The Sedimentary Rocks Page
All information on this page
was taken from the Geology 101 Laboratory Guide Fourth Edition - George
Mustoe
All pictures on this page
were taken from Earth, An Introduction to Physical Geology, Fifth Edition
- Edward J. Tarbuck and Frederick K. Lutgens
For individual credits for
the pictures or information, please consult these books
Before looking at each individual
rock, here are a few things you should know about Sedimentary rocks in
general:
Kinds of Sedimentary Rocks
There are two kinds of Sedimentary
rocks, Detrital and Chemical. Detrital (sometimes called Clastic
or Fragmental) rock particles are deposited by gravity, wind and water.
Chemical (sometimes called Depositional) rock particles are deposited by
chemical or biological processes.
Grain Type/Size
| Grain Type |
Diameter (millimeters) |
| Boulder |
> 256 |
| Cobble |
64 - 256 |
| Pebble |
4 - 64 |
| Granule |
2 - 4 |
| Course Sand |
0.5 - 2 |
| Medium Sand |
0.25 - 0.5 |
| Fine Sand |
0.06 - 0.25 |
| Silt |
0.002 - 0.06 |
| Clay |
< 0.002 |
Particle Shape
Rock fragments are worn down as
they are transported, so their shape provides us with some information
about their history. Freshly-eroded rocks have angular shapes, whereas
rocks that have been carried downstream for a long time have smoother grains.
Sorting
Sediment that hasn't traveled
very far is likely to be poorly sorted, which means it will probably consist
of a mixture of large and small particles. Well sorted sediment is
all the same size and shape. This can also tell us something about
the history of a rock.
How a Detrital Sedimentary
Rock is made:
Breccia - Conglomerate - Pebbly
Sandstone - Sandstone - Sandy Siltstone - Siltstone - Shale
Detrital Rocks
Breccia
(Breccia)
Contains angular rock fragments,
usually in a fine-grained matrix
Depositional environment:
landslides
Conglomerate
(Conglomerate)
Composed mostly of rounded pebbles
in a fine=grained matrix
Depositional environment:
fast-flowing rivers, beaches, glaciers
Sandstone
(Sandstone)
Sand-sized grains
Depositional environment:
by water - beaches, sandbars
by wind - sand dunes
Shale
(Shale)
Made up of silt and clay.
Not uncommon to have fossils embedded within Shale
Depositional environment:
quiet water-lakes, swamps and marine basins
Chemical Rocks
Limestone (can
be Organic and Inorganic - see below)
Organic Limestone
Chalk
Soft and powdery, possibly slightly
granular. White or cream colored. Reacts to acid.
Depositional environment:
shallow marine environments
Origin: composed of calcareous
shells of microscopic animals
Organic Limestone
Coquina
(Coquina)
Loose shell fragments, poorly
cemented together
Depositional environment:
shallow marine environments
Inorganic Limestone
Travertine
(Travertine)
Can be almost any color and texture
Depositional environment:
usually forms in marine environments, caves
Origin: Consists of calcium
carbonate deposited by chemical precipitation.
Diatomite
Soft and powdery, white color
Depositional environment:
lake beds
Origin: formed by accumulation
of silica shells of diatoms (microscopic plants)
Chert
(Chert)
Can be any color, hard, easily
scratches glass, conchoidal fracture, luster is porcelain-like
Depositional environment:
deep ocean basins
Origin: microcrystalline
quartz formed by chemical precipitation
Coal
Soft and brittle, black, may show
conchoidal fracture
Depositional environment:
swamps
Origin: Accumulation of
plant debris produces peat. After burial, heat and pressure convert
peat to coal
Gypsum
Granular to coarse crystalline,
usually colorless or white
Depositional environment:
shallow seas; lakes in arid regions
Origin: Gypsum, halite (rock
salt) and other water-soluble minerals are deposited by evaporation of
saline water
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