Geological Folding and Foliation Analysis (Lecture 13)

Introduction

• Focus: Superposition of folds in Katrila, notably in Harindungri and surrounding areas.
• Key topics: Form surfaces (S0, S1, S2), axial planar cleavages, relations among folds, stereonet disposition.

Key Features and Observations

Form Surfaces and Folds

• Form Surfaces (S0, S1, S2):
  • S0: Primary bedding surface.
  • S1: Axial planar cleavage from F1 folds.
  • S2: Developed foliations from further deformation.
• Folds (F1, F2):
  • F1 folds involve the bending of S0.
  • F2 affects the previous cleavage S1, creating a spaced foliation S2.
• **Asymmetric Folds: **
  • Mark fold axis for F1 and related cleavage (axial planar cleavage).
  • F1 displays asymmetric folds with traces of S0 appearing •granulated.

Analysis and Interpretations

Mutual Relationships of Folds

• Coaxial Folds:
  • Folds F1 and F2 are often found parallel or subparallel, indicating a limb of a major fold.
  • Foliation surfaces granulated, indicating an F2 over F1 scenario.
• Stereonet and Measurements:
  • F1 and F2 fold axes are coaxial.
  • Data collected through measurements and plots on stereonets confirm fold orientation and displacement.

Major Fold Effects on Cleavages

• Three-Dimensional Fold Representation:
  • S0 getting additionally deformed by F2, affecting hinge sections.
  • Ramsey's Type 3 superposition: Hook-like outcrop patterns.
• Non-cylindrical Folds:
  • Varied fold axis inclinations: Fairy God folds = gentle; Harindungri = reclined.
  • Regional fold non-cylindrical due to superposition of deforming forces.

Detailed Observations from Field Examples

Harindungri and Regional Comparisons

• Bedding and Initial Cleavage (S0 and S1):
  • Traces of bedding planes and initial cleavage surfaces.
  • Primary fold causes an S1 continuous cleavage.
  • Later generations of folds (F2, F3) manifest as space cleavages over S1.

Small Fold Superpositions

• Spatial Relations:
  • Detailed mapping exposed small fold interrelationships.
  • Traces of F1 and F2 observed on S1 confirming coaxial nature.
  • F3 space cleavages identified parallel to major fold directions.

Axial Planar Cleavage and Bedding Interactions

• Example of Tight Asymmetric Folds:
  • Distribution of axial planar cleavages consistent with textbook examples.
  • Influence of rock composition (quartz-rich, mica-rich) on the visibility of cleavages.

Conclusion

• Important Morphological Characteristics:
  • Continuous foliations are typically S1, while subsequent generations result in space cleavages.
  • Regional variations indicate significant fold complexity and non-cylindrical dynamics.
  • Further exploration and stereonet plotting essential for deeper insights.

Next Lecture

• Topic: Superposed Folding in Outcrop at a new location.
• Stay tuned for the exploration of more geological phenomena.

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