IMES
Lecture 5
Rocks: Records of Geologic Process
Sediments and Sedimentary Rocks
Dr. Sharon K. Reamer
Department of Earthquake Geology
Material modified from:
Peter Copeland and William Dupré
University of Houston
Sedimentary Rocks
• Sediments, the precursors of sedimentary rocks, are
found on Earth’s surface as layers of loose particles,
such as sand, silt and the shells of organisms.
• Weathering is all of the chemical and physical
processes that break up and decay rocks into
fragments of various sizes.
The fragmented rock particles are transported by
erosion, the set of processes that loosen soil and rock
and move them to the spot where they are deposited.
Sedimentary Rocks
•
a rock resulting from the consolidation of
loose sediment that has been derived from
previously existing rocks and accumulated in
layers.
•
a rock formed by the precipitation of
minerals from solution by either organic or
inorganic processes (chemical)
• Two types of sediments are produced by
weathering and erosion:
– Clastic sediments - physically deposited
particles such as grains of quartz and feldspar
derived from a weathered granite
» These sediments are laid down by running water,
wind and ice
» They form layers of sand, silt, and gravel
• Two types of sediments are produced by
weathering and erosion:
– Chemical and biochemical sediments - are new
chemical substances that form by
precipitation when some of a rock’s
components dissolve during weathering
» These sediments are laid down when rivers carry
the particles to the sea
» These sediments include layers of such minerals
as halite (NaCl) and calcite (CaCO3) which is
most often found in the form of reefs and shells
From Weathering to Sedimentary Rock
1. Particles of rock created by weathering
2... are transported
downhill by erosion
Igneous rock
Delta
Continental
margin
Beach
3... and deposited as
layers of sediment
Transportation:
water, glaciers or wind
4...where they form
parallel layers or bedding.
From Weathering to Sedimentary Rock
5. Clastic sediments are made
of deposited particles of
sand, silt and gravel
6. Chemical and biochemical
sediments are precipitated
from seawater or made by
coral reefs and shells.
7. Buried sediments lithify by
compaction and cementation
Genesis of Sedimentary Rocks
• Physical weathering: reduction in size
• Chemical weathering: change in composition
• Transportation:
– Solid particles: clastics by water, wind, ice
» primarily gravitational response - downhill
» river transport alone deposits 25 billion tons in the
ocean per year
– Ions in solution: Nature's cauldron
» weathering occurs intermittently between
transport cycles – can take 100s to 1000s of years
» earth's oceans have a "balanced" salinity
Transport will effect the sediment by: Sorting
•Variation in the range of grain sizes in a rock or sediment
•Poorly-sorted: either not far-removed from their source or
deposited by glaciers.
•Well-sorted: subjected to prolonged water or wind action.
Physical Effects of Transport - sorting, roundness,
and sphericity all increase with the amount of transport
short
larger, more angular
moderate
long
smaller, more rounded
Genesis of Sedimentary Rocks
• Deposition:
– same mechanisms as transport (water, wind, and ice)
– ultimately controlled by plate tectonic settings
» contintental collision results in mountain building
and is accompanied by alluvial deposition
» continental rifting and sea-floor spreading results
in sedimentary basins and continental shelf
deposits
Sedimentary Environments
Sedimentary Environments
Sedimentary Environments
Genesis of Sedimentary Rocks
• Burial and Diagenesis:
– sediments are built of buried layers of sedimentary
rocks
– these layers are called bedding sequences
– After burial, sediments continue to undergo physical
and chemical changes resulting in sedimentary rock:
this process is called diagenesis
– analysis of bedding sequences allows geologists to
reconstruct transport, deposition, and diagenesis
processes
From Sediment to Solid Rock
• Lithification - the chemical and physical processes
that convert sediments into solid rock:
– By cementation - minerals precipitate around
deposited particles and bind them together
(decreased porosity)
– By compaction - grains are squeezed together by the
weight of overlying sediment into a mass denser than
the original (decreased porosity and volume)
• Sediments are compacted and cemented after burial
under additional layers of sediment.
• Sandstone forms by the lithification of sand particles
• Limestone forms by the lithification of shells and other
particles of CaCO3.
Results of Lithification
How common are sedimentary rocks?
• sedimentary rocks are formed by surface processes
• they cover much of the Earth's land surface and seafloor
• their volume is small compared to igneous and
metamorphic rocks because they are difficult to preserve
What are sedimentary rocks made of?
Quartz*
Clay minerals*
Feldspar*
Calcite
Dolomite
Gypsum
Halite
*86%
• The most common minerals of clastic sediments are silicates*
- silicate minerals predominate in rocks that weather
• The most common minerals of chemically or biochemically
precipitated sediments are carbonates (e.g., calcite, the main
constituent of limestone)