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Rock Fabric - Structural Geology - Lecture Notes, Study notes of Geology

West Bengal State UniversityGeology

In these Lecture notes, Professor has tried to illustrate the following points : Rock Fabric, Secondary Planar, Linear Penetrative, Structures, Distributed Planar, Sedimentary Bedding, Imbricate Pebbles, Schistose Foliation, Slaty Cleavage, Gneissic Banding

Typology: Study notes

2012/2013

Uploaded on 07/22/2013

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seshadri_44het 🇮🇳

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I. Introduction
A. Rock Fabric = secondary planar and linear penetrative structures associated with
strain.
B. Foliation: homogeneously distributed planar structure in rock
1. examples
a. sedimentary bedding
b. imbricate pebbles in conglomerate
c. schistose foliation with parallel alignment of platy phyllosilicates
d. slaty cleavage
e. gneissic banding
C. Lineation: homogeneously distributed linear structure
1. surficial lineations: linear features on discrete surfaces only
a. slickenlines on fault
b. groove marks on sedimentary bedding plane
2. penetratitive: linear features occur throughout the body of rock
a. hinge lines of crenulations in foliation
b. preferred alignment of elongate minerals
D. Primary vs. secondary features
1. primary: occur as result of sedimentary or igneous process of rock
formation
2. secondary: originate as result of tectonic deformation or metamorphism
E. Other terms
1. S-surfaces: penetrative planar features designated as S1, S2, etc.
2. Rock cleavage: tendency for rock to break along surfaces of weakness
a. cleavage is a type of foliation in terms of rocks
3. banding: compositional zonation in rocks
4. Structural domains: units of rock with similar structural characteristics
5. Foliation morphology
a. spaced - foliations spaced at 10 um or more
b. continuous - fine, closely spaced foliation, < 10 um spacing.
II. Foliation
A. Compositional Foliations
1. Defined: banding = mineralogic zonation in rock
2. Types
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I. Introduction A. Rock Fabric = secondary planar and linear penetrative structures associated with strain. B. Foliation: homogeneously distributed planar structure in rock

  1. examples a.b. sedimentary beddingimbricate pebbles in conglomerate c.d. schistose foliation with parallel alignment of platy phyllosilicatesslaty cleavage e. gneissic banding C. Lineation: homogeneously distributed linear structure
  2. surficial lineations: linear features on discrete surfaces only a. slickenlines on fault
  3. b.penetratitive: linear features occur throughout the body of rock^ groove marks on sedimentary bedding plane a.b. hinge lines of crenulations in foliationpreferred alignment of elongate minerals

D. Primary vs. secondary features

  1. primary: occur as result of sedimentary or igneous process of rockformation
  2. secondary: originate as result of tectonic deformation or metamorphism E. Other terms
  3. S-surfaces: penetrative planar features designated as S1, S2, etc.
  4. Rock cleavage: tendency for rock to break along surfaces of weaknessa. cleavage is a type of foliation in terms of rocks 3.4. banding: compositional zonation in rocksStructural domains: units of rock with similar structural characteristics
  5. Foliation morphology a.b. spaced - foliations spaced at 10 um or morecontinuous - fine, closely spaced foliation, < 10 um spacing.

II. Foliation A. Compositional Foliations

  1. Defined: banding = mineralogic zonation in rock
  2. Types

a. Diffuse foliations: weakly zone mineral concentrations b. Banded foliations: strong compositional layering(1) e.g. gneisses

B. Disjunctive Foliations

  1. Defined: disjoined or detached foliations characterized by seams ofminerals a. microlithons: localized, seam-like accumulations of minerals of differing composition from rest of rock
  2. Types a. Stylolitic foliation: toothed, jagged cleavage common in b. limestones, and marblesAnastomosing foliation: wavy cleavage c.d. Rough foliation: blocky foliationsmooth foliation: e.g. slaty cleavage

C. Cenulation Cleavage

  1. crenulations: harmonic, small-scale chevron folds that develops in a pre-existing foliation

a. e.g. crenulated cleavage on a schistose foliation D. Continuous Foliations

  1. continuous foliation of mineral grains at microscopic level

E. Relationship of Foliation to Other Structures

  1. Fold-related foliation a. axial plane cleavage: foliation oriented parallel to axial surfaces offolds

F. Nomenclature of Foliations

  1. Slaty cleavage: fine continuous foliations characteristic of slates(comprised of finely crystalline phyllosilicates)
  2. Phyllitic cleavage: similar to slaty, only in coarser grained phyllites 3.4. Schistosity: foliation in coarse-grained mica-rich schistGneissic foliation: compositional banding in gneisses

III. Lineations A. Introduction

  1. Structural lineations: preferred orientation of linear structure in rock

A. Primary Processes

  1. Ductile flattening and elongation of rock a. rock squeezing complimented by stretching at 90 degrees(1) e.g. result in boudinage (pinch and swell structure)
  2. Mechanical rotation a. shear mechanisms rotate minerals into alignment
  3. solution and precipitation a. foliation via mineral mobility in rock (1) mobilization of mineral components by breakdown into solutions (2) e.g. stylotilization (a) common in limestones i) comprised of calcite plus insolubleimpurities (e.g. clay, carbonaceous matter) ii) Riecke's principle (pressure solution) a) calcite will go into solution at pointsof maximum compressive stress b)c) pressure-point solutionremineralize at minimum compressive stress points (b) stylolites i) lines where calcite dissolved and mobilized out of system (volume loss) ii) insoluble residuum accumulates alongpressure-solution boundary
  4. recrystallization a. process of mineral transformation, recrystallization duringmetamorphism (1) associated with attendant change in shape(a) development of foliation
  5. Slickensides and mineral fabric alignment a. process: microscopic alignment of mineral fibers in shear-fracture

zones b. aspirites: irregularities or rough bumps on fault surface that give rise to slickenlines via scratching. B. Developmental stages of Foliation (with increasing tectonic pressure

  1. cleavage stage=volume loss by porosity reduction (bedding fissility) and pressure solution) 2.3. pencil cleavagerough to moderate cleavage (handcock stage)
  2. strong cleavage stage = well dev. bedding-cleavage lineations andcrenulations
  3. passive folding (axial planar foliation) 6.7. transposition (rotation of layers into parallelism with cleavage)Recrystallization a. preferred orientation of deformation and new minerals