FOLIATION AND LINEATION.pptx

Geology- Foliation and lineation- types

1. FOLIATION AND LINEATION

2. Fabric The fabric of a rock is the geometric arrangement of component features in the rock. If there is no preferred orientation (i.e., alignment) of the fabric elements, then we say that the rock has a random fabric . Undeformed sandstone, granite, or basalt are rocks with random fabrics. Deformed rocks typically have a non-random fabric or a preferred fabric , in which the fabric elements are aligned in some manner and/or are repeated at an approximately regular spacing.

3. There are two main classes of preferred fabrics in rock. A planar fabric, or foliation , is one in which the fabric element is a planar or tabular feature. A linear fabric, or lineation , is one in which the fabric element is effectively a linear feature. Foliations and lineations can be subdivided into primary (pre-deformational) ones, such as bedding, and secondary (deformational) ones. If the fabric elements are closer than 1 mm (that is, below the resolution of the eye), the fabric is continuous. When there is an obvious spacing between fabric elements, we say that the fabric is spaced.

4. Fabrics that form as a consequence of tectonic deformation of rock are called tectonic fabrics , and fabrics that form during the formation of the rock are called primary fabrics. Rocks with a penetrative tectonic fabric are also called tectonites . When linear fabric elements dominate, the rock is called an L-tectonite, A rock with dominantly planar fabrics is called an S-tectonite. The rocks with both types of fabric elements are called LS-tectonites.

5. Foliation A foliation is any type of planar fabric in a rock, property of rocks whereby break along approximately parallel surfaces. Sedimentary rocks shows a property called bedding fissility in which rocks split parallel to stratification or along bedding planes. Foliation plane is usually reffered as S plane. The term has no genetic implications and may include sedimentary bedding or igneous layering, etc.

6. Metamorphic foliations are divided into cleavages , schistosity and gneissose structure. Cleavage or rock cleavage – it is the property of rocks whereby they break along approximately parallel surfaces of secondary origin. Cleavages are foliations of secondary origin and usually parallel to bedding or inclined to bedding. It is continuous cleavage or spaced cleavage. Slaty cleavage – it is generally shown by slate due to parallel arrangement of platy minerals lik mica, chlorite, etc. . Slaty cleavage

7. 2. Schistosity – secondary foliation developed by recrystallisation, in which rocks tend to split along parallel planes and is the general property of schist. Common in high grade metamorphic rocks. 3. Gneissosity / gneissic banding – in case of gneissosity, alternate silicic and mafic layers are common. Mafic is shown by ferromagnesian minerals and felsic by light coloured minerals.

8. Other types of foliation includes;- Fracture cleavage – these are closely spaced joints. The distance between fractures exceed a few cms. In fracture cleavage, an individual block are bounded by fractures called microlithon. Shear cleavage – it is synonym with slip cleavage. Fracture cleavage with displacement. In the other hand, shear cleavages are closely spaced fractures along with those has been some displacement. Bedding cleavage – in some bedding plane, cleavage is parallel to the bedding, it is called bedding cleavage/bedding schistosity. Axial plane cleavage– those foliations that are parallel to the axial plane of folds.

9. Lineations It is the result of the parallelism of some directional property in the rock, any linear feature that appears on the surface of the rock .

10. Primary lineation It is found in both sedimentary and igneous rock and are formed along with rock. Linear flow structure – these are formed in igneous rocks as a result of flowage of magma, longest axis of mineral align parallel to one another and will be parallel to the flow direction. Clots / aggregation – individual minerals when formed together clots may develop, clots may allign parallel to themselves during the flowage of magma. Mineral lineation- linear parallelism of equigranular minerals develop mineral lineation.

11. Secondary lineation These are superimposed on rock after they are originally deposited, erupted or intruded. May be imposed more than once, so several lineation may be present in a rock. May occur with or without foliation, but more commonly, associated with foliation. A rock without cleavage or schistosity possess lineation.

12. Elongated / stretched pebbles - They usually result from the elongation of minerals or mineral aggregates (stretching lineations). Stretched pebbles in deformed conglomerates is a common example. Intersection lineation - Intersection of bedding and cleavage produces a lineation. This is because the intersection of two planes is a line and if the rocks break parallel to the cleavage, the trace of bedding appears as parallel; streaks on cleavage.

13. 3. Slickenslides – polished striated surfaces that result from friction along the fault plane. Scratches clearly indicate direction of movement.

14. 4. Boudinage or sausage structure – I n cross section, a competent bed thickens and thins to form a string of sausages. It is parallel to bedding, each unit look like sausages lying side by side. Line of junction of individual units may be called boudin line and this is a lineation. Boudin line may be occupied by quartz, feldspar or other mineral. Boudinage a result of stretching at right angles to boudin line.

15. 5. Pencil lineation – it is a very distinctive linear structure associated with folded and cleaved mudstone and silt stones. It have a prefered orientation and forms a lineation in unmetamorphosed and very low grade metamorphic rocks. 6. Mullions - are linear deformation structure that are restricted to the interface between a competent and incompetent rock.