Penecontemporaneous Deformation Structures
Soft-sediment deformation structures are sometimes considered to be part of the initial diagenetic changes of a sediment, and include:
• Load structures (density contrasts between sand and underlying wet mud; can in extreme cases cause mud diapirs)
• Slump structures (on slopes)
• Dewatering structures (upward escape of water, commonly due to loading; "dish and pillar structures")
Wrinkles on Bedding Plane; Indication of Rapid Loading & Compaction
Flame Structures Flame Structures
|Ball and Pillow Structure|
|Convolute Bedding due to shear caused by slope instability or tractional drag||Convolute Bedding|
|Dewatering structures develop when a high-porosity water-rich layer such as a silstone is overlain by a layer with lower porosity such as a sandstone. Compaction of the silstone layer under gravity leads to fluid-overpressure. Fluids (muds) escape upwards forming pillar structures that represent fluid-path ways. In silica-rich fluid quartz (white area on that picture) can precipitate in the pillar.|
|The layering in the silstone (yellow lines) is deformed during dewatering. This deformation is said to be hydroplastic as it occurs when the sediment was water-rich and poorly consolidated. The red line marks the boundary between silstone and sandstone. The cuspate shape of the bedding in the silstone points toward the younging direction which is again upward on that outcrop.|
Mud and Sand Volcanoes and Other Similar Features
Many unusual deepwater sandstone geometries have been identified from 3D seismic and drilling in the North Sea and other basins worldwide as intrustive clastic bodies. Some of these intrusive sandstone bodies appear to be kilometre-scale in size. Sandstone intrusions of this magnitude have not yet been documented from onshore outcrops.
Stress state, burial depth, fluid pressure and the nature of the sedimentary host rock are crucial in controlling intrusion styles, geometry and scale. Seismicity, tectonically driven changes to the stress field and excessive build up of in situ pore pressure are the most commonly cited explanations for the occurrence of clastic intrusions. The addition of another fluid such as oil or gas is more rarely referred to as a triggering mechanism.
The scale of the intrusive complex is governed by the stress and the depth at which the intrusion occurs. At shallow depths, within a few meters of the surface, small irregular intrusions are generated, more commonly forming sills, whereas at depth dykes and sills form clastic intrusion networks.
|The largest clastic intrusion known in outcrop. This dyke (& sill) intruding Miocene mudrocks (white rocks in photo) in Santa Cruz is 10 m wide. These large intrusions in Santa Cruz have extensive bitumen staining||A sand volcano - sand that has "erupted" on to the seabed forming a mound ( Carboniferous; note coin at top right for scale )|
From Lonergan & Jolly, Imperial College, London.