Post depositional modification of sedimentary layers

Sediment is generally deposited as layers that may contain features such as cross-bedding, wave ripples or horizontal lamination formed during deposition: these are referred to as primary sedimentary structures. The original layering and these sedimentary structures may be subject to modification by fluid movement and gravitational effects if the sediment remains soft. Disruption of the sedimentary layers may occur within minutes of deposition or may happen at any time up to the point when the material becomes lithified. Soft-sediment deformation is the general term for changes to the fabric and layering of beds of recently deposited sediment. The deformation structures are mostly formed as a result of sediment instabilities caused by density contrasts and by movement of pore fluids through the sediment. When sediment is deposited in marine environments it is saturated with water, and many continental deposits are also saturated by groundwater. Burial by more sediment usually leads to gradual expulsion of the pore waters, except where the water gets trapped within a layer by an impermeable bed above. This trapped water becomes overpressured, and when a crack in the overlying layer allows fluid to be released it travels at high velocity upwards. Rapidly moving pore water causes fluidisation of the sediment, which is carried upwards with the moving water. Finer sediment can be more easily carried upwards, so the process of elutriation occurs, as fine sand is carried away by the fluid, leaving behind coarser, and more cohesive, material. Liquefaction is a shorter-term process that happens when a mass of saturated sediment is affected by a shock, such as an earthquake, and becomes momentarily liquid, behaving like a viscous fluid. There is usually only very localised movement of sediment and fluid during liquefaction. Soft-sediment deformation takes a variety of forms at various scales and can occur in any sediment deposited subaqueously that retains some water after deposition. They can be loosely grouped into structures due to sediment instabilities, liquefaction, fluidisation and loading, although these are not mutually exclusive categories.


Structures due to sediment instabilities



Slumps and slump scars
Slumps and slump scars form as a result of gravitational instabilities in sediment piles. When a mass of sediment is deposited on a slope it is often unstable even if the slope is only a matter of a degree or so. If subjected to a shock from an earthquake or sudden addition of more sediment failure may occur on surfaces within the sediment body and this leads to slumping of material. Slumped beds are deformed into layers that will typically show a fold structure with the noses of the anticlines oriented in the downslope direction. The surface left as the slumped material is removed is a slump scar, which is preserved when later sedimentation subsequently fills in the scar. Slump scars can be recognised in the stratigraphic record as spoon-shaped surfaces in three dimensions and they range from a few metres to hundreds of metres across. They are common in deltaic sequences but may also occur within any material deposited on a slope.




Growth faults
There is a continuum of process and scale between slump scars and growth faults, which are surfaces within sedimentary succession along which there is relative displacement. Growth faults are considered to be synsedimentary structures, that is, they form during the deposition of a package of strata. They are most commonly found in delta-front successions, where the depositional slope and the superposition of mouth-bar sands on top of delta-front and prodelta muds results in gravitational instabilities within the succession. Failure occurs on weak horizons and propagates upwards to form a spoon-shaped fault (a listric fault) within the sedimentary succession. Movement of the beds above the fault over the curved fault surface results in a characteristic rotation of the beds. Growth faults can be distinguished from post-depositional faulting because a single fault affects only part of the succession, with overlying beds unaffected by that fault.

Structures due to liquefaction



Convolute bedding and convolute lamination
The layering within sediments can be disrupted during or after deposition by localised and small-scale liquefaction of the material. The structures range from slight over-steepening of cross-strata, to the development of highly folded and contorted layers called convolute lamination and convolute bedding. These structures form where the sediment is either deposited on a slight slope or where there is a shear stress on the material due to flow of overlying fluid. The folds in the layering tend to be asymmetric, with the noses of the anticlines pointing downslope or in the direction of the flow. Convolute lamination is particularly common in turbidites, where it can be seen within the laminated and cross-laminated parts of the beds.


Overturned cross-stratification
Sands deposited by avalanching down the lee slope of subaqueous dunes are loosely packed and saturated with water. They are easily liquefied and can be deformed by the shear stress caused by a strong current over a set of cross-beds. Shearing of the upper part of the cross-beds creates a characteristic form called recumbent cross-bedding or overturned cross-stratification.

Structures due to fluidisation



Dish and pillar structures
Soft-sediment deformation structures formed by fluidisation processes are often called dewatering structures as they result from the expulsion of pore water from a bed. Dish structures are concave disruptions to the layering in sediments a few centimetres to tens of centimetres across formed by the upward movement of fluid. They are often picked out by fine clay laminae that are the cause of local barriers to fluid flow within the sediment. In plan view the dish structures form polygonal shapes. Pillar structures, also known as elutriation pipes, are vertical water-escape channels that can be simple tubes or have a vertical sheet-like form. Dish and pillar structures often occur together, although they can form separately.



