Welcome to GLG101C Introduction to Geology Fall 2004 Professor James Tyburczy
Department of Geological Sciences
	Chapter 8 Notes

Chapter 8. SEDIMENTS AND SEDIMENTARY ROCKS

• Contain information on history of life, history of climate, history of surface conditions
• Economic uses of sedimentary rock - gravel and sand (building materials); building stone (sandstone, limestone); coal; reservoir for petroleum, natural gas, and groundwater; salt; gypsum; phosphates (fertilizer, chemicals); cement; iron, gold deposits, ceramics (pottery, brick, tile, china)

Formation of sedimentary rocks

• Weathering and erosion - Recall solid products of weathering versus dissolved products of weathering
• Solid products of chemical weathering-> clay minerals, quartz, iron oxides, aluminum oxide
• Dissolved products -> ions in solution (for example K+, Na+, Ca+2, etc.), plus dissolved silica (SiO2). Acid rain speeds up dissolution of monuments and statues.
• Transportation of clasts- currents (wind, water) and glaciers reduces particle size, causes rounding and sorting
• Transport of dissolved material - collects in oceans or lakes
• Deposition

  Clastic deposition - occurs when velocity of current is reduced - particle size

  Chemical deposition - precipitation when chemical conditions change (temperature, acidity, concentration) - mostly in oceans, sometimes in salt lakes

  Biochemical precipitation - shells, skeletons of organisms - mostly in oceans Diagenesis (lithification) - compaction plus cementation (calcite, silica, and/or iron-oxide cement)

• Burial

  Temperature and pressure go up as you go deeper in to the earth. At depths of 10 km, the pressure can be 3000 times the pressures at the earth's surface, T can be 250 to 300 degrees C (482 - 572 degrees F)

  In some places there are large (hundreds of km) sedimentary basins that can be very thick (10 km!), caused by bending of the crust in response to the heavy load of sediments. These areas are of great interest in oil exploration.

• Diagenesis (Lithification) - compaction, cementation, recrystallization

Environments of deposition -

• Geologic processes
• Agent of transport - water, wind, glacier
• Plate tectonic setting
• Environment
  • Kind & amount of water
  • Topography
  • Biological activity
  • Climate

Types of environments - Clastic Sediments

• Continental - glacial, lake, flood plain, river channel, alluvial fan, dune, playa lake, swamp Shoreline - delta, tidal flat, beach, estuary
• Marine - continental shelf (shallow),continental slope (deeper), deep sea, reef, turbidity current

Types of environments - Chemical and Biochemical Sediments

• Carbonate environments - warm tropical shallow oceans
• Marine Evaporite environments - modern analogy is the Mediterranean Sea Calcite (CaC)3), gypsum (CaSO4o2H2O), halite (NaCl), other salts
• Siliceous environments - microorganisms with silica shells and skeletons, deep sea, chert, flint Swamp environments - vegetable matter - peat, coal

What are the characteristics of sediments deposited in each of these environments? - see Tables 7.2, 7.3, 7.4 and 7.5 in your text (Press and Siever, 3rd edition)

Sedimentary Structures

• Bedding - initially horizontal
• Cross-bedding - water currents and wind, can tell direction
• Graded bedding - continental slope, turbidites (turbidity currents) - deep sea 'landslides'
• Ripples and ripple marks - indicate direction of current
• Bioturbation, - rework existing sediments by organisms - burrows, roots, etc.
• Mud cracks - shallow water, dries out, exposed to air
• Bedding sequences - lateral & vertical variation in beds can tell of environment

Types of clastic sedimentary rocks - based on particle size

• Conglomerate - large, rounded clasts
• Sedimentary Breccia - large, angular clasts (talus slopes, glacial deposits)
• Sandstone - sand-sized grains, can be well- or poorly sorted, rounded or angular clasts, fraction of quartz versus other minerals or rock fragments. Often good oil and gas reservoirs
• Shale (and siltstone, mudstone) - clay-sized grains clay minerals aligned - causes low porosity, little cement (crumbly rocks), oil shales (future energy source?)

Chemical Sedimentary Rocks - inorganic origins and biochemical origins

• Limestone - CaCO3 - biochemical origins most common, some inorganic - recrystallization during compaction & cementation - can destroy evidence of origins
• Dolostone, dolomite - replace some Ca with Mg to form CaMg(CO3)2 - marine environment
• Chert - silica (SiO2) - biochemical (most common) and inorganic types - deep sea, chert nodules
• Evaporites - chemical - evaporation of salty water - halite, gypsum
• Coal & peat - vegetable matter from swampy (land) envrionment
• Oil and gas - deposits of organic matter in oceans, diagenesis
• Iron formations - iron deposited in oceans, very old (2.6 billion years), before earth's atmosphere became oxidizing