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)
What is the Great American Biotic Interchange? (GABI)
The response of biota to changing paleogeography between North and South America, specifically the establishment of the Isthmus of Panama 3mya
What is the Isthmus of Panama?
An area of land that connected North and South America (land-bridge corridor) and separated the Atlantic and Pacific oceans (formed as a result of tectonic processes)
What did the Isthmus of Panama do?
Removed a barrier to faunal distribution - allowed the exchange of species between North and South America which had been separated the previous 70my (there were distinctive faunal provinces in each before the Isthmus of Panama)
Biology altered by geological processes
What terrestrial and freshwater fauna moved into the North and South?
Into the North: armadillo, possum, porcupine
Into the South: jaguars, mastadons
What does the ‘classic model’ argue about extinctions in South America?
That it was the result of invaders but it is more complex than that
What did Tuzo Wilson (1966) argue for using palaeontological evidence?
A proto-Atlantic ocean (ancient seaway in Northern hemisphere in Early Palaeozoic (Cambrian - Silurian)
What are the 4 parts of the argument for a proto-Atlantic ocean?
1) Fossil assemblage distinct in North American (Laurentia, equator) and European (Avalonia, high latitude)
2) Separated by major ‘suture’ corresponding to Appalachians and Caledonian mountain belts (how seen today)
3) Suture marked the site of an ocean (Iapetus ocean) that had closed
4) Atlantic later reopened in roughly the same place (not precisely same line)
What does cluster analysis tell us about the proto-Atlantic ocean?
Certain fossils in the East of Ireland are similar to those in Anglesea (UK) and one in the West of Ireland have affinities with Newfoundland (Cowhead) which tells us the line of the ancient ocean (Iapetus) must lie between these areas (West of Ireland part of Laurentia and East part of Avalonia)
What was the evidence for the proto-Atlantic ocean based on?
Fossils being restricted paleogeographically, comparing them with each other for similarities and differences and reconstructing the paleogeography
What was the name of the ocean that closed?
The Iapetus Ocean
What is the nature of the paleontological evidence for the proto-Atlantic ocean?
Changes over time in what taxa and ecologies are shared between Laurentia and Avalonia. (graph) shows ecology change as ocean closes
List the changes that occur as the Iapetus Ocean closes (from oldest (bottom of graph) to youngest (top of graph).
1) Dictyonema
2) Didymograptus bifidus
3) Trilobite and Brachiopod GENERA
4) Trilobite and Brachiopod SPECIES (genus to species)
5) Benthic Ostracods (plnaktonic to benthic)
6) Freshwater fish (marine to freshwater)
7) Closure of ocean in Norway
How did the closure of the Iapetus Ocean influence the Geology of Scotland?
In southern Scotland: strip-like rock belt of Ordovician - Silurian rocks in the Southern Uplands of Scotland (represent accretionary prism) deposited on ocean floor as sediments - all formations run NE-SW and boundaries are all faults
Describe the geology of SW Scotland
Narrow strips of steeply dipping sediment separated by faults
Faults: thrusts - produced by compression (squeezing)
Each slice dips and young to the NW
What does each slice of rock consist of in SW Scotland geology?
Black (low O2) muds at the base of the slice (graptolitic - plankton falling down - pelagic sediments characteristic of open water conditions away from a land mass)
Turbidites at the top of each slice (coarse-grained event beds - beds fine upwards as coarser sediments deposited first - erosive beds = signals for event bed - sediment remobilised from shallower water depths - happens along margins of continents)
How did graptolite assemblages in Scotland help us understand the transition across thrust slices? (graph?) - may not need to know as well
Graptolites from shales.
Biozone identified.
Sediment column for each slice positioned in correct horizontal space (NW - SE) and age on vertical axis.
Longer times in the SE sections.
Pattern: turbidites at top and black shales underneath, top is younger to SE
Describe how an accretionary prism, just like that in Scotland, forms.
Accretionary prism = package of sediments.
Iapetus ocean closing as oceanic crust (Avalonia?) subectued under Laurentia.
Open ocean accumulated black shales and closer to land turbidites formed so as ocean closed turbidites began going further out over black shales as land moved. (out further there is thicker layer of graptolitic black shales as they had a longer time to form).
The material that was scraped off as the crust subducted got plastered to the bottom of the accretionary prism.
Younger to SE overall
Give another example of an accretionary prism
Nankai Accretionary Wedge
