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 =90)

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

• 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

• 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.

• 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?