1. The Early Paleozoic Era
Geology 103

2. Precambrian Washington…doesn’t exist
In fact, you’d have to go to the Idaho-Montana border to see rocks of Precambrian age
Typical rocks are the Belt Supergroup, which consist of rocks of a clastic wedge developing off the edge of the then-North American craton

3. What’s a clastic wedge?
A clastic wedge comprises the rocks that represent the deltaic/fluvial deposits made by large stream systems
Sand, silt and clay
Tells you which direction the uplands were
Belt supergroup represents a clastic wedge 1.45 by old

4. Precambrian life (besides Ediacaran fauna) is mostly stromatolites
Basically, they are algal mats of various bacteria that produce a biofilm that binds sedimentary grains that help preserve the mat; later the structure is mineralized, so there is no organic stuff

5. At the start of the Paleozoic…
The supercontinent Pannotia has broken up, and the Earth is leaving extreme ice (“Snowball Earth”) conditions
Most reconstructions show two continents: Laurentia (modern North America and Europe) and Gondwana (everything else)

6. The continued rifting of continents increases continental shelf area
Continental shelf seas are shallow, so plenty of light for photosynthetic organisms and all that feed on them and each other
Due to the Sauk transgression, the ocean invaded low-lying continental areas: “epeiric seas”

7. The base of the Cambrian
What defines the start of the Cambrian period?
Used to be where trilobites were found, but “small shelly fauna” (SSF) was found under the trilobites
Many modern phyla are represented
Defined now as the first appearance of Trichophycus (formerly Phycodes) pedum
Ediacaran fauna is gone

8. Cambrian “explosion”
For reasons unclear, 13 million years after the start of the Cambrian (so 530 my), there was a tremendous diversification of marine life forms, some of which have never been reproduced. Stephen Jay Gould’s Wonderful Life accounts for events surrounding the Burgess Shale, the most famous outcrop of the Cambrian explosion.

9. The Burgess shale Lagerstatten
Burgess shale quarry was discovered by Alexander Wolcott in the early 1900s
The diversity of Cambrian fossils there is due to a Lagerstatten – an exceptionally well-preserved fossil locality
Depositional environment: bottom of an algal reef

10. Other Cambrian life
Trilobites – type of arthropod (“jointed feet”), major predator, swimmer (nekton)
Archaeocyathids – related to sponges
Inarticulate brachipods (“lamp shells”)
End of the Cambrian mass extinction got rid of many trilobites and all archeocyaths; cause may be the end of the Sauk transgression

11. Tippecanoe transgressive sequence
Ordovician period begins with this sea level rise
In North America, first major Paleozoic orogeny on eastern margin – the Taconic orogeny, which was the result of the Iapetus (proto-Atlantic) Ocean closing
Queenston clastic wedge lays down sandstones in Ohio, evidence for 4000 meter mountains along North American east coast

12. Reef-builders begin, a new ecosystem
In the Cambrian (and before), algae and archeocyathids made large carbonate structures, not particularly extensive
In the Ordovician, tabulate and rugose corals developed and made extensive “patch”reefs (not like today’s scleractinian corals that make linear reefs)

13. Ordovician life
Graptolites (Graptolithnia) are creatures that made their first appearance in the Cambrian but are considered index fossils of the Ordovician
Look like “rock writing” , hence their name
Are hemichordates!
Go extinct during Carboniferous

14. First land “plants”
Some recent research (2001) has given some evidence that there were some land-based lichen or fungi as early as 1.1 by
More conventionally, there is good evidence for liverwort-like plants during the Ordovician - no vascular tissue, so short, near water
The presence of plants will alter not only atmosphere chemistry, but also the rate of weathering

15. End of Ordovician extinction
Second largest mass extinction (except for the end of Permian)
450 to 440 my
Cause: massive ice age as Gondwana moves over South Pole
Sea level falls as glaciers grow, and expose much shelf area

16. Mid-Paleozoic climate
Except for the 30 my long ice age at the end of the Ordovician, both the Ordovician and Silurian were greenhouse Earth times – sea level was 200 m higher than today

17. Silurian reefs and basins
In what is now Michigan, coral reefs developed toward the north and south (and west, though these are gone), trapping sea water in between
Subsidence dropped the level of this basin, allowing deposition of various salts as the restricted basin’s seawater evaporated

18. What did these reefs look like?
Similar to modern structures, since reefs exist in clear continental shelf water, at the edge of much deeper water
Can restrict the flow of ocean currents, strong enough to withstand even storm-driven waves

19. Evaporite basin
Repeated inundation by seawater and plenty of time resulted in 600 m of halite deposits in the center of the Michigan basin
Maximum deposition during the late Silurian

20. Eurypterids are the major marine predator
Mistakenly called “sea scorpions”, they are not true scorpions or entirely marine, but they are among the largest arthropods ever on Earth

21. Silurian land plants
Transition between non-vascular and vascular plants, could be a bit further from water’s edge
Cooksonia is the name of this extinct group of plants; fossils found globally
Used spores for reproduction

22. Acadian-Caledonian orogeny
At the end of the Silurian, the Tippecanoe transgression comes to an end due to the accretion of the Avalonia terrane to eastern North America (Laurentia)
Called the Acadian orogeny in North America, it’s responsible for much of the northern Appalachian uplift
In Britain, called the Caledonian orogeny (Scottish highlands)

23. The Devonian period starts the Kaskasia transgressive sequence

24. Devonian life
Though jawless fishes appear in the late Cambrian and jawed fishes in the Silurian, fishes (including sharks and bony fishes) diversify during the Devonian
Also, first amphibians and insects

25. First forests
Restricted to the water’s edge, the first true trees were comparable to modern trees in height
Lycopsids and progymnosperms (both extinct) dominate, look like tree ferns, but they are not ferns (structures are a result of convergent evolution)
No flowers or seeds yet

26. West coast of North America finally stops being passive – Antler orogeny