SEDIMENTARY ROCKS
- secondary rocks formed by the weathering and erosion of preexisting rocks, their transport and deposition or precipitaion elsewhere as a new rock.

Sedimentary rocks can be subdivided into two major categories.

CLASTIC ROCKS

-formed from the bits and pieces of rocks and minerals released by the physical breakdown of the weathered rock

- are typicaly named based on the size of the grains in the rock.

Conglomerate - contains gravel or larger sized grains
Sandstone - contains sand sized grains
Siltstone - contains grains too small to see, but which you can feel
Shale - contains grains microscopic in size, you cant feel or see

CHEMICAL/BIOCHEMICAL ROCKS

- form from the precipitation of ions dissolved from other rocks or the presence of biological activity

Three Main Types of Chemical Rocks

Evaporites - rocks which form from evaporative pools or bodies of water which evaporate

Rock Salt
Rock Gypsum
Fine-Grained Limestone

Cherts - rocks which form from the precipitation of quartz (silica) in water

Bedded Chert
(Nodular Chert)

Limestones - Rocks which form from the precipitation of the mineral calcite (CaC03)

Fossiliferous Limestone
Fine-Grained Limestone
Diatomite
Travertine

Organic-Derived

Coal
(Fossiliferous LS?)


IDENTIFYING CLASTIC ROCKS

The Three S’s -

Size - gravel, sand, silt, clay
Shape - angular or rounded
Sorting - well or poorly sorted


Three Rules of Thumb for Clastic Rock Textures

1. The larger the grain size, the larger the current needed to move the grain.

2. The larger the grain size, the larger the current, then therefore the higher the energy of the system. Low energy systems tend to have finer grain sizes than high e. systems.

3. Currents may be waves or wind, which may form structures which you can identify.


SEDIMENTARY STRUCTURES

Ripples

- indicate paleocurrent direction when preserved in rock

the larger the ripple the more powerful the current and the longer the current flowed there.

- ripples may be symetrical or assymetrical. asymetrical ripples indicate current flow toward the steep side.