Chay, at the northern boundary of the Jurassic Biscay Basin (France). At Belette outcrop, in some layers the mean joint spacing of the N120° joint set is close to mean layer thickness. There, the classical spacing to thickness relationship appears to be valid in the competent carbonate layers that are included in a more argillaceous matrix. At Pillar outcrop, the N10° joint set is characterised by a high level of joint density and a non-saturated spacing distribution as indicated by the mode/mean ratio values and the C v values; C v is the ratio of standard deviation to mean fracture spacing. The classical relationship between layer thickness and fracture spacing has not been observed at the Pillar outcrop. Joint aperture reaches larger values at the Pillar outcrop than at the Belette outcrop where aperture is more homogeneously distributed. Almost all the joints are opened with moderate aperture values at Belette outcrop whereas most joints with large vertical dimension have large apertures at Pillar outcrop, and smaller fractures are closed or poorly opened. From two outcrops that have been subjected to the same geological conditions, apertures of non-stratabound joints appear to be controlled by the vertical dimension of the joints whereas stratabound joints are more regularly spaced and opened.