PHYSICAL ATTRIBUTES OF ROCK JOINTS Part 7 PHYSICAL ATTRIBUTES OF ROCK JOINTS

Plan view illustrating spatial arraignment of systematic regional joints. Note overlapping nature and tendency to form clusters Block diagram showing systematic joint clusters of one joint set, or suite Block diagram illustrating how joints of the same set or suite may occasionally intersect one another Block diagram illustrating how joint sets intersect one another. These appear regular plan, but cross one another in section.

Schematic block diagram showing observed variables in spacing of primary systematic regional joints. Note how the spacing between joints decreases with decreasing bed thickness, likely due to variances in layer stiffness.

Systematic joints on a 1600 foot high exposure of the Navajo Sandstone near Hildale, Utah. Joint spacings are between 85 and 125 feet.

Intensity of joints Joint intensity refers to how numerous joints are; e.g. the physical separation, or, spacing, between adjacent joints The stiffness of a rock mass depends on the stiffness of the rock fabric AND the joint intensity, aperture, and infilling.

Joint Spacing Less stiff Stiff units tend to be more brittle, and spawn fractures on close spacings, as viewed here Slickrock Member of the Entrada Formation, Broken Arch at Arches National Park Stiff & brittle Less stiff

Upper sketch – Block diagram showing plumose structures on the face of a joint Lower sketch – Sketch showing plumose and conchoidal structure on the complete face of a joint, as they formed (seldom visible in its whole). The conchoids likely represent the physical extent of fracture extension during formation of the joint trace

Surface Roughness Calcite accumulations on a joint face are often misinterpreted as slickensides. These accretions are ascribable to groundwater percolation. Calcite striae can form parallel to the cross bed laminae or the direction of seepage.

Mineral Halos and “healed joints” Groundwater preferentially flows through joints in rock The chemistry of that groundwater determines whether the joints experienced solutioning or infilling. This shows a healed joint with mineralization halos

Undulating conchoidal ridges on the lower face of a large systematic joint in the Navajo Sandstone at Zion National Park. This conchoidal ridge (next to the park ranger) has an amplitude of almost 3 feet.

Joint Plane Surfaces: 1) main joint face; 2) joint fringe; 3) plumose structure; 4) fringe joints (B-planes); 5) C fractures; 6) shoulder of joint plane; and 7) trace of main joint face (taken from Hodgson, 1961)

Other Attributes of Rock Joints Joint aperture refers to the spacing between opposing faces of the same joint

Joint aperture affected by dilation Joints are zero tension boundaries, which can dilate, or open up, in response to changing loads or tectonic deformation This shows the opposing faces of a single joint in the grabens of Canyonlands, caused by solution underlying salt diapirs

Joint Control Most cliffs are joint controlled, by one, two, or three sets of primary regional systematic joints. Note dilation of joints seen here These joints are often open behind the cliff face, as shown here (Coconino Sandstone beneath Monument Point, North Rim of the Grand Canyon)

A common problem with mapping discontinuities exposed in outcrops are secondary fractures that tend to parallel the valley side, known as valley-side joints. These joints are not often visible and are, generally, the most deadly features.