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Download Chapter 23 The Geology of the Mesozoic Era
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Mesozoic Geology Introduction • The Mesozoic began 248 mya and ended 65 mya – Three periods - Triassic, Jurassic, Cretaceous – breakup of Pangaea was the major geologic event – tectonism and sedimentation are used to classify the Mesozoic in N. America – Note the overlap in three styles of Cordilleran Orogeny Tectonism and Sedimentation Seaway drains 1. The Breakup of Pangaea • The movement of continents during and after the breakup affected global climates • Sea-level changes due MOR heating and growth, then cooling Pangaea - Early Triassic Pangaea ran from pole to pole and straddled the equator. The Panthalassa Ocean allowed equatorial waters to warm polar lands. The East coast indent is called the “Tethys Sea” Tethys Panthalassa Panthalassa Late Triassic – Rifting E Orogeny W We will consider mostly North America for this lecture Orogeny Rift Better look at Tethys Pangaea – Early Jurassic E Jurassic – Atlantic Rift Shallow Note offshore Terranes E Jurassic – Another Look Orogeny New Sea Wrangellia Pangaea: mid-Jurassic Latest Jurassic – Early Cretaceous Atlantic Connected with Tethys Africa rotation closes Tethys Tethys Atlantic Late K – Epeiric Sea until 70 mya Mesozoic Global Climates • • • • • Carbonates (for example the stable isotope ratio d13C) reveal large concentrations of carbon dioxide present in the Mesozoic atmosphere. This suggests a greenhouse climate. No glaciers, no coal, so CO2 abundant. Greenhouse gasses pass sunlight which hits the land and sea. Re-radiate heat (IR) Greenhouse gasses hold the heat, not lost to space as quickly. Warmer equilibrium. Global Climates in the Mesozoic Mesozoic climates were more equable than today, lacked the strong north-south climate zones. Mesozoic plant fossils indicate subtropical conditions in high latitude locations Seasonal differences were monsoonal Cycads Next: Mesozoic Tectonics NA • Cretaceous : global rise in sea level until 75 -70 mya, vast MOR • Jurassic: –Atlantic opens E, –began building the Cordillera W, – Gulf of Mexico begins to form and experiences evaporite deposition • Late Triassic: Begin rifting in East Start here Late Triassic: Rifting opens the Atlantic • The Newark Supergroup documents the rifting of Pangaea to form the Atlantic • Early Triassic saw coarse detrital sediments deposited from the erosion of Appalachian highlands – fault-block basins developed as N. America separated from Africa and filled with nonmarine sediment plus dikes and sills – eroded to a flat plain by the Cretaceous Mesozoic rift basins Kean University Structure of the Newark basin Note how faulting offsets sedimentation Lake cycles, East Berlin formation Alternating wet and dry climate due 21000y Milankovitch cycle of tilt axis wobble Also 100,000 year cycles due to orbit eccentricity E. Jurassic Gulf Coast Evaporites 200 mya is just outside our door Restricted Basin Lots of evaporation Gulf Coastal Region • First, as continents separate, restricted basin, thick evaporites formed in the Gulf • Normal marine deposition returned to the Gulf by Late Jurassic, with transgressions and regressions – thousand of meters of sediments were deposited Does this cross-section show a transgression or regression? Gulf Coast continental margin Rising Salt Domes tilt sediments Concentrate petroleum Discussion: Petroleum exploration around salt domes Squint: The Petroleum is in the bright spots, at the boundary of the salt and pushed up sediment. Tells us where to look, saves money, lowers cost of fuel. Next: Western North America Tectonics • Building the western margin of North America and the Cordillera Displaced terranes – Western Cordillera These terranes overlap in age but have different rock types, paleolatitudes and fossils. However, we can deduce when they accreted from this map. Exercise: Arrange the following terranes by oldest to youngest time of accretion onto the west coast: Alexander, Cache