Sedimentary rocks Geology 115

Making sediment Weathering = rock breakdown into smaller rock, or minerals, or chemicals Sediment = result of weathering of rocks Erosion = movement of sediment

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

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.

Lithification Sedimentary rocks become lithified when they are compacted then cemented

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

Chemical sed rx Evaporites: limestone, rock salt Oxidation product: taconite (iron ore)

Biological sed rx Plant remains: coal Animal remains: limestone, chert

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

Depositional environment Is the term given to the area where the sediment was originally deposited Must be inferred from the rock identity

Sedimentary structures Macroscopic features in the rock that allow you to infer climate, current direction or other attributes of the depositional environment

How to make cross-beds And notice that you can infer the wind (or water direction as well!

Cross-beds allow you to infer the depositional environment; meter-scale cross-beds are eolian

Bioturbation – trace fossils burrows get filled in with sediment filled burrows can tell you which way is up in a layer!