Sedimentary rocks Geology 103. Making sediment Weathering = rock breakdown into smaller rock, or...
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Transcript of Sedimentary rocks Geology 103. Making sediment Weathering = rock breakdown into smaller rock, or...
Making sediment• Weathering = rock breakdown into smaller
rock, or minerals, or chemicals• Sediment = result of weathering of rocks• Erosion = movement of sediment
Sediment stages1. Weathering 2. Erosion
3. Transportation via water, glaciers and wind
4. Deposition
5. Burial and compaction
6. Diagenesis
Physical weathering
• Physical: Breaking apart of rocks by a physical force
• Also called “mechanical” weathering
Chemical weathering• A chemical reaction
alters the composition of the minerals in the rocks (e.g., dissolving halite or altering feldspars into clay minerals)
• Dissolving limestone leads to karst topography and caves
Caves are the product of varying water tables and limestone dissolution; cave formations (speleothems)
are the result of calcite precipitation
Or a combination
• Physical weathering may break up bedrock, then chemical weathering may break down the pieces into a soil
A quick word on soils
• Soils are the weathered material at the surface that include both organic and mineral components
• Soils differ due to the parent material, time of weathering and water content
Diagenesis is also a term used to describe the production of sedimentary rocks from sediments
Compaction squeezes out the water
Precipitation: addition of new minerals cements the sediment particles
Classification of sed rx
• Clastic = “broken”; sed rx made of broken-up parts of other rocks
• Chemical sed rx are made from the precipitation of chemicals from water (“evaporites”) or the oxidation of chemicals
• Biological sed rx are “born”; that is, they derive from the remains of creatures
Clastic sed rx
• Classified by dominant grain size• Scale: “boulder”, “cobble”, “pebble”, “granule”,
“sand”, “silt”, “clay”• Texture and composition are secondary
considerations (e.g., “shale”)
Texture: Grain roundedness
• The roundedness (that is, how sharply defined the “corners” of individual grains are) is used to determine the transport distance
• Proximal = near; distal = far
Texture: Grain sorting
• The variation between coarse and fine particles in the sediment is called sorting
• Sorting is used to infer transport distance; well-sorted sediments have come far
Layering in clastic sed rx
• Layers are called beds, unless they are thin, in which case they are called laminae
• Beds and laminae represent distinct depositional events, like floods
How evaporites are deposited 1. During the Miocene epoch, the Mediterranean Sea became a shallow evaporite basin when the Strait of Gibraltar was above sea level
2. Reduced water exchange with the Atlantic
3. Evaporation removed water
4. Fresh water inflow was limited
5. Gypsum and halite crystalize first forming evaporites
The Mediterranean dried up repeatedly between 5.6 and 5.33 Ma; the Zanclean Flood refilled the basin within two years
Garcia-Castellanos et al., “Catastrophic flood of the Mediterranean after the Messinian salinity crisis”, Nature 462 (2009), 778-781
Carbonate platform formation
Within the reef lagoon, growth of carbonate-secreting organisms, including forminefera, coral, algae and molusks, is rapid, and carbonate sediments form quickly
If sea level rises, the reef continues to grow towards the light and lagoon sedimentation outpaces sedimentation in the open ocean
Eventually a carbonate platform grows with steep sides towards the sea