Main content area

Can once lithified rocks later undergo soft-sediment deformation?

van Loon, A.J., Mazumder, R.
Sedimentary geology 2011 v.238 no.1-2 pp. 101-105
cement, deformation, gneiss, sediments, temperature, India
Soft-sediment deformation structures (SSDS) have received much attention from sedimentologists, but they use the term ‘SSDS’ commonly in a loose way. In practice, the term is used, as a rule, for deformation structures that were formed before the sediment has become lithified. This usage is unfortunate because lithification is a gradual process, so that no sharp boundary between soft sediments and lithified rocks exists; moreover, cement can be dissolved again, changing a lithified rock back into a soft sediment. The term ‘SSDS’ is also sometimes restricted to deformations in sediments that are in a non-brittle state. This usage is unfortunate, because the nature of the deformation (brittle or plastic) depends on the deformation velocity. The type of deformation depends also, however, on temperature and pressure. An exceptional situation is illustrated by a deformation structure in the Proterozoic Chotanagpur Gneissic Complex in eastern India. The gneisses have been intruded after metamorphism by pegmatitic veins, so that the gneisses became plastic again due to the high temperature of the intruding magma. In this plastic state, they became deformed in the same way as soft sediments. The resulting deformation structures can physically not be distinguished from fluid/gas-escape structures in unconsolidated sediments. It seems therefore advisable to distinguish between SSDS (the ‘classical’ SSDS) and NTDS (non-tectonic deformation structures), which include SSDS formed under extraordinary conditions.