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BEDDING CLEAVAGE RELATIONSHIP This is a classic elementary structural geology photograph by Prof O Tobisch that shows that the axial planar cleavage of the fold dips at a gentler angle than the bedding, the only case when the strata can be considered to be overturned or "Downward Facing" (term that was used by late Prof R M Shackleton in his classic paper "Downward facing structures of the Highland border", 1958). Hence in this photograph, it is obvious that the strata 'young' to the right hand side or the synclinal axis lies to the right, i.e. the direction opposed to the one in which axial planar cleavage dips. In geological situations where the cleavage or schistosity is a simple direct result of strain development during the fold forming process, the overall geometric relationships between the orientation and intensity of cleavage are systematically related to fold form. Cleavage refraction on fold limbs and convergent or divergent cleavage fans relate directly to differences in competence between the layers. In these circumstances the relationship between
cleavage and lithological layering in a rock outcrop can provide an
extremely useful guide in the field to indicate where the particular
outcrop being viewed is situated in the overall major structure. The
following features are important:
I. On the normal fold limb the cleavage has an overall higher angle of dip
than the layering. 2. On the overturnedfold limb the cleavage has an overall lower angle
of dip than the layering. 3. At the fold hinge the cleavage and layering are perpendicular and
there is no cleavage refraction. Here the cleavage is practically always
parallel to the axial surface of the fold. 4. On both fold limbs cleavage refraction occurs, the smaller
cleavage-bedding angle is always characteristic of the less competent
layers, the greater angle is found in the more competent layers. The
geometric relationships described above hold true irrespective whether the
fold is an upward or downward facing structure. If the polarity of the
layers in a fold can be determined (e.g. with sedimentary structures such
as cross bedding or lithological grading) then these observations can be
combined with cleavage bedding relationships to determine the jacing
direction of a fold. Once this facing direction has been established then
the cleavage-bedding relationships at individual outcrops can be used to
determine the stratigraphic polarity of the beds at the locality, even
when primary indications of bedding polarity are absent. The line of intersection of cleavage and bedding is parallel to the
hinge line of folds to which the cleavage is related, being generally
parallel to Y. |