Page 254 - Microtectonics

8.3 · Primary Structures in Sedimentary Rocks 245

Fig. 8.9. Pseudo chevron folds in carbonatic rock. The apparent folds were not generated by a process of folding but by differentiated
growth of elongated carbonate crystals. Origin of sample unknown. Width of view 1.2 mm. CPL
ated in soft sediment by earthquakes. This structure can Another case of apparent folds is the one illustrated
be misinterpreted as early phase isoclinal folding. An in Fig. 8.9, where elongated carbonate crystals generated
obvious difference is that no axial plane cleavage should pseudo chevron folds, due to their differentiated growth
be present, but diagenetic foliation may have formed, (compare also Figs. 6.12, 11.5).
parallel to bedding and axial planes. In the same la- Rounded detrital grains in a sediment are commonly
custrine tectonically undeformed sediments some struc- masked by overgrowth during diagenesis or low-grade
tures occur with the appearance of early thrust folding metamorphism. However, with special methods such as
(Fig. 8.8). These have also been interpreted as related to cathodoluminescence or Raman mapping, the original
earthquakes. contours of the grains can still be recovered (Figs. 5.1, 10.9).