The Early Paleozoic Era

Geology 103

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

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

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

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)

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”

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

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.

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

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

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

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)

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

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

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

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

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

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

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

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

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

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)

The Devonian period starts the Kaskasia transgressive sequence

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

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

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