F3 key idea 1 Flashcards
(14 cards)
lithofacies
body of rock with specific physical and chemical characteristics that reflect its depositional environment
in terms of grain size, rock type, sed. structures and mineral composition
biofacies
body of rick characterised by its fossil content especially the types and abundances
in terms of type of fossil, abundance and diversity and preservation
what makes good zone fossils
Identifiable species
distribution is wide
abundance is high
time period is specific
evolution is rapid
benthonic mode of life
organisms that live on or in the sediment on the sea floor
infaunal - in burrows
epifaunal - live on the sediment
vagrant - moves around on the sea floor
sessile - does not move around, may be attached
pelagic mode of life
lives in the water column
planktonic - floats with the current
nektonic - actively swims
the fossil record
biased and incomplete
In favour of factors that improve fossilisation
natural processes can distort or distort fossil evidence
factors effecting fossilisation
ostriches eat tasty raspberries and some drink coffee
original composition - hard parts made of silica preserved unaltered, calcite can be altered easy
energy levels - higher energy produces more fragments due to collisions causing breakage
transport distance - fossils fragmented during transport due to abrasion
rapidity of burial - faster burials means more chance of of whole body fossils being preserved
amount of oxygen - oxygen accelerates the breakdown due to bacterial decay
sediment - fine sediment preserves organisms better
diagenesis - composition and acidity of percolating groundwater, dissolving or replacing fossil
compaction - may cause fossils to be flattened
derived fossils
fossil that has been re eroded from original layer
and redeposited into a younger sedimentary layer
fossil is older than the rock found in
replacement
occurs when original material is dissolved atom by atom and substituted with another mineral
(silicification, calcification, pyritisation)
silicification
occurs when percolating groundwater rich in silica dioxide moves through the rock
the minerals dissolved in the groundwater crystallise out of solution and fill any pores present
calcification
ground water with calcium carbonate precipitate additional calcium in the spore spaces
pyritisation
replacement of original material by iron pyrites
occurs in anoxic conditions and the only live organisms are suffer bacteria (using sulpher to respire, reducing surfer to bisulphate)
bisulphate reacts with any iron in the environment to form iron pyrites, replacing the fossil material
carbonisation (mould/casts)
moulds are formed when fossils are dissolved out of the rock they’re in, leaving a hole in the rock
if the fossil was filled with sediment before complete burial, the sediment may reveal and internal mould showing internal structure
cast is formed when the hole is filled with another material such as iron pyrites or silica
trace fossils
tracks/ trails along the surface - soft or muddy fine grained sediments, marine or terrestrial (walking, usually the imprints of legs or feet)
burrows - may have been for protection or feeding indicating a soft substrate, burrowing also mixes layers of sediment by bioturbation
borings - formed in rock or wood, indicating a hard substrate
excrement - evidence of feeding, needing to be covered quickly before they break up, evidence of size of animal
root structures - woody-looking impressions showing shallow marine or terrestrial conditions, often branching from trees or other plants
