Natural Fractures

Fracture Identification Flow-Chart

The following series of nested charts will help you identify and describe the different types of natural rock fractures. Lists of criteria for identifying features (boxes) include criteria that are available in image logs and/or core and/or outcrop. All of these features except slickolites and tail joints are identified in image logs under favorable circumstances, however compaction and deformation bands cannot be differentiated from each other in image logs.

To identify and describe a fracture, work through the flowchart from the beginning. Each chart leads to a box that contains a button outlined in blue. Click on the button outlined in blue to go to the next step in the process of classifying and describing the fracture.

Chart 1: Start

Chart 2: Stylolites

	Describe a stylolite or slickolite



	In image logs, core or outcrop record the following: Orientation (dip-angle/dip-azimuth) of the overall plane of the feature. If the feature is a slickolite, then measure the orientation (plunge-angle/plunge-azimuth) of the teeth. Maximum tooth height. Thickness of fill (insoluble residue). Evidence of relative age such as abutting and crosscutting relations. Any evidence of reactivation. This completes the classification and description of a stylolite or slickolite. Return to START to classify and describe another fracture.

Chart 3: Fractures

	Describe a natural fracture.



	In image logs, core or outcrop record the following: Orientation (dip-angle/dip-azimuth) Evidence of relative age such as abutting and crosscutting relations. Overall shape, planarity or any other distinctive feature. Any available information on size and shape. Host lithology.

	Describe a compaction band.



	In image logs, core or outcrop record the following: · Orientation (dip-angle/dip-azimuth) Evidence of relative age such as abutting and crosscutting relations. Overall shape, planarity or any other distinctive feature. Any available information on size and shape in the plane of the compaction band. Host lithology. The orientation of sedimentary layering inside and outside of the band, if the layering is visibly deflected. Deformation bands and compaction bands have very little porosity (<3%) and no large voids so that any cement or mineral fill is not readily visible and isn't described. This completes description of a compaction band. Return to START to classify and describe another fracture.

Chart 4: Joints

	Describe a joint



	In image logs, core or outcrop record the following: All joints: Orientation (dip-angle/dip-azimuth) Evidence that the feature is a joint. Different evidence is available in image logs, core and outcrop. Evidence of relative age such as abutting and crosscutting relations. Gross morphology. Any available information on size and shape. In outcrop, sometimes in core: Surface morphology. Any evidence of reactivation. Record additional information on joint subtypes: *Contained joints* Nature and lithology of the feature that contains the joints. In other words, is the joint contained within a single bed, between two older joints, or some other feature? Thickness/spacing of bed, strata, or older fractures that contain the joint. *Pinnate joints* Orientation of parent fault and other relevant information. Planarity and any available shape information. *Tail joints* Orientation and nature of parent fracture.

Chart 5A: Faults

	Describe a fault or fault zone

	Describing a fault is a four-part process: Step 1: Name the fault type based on the slip-sense and slip-direction Step 2: If your fault is a fault zone, then describe the fault-rocks Step 3: Record the key characteristics Step 4: Describe the mineral fill
	Fault description - Step 1: Name the fault type based on the slip-sense and slip-direction.
	At this point, you should name your fault either: Normal Reverse Right-lateral wrench Left-lateral wrench or give it a compound name such as *right-lateral reverse fault. If you are comfortable with fault names, then name your fault and continue to the next steps, which are recording the key characteristics and describing the fault rocks. If you are uncomfortable with fault names, then read about fault nomenclature* first.


	Fault description - Step 2:

	Fault description - Step 3: Record the key characteristics.

	In image logs, core or outcrop record the following: *All faults:* Orientation (dip-angle/dip-azimuth) Evidence that the feature is a fault. Different evidence is available in image logs, core and outcrop. Slip parameters. Evidence of relative age such as abutting and crosscutting relations. Thickness of damage zone and type of damage. Gross morphology. Any available information on size and shape. *Fault zones:* Thickness of fault rocks. Type of fault rocks. Internal structure. In core and outcrop record this additional information: Surface morphology. Any evidence of reactivation.

Fault description - Step 4

	Describe the mineralization

	Describing the mineral fill, or the lack of it, is the last step in classifying and naming a rock fracture. Classify the fill type and name your fracture. Void in fault rocks become mineralized just like the void spaces in open fractures. Examples of fracture names follow the table. Note that deformation bands and compaction bands have very little porosity (<3%) and no large voids so that any cement or mineral fill is not readily visible and isn't described.

Here's some examples of fracture names:

Unmineralized reverse fault.
Partially mineralized contained joint.
Fracture.
Partially filled fracture.
Mineralized, brecciated, fault zone.

Return to START to classify and describe another fracture.