Cleavage

Describing the tectonic foliation for Structural Geology 2019.

Kate Flynn

What is cleavage?

Cleavage is a tectonic foliation, meaning it is a sub-planar, often parallel, preferred fabric that develops in low-grade rocks. A fabric is any geometrical arrangement of features in the rock. As a tectonic foliation, cleavage occurs as a response to tectonic activity. However, it forms without the loss of cohesion and is not a fracture. Cleavage is often used to indicate the long axis of the strain ellipse, as it forms perpendicular to the maximum shortening direction.

How does cleavage form?

Diagram of cleavage formation. Image from Foliations/Lineations slides (2019).

Cleavage is a dissolution feature, meaning that minerals easily dissolved will be removed in the presence of a directed stress. This process requires H2O and less than 10% clay. The residual clay minerals will become aligned perpendicular to the directed stress, and form the cleavage domain. In some cases, no specific stress field is necessary and mineral will dissolve in the presence of water. This is called free-face dissolution (Wilkerson, class notes, 2019). 

The Wilkerson diagram explains how the area of greater clay concentration becomes aligned perpendicular to a directed stress, or the maximum shortening direction.

Cleavage typically forms at a depth of around 10-15km below the surface. This is the depth where pressure solutions act on the rock to form cleavage in a ductile setting, so there is no fracturing. It is a low grade metamorphism.

Cleavage Elements

  • Cleavage domain: the narrow planar region which has a concentration of insoluble minerals, such as a clay.
  • Microlithon: band of unaffected rock between domains. Spaced cleavage will have microthons visible between the cleavage, while in continuous cleavage the microlithon can be difficult to point out.
  • Fabric: fabrics can be continuous or spaced. In continuous fabrics, cleavage is pervasive at all scales. In spaced fabrics, rock contains fabric and regions missing fabric.

Not all cleavage looks the same. Cleavage can be described using domain morphology and the spacing of domains. Domain morphology describes whether the cleavage is sutured, like a styolite, or non-sutured. If it is non-sutured, the cleavage can be planar, wavy, or anastamosing.  

Spacing and morphology are determined by clay content and strain. The more clay present in a rock, the more susceptible it will be to cleavage formation as the clay can act as a conduit for water. Lower strain rates also give wider spacing. Check out some examples below.

Styolites

Styolitic cleavage is a form of sutured cleavage domain. Styolites are formed when mineral materials are removed by pressure dissolution.

Image from  Wikipedia .

Non-sutured cleavage can be wavy or planar.

This image is an example of planar cleavage. Sample from Scott Wilkerson, image by Kate Flynn.

Anastamosing cleavage is another non-sutured domain morphology.

More cleavage types:

Compactional cleavage is the reworking of mineral grains as pore spaces collapse during compaction. It occurs in sedimentary rocks as bedding is reworked.

Through compaction cleavage, a clay or a claystone will become a shale with distinct cleavage.

Disjunctive cleavage occurs when cleavage cuts across, but does not reorient previous foliations. Styolitic and anastamosing cleavage are also examples of disjunctive cleavage.

Photo by Kate Flynn

The image is from Van Hise Rock in Rock Springs, WI. The beds pictured here at Van Hise Rock are phyllite (left) and quartzite (right). The phyllite layer appears to be slightly wavy and continuous while the fabric of the quartzite layer is mostly-spaced and shows more of the microlithon. Van Hise Rock is also an example of cleavage refraction.

Refracted cleavage occurs when it is reoriented across lithologic boundaries due to mechanical differences. Refraction is influenced by simple shear, because clay-rich rocks will be display more rotation of the strain ellipse, and strain accommodation between the beds.

At Van Hise Rock, the cleavage in the phyllite layer will be co-planar with the fold anywhere in the region of the Baraboo Syncline. Cleavage in the quartzite will fan out in comparison to the fold (Wilkerson, pers. com., 2019).

Some cleavage is not bedding parallel, and forms parallel to the axial plane of a fold. This is called axial planar cleavage. Similar to this is transecting cleavage in which the cleavage planes cut across the axial plane of the fold.

Image from Wilkerson (2019). This diagram shows the planes of cleavage that are parallel to the axial plane of the fold. The bedding, or the "S0", is the original structure and the axial planar cleavage is the "S1" structure that intersects it.

Diagram of transecting cleavage from Wilkerson's Foliations and Lineations slides (2019).

Crenulation cleavage is created when cleavage planes cut across and reorient foliations, causing folding.

It generally forms in finely laminated sequences and can be symmetric or asymmetric. In some cases, transposition can occur, where the crenulation cleavage is created progressively.

Crenulation Cleavage. Image from  Wikipedia .

Pencil Cleavage occurs when two spaced fabrics intersect, causing parting in pencil-like pieces. It commonly develops with one tectonic cleavage and one compactional cleavage in a clay-rich rock.

Pencil cleavage begins spaced, but eventually can develop into continuous or slaty cleavage with increased strain (Wilkerson, 2019).

Pencil Cleavage.JPG from  Wikipedia .

Cleavage-bedding relationships:

Cleavage-bedding relationships can be used to determine orientation of structures like synclines and anticlines using their angular relationships. This is helpful to a geologist out in the field especially in a place with poor outcrops.

One thing that geologists might use this for is determining closure direction of a fold. This can be done by extending the cleavage through bedding and rotating towards the acute angle. When the angle is steeper than the beds, we have an upright fold. If it is shallower, the fold is overturned.

Image from Rey (2019).

Cleavage relationships can also be used to visually infer the location of a fold closure. Cleavage diverges away from the fold closure. In a syncline, cleavage diverges up as the beds also shear up out of the core of the syncline. The only possible geometry for the fold in this example is an anticline, as the cleavage diverges down out of hinge (Ray, 43).

Van Hise Rock (pictured above) is an excellent example of using cleavage bedding relationships to determine the surrounding fold closures, and to locate the outcrop in the fold system.

By determining the relationship between cleavage and bedding at Van Hise Rock, it can be inferred that it sits on the North Range of the syncline, where the beds shear up out of the core of the syncline.

IM=mage from Maitai (2008).

References:

Diagram of cleavage formation. Image from Foliations/Lineations slides (2019).

Photo by Kate Flynn

Image from Wilkerson (2019). This diagram shows the planes of cleavage that are parallel to the axial plane of the fold. The bedding, or the "S0", is the original structure and the axial planar cleavage is the "S1" structure that intersects it.

Diagram of transecting cleavage from Wilkerson's Foliations and Lineations slides (2019).