Lecture 6: Sedimentary Rocks and Continental Margins

Outline

1. Weathering, Transport, Deposition, Lithification
   • Sedimentary Facies, Environmental Analysis, Resources

   
   

2. Review of mechanical and chemical weathering

   • Definitions:
     • Mechanical weathering - reduction into physically smaller particles
     • Chemical weathering - chemical reactions in presence of water, oxygen and carbon dioxide
   • Examples:
     • Granite (mostly quartz + feldspar + amphibole or biotite mica):
       • quartz weathers mechanically to produce sand
       • feldspar weathers chemically to produce clay and dissolved silica, calcium and sodium
       • Fe-Mg minerals weather chemically to produce iron oxide and dissolved silica, calcium, magnesium, and sodium.
     • Basalt (mostly pyroxene + feldspar + olivine):
       • pyroxene weathers chemically to produce iron oxide and dissolved silica, calcium, magnesium, and sodium.
       • feldspar weathers chemically to produce clay and dissolved silica, calcium and sodium
       • olivine weathers chemically to produce iron oxide and dissolved silica and magnesium

   
   

3. Formation of sedimentary rocks - sediments must be deposited in a deep hole (or a hole that is deepening), otherwise they will be eroded again.

   • One such deep hole is the deep ocean basin (or abyss), at the edge of continents (Fig 6.5)
   • Delivery of sediments to the sea by rivers (Table in Lecture 5)
     • Bed load and suspended load (Fig. 15.13)
     • Dissolved load
   • Redistribution of sediments by waves
     • Concept of wave base: (see Waves in Chap. 19) - deepest water depth where sediments are moved by wave action
     • Wave base is determined by the largest storms

   
   

4. Delivery of sediments to the abyss

   • Turbidites: rapid flow of sediments down submarine canyons, like an avalanche (Figs. 6.19, 11.10)
     • Coarse sediments moved quite far off shore
     • Telephone cable breakage after the Grand Banks earthquake (1929) indicated that the flow velocity was about 80 km/hr
   • Suspended sediment: fine particles
   • Wind blown sediment: fine clay brought to the middle of the ocean
   • Dissolved sediments: especially limestone, deposited by corals and other organisms that make limey shells.
     • Form mainly where there is little clastic deposition (clear water)
     • Florida Keys and Bahama Banks are two areas of the US where limestone are being laid down
     • Corals may be ground up and reworked into lime sand before deposition

   
   

5. Environments of deposition

   • Biogenic Carbonate deposits (Limestone)
     • Warm, shallow water, absence of clastics.
     • Examples:
       • Florida shelf
       • Yucatan shelf
       • Bahama banks
     • map symbol: brick pattern
   • Fine-grained sediments (shale) (e.g. Fig. 6.15)
     • Deposited in quiet water - below wave base
     • Often some distance from shore
     • Examples:
       • Prodelta sediments
       • Marsh environments
       • Deep water deposits
   • Coarse clastics
     • Higher energy environments - near shore- above wave base, wind deposits
     • Examples:
       • Delta mouth bar
       • Point bar
       • Beach deposits
       • Desert deposits: fine material blown away by dunes
   • History in the sequence of sediments - more detail in upcoming lecture on Geologic Time
     • Lithology - facies - paleoenvironments
     • Superposition of sediments - a sequence in time (e.g. Fig. 6.16)
       • Idea first developed by Nikalaus Stensen (Steno 17th century)
     • Fossils - their evolution provides a unidirectional vector in time
       • William Smith (early 18th century) showed that the ages of rocks could be characterized by the fossils that they contained
       • Called the law of Faunal succession
     • The Geologic Column (see Fig. 8.2)
       • Division of the geologic record
       • Eons, Eras, Periods, Epochs
     • The major geologic Eras (Paleozoic, Mesozoic, Cenozoic) cover only the most recent 12% of the earth's age, but this time is important because it spans the time when multicellular life has flourished.
     • You should learn the Eons and Eras and the ages of the boundaries of each.
   • Age of ocean margin sediments
     • Discussion of sediment cross-sections from the two sides of the Atlantic (near Senegal, Africa and Maryland, US).
       • Similar sediment sequence on the two sides of the Atlantic
       • Oldest sediments: Triassic-Jurassic
         • Why are there no earlier sediments? (There was no ocean then).
       • Earliest rocks are limestones. Why?
         • Thermal bulge at the beginning of the opening of the ocean caused high elevations at edge of rift
         • Little clastics deposited, since streams tend to flow away from high topography of bulge
       • Shallow water limestones now at several kilometers depth
         • Imply subsidence
         • Due to cooling in this case