Dewatering Structures

Dewatering structures encompass a variety of features that form when fluids escape sometime following deposition.  They commonly develop in coarser sediment, usually sandstone, that is rapidly deposited.  Thus, they are commonly found in turbiditic successions.  Some of these features are useful in determining depositional stratigraphic relationships.

	Dish Structures.  This image shows what can be considered the early stages of the development of dish structures.  The muddy laminae have become irregular and some are beginning to break apart, but well-developed dish structures have not formed.  The image below shows the structure that develops when dewatering proceeds further than it has in this example.
	Dish and Pillar Structures.  This image shows well-developed stuctures.  The presence of dish structures are evidence of rapid sediment deposition.  They form by the escape of water during consolidation of the sediment.   The initial bed consists of sand interbedded with thin, lesser-permeability mud laminae.  As fluids move upward during compaction and dewatering, they are impeded by the muddy laminae, causing them to travel horizontally until an easier path for escape is encountered.  The muddy laminae create the convex-up dishes, while the pillars serve as the fluid escape paths.
	Dish Structures.  Note the convex-up dish structures scattered across this outcrop.  These structures are not as pronounced as in the above image, perhaps because compaction and dewatering has proceeded to such an extent that some of the previously formed structures have become deformed.
	Fluid Escape Pipes.  These structures form in a similar fashion to dish structures with the exception that the original sediment was probably more homogeneous and lacked a significant muddy component.  As fluids dewatered, the escape paths were preserved as vertical pipes, seen here. When you see vertical structures like these, you've got to be careful not to confuse them with vertical burrows—some organisms, particularly Skolithos, create features that are strikingly similar.