Creek, Chugach, Eastern, Stikine, Taku, Tracy Arm, Wrangellia, Yukon-Takana Western Region • Cordilleran Orogeny – Laramide – Vertical blocksbuilt the present day Rockies KTertiary – Sevier – J-K thrust faulting to the east – Nevadan Jurassic batholith intrusion in the Sierra Nevada and elsewhere on the western edge Western Margin during Orogens North America drifting west due opening of Atlantic Westward subduction zones stopped when continental crust arrived. Remember the late Permian Sonoma? It continued into the Early Triassic Sonomia docking Late Pm –Early Triassic Late Triassic on, eastward subduction of Farallon oceanic crust continues Cordilleran Orogeny Late J Early K Nevadan Batholiths Nevadan Orogeny east subduction Farallon Sierra Nevada Mountains Nevadan Orogeny: Subduction formed batholith cores of continental volcanic arc once as tall as Andes Mesozoic orogenic events Thin-skinned tectonics Cretaceous Sevier Wrangellia docking? Later moved by transform fault? K-T Laramide Continental Overide Bouyant Subduction Buoyant Subduction Laramide Orogeny Vertical block uplift Normal, thin-skinned Now we understand weird looking Tetons Approaching Continent pushes accretionary wedge sediments into forearc sediments Sevier thin-skinned deformation Using the layer colored sky blue, look at the faults. Is the hanging wall mostly up or down? What kind of faults are these? Sevier thrust belt Precambrian and/or Paleozoic Sediments thrust over younger Mesozoic rocks let’s look down here Look in detail at western plate margin This area has much simpler geology Franciscan Range, Great Valley Group, and Sierra Nevada Volcanics and Plutonics Next: Mesozoic Sedimentation on the Craton • Cretaceous – extensive marine deposition until 70mya, thins to the east • Jurassic Foreland Basin! – clean cross-bedded sandstones – marine sediments in the Sundance Sea • Triassic – shallow-water marine clastics – red beds North America - Triassic Marine deposition limited to western margin Volcanic Arc sends frequent ashfalls eastward Pollen similar Chinle Note Equator Newark Late Triassic Chinle Fm. Mudstones and Sandstones of stream deposits, volcanic ash, with fossil trees (the Petrified Forest!) Texas, New Mexico, northern Arizona, Nevada, Utah, and western Colorado Pollen studies show that the Chinle is the same age as early Newark Supergroup http://en.wikipedia.org/wiki/Petrified_Forest_National_Park Triassic caliche paleosol- Nova Sc. Source of carbonates for d13C measurements. Results suggest high CO2 in atmosphere Similar in Newark Supergroup North America - Jurassic period Dry region in the rain shadow of the beginning Nevadans Zuni Transgression Sedimentation Seaway drains Evaporites Jurassic Eolian sandstone Navaho SS, S. Utah http://rainbow.ldgo.columbia.edu/courses/v1001/morisson14.html Jurassic Morrison Formation Paul Olsen's Dinosaur Course Stream Deposits, huge sauropods Apatosaurus, also Stegosaurus, carnivore Allosaurus Fossils of Jurassic dinosaurs Morrison Formation sandstones, DNM, Vernal, Utah Giant sauropod and Allosaurus bones, Mossison Fm., DNM, Vernal Utah Late Cretaceous really big epeiric sea Dinosaurs on the North Slope Until 70 mya Land Land Did the Sevier Orogenic Belt form before or after the Navaho SS, lower left? Did the Sevier Orogenic Belt form before or after the Fox Hills SS, upper right? Fox Hills SS Western Interior Seaway Regression Western Interior Seaway Transgression Dakota SS is bottom right Dakota Sandstone Early Cretaceous shallow sea sediments gently folded by Sevier Orogeny. Then, at 75-70 my, Regression In Montana the sequence is similar. Above the marine Pierre Shale (ammonites) and Claggett Sandstone (nearshore and beach) is the Late Cretaceous Judith River Fm. containing dinosaur bones and conifers in stream deposits. Is this sequence a transgression or a regression? Western Interior Seaway Regression Western Interior Seaway Transgression Pierre Shale 75 mya Regression Mesa Verde Sandstones over Mancos Shale: Coarsening Upward K-T Boundary End of Mesozoic Geology
