1.7 - Fossils As A Tool In Reconstructing Geological Processes Flashcards
How do fossils inform us on paleocommunities?
Various spatial and temporal scales including their evolution through time.
Eg. decreasing oxygen levels - how organisms interacted with each other - rich or poor in fossils
Can fossils inform us about depositional environments?
Yes.
Can reconstruct communities, composition will vary as environmental conditions change (1) oxygen level change 2) response to sea level fluctuations)
Which fossil phylum are good environmental indicators?
Brachiopods
How are brachiopods useful as environmental indicators?
Different brachiopod taxa track changing water depths (eg. Llandovery Welsh basin).
Used to define onshore-offshore gradient tracking water depth.
What are brachiopods?
2 mineralised valves, abundant in the Palaeozoic
What event occurs in the Llandovery basin that is recorded by the brachiopods?
Transgression event
List the 6 types of brachiopods that change with increasing water depth (from shallowest to deepest environments)?
Lingula Eocelia Pentamerus Stricklandia Clorinda Graptolitic black shales (LEPSCG)
How do you identify the transgression event by looking at the different brachiopods?
As the water transgresses, you get lingula in the nearshore environment, the different types come further over what used to be land (changes with water depth)
What did lava flows have to do with the change in brachiopod phylum?
Either side of a lava flow (above and below) there were different brachiopod communities because water depth has changed (shallower) after a lava flow (different brachiopods exist at different water depths)
What are lingula?
A living fossil - type of brachiopod (lophophore)
2 valves
Semi-infaunal lifestyle (lives inside water column)
Fleshy stalk to hold it to the substrate
What environment are lingula found in?
Diagnostic of shallow water close to land (nearshore marine)
Stressed environments (low O2)
Always in the nearshore habitat as transgression occurs
How do lingula act as a facies fossil?
They track the environment with time in local settings.
Associated fossils allow fossil assemblages and palaeocommunities to be defined.
In the Welsh basin they track the shift of the nearshore environment eastward and southward over time.
What are outside of brachiopod communities?
Graptolitic black shales
What do the Lower Silurian strata in the Welsh basin show?
Communities changing laterally and vertically in response to water depth
What are the Silurian Graptolite Assemblage and how are they preserved?
No benthos organisms (bottom living) = no infauna and epifauna
Fossil content dominated by plankton and nekton
Distinctive lithology (black shales) - dark coloured (high organic content due to low O2) fine grained sediments
Plankton fall from water column to seafloor and are preserved
What direction is the transgression in the Welsh basin?
NW to SE
What is the taphonomy of fossils?
The processes involved in preservation
What does the taphonomy of fossils inform us on?
The depositional processes and help us to reconstruct the palaeoenvironment
The bioclasts present will be dependent on the processes they are subjected to
What is a crinoid?
An echinoderm
Has a multi-element skeleton (when the crinoid dies this disarticulates)
What happens to a crinoid that can inform us about the environment at the time?
Use the nature of the fossil to reconstruct a geological record
If a crinoid was highly separated, it may have been a high energy environment
If a crinoid is articulated it was likely a low energy environment
What did palaeobiogeography play a key role in?
The concept of plate tectonics
When did the concept of plate tectonics originate and as what theory?
20th C as the concept of continental drift
Who showed evidence that continents were not fixed on the surface of the Earth?
Alfred Wegner
What evidence was used by Alfred Wegner to demonstrate that continents were not fixed on the surface of the Earth?
The distribution of distinctive animal and plant fossils in rocks of a certain age.
Evidence: Carboniferous-Permian age, Glossopteris flora (plant fossil) and Mesosaurus fauna (animal fossil) suggested South America, Africa, India, Antarctica and Australia were closely juxtaposed and had drifted apart since the Permo-Triassic (found on either side of the ocean suggesting there was once a single large land mass)
