lithostratigraphic correlation Flashcards
What does lithostratigraphic correlation rely on?
recognising and matching a rock type, or more often a sequence or succession of beds.
What is a marker bed/ horizon?
These beds are unusual so easily recognised. They are often due to volcanic eruptions over large areas. These can be dated absolutely.
How do varves form?
In the summer glacial lakes melt- the increased flow takes silt down to the glacial lake and deposits a pale layer. In winter the lake freezes over. This gives a thin dark layer. One year = a pair of layer. Their thicknesses can be matched.
what are the problems with lithostratigraphic dating? X4
lateral variation makes matching beds hard
beds vary in thickness horizontally depending on how close they are to source of sediment
beds change completely like a point bar next to a flood plain clay
Diachronous beds- refers to a bed laid down over thousands of years like a river delta- all the rocks are different ages.
biostratigraphic dating
if two rock units have identical fossils they are then assumed to be the same age
biozone
characterised by a certain assemblage of fossils
assemblage
a group of fossils in a rock unit
what makes a good zone fossil? x6
evolved rapidly abundant/ common easily identifiable found in lots of rock types widespread geographical distribution strong hard shells so well preserved
what are the problems with biostratigraphic correlation? x7
many fossils are restricted to one environment
some fossils are very long ranged
some fossils are very delicate
derived fossils
not all rocks have fossils
some evolutionary changes take time to move around the world
first and last appearance of fossil groups are still being updated
lateral variation
means there are changes in thickness or lithology in beds laid down at the same time
