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)  

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

Joint Sets- a group of systematic joints

Joint System-

    -defined by two or more joint sets

   - joint sets intersect at constant angles.  

Dihedral Angle- angle between two joint sets in a joint system

    -Orthogonal System- mutually perpendicular joint sets

                    (dihedral angle =90⁰)

-Conjugate System- dihedral angle ~ 30⁰-60⁰.

Cross-Cutting Relations-  

    determining relative timing of joint sets

Mutually cross cutting-  

            �+� or �x� intersections; uncertain age relationships.  

        May be conjugate pair

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 (>60⁰) to bedding strike

Sheeting Joints (Exfoliation Joints)-

        -Common in intrusive/metamorphic rocks

        -Particularly in rocks that lack strong foliations.

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

Columnar Joints-

    -Cooling joints;               

    -Develop from the top and base towards the middle.

Photo: Devil's Tower, Wyoming

Plumose structures-

-Feathery Mode I joint surfaces

-Develop due to rock inhomogeneity

-Variable stress distribution as the joint grows.

En Echelon Joints-

   -Sub parallel joints

   -Offset to each other

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

Origin and Tectonic Interpretation of Joints:

                Uplift and Unroofing

                Hydraulic Fracturing

                Tectonic Denudation

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?

Data Projection:

                Stereonets

                Histogram

                Rose Diagrams

                Joint Trajectory map

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