Microstructures

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Synsedimentary Deformation

Click image to enlarge

11. Synsedimentary microfold – This completely unmetamorphosed laminated siltstone of the Middle Devonian Orcadian Basin of northern Scotland (sample courtesy of Prof. R.N. Donovan) contains a zone of recumbent folds that formed by movement of partially consolidated sediments down a steep slope at the basin's edge. In this fold hinge, axial planar cleavage in the fine-grained mudstones (brown) is defined by aligned elongate detrital micas and planar concentrations of organic carbon (dark brown to black). Cleavage in the coarser-grained siltstone layer forms a convergent fan, defined by aligned detrital micas, elongate quartz grains and thin zones of mudstone. Disruption of the siltstone layer occurs in the inner hinge region, where there is also a greater concentration of mud-sized particles than elsewhere in the layer. There is no evidence for dissolution of quartz grains, and the cleavage planes are interpreted as channels for fluid flow during folding of the laminites. If this rock were now tectonically deformed, the synsedimentary cleavage could be misinterpreted as representing an early deformation phase of regional significance! See also image #12.

FOV 3.6 mm, Plane Light.

12. Detrital mica in synsedimentary cleavage zone – A close-up of a hinge region of folded laminite from the same locality as image #11 shows the weakly anastomosing cleavage (horizontal) defined by aligned detrital micas and thin seams of carbon particles. In the image center is a detrital mica grain that has remained perpendicular to cleavage. The mica is not deformed, and the cleavage geometry is not significantly affected by its presence.

FOV 0.5 mm, Plane Light.

Synsedimentary Deformation	Click image to enlarge
11. Synsedimentary microfold – This completely unmetamorphosed laminated siltstone of the Middle Devonian Orcadian Basin of northern Scotland (sample courtesy of Prof. R.N. Donovan) contains a zone of recumbent folds that formed by movement of partially consolidated sediments down a steep slope at the basin's edge. In this fold hinge, axial planar cleavage in the fine-grained mudstones (brown) is defined by aligned elongate detrital micas and planar concentrations of organic carbon (dark brown to black). Cleavage in the coarser-grained siltstone layer forms a convergent fan, defined by aligned detrital micas, elongate quartz grains and thin zones of mudstone. Disruption of the siltstone layer occurs in the inner hinge region, where there is also a greater concentration of mud-sized particles than elsewhere in the layer. There is no evidence for dissolution of quartz grains, and the cleavage planes are interpreted as channels for fluid flow during folding of the laminites. If this rock were now tectonically deformed, the synsedimentary cleavage could be misinterpreted as representing an early deformation phase of regional significance! See also image #12. FOV 3.6 mm, Plane Light.
12. Detrital mica in synsedimentary cleavage zone – A close-up of a hinge region of folded laminite from the same locality as image #11 shows the weakly anastomosing cleavage (horizontal) defined by aligned detrital micas and thin seams of carbon particles. In the image center is a detrital mica grain that has remained perpendicular to cleavage. The mica is not deformed, and the cleavage geometry is not significantly affected by its presence. FOV 0.5 mm, Plane Light.