Sedimentary rocks

Geology 103

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

Lithification

• Sedimentary rocks become lithified when they are compacted then cemented

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

Relative abundance of sediments

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

Mineralogy of sandstone and depositional environments

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

Chemical sed rx

• Evaporites: limestone, rock salt• Oxidation product: taconite (iron ore)

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

Biological sed rx

• Plant remains: coal• Animal remains: limestone, chert

Most biological sed rocks are deposited on (and make) carbonate platforms

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

Bora Bora atoll, South Pacifc

Coral reefs can form an atoll around a mid-ocean volcano

Coral reefs and atolls creation

Process first described by Charles Darwin

Water energy

• The speed of the depositing medium (usually water) can be inferred from clastic and some other sed rx

• This is because coarser grains settle first in a suspension