Sedimentary Rocks

All places on Earth are, at any moment, either

EROSIONAL or DEPOSITIONAL• High places are erosional• Low places are depositional

Sedimentary Rocks• Are made of pieces of other rock

cemented together.• Cover about 75% of Earth’s

surface, BUT-• Account for only about 5% of

Earth’s crust (They make a very thin layer.)

• Are the key to Earth’s history.

I. Nature of Sediments

• A. Clastic = fragments of rock from weathering

• B. Chemical = dissolved rocks that crystallize

• C. Organic = once living materials (shells & leaves)

II. Types of sedimentary rocks

• A. Subclass clastic - Classified based on size of fragments (clasts)

.166

1. Gravel - size fragments (or larger = >2mm)

• Require a lot of energy to transport, for example: fast rivers, glaciers, land slides– a. rounded = Conglomerate - transported a

long distance by fast-flowing water (= fast fluvial)

– b. angular = Breccia - transported a short distance by:

• Glaciers - deposited when ice melts• Gravity - an alluvial environment or talus (scree)

2. Sand-sized pieces (.166-2mm) = Sandstone

• Usually majority quartz. The farther it has been transported - the closer it will be to pure quartz.

– a. river bank (fluvial)

– B. beaches

– C. windblown

Silt & Clay - sized pieces( <.166mm) = Shale

• Products of feldspars & micas weathering

• Deposited:– a. where mud piles up (flood

plains, basins, deltas, offshore)– b. quiet water - slow rivers

B. Subclass Chemical

• From dissolved minerals• Often single mineral rocks (so rocks

are not always mixtures of minerals.)

• Two ways for formation:– Evaporites– Precipitates

– (see next two slides)

1. Evaporites - water totally gone:

• A. halite (NaCl) - forms Rock Salt

• B. gypsum (CaSO4) - Rock Gypsum

2. Precipitates -minerals crystallize out of solution (without body of water drying up)

• a. calcite (CaCO3) - Limestone– 1. deep ocean (cold!!)– 2. underground - stalactites & stalagmites)– 3. at surface - Travertine (Tonto Natural

Bridge or Havasupai Canyon)b. quartz (SiO2) - requires hydrothermal (hot)

water!1. small hollow sphere = Geode2. veins in cracks with secondary minerals

C. Subclass Organic - once living organisms

• 1. Ocean critters (Clams, oysters, snails, etc.) - Limestone

• 2. Leaves, wood, swamp material:– a. solid, soft - Coal

– b. liquid, viscous - Petroleum

– c. volatile gasses - Natural Gas

III. Sedimentary Environments:

Oceans:

IV. Sedimentary Structures:

• A. Stratification = continuous horizontal layers. Most often deposited in water.– 1. Small scale - lakes, riverbeds, deltas

– 2. Grand (large) scale - ocean basins

C. Graded Bedding (storm layers)

D. Surface impressions:• 1. Ripple marks -

• 2. Mud Cracks -

• 3. Tracks -