Dip Isogon Patterns and Fold Classes in Banded Iron Formation of Donimalai Area, Sandur, Karnataka

Folds belonging to two generations are identified which have been named as early and late structures. It is possible that the early structures may include folds of two different generations, the more compressed (F_1A) and less compressed (F_1B). The late structures are referred to as (F₂).

The F_1A folds show parallel isogon pattern, while F_1B folds have weakly converging isogon pattern, suggesting their flexural nature. The late generation folds F₂ have bands with convergent isogons alternating with divergent isogons. The isogons' analyses did not reveal any systematic difference in competency between hematite and chert. Gaps between two layers with similar curvature, however, are filled by hematite, suggesting that hematite was less competent compared to chert.

t_α (t_α/t_o) measurements were carried out on different layers of all folds. The observations are; (1) the layers in F_1A folds predominantly belong to class 3 or 1C (2) the F_1B fold layers belong to class 1A, 1C and 3, (3) the F₂ folds predominantly belong to class 1A though other classes are also present. The F_1A are most flattened and F₂ are least compressed. The fold geometry is very complex, as the same layer falls under different classes of folds at different values. Apart from the complexity of folding, this may be partly due to the original feature of banding.

Presence of disharmonic folds and variation of wave length with thickness of layers suggest that the folds were initiated by buckling. The F_1A folds underwent considerable flattening subsequently due to compresssion.