Sedimentary Rocks
Sedimentary Rocks
• Formed by the compaction and cementation
of sediments (small pieces of rock and living
things).
Depositional Environment
• Sedimentary rocks can tell you what the
environment was like when they formed. This
is called the “Depositional Environment.” For
example, a sandstone’s depositional
environment was likely a beach or a desert
since that is where you find sand today.
Where did these rocks form?
Sandstone with cross-bedding
Fossil Fish
Mudstone with ripple marks
Fossil Plant Impressions
Types
• 2 major types:
– A. Clastic: Made of smaller pieces of rocks called
clasts. (sandstone for example)
– B. Nonclastic (Either Chemical or Organic): Made
of once living things or from
evaporated/precipitated minerals. (Coal and
Limestone for example).
Examples of Clastic Sediments
Breccia
Sandstone
Conglomerate
Shale
Texture of Sedimentary Rocks
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Grain Size
Grain Type
Sorting
Grain Shape
Clastic
• Clastics - made from fragments of other rocks
called clasts. These clasts are either
compacted when sediments build up on top of
them or they are cemented together by some
sort of solution that acts like glue. Form layers
that are easily seen with the naked eye.
Size
• Fine grained: Small, can’t see individual grains
• Medium grained: Sand sized, can see grains
• Coarse grained: Pebble sized, can easily see
individual grains
Grain Shape
Sorting
• Well Sorted: Grains are similar shape and size
• Poorly Sorted: Grains are all shapes and sizes
Well or poorly sorted?
Well or Poorly Sorted?
Fossils
• Sedimentary rocks are the most likely rock to
have fossils. The fossil can tell you where the
rock formed.
Brachiopod fossils in limestone
Imprint of
a leaf
Trilobites
Trilobite fossils – over 250 MILLION years old!!!
Nonclastics
• Made from chemical or organic processes
• A. Chemical:
Precipitates - Minerals combine and settle
out in the ocean.
• Examples include: limestone (CaCO3),
dolomite (MgCO3), and chert (SiO2) & some
ores like hematite concretions.
• -Limestone and Dolomite form in shallow,
warm, tropical oceans. Chert forms in colder,
deeper ocean waters.
• Evaporites - left behind when water
evaporates. Rock Salt (NaCl)
Rock Salt
Nonclastics
• B. Organic:
• Limestone – Formed in shallow, warm,
tropical oceans from shells of organisms
and corals reefs that are cemented or
compacted together.
• Bituminous coal – Form from build up of
dead plants. Formed in swamps and
marshes.
• Coquina – Formed from cemented shells.
Likely formed in a beach environment.
Coal
Limestone
Coquina
Tectonic Boundaries
• Sedimentary rocks form in areas that lack
tectonic plate boundaries (passive margins)
like the east coast of the United States,
including Virginia.