F3 key idea 2 Flashcards
(12 cards)
death assemblages
when organisms die, their shells are usually transported and deposited elsewhere
called a death assemblage as the fossils are not found in their original life positions
recognised by fragmented shells that are sorted by size or aligned by a current
made up of disarticulated shells
life assemblage
fossils found fossilised in their living positions form a life assemblage (e.g. organisms preserved in their burrows) also known as geopetal structures
high energy continental shelf (shallow marine)
fragmentation suggests high energy, supported by thick-shelled fauna
low energy shallow marine
wide shells suggest the need to spread weight across a soft sediment as well as burrowers suggest a softer sediment
low energy deep marine
no bottom dwellers, meaning conditions were anoxic
complete specimens suggest low energy as they have not been broken up
low energy terrestrial
presence of plant material, signifying close proximity to land
relative dating
means they are put into age order, separated by events such as metamorphism and deformation
limits understanding of how long it took to lay down the rocks and subject them to different processes
rely on observation of present day processes
sequences may be difficult to sort in events of highly deformed strata (inverted) or missing due to erosion
stratigraphic methods
laws and principles
original horizontality - assumed that beds are commonly laid down horizontally, so tilting shows they have been moved
superposition - rocks at bottom of sequence will always be oldest (can use way up structures)
included fragments - fragments from an older rock can be found within a younger rock (xenoliths, derived fossils, conglomerates)
cross-cutting relationships - features which cut through rocks must be younger than the rocks they cut
radiometric dating (absolute dating)
naturally occurring radioactive isotopes in rocks are unstable and break down (decay) at a constant rate
this rate of decay can be measured and is often expressed as the half life of the isotope
time taken for half of the unstable parent atoms to break down into stable daughter atoms
half life
if the relative amounts of parent to daughter atoms can be measured
then we know how many half lives have passed since the parent was formed
potassium-argon (K-Ar) dating
found in micas, hornblende
most widely used method of radiometric dating
potassium is a component in many common rock-forming minerals (three isotopes in potassium)
overtime potassium-40 decays into argon-40, a noble gas that stays trapped inside the crystal structure and accumulates unless reheated
by measuring the ratio of potassium-40 to argon-40, we can calculate how much time as passed since the rock cooled and solidified
problems with radiometric dating
many intrusions takes tens of millions of years to cool, reaching closure temperature at different times
most reliable igneous rocks for dating are shallowly emplaced minor intrusions and extrusive rocks that cool rapidly (lava flows)
sedimentary rocks are made up of fragments of older rocks, these blasts could have a range of ages, older than when the rock was formed (included fragments)
sedimentary rocks tend to be more porous and prone to weathering which can alter the ratio of parent-daughter atoms
in metamorphic rocks, different minerals become closed systems at different temperatures
parent or daughter atoms can be lost or gained during heating events
