Sediments and Sedimentary Rocks
Texture, mineralogy, structure tells us about source of sediment and kind of environment in which sediments formed. Sedimentary Rocks illustrate Uniformitarianism. Sedimentary Rocks primary source of Oil, Gas, and Coal.
Sedimentary Rocks and the Rock Cycle
Weathering - breakdown or dissolve rocks
Erosion/Transportation - carry weathered material downhill/downstream
Deposition/Burial - Detrital particles settle out, chemical and biochemical precipitates. New sediments deposited buries older sediments
Diagenesis - sediments altered and lithofied by heat, pressure and chemical reactions (e.g., convert sand grains into a sandstone)
Weathering and Erosion Products
Clastic Sediments - physically transported solid fragments of weathered rock. Most common
Chemical and Biochemical Sediments - precipitation of new minerals (e.g., calcite).
Transportation and Deposition - wind, rivers, ocean currents, glaciers.
Particle Size and Sorting
Sorting - well sorted implies all the particles are the same size
Physical weathering reduces size, rounds angles
Dissolved Materials in Solution - chemical reaction NOT related to velocity of fluid
Oceans: chemical mixing vats - on our time scale, amount of water and salinity is constant.
Calcium is weathered out of silicates then taken out of solution by organisms
NaCl precipitates out of sea water in warm, shallow seas which balances fluxes from rivers.
Vegetation (land plants) deposited in swamps -> peat -> coal
Vegetation (algae) deposited in lakes and oceans -> oil and gas
Sedimentary Environments- A particular set of geological and environmental conditions
Continental Environments
Alluvial - river, widespread deposits
Desert - wind
Lake - chemical sedimentation (Great Salt Lake)
Glaical - moving ice
Shoreline Environments
Deltas - river enters ocean or lake
Tidal flats - areas exposed at low tide
Beach - wave action distributes sediment
Marine Environments
Continental Shelf - gentle currents in shallow water, clastic or chemical
Continental Margin - deeper water, turbidity currents
Organic reefs - carbonates found in continental shelf or volcanic islands
Deep-sea - turbidity currents and occasional oceanic currents
Clastic Sedimentary Environments - continental alluvial (streams), deserts, lake, and glacial, deltas, beaches, and tidal flats
Chemical and Biochemical Sedimentary Environments - carbonates (tropical or subtropical oceans), evaporites (warm, shallow, restricted sea), Siliceous (deep-sea)
Sedimentary Structures
Bedding or stratification - parallel layers of different grain size or compositions
Cross-bedding - sets of sand beds inclined at angles to the horizontal, indicates wind or moving water
Graded Bedding - grain size changes from coarse at bottom to fine at top. Turbidity current
Ripples - small dunes of sand or silt from wind or moving water (rivers or ocean waves)
Bioturbation Structures - worm holes in soft sediment
Bedding Sequences - interbedding of sandstone, shale, and other sedimentary rocks
Burial and Diagenesis from Sediment to Rock
Subsidence and Accumulation - broad area of crust sinks, it fills with sediment, weight of sediment causes it to sink some more.
Diagenesis - heat, pressure, and chemical reactions
Compaction - weight of overlying sediments decreases volume and porosity.
Cementation - Temperatures are higher at depth causes chemical reactions. Calcite (or quartz) precipitates which "glues" sediments together
Classification of Clastic Sedimentary Rocks - (75%) by Particle Size
Gravel -> Conglomerate or Breccia (mountain streams, rocky beaches, glaicers)
Sand -> Sandstone (rivers, ocean waves, wind)
Mud -> Siltstone (tides and rivers)
Clay -> Shale (breaks along parallel planes) or Claystone (wind blown dust)
Siltstones, Shales and Claystones are 75% of clastic rocks
Classification of Chemical/Biochemical Sedimentary Rocks - (25%) by Chemical Composition
Carbonates (CaCO3) - foraminifera (single cell organisms in shallow water), coral reefs, and inorganic precipitation (Bahamas)
Limestone - calcite from shells or precipitate
Dolostone - altered calcite from mineral to seawater exchange of Mg2+ and Ca2+
Evaporites - Gypsum , Anhydrite, and Halite from evaporation of sea water
Siliceous - chert - organisms die, shells sink to bottom, buried and cemented into chert
Iron Oxides - formed early in earth's history before atmosphere had free oxygen. Iron ore