Ichnology Flashcards
What are ichnofossils and how are they formed?
Trace fossils formed by the movement of organisms
What do ichnofossils show and what can they be used to determine?
They represent activity of soft-bodied organisms and can be used to determine current velocity, sedimentation rate, and strength of substrate.
Where are boring ichnofossils usually found?
High energy, rocky coastlines
Where are simple burrow ichnofossils usually found?
Sandy shores
Where are organisms that live and graze on the surface usually found?
Low energy offshore and deepwater environments
How are ichnofossils classified?
Ichnogenus, ichnospecies (Linnean binomials)
How can one organism produce different types of ichnofossils in its life?
Different activities and different substrates
What is the name for locomotion traces?
Repichnia
What is the name for resting traces?
Cubichnia
What is the name for dwelling traces?
Domichnia
What is the name for feeding burrows?
Fodinichnia
What is the name for grazing traces?
Pascichnia
Describe dwelling traces
Cylindrical tubes with minor branching
Name and describe the two different types of dwelling traces
Borings made into hard or semiarid substrate (rock). Burrows made into soft substrate (unconsolidated sand).
What can feeding burrows do and what can they show?
They can cross bedding surfaces and the complex patterns represent mining behaviour
What do repeating patterns in grazing traces show?
A systematic feeding strategy
FOSSIL ROOTS ARE NOT TRACE FOSSILS
Got that?
What are fossil roots diagnostic of?
The terrestrial depositional environment
Describe the morphology of fossil roots
Highly irregular and downward branching
What can bright colours in sedimentary rocks indicate?
Clays (broken down micas and feldspars) and organic material
Define diagenesis
The process of loose sediment becoming rock.
What does diagenesis destroy?
Rock
Describe the conditions of diagenesis
Begins at surface temperature and pressure and continues as T and P increases.
Give the names for early and late diagenesis
Eodiagenetic and mesodiagenetic
What is bioturbation?
The disturbance of sedimentary structures by organisms
Give an example of bioturbation and what it results in
The consumption and excretion of sediment that can change the composition of some clays
Describe a eodiagenetic bioturbation
The mechanical and chemical activities of organisms buried near the surface sediment
What does the churning and homogenising of sediment result in?
The reduction of deposition barriers to flow in reservoirs
What causes mechanical compaction?
Overburden
What are two eodiagenetic processes that occur in mechanical compaction?
Grains are rotated to reduce volume and water is expelled upwards
Describe the formation of sandstone dykes
Water is explosively expelled upwards after a sudden, large compaction
Why types of grains can be highly compacted and why?
Silts and clays have platy morphology and can be compacted up to 90%.
What type of grain cannot be highly compacted and why?
Sand because it is mostly quartz, which is often round.
What mesodiagenetic process allows further compaction after the rotation of grains?
Mechanical fractioning of the grains
What type of process is pressure solution?
Mesodiagenetic
Describe the process of pressure solution
The material at the grain contacts (perpendicular to compaction) dissolves. This reprecipitates as cement at contacts that are perpendicular to compaction. Porosity is lost.
What causes dissolution?
Changes in temperature and pressure and/or composition of porewater
When is a mineral more stable?
In a solution
Where does dissolution without reprecipitation occur?
In systems that are subject prolonged pumping of exotic porewater with an abundant fresh supply
Describe the process of cementation
Chemical precipitates, forming new crystals in the mores of a sediment/rock, binding the grains together
What types of sources can the precipitate have during cementation?
Local or external
What are concretions?
Spherical masses of sediment that are more strongly cemented than the surrounding volume
What grain size do concretions have compared to the surrounding?
The same
What are concretions associated with?
Organic material that reacts with the surrounding chemistry
Give an example of a concretion produced by organic material
Iron found in organic matter that forms iron carbonate concretions
What is recrystallisation?
The reorientation of the same crystal lattices in mineral grains
What processes are involved in recrystallisation?
Solution and reprecipitation of the mineral phase already present
Where is the process of recrystallisation commonly seen?
Carbonate shells
What is mineral replacement?
Where a newly formed minerals replace preexisting ones in situ
How can the new minerals be different to the original mineral?
It can be a polymorph
How does mineral replacement affect pore space?
The new structure can have a different volume
