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 = Geode
2. 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 -