Clastic dykes
Fluidisation of a large body of sediment in the subsurface can result in elutriation of sediment and the formation of vertical clastic dykes centimetres to tens of centimetres across. These sheet-like vertical bodies are typically made of fine sand and they cross-cut other beds. They form when a fracture occurs above an overpressured bed and the upward rush of pore waters carries sediment with it into the crack. The sand may show some layering parallel to the walls of the dyke but is otherwise structureless. A distinction must be drawn between clastic dykes, which are injected from below, and fissure fills formed by the passive infill from above of fissures and cracks in the underlying layers. Fissure fills form where cracks occur at the surface due to earthquake activity or where solution opens cracks in the process of karstic weathering. They can usually be distinguished from clastic dykes because they taper downwards, can be filled with any size of clast (breccia is common) and can show multiple phases of opening and filling where they are earthquake related. The term 'Neptunian dyke' has been used in the past for these fissure fills.



Sand volcanoes and extruded sheets
Liquified sediment brought to the surface in isolated pipes emerges to form small sand volcanoes a few tens of centimetres to metres across. These eruptions of sand on the surface can be preserved only if lowenergy conditions prevent the sand being reworked by currents. Sand brought to the surface through clastic dykes can also spread out on the surface, usually as an extruded sheet of sandy sediment. These sheets can be difficult to recognise if the connection with an underlying dyke cannot be established. Intrusions forming 'sills' of sand can form, but can also be difficult to identify.

Structures related to loading



Load casts

If a body of material of relatively low density is overlain by a mass of higher density, the result is an unstable situation. If both layers are relatively wet, the lower density mass will be under pressure and will try to move upwards by exploiting weaknesses in the overlying unit, forcing it to deform. Load casts form where the higher density sand has partially sunk into the underlying mud to form downward-facing, bulbous structures: the mud may also become forced up into the overlying sand bed to form a flame structure. As sand is forced downwards and the mud upwards, load balls of sand may become completely isolated within the muddy bed. These load-cast features are sometimes referred to as 'ball-and-pillow structures'. They are common at the bases of sandy turbidite beds and other situations where sand is deposited directly on wet muds.



Diapirism

In cases where the instability due to density differences between layers of unconsolidated sediment results in movements of material on a large scale, the process is known as diapirism. This process can occur in a range of rock and sediment types in a variety of geological settings, but it is most commonly observed where the density contrast is large and the low-density material is relatively mobile. The bulk density of a layer of rock or sediment is determined by two factors: 
  • The density of the minerals and
  • The proportion of the material that is occupied by pore spaces filled with gas or liquid. 
Two types of diapirism are commonly seen in sedimentary successions, salt diapirism and mud diapirism, and they have two important implications for sedimentology and stratigraphy: first, diapiric structures can create local highs on the sea floor that may become the locus for carbonate development and second, diapirism can create subsurface structures that can be traps for hydrocarbons. Halite (NaCl) has a mineral density of 2.17g cm 3, which is considerably lower than most sandstones and limestones, even if they are moderately porous. Halite is solid, but in common with all geological materials it will behave in a plastic manner and deform if put under sufficient heat and/or pressure. The pressure required to cause halite to behave plastically can be generated by only a few hundred metres thickness of overlying strata (overburden) and, due to its lower density, the halite mass will start to move up in areas where the overburden is thinner or weakened by faults. The diapiric movement of salt deforms the overlying strata, a phenomenon that is known as 'salt tectonics'. The effects range from creating swells in the layer of salt, to creating domelike bodies that intrude into the overlying strata, to places where the salt mass breaks through to the surface. In very arid regions the extruded salt may form a mass of halite in the landscape like a very viscous volcanic flow.
The second main form of diapirism occurs where a layer of sediment has a high porosity and its density is reduced due to the presence of a high proportion of water mixed with the sediment. This tends to occur where muddy sediment is deposited rapidly. Mud freshly deposited on the sea floor has about 75% of its mass composed of water. As more sediment is deposited on top, the water is gradually squeezed out, but clay-rich deposits, although they may be porous, have a low permeability because the platelike clay minerals inhibit the passage of fluids through the material. Therefore water tends to become trapped within muddy layers if there is insufficient time for the water to escape. This creates a layer of water-rich, low-density material that may be overlain by denser sediment. This situation most commonly occurs in deltas where fine-grained prodelta facies are overlain by sands of the delta front and delta top as the delta progrades. Mud diapirism (also sometimes called shale diapirism) is therefore a common feature of muddy deltaic successions.
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