2 Making sedimentWeathering = rock breakdown into smaller rock, or minerals, or chemicalsSediment = result of weathering of rocksErosion = movement of sediment
3 Physical weatheringPhysical: Breaking apart of rocks by a physical forceAlso called “mechanical” weathering
4 Chemical weatheringA 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
5 Caves are the product of varying water tables and limestone dissolution; cave formations (speleothems) are the result of calcite precipitation
6 Or a combinationPhysical weathering may break up bedrock, then chemical weathering may break down the pieces into a soil
7 A quick word on soilsSoils are the weathered material at the surface that include both organic and mineral componentsSoils differ due to the parent material, time of weathering and water content
8 Charles Darwin and soils Among other work, Darwin in 1854 determined that earthworms, through their castings, produced about an inch of soil per year.From this rate, he realized that a great deal of geologic time was needed even to produce a fertile soil.
9 LithificationSedimentary rocks become lithified when they are compacted then cemented
10 Classification of sed rx Clastic = “broken”; sed rx made of broken-up parts of other rocksChemical sed rx are made from the precipitation of chemicals from water (“evaporites”) or the oxidation of chemicalsBiological sed rx are “born”; that is, they derive from the remains of creatures
11 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”)
12 Texture: Grain roundedness The roundedness (that is, how sharply defined the “corners” of individual grains are) is used to determine the transport distanceProximal = near; distal = far
13 Texture: Grain sorting The variation between coarse and fine particles in the sediment is called sortingSorting is used to infer transport distance; well-sorted sediments have come far
14 Layering in clastic sed rx Layers are called beds, unless they are thin, in which case they are called laminaeBeds and laminae represent distinct depositional events, like floods
15 Chemical sed rx Evaporites: limestone, rock salt Oxidation product: taconite (iron ore)
16 Biological sed rxPlant remains: coalAnimal remains: limestone, chert
17 Water energyThe speed of the depositing medium (usually water) can be inferred from clastic and some other sed rxThis is because coarser grains settle first in a suspension
18 Depositional environment Is the term given to the area where the sediment was originally depositedMust be inferred from the rock identity
19 Sedimentary structures Macroscopic features in the rock that allow you to infer climate, current direction or other attributes of the depositional environment
20 How to make cross-bedsAnd notice that you can infer the wind (or water direction as well!
21 Cross-beds allow you to infer the depositional environment; meter-scale cross-beds are eolian
22 Bioturbation – trace fossils burrows get filled in with sedimentfilled burrows can tell you whichway is up in a layer!