The document discusses joints, which are fractures in rock with no measurable shear displacement. There are two main types of joints: systematic joints which are parallel and evenly spaced, and non-systematic joints which are irregular. Joint sets are groups of systematic joints that intersect at consistent angles, forming joint systems such as orthogonal and conjugate systems. The origin and characteristics of different joint types like sheeting and columnar joints are described. Factors that control joint spacing and the data to collect during joint analysis are also outlined.

1. JOINTS
inherently important in the study of fluid flow
through rocks.
JOINTS-
-Fractures
-no measurable shear displacement
-Thought to represent Mode I fractures
(tensile stress features)

2. • Systematic vs. Non-Systematic Joint Arrays
• Non-Systematic Joints-
• -irregularly oriented joints
no obvious spatial relationship
-.
• Systematic Joints-
• -group of parallel to sub-parallel
joints
• - evenly spaced to one another

3.
• Joint Sets- a group of systematic joints
• Joint System-
• -defined by two or more joint sets
• - joint sets intersect at constant angles.

4. • Dihedral Angle- angle between two joint sets
in a joint system
• -Orthogonal System- mutually perpendicular
joint sets
• (dihedral angle =90)

5. • -Conjugate System- dihedral angle ~ 30-60.

6. • Joint types:
• Master Joints and Cross Joints- master joints
are long uninterrupted joints linked by truncated
cross joints
• Strike Parallel Joints- joints parallel to strike
of bedding
• Cross-Strike Joints- joints at high angle (>60)
to bedding strike

7. • Sheeting Joints (Exfoliation Joints)-
• -Common in intrusive/metamorphic rocks
• -Particularly in rocks that lack strong foliations.

8. • Exfoliation Domes- -Spherical joint surfaces of
intrusive/metamorphic rocks; Common in steep
topography

9.
• Columnar Joints-
• -Cooling joints;
• -Develop from the top and base towards the
middle.

10. • Plumose structures-
• -Feathery Mode I joint surfaces
• -Develop due to rock inhomogeneity
• -Variable stress distribution as the joint grows.

11. • En Echelon Joints-
• -Sub parallel joints
• -Offset to each other

12. • Joint spacing-
• -Average distance between joint surfaces;
• dependent upon:
• 1)Bed thickness-
• -Joints closely spaced in thinner beds
• 2)Lithology-
• -Stiffer lithologies ~ closer spacing
• -Less stiff rocks ~wide joint spacing.
• 3)Tensile strength of the rock-
• -Lower tensile strength ~ more joints
• 4)Magnitude of extensional strain

13. • Origin and Tectonic Interpretation of
Joints:
• Uplift and Unroofing
• Hydraulic Fracturing
• Tectonic Denudation

14. • Joint Analysis:
• Points to consider & data to collect in the analysis of
joints.
• Are the joints systematic or non-systematic?
• Determine the orientation of joints present
• How many different joint sets are present?
• Determine cross-cutting relationships
• Describe the appearance of the joints.
• Describe the dimensions of the joints
• Describe joint spacing and density
• Describe relationship of joints & lithology
• Do joints vary with changes in lithology.
• Are joints isolated or connected to regional network?
• How do joints relate to geologic structures?