Geology 111G/Lecture 8

Sedimentary Rocks

Processes Leading to Formation

Clastic

Chemical

Sedimentary Basins

Classification

I. Sedimentary Rocks: Lithified deposits formed on land or in the sea. A number of processes are important to their formation:

A. Weathering. Creates sediment, either as fragments or alteration products, such as clay, of former rocks, which we call clastic sediment, or chemical sediments which are dissolved products of weathering, primarily ions in solution.

B. Erosion. Initial displacement of the particles away from place of origin. More important in clastic sediment.

C. Transport. Movement of particles from site of formation to site of deposition.

D. Deposition (sedimentation). Settling of particles as current or wind energy slackens or decreases.

E. Burial.

F. Diagenesis: The processes of compaction and cementation that convert sediment to sedimentary rock.

II. Primary types: Clastic vs. Chemical. The two types of sediment create two very different kinds of sedimentary rock.

A. Clastic sedimentary rocks. These are dominated by fragments of pre-existing rocks. Sand grains, pebbles, clay.

B. Chemical sedimentary rocks. Precipitated from ions dissolved in water.

1. Chemical precipitation. Evaporation or supersaturation in water (brine).

2. Biological precipitation. Carbonate-secreting organisms are very important contributors to formation of rock.

III. Sedimentary basins. Sediment accumulates in large-scale subsiding structures known as sedimentary basins. Essentially the opposite of mountains. Here sediment is buried to great depth and lithified via diagenesis. Important locations for resources, including oil, gas and many mineral deposits.

A. Example in book is East African rift valley, but we live in a sedimentary basin.

IV. Classification.

A. Clastic rocks are classified according to grain size.

1. Conglomerate.

2. Sandstone

3. Siltstone; particles <1/16 mm diameter

4. Mudstone, shale, claystone: particles <1/256 mm diameter.

a. The reason this is a little confusing is because there is a dual meaning for the term clay--a size definition and a mineral definition.

B. Chemical rocks are classified by their chemical composition, or primary mineral present.

1. Limestone: Composed mostly of CaCO₃, calcium carbonate.

2. Dolostone. Composed mostly of CaMg(CO₃)₂, dolomite.

3. Evaporite. Formed by inorganic precipitation, usually by evaporation. Typical rocks are composed of gypsum or halite.

4. Chert. Rock composed of microcrystalline silica. This commonly comes from marine micro-organisms, diatoms or radiolarians.

5. Coal. Sedimentary rock composed entirely of organic carbon originally derived from plants and converted by diagenesis. Formed in marshy areas where organic productivity is high and subsequently buried. Tends to occur in beds or seams.

6. Petroleum (oil and gas). Result of diagenesis of marine microorganisms buried in mudstone or shale.