Jurassic Period Notes

Jurassic Period (201-145 Ma)

Chronostratigraphy

• The Jurassic Period is a part of the Phanerozoic Eon, Mesozoic Era.
• It is further divided into epochs:
  • Upper Jurassic:
    • Tithonian: $145.0 ± 0.8$ Ma
    • Kimmeridgian: $152.1 ± 0.9$ Ma
    • Oxfordian: $157.3 ± 1.0$ Ma
  • Middle Jurassic:
    • Callovian: $163.5 ± 1.0$ Ma
    • Bathonian: $166.1 ± 1.2$ Ma
    • Bajocian: $168.3 ± 1.3$ Ma
    • Aalenian: $170.3 ± 1.4$ Ma
  • Lower Jurassic:
    • Toarcian: $174.1 ± 1.0$ Ma
    • Pliensbachian: $182.7 ± 0.7$ Ma
    • Sinemurian: $190.8 ± 1.0$ Ma
    • Hettangian: $199.3 ± 0.3$ Ma
• The Jurassic Period ended $201.3 ± 0.2$ Ma.
• Chronostratigraphic units, ranks, names, and formal status are approved by the International Commission on Stratigraphy (ICS) and ratified by the International Union of Geological Sciences (IUGS).

Paleogeography

• The breakup of Pangea continued during the Jurassic.
• Gondwana began to break up, separating Antarctica/India/Australia from South America/Africa.
• Diagrams illustrate the positions of continents and oceans during the Middle Triassic (240 Ma), Late Triassic (220 Ma), Middle Jurassic (170 Ma), and Late Jurassic (150 Ma).
• Key features include the Panthalassic Ocean, Tethys Ocean, major triple junctions, ancient sutures, and future break-ups/major rifts.

Sea Level

• Sea level began to rise in the Zuni Sequence.
• The Exxon sea level curve shows global sea level fluctuations throughout geological time.
• The graph plots millions of years ago (0-542) against sea level change (m), showing the Absaroka, Kaskaskia, Tippecanoe, and Sauk sequences.

Nevadan Orogeny

• Began in the Late Jurassic and reached its maximum in the Early Cretaceous.
• Affected the western Cordilleran margin with folds, faults, and metamorphism.
• Formed the Sierra Nevada orogenic belt and batholith; also recorded in the Klamath Mountains.
• Converging plates trapped altered rock sequence called mélange in between them.

Cordilleran Accretionary Tectonics

• Subduction zones were active along the western margin of North America.
• Oceanic crust was subducted.
• Fragments of continents and volcanic arcs/islands collided with North America in several episodes of accretion.
• These are identified as exotic accreted terranes.

Exotic Terranes

• More than 50 exotic terranes have been identified in the Cordillera.
• They make up approximately 70% of the Cordillera, forming a tectonic collage.
• Obduction is the process where land masses on moving plates ride up onto another land mass.

Cordillera

• The name is derived from Spanish, meaning "little rope."
• It refers to an extensive chain or network system of mountain ranges that run approximately parallel to each other along with plateaus and other intervening features.
• Examples include western North America and the Andes.

Jurassic Deposition

• Many terrestrial areas existed with alluvial and aeolian deposits and red beds.
• Mountains were eroding along the east coast.
• The Nevadan Orogeny occurred along the west coast.
• The Sundance Sea was present in the mid-late Jurassic.
• The Sundance Sea was topped by the Morrison Formation in the Late Jurassic.

Navajo Sandstone

• Consists of dunes with cross-beds, representing a coastal dune environment.
• The Navajo Sandstone is part of the Glen Canyon Group.
• The San Rafael Group and Morrison Formation are younger.
• The sequence includes:
  • Morrison Formation
  • Summerville Formation
  • Curtis Sandstone
  • Entrada Sandstone
  • Carmel Formation
  • Navajo Sandstone
  • Kayenta Formation
  • Wingate Sandstone

Sundance Formation

• Consists of sands and silts with marine reptile fossils, formed in the Sundance Sea.

Morrison Formation

• Composed of terrestrial sands, silts, and mudstones formed in swampy river floodplains on top of the Sundance Formation.
• Rich in dinosaur fossils, with the Dinosaur National Monument being a prominent fossil site.

Jurassic Climate

• The climate was warm and temperate.
• Vast seas had warm surface temperatures.
• There were no glaciers; coal deposits are found in Antarctica.