Part 1: The Vendian, Cambrian, and Early Ordovician Periods Earth History GEOL 2110 The Paleozoic Era Part 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.

Assembly of Rodinia 1200 – 750 Ma Li et al., 2008

Li et al., 2008

of Rodinia by a Superplume Disassembly of Rodinia by a Superplume 750 Ma Li et al., 2008

Laurentia Becomes Modern-day Africa Surrounded by Rifted Passive Margins

Mid-Cambrian Plate Reconstruction Laurentia becomes Isolated and Flooded Siberia Iapetus Ocean Gondwanaland Taconic Arc Baltica glaciation

Passive Margin Sedimentation Western 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 MN/WI Strat Column 95-99% Quartz Well Rounded Well Sorted Jordan SS

Depositional Environment Shallow Marine or Eolian? Low –angle cross stratification -Marine Both Eolian – early transport Marine – final deposition Abraided quartz grains - Eolian Mod–angle cross stratification -Eolian

How Shallow is Shallow Marine? Stromatolitic Limestone Fossilized Algal Mats Tidal Oolitic Carbonates Agitated Water Flat-pebble Conglomerates Storm Rip-ups of the Seafloor Mud Cracks Periodic Drying

Modern Day Analog to the Sauk Sea Gulf 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 Era Part 2: Late Ordovician Period Limestone, Limestone, and more Limestone and The Emergence of the Appalachians