Cambrian Period Notes

CAMBRIAN

• History of Earth through Deep Time.

PALEOZOIC

• The Paleozoic Era includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods.
• Timeline:
  • Cambrian: $540$ MYA
  • Ordovician: $485$ MYA
  • Silurian: $444$ MYA
  • Devonian: $419$ MYA
  • Carboniferous: $359$ MYA
  • Permian: $299$ MYA

LATE PROTEROZOIC

• Breakup of Supercontinent Pannotia in the late Proterozoic into the Cambrian Period.
• In the Cambrian Period, six large continents formed:
  • Laurentia: North America Greenland, Ireland, and Scotland
  • Baltica: Russia and most of northern Europe
  • Kazakhstania: Middle East
  • Siberia: Russia and Mongolia
  • China: China, Indochina, and Malay Peninsula
  • Gondwana: South America, Africa, India, Australia, and Antarctica

SEQUENCES

• Cratonic sequences (Sloss sequences) are large-scale lithostratigraphic units.
• They represent major global transgressive-regressive cycles.
• These cycles are bounded by craton-wide unconformities.
• Sedimentary rocks of North America are divided into six cratonic sequences.

TRANSGRESSION AND RETROGRADATION

• Transgression is the rising sea level due to glacial melting, tectonic subsidence, and high MOR spreading.
• Retrogradation is the deposition of sediments landward due to a rise in sea level.

REGRESSION AND PROGRADATION

• Regression is a fall in sea level due to the formation of glaciers, tectonic uplift, and low MOR spreading.
• Progradation is the deposition of sediments seaward; land builds seaward due to a fall in sea level.

CAMBRIAN (541-485 MA)

• Cambrian Period is divided into systems, series, and stages.
• The series include Terreneuvian, Cambrian Series 2, Miaolingian, and Furongian.
• Stages within these series are defined by the First Appearance Datum (FAD) of specific fossils (GSSPs).
• Key stage boundaries and their corresponding FAD markers include:
  • Base of the Cambrian (Fortunian): FAD of Trichophycus pedum.
  • Base of Cambrian Stage 2: FAD of Watsonella crosbyi.
  • Base of Cambrian Stage 3: Undefined
  • Base of Cambrian Stage 4: Undefined
  • Base of Wuliuan: FAD of Oryctocephalus indicus.
  • Base of Drumian: FAD of Ptychagnostus atavus.
  • Base of Guzhangian: FAD of Lejopyge laevigata.
  • Base of Paibian: FAD of Glyptagnostus reticulatus.
  • Base of Jiangshanian: FAD of Agnostotes orientalis.
  • Base of Cambrian Stage 10: FAD of Iapetognathus fluctivagus.

CAMBRIAN EVENTS ON LAURENTIA

• Early Cambrian erosion occurred due to low sea level, exposing much of the continent until the Sauk transgression.
• Laurentia was fairly tectonically stable.
• Sedimentary rocks formed from sands/clays eroded from older metamorphosed granites.
• Limestones were abundant and fossiliferous.

CAMBRIAN EPICONTINENTAL SEAS

• The rise in sea level flooded continents.
• A series of transgressions and regressions left distinct rock sequences of alternating sand, mud, and carbonates.
• Shallow warm seas were favorable for marine life.

CAMBRIAN SEAS

• Sauk transgression formed shallow epicontinental seas and laid down sedimentary rocks on continents.

SAUK SEQUENCE

• Transgression began in the late Proterozoic.
• For the first $50$ million years of the Cambrian Period, most of the continent was exposed and undergoing weathering.
• The maximum transgression occurred in the late Cambrian, covering Laurentia from Montana to New York.
• Seas were warm and clear without suspended muds.
• Calcium carbonate precipitation occurred by marine organisms.

EARLY PALEOZOIC

• Coastline migration eastward through the Late Proterozoic, Cambrian, Ordovician, and Silurian periods
• Key sequences: Sauk and Tippecanoe.
• Orogenies: Taconic and Acadian-Caledonian.

EVOLUTION OF LIFE

• Eukaryotes evolved ~$2.1$-$1.6$ Ga.
• Biodiversity had greatly expanded in shallow marine environments by the end of the Proterozoic.
• Possible evolution of almost all major phyla (as measured by molecular clock estimates) and some fossil data.

LIFE

• Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.
• Listing of phyla, their common names, distinguishing characteristics, and number of described species:
  • Porifera (Sponges)
  • Cnidaria (Jellyfish)
  • Arthropoda (Insects, spiders, crustaceans)
  • Brachiopoda (Lampshells)
  • Bryozoa (Moss animals)
  • Annelida (Segmented worms)
  • Mollusca (Snails, clams, squids)
  • Chordata (Vertebrates)
  • Echinodermata (Starfish, sea urchins)

CAMBRIAN EXPLOSION

• Sudden appearance in the fossil record of most phyla in the early Cambrian.
• This may represent a true “explosion” of rapid diversification of life at the phylum level.
• It may reflect the appearance of fossilizable hard parts in already existing phyla.
• It could also be due to a lack of appropriate older rocks to study.

LAGERSTÄTTEN

• Lagerstätten are deposits with exceptional fossil preservation.
• They are very important for the preservation of soft parts.
• They provide “windows” into early life.
• They are more common in the Cambrian due to less burrowing.
• Cambrian Lagerstätte fossils represent major phyla.

BODY PLAN

• Fossils showing the body plan characteristics of most phyla that have living members are found in the early Cambrian.
• Members of a phylum share a set of morphological features (observable physical characteristics) that are distinct from the features of other phyla.
• This set of features defines the body plan.
• Example: Chordates are bilaterally symmetrical and have a notochord, hollow dorsal nerve cord, v-shaped muscle fibers, pharyngeal slits, and a post-anal tail.

BURGESS SHALE

• The Burgess Shale represents a vibrant community with many arthropods.

MARINE FAMILIES

• The number of marine families through geologic time shows distinct phases: Cambrian, Paleozoic, and Mesozoic-Cenozoic.
• The graph illustrates the number of families versus geologic time (mya).

GEOLOGICAL TIME PERIODS

• Distributions of Cambrian fauna, Paleozoic fauna, and Modern fauna over geological time.
• Illustration of The Big Five mass extinctions:
  • End-Cretaceous
  • End-Triassic
  • End-Permian
  • End-Devonian
  • End-Ordovician