Chapter 10Early Paleozoic Events

Cambrian limestones on Lake Superior’s south shore.

The Phanerozoic Eon • Consists of three eras (from oldest to

youngest):

– Paleozoic = "ancient life" (542-251 m.y. ago) – Mesozoic = "middle life" (251-65.5 m.y. ago) – Cenozoic = "recent life" (65.5 m.y. ago - present)

Paleozoic EraPaleozoic can be divided into:

• Early Paleozoic = Cambrian, Ordovician and Silurian

• Late Paleozoic = Devonian, Mississippian, Pennsylvanian, and Permian

Paleozoic overview

Paleozoic orogenic belts

Orogenic BeltsOrogenic belts are present along the edges of

the continent.

In the orogenic belts, strata are intensely deformed, with folding, faulting, metamorphism, and igneous intrusions.

Deformation occurred as a result of

continental collision.

OrogeniesIn the Appalachian region, there were three

Paleozoic mountain-building events (or orogenies):

–Taconic orogeny –Acadian orogeny –Alleghanian orogeny

Paleozoic Rocks of the Platform Across the

platform, in the continental interior, Paleozoic strata are relatively flat-lying to gently dipping, and warped into basins, domes, arches, and broad synclines.

Paleozoic Paleogeography • Paleogeography = "ancient geography." The

ancient geographic arrangement of the continents.

• Reconstructing the paleogeography requires paleomagnetic, paleoclimatic, geochronologic, tectonic, sedimentologic, and biogeographic fossil data.

Paleozoic Paleoclimates• Paleoclimatic evidence comes from

environmentally-sensitive sedimentary rocks (glacial deposits, coal swamp deposits, reef carbonates, evaporites).

• Early Paleozoic climate was affected by several factors: – The Earth spun faster and had shorter days. – Tidal effects were stronger because the Moon was

closer to Earth. – No vascular plants were present on the land.

Neoproterozoic Paleogeography

Just before Paleozoic began, the Precambrian supercontinent, Rodinia, had rifted apart to form six large continents and several smaller continents.

1. Laurentia (North America, Greenland, Ireland, and Scotland)

2. Baltica (Northern Europe and western Russia)

3. Kazakhstania (between the Caspian Sea and China)

4. Siberia (Russia east of the Ural Mtns and north of Mongolia)

5. China (China, Indochina, and the Malay Peninsula)

6. Gondwana (Africa, South America, India, Australia, Antarctica)

Early Paleozoic Continents

When continents are located on a pole, if conditions are right, glaciers will form.

During glaciations, sea level is lowered worldwide because the water is tied up in the ice sheets.

Shallow epicontinental seas are unlikely during glaciations.

By Late Cambrian, the continents moved off the pole. Some continents lie on the equator.

Glaciers melted, sea levels rose, and shallow epicontinental seas flooded the continents.

Transgressions and Regressions Shallow epicontinental seas transgressed

across the Laurentian (North American) craton during Early Paleozoic as the glaciers melted and sea level rose. The seas regressed as the glaciers enlarged and sea level dropped.

Transgressive-Regressive Sequences The transgression and regression of the

seas deposited sequences of sedimentary rocks that reflect the deepening and shallowing of the waters. These are called transgressive-regressive sequences.

Epicontinental Seas

Wave-washed sands, muds, and carbonates were deposited in the shallow epicontinental seas.

The epicontinental seas were sites of major diversification of marine life.

Unconformities During regressions, the former seafloor was

exposed to erosion, creating extensive unconformities that mark the boundaries between the transgressive-regressive sequences.

Cratonic Sequences• The unconformities can be used to correlate particular

sequences from one region to another.

• The unconformity-bounded sequences are sometimes called cratonic sequences.

• Two major transgressions occurred during Early Paleozoic in North America: – Sauk sequence (older - primarily Cambrian) – Tippecanoe sequence (Ordovician-Silurian)

North American cratonic sequences

Green = sedimentary deposits

Yellow = missing strata associated with unconformities

Worldwide Sea Level Change• Similar transgressive-regressive sequences are found

on other continents, suggesting that worldwide sea level change caused the transgressions and regressions.

• Worldwide sea level changes were probably related to glaciations and/or sea floor spreading.

• During times of rapid sea floor spreading, mid-ocean ridge volcanism displaces sea water onto the continents.

• Laurentia is nearly covered by shallow epicontinental seas.

• Laurentia lies on the equator, so water is warm.

• Deposition of sand & carbonate sediments

• Water deepens toward edges of continent, where shale is deposited

Cambrian Paleogeography

The Base of Cambrian• The base of Cambrian was formerly identified by the

first-occurrence of shell-bearing organisms such as trilobites.

• During the 1970s, small shelly fossils were found below the first trilobites, and dated at 544 m.y. The small shelly fauna includes sponge spicules, brachiopods, molluscs, and possibly annelids.

The Base of Cambrian The base of Cambrian is now placed at the oldest occurrence

of feeding burrows of the trace fossil Trichophycus, and dated at 542 m.y.

Nice outcrop, eh? It’s composed of Lower Ordovician sedimentary rocks, formed more than 450 million years ago, and is on Bell Island, just offshore from St. Johns, Newfoundland.

Multiple specimens of Trichophycus in Lower Ordovician rocks of Newfoundland, Canada, preserved as natural casts of the burrows. See all of those scratchmarks on the burrow walls? These were also made by trilobites.