Part 7Part 7

PHYSICAL PHYSICAL ATTRIBUTES ATTRIBUTES

OF ROCK OF ROCK JOINTSJOINTS

Plan view illustrating spatial arraignment Plan view illustrating spatial arraignment of systematic regional joints. Note of systematic regional joints. Note overlapping nature and tendency to form overlapping nature and tendency to form clusters clusters

Block diagram showing systematic Block diagram showing systematic joint clusters of one joint set, or suite joint clusters of one joint set, or suite

Block diagram illustrating how joints Block diagram illustrating how joints of the same set or suite may of the same set or suite may occasionally intersect one another occasionally intersect one another

Block diagram illustrating how joint Block diagram illustrating how joint sets intersect one another. These sets intersect one another. These appear regular plan, but cross one appear regular plan, but cross one another in section. another in section.

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

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

Intensity of jointsIntensity of joints

• Joint intensityJoint intensity refers to refers to how numerous joints how numerous joints are; e.g. the physical are; e.g. the physical separation, or, spacing, separation, or, spacing, between adjacent jointsbetween adjacent joints

• The stiffness of a The stiffness of a rock rock massmass depends on the depends on the stiffness of the stiffness of the rock rock fabricfabric AND the AND the joint joint intensity, apertureintensity, aperture, and , and infillinginfilling. .

Joint Joint SpacingSpacing

• Stiff units tend to Stiff units tend to be more brittle, be more brittle, and spawn and spawn fractures on fractures on close spacings, close spacings, as viewed hereas viewed here

• Slickrock Slickrock Member of the Member of the Entrada Entrada Formation, Formation, Broken Arch at Broken Arch at Arches National Arches National Park Park

Stiff & Stiff & brittlebrittle

Less stiffLess stiff

Less stiffLess stiff

• Upper sketch – Block diagram showing Upper sketch – Block diagram showing plumose structuresplumose structures on on the face of a jointthe face of a joint

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

Surface Surface RoughnessRoughness

• Calcite accumulations on Calcite accumulations on a joint face are often a joint face are often misinterpreted as misinterpreted as slickensides. These slickensides. These accretions are ascribable accretions are ascribable to groundwater to groundwater percolation. percolation.

• Calcite striae can form Calcite striae can form parallel to the cross bed parallel to the cross bed laminae or the direction of laminae or the direction of seepage. seepage.

Mineral Halos and Mineral Halos and “healed joints”“healed joints” • Groundwater Groundwater

preferentially flows preferentially flows through joints in rockthrough joints in rock

• The chemistry of that The chemistry of that groundwater groundwater determines whether the determines whether the joints experienced joints experienced solutioning or infilling.solutioning or infilling.

• This shows a healed This shows a healed joint with joint with mineralization halos mineralization halos

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

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

Other Attributes of Rock JointsOther Attributes of Rock Joints

Joint apertureJoint aperture refers to refers to the spacing between the spacing between opposing faces of the opposing faces of the same joint same joint

Joint aperture Joint aperture affected by affected by

dilationdilation• Joints are zero tension Joints are zero tension

boundaries, which can boundaries, which can dilate, or open up, in dilate, or open up, in response to changing response to changing loads or tectonic loads or tectonic deformationdeformation

• This shows the opposing This shows the opposing faces of a single joint in faces of a single joint in the grabens of the grabens of Canyonlands, caused by Canyonlands, caused by solution underlying salt solution underlying salt diapirs diapirs

Joint ControlJoint Control

• Most cliffs are joint Most cliffs are joint controlled, by one, two, controlled, by one, two, or three sets of primary or three sets of primary regional systematic regional systematic joints. joints.

• Note dilation of joints Note dilation of joints seen hereseen here

• These joints are often These joints are often open behind the cliff open behind the cliff face, as shown here face, as shown here (Coconino Sandstone (Coconino Sandstone beneath Monument Point, beneath Monument Point, North Rim of the Grand North Rim of the Grand Canyon)Canyon)

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