Post Depositional Structures Flashcards
the structures formed during deposition
Primary sedimentary structures
- is the general term for changes to the fabric and layering of beds of recently deposited sediment.
Soft-sediment deformation
is the process of whereby a granular material is converted from a static solid like state to a dynamic fluid like state
Fluidization
is a process for separating particles based on their size, shape and density, using a stream of gas or liquid flowing in a direction usually opposite to the direction of sedimentation
Process of elutriation
- 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
Liquefaction
are deformed into layers that will typically show a fold structure with the noses of the anticlines oriented in the downslope direction
Slumped Beds
The surface left as the slumped material is removed and is preserved when later sedimentation subsequently fills in the scar
Slumped Scars
synsedimentary extensional faults that initiate and evolve at the margins of continental plates. They extend parallel to passive margins that have high sediment supply.
Growth Faults
- are concave disruptions to the layering in sediments a few centimeters to tens of centimeters across formed by the upward movement of fluid
Dish
- also known as elutriation pipes, are vertical water-escape channels that can be simple tubes or have a vertical sheet-like form.
Pillar
- Fluidisation of a large body of sediment in the subsurface and can result in elutriation of sediment and the formation of vertical structure centimetres to tens of centimetres across
Clastic Dykes
are formed from the extruded liquefied sediment brought to the surface in isolated pipes
Sand Volcanoes
sand brought up to the surface through clastic dykes that are spread out on the surface in thin sheet like
structure
Extruded Sheet
form where the higher density sand has partially sunk into the underlying mud to form downward-facing,
bulbous structures
Load Cast
the mud may also become forced up into the overlying sand bed
Flame Structure
are masses of clastic sediment that take the form of isolated pillows or protruding ball structures
Ball and Pillow Structures
where the instability due to density differences
between layers of unconsolidated sediment results in
movements of material on a large scale
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
Mud Diapirs
The physical and chemical changes that alter the characteristics of sediment after deposition
These processes occur at relatively low temperatures, typically below 250° and at depths of about 5000m
Diagenesis
is the process of transforming sediment into sedimentary rock, and involves both chemical and
physical changes that take place at any time after initial
deposition
Lithifaction
happens when sediments are deeply buried, placing them under pressure because of the weight of overlying layers. This squashes the grains together more tightly
Compaction
occurs as one part of a sediment pile compacts more than the part adjacent to it
Differential Compaction
subjected to very little overburden pressure the clasts will be in contact mainly at the point where they touch
Point Contacts
grains are rotated and pushed closer together, and pore space is reduced
Long Contacts
softer grains are compacted around harder ones, and grains start to dissolve in the pore waters at their contacts
Concavo-Convex Contacts
irregular grain boundaries
Sutured Contacts
-is a deformation mechanism that involves the dissolution of minerals at grain-to-grain contacts into an aqueous pore fluid in areas of relatively high stress
Pressure Solution
– forms very soon after deposition
Eogenetic Cement
chemical changes occur in sediment that is buried and saturated with pore waters
Mesogenic Cements
cement formation occurs during uplift
Telogenic Cementation
The nucleation and growth of crystals within pore spaces in sediments
Cementation
are minerals precipitated within pore spaces during diagenesis
Cement
Cement growth is in optical continuity with the adjacent grain
Overgrowth Cement
Grains are enveloped in large crystals of cement
Poikilotopic Cement
Cystals of cement grow uniformly into pores
Isopachous Cement
Cement develops from drops of pore water attached to grains
Meniscus Cement
mineral deposit formed by chemical processes that dissolve a rock and deposit a new assemblage of
minerals in its place
Replacement
- The introduction of, or replacement by, silica, generally resulting in the formation of fine-grained
quartz, chalcedony, or opal, which may fill pores and replace existing minerals.
Silicification
is a mass, or lump of a mineral or mineral aggregate that typically has a contrasting composition,. Forms as
irregular cemented patches
Nodules
Large nodules are referred as
Doggers
- form as minerals within a rock and begins to precipitate within cracks and cavities. It has a symmetrical, round or discoid features
Concretions
The interiors of some carbonate concretions in mudstones display an array of cracks that are often filled with sparry calcite. “turtle-back” appearance.
Septarian Concretions
Chert form directly from siliceous ooze deposited on the sea floor.
Primary Chert
may also form in concretions or nodules as a result of the concentration of silica during diagenesis
Secondary Chert
is the specific name given to nodules of chert formed in the Cretaceous Chalk
Flint
nonmarine calcite cement that forms above the water table
Vadose Zone
Calcite cement that forms on aquifers
Phreatic Zone
The diagenetic replacement of a mineral by a different crystal form of the same mineral
Neomorphism
Cement that forms in Phreatic Zone
Meniscus Cement
The process by which limestone is altered into dolomite
Dolomitization
Carbonate cement that form in vadose zone
Isopachous Cement
triggered by an evaporation phase in lagoonal and/or shallow marine settings
Reflux Model
proposes that where fresh water mixes with marine waters then dolomitisation would occur
Mixing-Zone Model
–involves prime mechanism which is the dewatering of basinal mud rocks due to compaction and removal of Mg-rich fluids into neighboring shelf edge.
Burial Model
Thermally driven circulation, either by a geothermal heat source or by temperature differences between the interior of a platform and seawater
Seawater Model
The process in which magnesium is removed from the mineral dolomite
Dedolomitization
waterlogged areas of accumulation of organic material
Mires
coal formed in areas where most of the input of water is from rainfall
Ombotrophic Mires
or
Bogs
coal formed in places where there is a through-flow of groundwater
Rheotrophi Mire
Or
Swamps
rheotrophic mires that have an input of clastic sediment, and these are referred to as
Marshes
Coals that originate as peat deposits are known as
Humic Coals
deposits of aquatic algae that accumulate in the bottoms of lakes are known as
Sapropelic Coal
