Time and change 2 (laws and principles, sedimentary structures, unconformities and graptolites)

Principle of Superposition

The oldest rocks are on the bottom and the rocks become progressively younger toward the top

Law of cross cutting relationships

An intrusion is younger than any other body of rock that it cuts through

Law of included fragments

Pieces of one rock found in another must be older than the rock in which they are found

Law of faunal succession

Fossils are found in order of youngest at the top

Metamorphism

Metamorphic rocks are always older than sedimentary rocks, Metamorphics have just been heated

Principle of lateral continuity

Rock layers should continue across where eroded

Principle of original horizontality

Layers are deposited horizontally so bottom rock must be oldest

Unconformities

A gap in the geologic record

Graded bedding

Coarser sediments is deposited first so is the oldest

Borrows and trace fossils

As the burrow deepens, the rocks get older

Desiccation cracks

V points towards the older rock

Flame and load structures

Flame points in the direction of younging

Pillow lavas

Rounded top points in the direction of younging

Ripple marks

Points are towards the direction of younging

Cross bedding

Truncated top is younger

Why are unconformities created?

Layers are eroded or sediment isn't deposited for a long period

Angular unconformity

Tilted rocks are overlain by flat lying rocks

Nonconformity

Layered sedimentary rocks lie on an erosion surface cut into igneous or metamorphic rocks

Disconformity

A break in a sedimentary sequence that does not involve a difference of inclination between the layers on each side of the break

Paraconformity

An unconformity for which there is no evidence but some geologists think should exist

Graptolite stipe evolution

Deceased from 8 to 1

Graptolite position of thecae evolution

From only found on one side to both sides with wider spacing in between

Graptolite stipe orientation evolution

From orientated downwards (pendant) to upwards (scandent)

Graptolite thecae shape evolution

From simple cup like shape to elaborate hooked and curved shapes