Transcript Sedimentary Rocks

Sedimentary Rocks
Every stone a story, like a rosary.
---Joan Osborne
Weathering
The sediments that make up sedimentary rocks are produced by:
Mechanical & Chemical Weathering
Sedimentary Rocks and Processes
Clastic
Non-Clastic
Sediments are derived from
fragments of pre-existing
rocks (clasts).
All sedimentary rocks not
composed of clasts
Categorized primarily
by clast size
Categorized primarily by
composition
Clastic Rocks
e.g., conglomerate, breccia, sandstone, siltstone, shale
clast size
Formed from clasts (bits and pieces of pre-existing rocks).
The major processes in clastic rock formation are:
Weathering of clasts from pre-existing rock
Transport of clasts
Deposition of clasts
Lithification of clasts
Weathering processes continue to alter the clasts until
deposition. In general, both physical and chemical weathering
tend to make clasts smaller and rounder.
Sorting of clasts by size occurs mostly during transportation.
Sedimentary clasts
are classified by
grain size (the size
of the individual
clasts) using the
Wentworth Scale:
Wentworth Scale
Boulders
Larger clasts require
more energy to
transport
Cobbles
Gravel (pebbles)
Sand
Smaller clasts require
less energy to
transport
http://www.glossary.oilfield.slb.com/
Silt
Clay
Clast Sorting
The farther the clasts have traveled from the
source, the more well sorted they tend to be.
Clast Rounding
As clasts are rolled around
during transportation, sharp
corners tend to be knocked off,
and the clasts become more
rounded the farther they are
transported.
Angular grains are usually
found only near the source
rock.
Deposition
Proximal Depositional
Environments
Near the source rock, the clasts
have not experienced much
physical or chemical weathering.
These areas contain poorly rounded
and poorly sorted clasts.
Clasts composed of chemically
unstable minerals (e.g., feldspar)
are most common close to the
source rock.
Distal Depositional
Environments
Clasts experience both chemical
and physical weathering as they
are transported. Thus, the further
they have traveled from the
source the more altered the
clasts are.
http://www.flmnh.ufl.edu/alligatorfund/potm-nov04.html
http://www.ufz.de/index.php?de=5366
Distal areas contain well rounded
and well sorted clasts. Large
clasts (e.g., boulders) are rare
(they have either been left
upstream, or have weathered into
smaller clasts).
Clasts composed of chemically
unstable minerals (e.g., feldspar)
are very rare in distal settings.
Clay minerals derived from
feldspars are common.
Clast size indicates ancient relative current velocity
weak moderate
>=1.8 km/hr (strong currents)
Chemical Sedimentary Rocks
Clastic Sedimentary Rocks
Formed from clasts (bits and pieces
of pre-existing rocks) transported,
deposited,and lithified
Chemical Sedimentary Rocks
Formed from precipitation of minerals
within the depositional basin. Includes
biominerals, evaporites, coal, chert, etc.
Evaporites
minerals precipitated from
evaporating water
Siliceous Rocks
Silica and opalline silica
precipitate either organically or
inorganically
Coal and Peat
Carbon-rich deposits formed from
build-up of plant material in
swamps
Carbonates
Variety of limestones composed
of calcite, aragonite, and dolomite
precipitated either organically or
inorganically
http://csmres.jmu.edu/geollab/Fichter/SedRx/chembiokey.html
Common Minerals:
Carbonates - Limestone
Biochemical (corals)
Bioclastic (chalk, coquina)
Inorganic – chemical precipitate
travertine
tufa
oolitic limestone
Calcite CaCO3
Aragonite CaCO3
Dolomite (CaMg)CO3
Corals and Snails
Chalk
Limestone formed from
the microscopic calcite
skeletons of tiny animals.
Siliceous Sedimentary Rocks
Microcrystalline quartz and opal precipitated from dissolved ions in
sedimentary environments. Can be precipitated organically or inorganically.
Microcrystalline quartz can also precipitate in pore spaces to form cement.
http://www.diskworks.com/octoberopals/opal1.html
Scanning Electron Micrograph (SEM) of tiny, siliceous
skeletons of radiolarians, which are a source silica chert
Ionic Composition of Seawater
Cations
Anions
Na+
10,556
Cl-
Mg2+
1,272
(SO4)2- 2,649
K+
380
(HCO3)- 140
Ca2+
400
parts per million
Common Mineral Salts
NaCl - Halite
KCl - Sylvite
CaSO4•H2O - Gypsum
18,980
Total
Ions:
H2O:
34,387 ppm
965,613 ppm
35 o/oo
normal marine salinity
Other Mineral Salts
KMgCl3•6H2O - Carnalite
MgCO3 - Magnesite
CaMg(CO3) - Dolomite
CaSO4 - Anhydrite
Fossil example of the past activity of organisms
mixing sediment --- an example of fossil
BIOTURBATION