Geologic Time

Absolute time

Absolute time

• “I am ___ years old” (absolute sense of age)

• Fixed time frame 

• Time measured as a numerical age

• Time of event: ex: metamorphism occurred 80 million years ago 

• Age of a rock or age of a particular process 

• Know the age of the rock 

Relative Time

• Older brother and younger sister (relative ages-just know they are older and younger)

• Time measured relative to another event/time marker 

• 2 different rocks in same area, tell if one is older or younger than the other

Principle of original horizontality 

• Sedimentary rocks (particularly clastic)—Idea is that when sediments are initially laid in a sedimentary basin, they are deposited as horizontal layers 

• Bunch of rocks now tilted=later event that has now tilted the rocks from an original position (some tectonic or any event)

Principle of superposition

• In undisturbed sequence/layers of rocks—Oldest at bottom, youngest at top

• Layers get flipped-as a geologist we can tell its upside down by if it has graded bedding (coarser at bottom and finer at the top) 

  • Coarser will be on top if it is upside down 

• Vesicles-gas bubbles from magma in a rock 

  • Concentrated near the top, coming out of magma and escaping from lava flow 

    • Indication which way rock is up, vesicles on one side–that is up 

Principle of cross-cutting relationships 

Tells us that if a geological feature cuts across other geologic feature, the feature that is cutting across is younger than the other layers 

• In picture on slide 6

  • Blue is the oldest-A

  • Light brown is next-b

  • Dark brown-c

  • Yellow-d

  • Red-e-youngest 

Fault

A fracture along which the rocks have been displaced 

• Ex on slide 7

  • C, B, D, A, D, E 

  • C, B, A-deposition of elements

  • D-dike intruded 

  • E-something cause faulting 

  • What is E? 

Hanging wall

Rock layers above the fault (hanging above the fault surface)

Foot wall

Rock layers below the fracture

Fault surface

line between the hanging wall and foot wall

Normal fault

• hanging wall moves down relative to the footwall-need tension (forces are moving apart) release of pressure 

  • Mid Ocean ridge-plates moving away

Reverse fault

•  hanging wall moves up relative to the footwall (not a normal sense of motion)-need compression to produce 

  • Collision and subduction 

  • Can be called a crust or thrust fault

• Repeat layers by the thrust fault–can tell there was some movement 

Outcrop

 exposure of rock at the earth's surface

Bedrock

 intact rock 

Laws of inclusions 

• Inclusions-rocks that are included/surrounded by other rocks (xenolith-foreign rock)

• The inclusions are always older than the rock surrounding it (in which they were found) 

  • If a rock is already formed-impossible to put a rock inside 

Principle of Faunal succession 

• Sedimentary rocks contain fossils 

  • Lived and died during deposition of sediments 

• Record of fossils in rock reflect that life forms evolve in a certain direction-predictable way that fossils occur in rock 

  • Succession of fossils in rock layers in a predictable manner 

• An assemblage of fossils can also give you a relative age

  • Contains ___ and ___ so older than this but younger than this

• Undisturbed succession–older at bottom and younger at the top 

• Ex: A rock containing trilobite and one not containing will give you a relative age 

• Ammonites–range of time where they lived, small time frame (find this, that is useful because they lived for a short period of time)

Body fossils

remains of body parts preserved 

trace fossils

activities of organisms preserved in the rock (burrows)

Index fossil

fossils that are good time markers due to the short time the organism lived (time it existed on earth)

• Ammonites–range of time where they lived, small time frame (fins this, that is useful because they lived for a short period of time)

Fossil assemblage

multiple fossils

Stratigraphic correlation

• establishing age relationships between different locations

  • Can be made using properties of the rocks (features the same in both areas)

Lithostratigraphic correlation

matching up rock units in the basis of their lithology (composition, texture, color etc) and stratigraphic position 

• Limestone: chemical sedimentary rock (both areas underwater at the same time in order to precipitate)

• Composition, texture etc. 

• Can use fossils (ammonite fossils are the same in both rocks at the diff locations)

Biostratigraphic correlation

matching up rock units in the basis of the fossils they contain

• Can use fossils (ammonite fossils are the same in both rocks at the diff locations)

Chronostratigraphic correlation

matching up rock units on the basis of age equivalence, as determined by radioactive dating methods 

• Using actual age of rock 

• Figure out time of deposition 

• Connection using the real age if the rocks 

  • Ex: shale–400 million years old and the other shale is the same age 

  • Similar properties as well–infer they deposited at the same time 

• Use over large scales (ex: western canada sedimentary basin)