3 Joints
The photograph in Figure 7 shows nearly vertical joints in sandstone beds. The pencil parallel to flat-lying bedding is included to provide scale.
Figure 7 – a) A joint in aeolian sandstone, commonly perpendicular to beds. The pencil along the bedding interface is for scale. b) A field diagram showing joint segments confined within beds indicating the truncation or abutting at bedding interfaces. c) A steep erosional surface showing joints and sheared joints in Entrada Sandstone, northwest of the Devil’s Garden parking lot, Arches National Park, Utah. Nils Johnson when he was about 8 years old appears for scale.
Joints are generally planar elements made by subparallel fracture walls that moved predominantly away from each other, resulting in an aperture value that might vary across the fracture. The field map in Figure 7b is a sketch depicting joints on an erosional surface of an outcrop with sub-horizontal layers, as in the photograph in Figure 7a. Natural joints, at least initially, commonly form orthogonal to bedding and are bed-confined (Figure 7). Bed confinement is due to truncation or abutting of joints at bed interfaces, or against any other pre-existing discontinuity, impeding the propagation of joints across (Helgeson and Aydin, 1991; Pollard and Aydin, 1988). Figure 7c shows a photograph of a steep erosional surface with a few well-developed lineaments that are joints in Jurassic Entrada Sandstone at Arches National Park, Utah, USA. The small boy in view is the younger son of A.M. Johnson, one of the pioneers of geomechanics, who along with his students studied these joints (Dyer, 1983; Cruikshank and Aydin, 1995). This site will be used later when introducing joint sets, the spacing of joints in a set and the shearing of fractures originally formed in opening mode. Often, once formed, joints are subsequently sheared and accommodate small shear offsets, the amount and kinematics of which may vary from location to location (Figure 7). Joints, if not filled by relatively strong material (in which case they are known as veins), are generally planes of weakness in rocks and are often prone to shearing. The products of this shearing are called sheared joints or simply small faults.
Aside from the aperture values, other features indicate opening across joints. These are known collectively as plumose structures, which are characteristic ornaments of joint surfaces including rib marks (Figure 8a) and hackle marks (Figure 8b). Rib marks represent pauses in the advance of the main fracture. Hackle marks are lineaments on joint surfaces that are easy to identify. Hackle marks indicate local propagation direction and, in some cases, the relative magnitude and position of the driving principal stress with respect to the axis of symmetry in the plumose pattern. These are well known in structural geology textbooks and are included here to complete the treatment of the subject for the benefit of those in allied fields.
Figure 8 – Joint surface morphologies, known as plumose structures, indicate opening or dilation perpendicular to the discontinuity when it first formed. a) Rib marks in Aztec Sandstone, Valley of Fire State Park, Nevada, USA. b) Hackle marks in a siltstone layer in Chilean Patagonia. Hackle marks can be used to infer the initiation and propagation direction of the joints as well as the symmetry of the driving stresses.