Earth History GEOL 2110

The Paleozoic EraPart 1: The Vendian, Cambrian, and Early

Ordovician Periods

Major Concepts• The long-lived supercontinent of Rodinia created a long period

between 1,000 – 600 Ma with little deposition in the interior of most continents.

• The break-up of Rodinia, which started around 750 Ma resulted in great amounts of sediment deposited on the passive margins of the disassembled continents.

• In North America (Laurentia) great thicknesses of Vendian (610-550 Ma) to Cambrian (550-490 Ma) sediments accumulated along it continental edges.

• Between middle Cambrian time and early Ordovician time, the dispersal of the Rodinian plates resulted in a global rise in sea level, which flooded the continents with shallow seas.

• In North America, this is called the Sauk transgression and produced sedimentation of ultrapure quartz sands and later carbonates.

Assemblyof Rodinia

1200 – 750 Ma

Li et al., 2008

Li et al., 2008

Li et al., 2008

Disassemblyof Rodinia by a

Superplume750 Ma

Laurentia Becomes Modern-day AfricaSurrounded by Rifted Passive Margins

Mid-Cambrian Plate ReconstructionLaurentia becomes Isolated and Flooded

Laurentia

Baltica

Siberia

Gondwanaland

glaciation

Iapetus Ocean

Taconic Arc

Passive Margin SedimentationWestern US

Rift Basalts(750 Ma)Glacial Deposits

(Snowball Earth)

Thinning of Cambrian Sediments across the Laurentian Craton

Belt Supergroup sediments preserved in rift grabens (aulocogens)

Distribution of Cambrian Sediments over Laurentia

Warping of the Craton• Broad horizontal

tectonic stresses related to plate motion

• Sediment loading• Isostatic adjustments

due to different densities within the crust

MCR

Was Craton Warping Syn- or Post-Depositional?

Syn-depositional Warping

Post-depositional Warping

How do we tell Structure of the Crust?

Sedimentary Facies and Paleogeography of Late Cambrian Sedimentary Deposits

Ultrapure Quartz Cambrian Sandstone

95-99% Quartz

Well Rounded

Well Sorted

Jordan SS

MN/WI Strat Column

Depositional Environment

Shallow Marine or Eolian?

Low –angle cross stratification -Marine

Mod–angle cross stratification -Eolian

Abraided quartz grains - Eolian

BothEolian – early transport

Marine – final deposition

How Shallow is Shallow Marine?

Oolitic CarbonatesAgitated Water

Stromatolitic LimestoneFossilized Algal Mats

Tidal

Mud CracksPeriodic Drying

Flat-pebble ConglomeratesStorm Rip-ups

of the Seafloor

Modern Day Analog to the Sauk SeaGulf of Mexico

• <200 Meters Deep

• Carbonate deposition in detrital sediment starved areas

Differences• ¼ the area of Sauk Sea• Coral reefs not present until Silurian• No land vegetation in Cambrian - fine sediment winnowed from land by wind

Actualism Deposition accomplished mostly

by Hurricanes

“Fossil Hash” -Mass-kills from Hurricanes

Next Lecture

The Paleozoic EraPart 2: Late Ordovician Period

Limestone, Limestone, and more Limestone and The Emergence of the Appalachians