Dating Rocks, Fossil Succession, and Fossilization Flashcards
What’s the significance of deep time?
- Earth is very old, and the geologic time scale is vast
- 4.6 billion years
What is relative dating and absolute dating?
Relative:
- order of deposition of a body of rock based on position > uses lithostratigraphy, biostratigraphy, magnetostratigraphy, chemostratigraphy
- can answer what age the rocks are from or compare ages of rocks (this one came before this one)
Absolute:
- a number representing the time a body of rock was deposited > radiometric dating
- can answer how old the rocks are
What is lithostratigraphic correlation?
- correlation based on rock types and sequences > rocks correlated without regard to their age
What is an index fossil (guide fossil)?
Any easily identified fossil with a wide geographic distribution and short geologic range; useful for determining relative ages of strata in different areas
Explain the principles of fossil succession?
(what is biostratigraphy?)
- vertical ordering of fossils in geological record (oldest at bottom of scale)
> rocks with shorter age ranges of evolution with a shorter vertical black line is better for the index because it helps us narrow down the age range - fossil assemblages (groups of fossils) vary through time
- relative ages of fossil assemblages determined using superposition
- occurrence of fossils can be independent of rock type:
> divided into biozones that do not necessarily correspond to those of lithostratigraphic uits
> biostratigraphy: uses fossils to establish relative ages of rocks and correlate successions of sedimentary rocks within and between depositional basins
Explain radioactive decay
- radioactive molecules (parent isotopes) of certain elements are locked into igneous rocks during cooling
- after crystallization, parent isotope begins to decay (break down)
- this decay produces the daughter isotope (stable form of element)
- the amount of decay indicates the time since rocks was formed
Explain isotopic dating - What’s a half-life?
- absolute age dating
- radioactive isotopes decay at constant exponential rate
- eventually, half of the parent present will survive and half will decay to daughter
- half-life = interval of time for half of parent to decay
- 50% = half life –> 50% daughter isotopes replacing parent isotope
- after the maximum age for dating there isn’t enough parent isotope left to detect
Explain radiocarbon dating (type of isotopic dating)
aka carbon-14 dating
- when an organism dies, C14 converts back to N14 by beta decay
- half life = 5730 yrs
- maximum age for dating is about 70 000 years
- bone, teeth, wood
What are 5 of the principal long-lived radioactive isotope pairs used in radiometric dating?
parent > daughter
Uranium 238 > Lead 206
(10 million-4.6 billion)
Uranium 235 > Lead 207
Thorium 232 > Lead 208
Rubidium 87 > Strontium 87
(10 million to 4.6 billion)
Potassium 40 > 1.3 billion
(100,000 to 4.6 billion)
What is Chronostratigraphy?
- combines relative dating (lithostratigraphy and biostratigraphy) with radiometric dating methods
- integration of multiple methods generates the most robust signal, to put dates on rock strata (layers)
- sequences of rocks are broken up into blocks of time based on the fossils they contain
- volcanic layers (geologically ‘instantaneous’), given the absolute context
*introducing element of time to our stratigraphy of rocks and combining relative and absolute dating
Explain how things like lava flows and volcanic ash falls can contribute to absolute age dating?
- some physical events of short duration (like lava flows and volcanic ash falls) can demonstrate time equivalence between two or more widely separated rock bodies
Explain how association with fossil-bearing rocks can be helpful
Ash falls, plutons, lava flows, and metamorphic rocks associated with fossil-bearing sedimentary rocks
- they’ve provided numerical ages for the geologic time scale
Igneous and metamorphic rocks associated with fossil-bearing rocks
- may provide age ranges for the fossils
What are fossils?
Fossils are prehistoric remains or traces usually preserved in sedimentary rocks
- provide information on ages, depositional environments, and evolution
- provide relative ages of strata in separated columnar sections of rock - biostratigraphy
What are the two types of fossils? (+ third?)
Body fossils:
- actual parts of the organisms - unaltered remains
- freezing environments can preserve a lot
- organisms trapped in amber (tree sap)
- altered remains > petrified wood (permineralization - organic structure of wood replaced by a mineral)
- examples: shells, bones, teeth
Trace fossils:
- any indication of organic life
- shell > something bore through the shell to get at the thing inside (little hole)
- ex. bioturbation → Dolomite Limestone in Winnipeg → lighter parts = original limestone → darker parts = shrimp burrowing into the stone - end up with two different types of mineralization
- exmaples: tracks, trails, burrows, nests, feces
Molds and Casts
- molds form around shells and other hard parts - leave impressions in the surrounding sediment
- cast forms if they shell is dissolved and its imprint is filled in with sediment or other minerals > mold filled with sediment forms a cast
What is taphonomy?
The study of all factors that affect an organism from the time of death to the time of discovery as a fossil (study of the fossilization process)
- death and decay –> burial –> after burial
(ex. did the animal die where it was living?)
What are examples of things we have good and bad fossil records for and why?
Good fossil record or corals, clams, and brachiopods
- have hard shells
Poor fossil record of jellyfish and bats
- have lots of soft or delicate parts
- soft tissue is rarely preserved like hair, feathers, scales
What are 3 favourable conditions for fossilization (relating to the organism)?
- organism had a durable skeleton
- lived where burial in sediment was likely
- avoided decay, scavenging, and metamorphism
What are the 3 basic requirements for fossils forming? - related to environment
- oxygen-poor environment
- low-energy setting
- rapid burial in fine-grained sediment
List the factors that affect preservation
- scavenging
- depth - deeper the body is in a basin the better
- salinity - higher salinity is good bc it draws out the moisture and dries out the remains
- pH - highly acidic environment is bad bc it would dissolve the bones
- oxygenation - low oxygen good bc it slows down decay process
- climate (temperature, moisture)
- depositional environment: materials and rates - body can be buried before any decay
- wave or current action: transport and abrasion - can break up the body and bones
- water covering bones can help protect them
*impact of each of these factors is in part determined by the length of time between death and burial
Explain the processes/conditions that are able to fossilize unaltered body fossils and altered body fossils
unaltered:
- freezing
- mummification
- preservation in amber
- preservation in tar
altered:
- permineralization
- recrystallization
- replacement
- carbonization
Explain the good and bad aspects of each environment for fossilization: continental, shore, reefs, deep marine
continental:
- good = rivers, mummification in the dessert, volcanic activity (lots of ash that adds a layer on top of everything and causes rapid burial)
shore:
- lots of wave action = bad
- if you have fine sediments and relatively rapid burial it can be good
reefs:
- excellent fossil record of corals
- Corals secrete Calcium carbonate - hard material resistant to weathering > forms their hard skeleton which is the fossil
- Large structures that are very hard
deep marine:
- rapid burial and not much oxygen = good
- Common in the fossil record are radiolaria - siliceous shell - give us indications of past water temperatures and lots of other stuff
