Lab Six: Sedimentary Rocks, Textures and Structures

Sediment: loose grains and chemical residues of earth materials- rock fragments, mineral grains, parts of plants and animals and rust.

· Two Types of Weathering:

Chemical- decomposition or dissolution of earth materials

ex- feldspars weather to clay

Physical- physical breakdown of earth materials ie. breaking, crushing

ex- A huge boulder is slowly eroded into sand grains by wind

*Sedimentary rocks form when sediments are compressed together, or hardened, and “lithified” usually by the precipitation of some chemical cement

· Texture of Sedimentary Rocks

Grain Shape: The degree of roundness observed in a grain. For example, a cubic grain would be considered angular, while a spherical grain would be considered rounded. Fig 6.1

*The degree of roundness can indicate distance from source and transport mechanism…an extremely angular grain would not have traveled as far as a rounded grain- because during transport all the edges get broken off.

Sorting: The degree a sample of sediment has been segregated based on its size and density. Fig 6.1

*Also a good indicator of distance to source and paleo-environment. For example, a beach setting produces extremely well rounded and well sorted sedimentary deposits, while an avalanche produces angular and poorly sorted deposits.

Inorganic Chemical Sedimentary Rocks-

When a sedimentary rock is created from chemical precipitation, like halite, identified by its microcrystalline texture.

BioChemical Sedimentary Rocks-

Formed from parts of now dead organisms or plants, for example, coal was once peat material formed from plants.

Sedimentary Structures

Mold: The original depression formed in the sediment

Cast: A mold filled with sediment and preserved

Stratification- Layering of sediments

Cross-Bedding (Cross Stratification)-

Strata deposited when moving wind or water causes migrating dunes, forms inclined strata like the pictures on Fig 6.11

Asymmetrical Ripples- wind or water- Indicate a Flow direction

Symmetrical (Oscillation) Ripples- Only in water, tidal waters move in and out

Mudcracks- mud shrinks as it dries forming polygonal cracks

*Check out many more sed structures and environments on pg 126*