The Rock Cycle
Sedimentary Processes
1
2) & 3)
Sediments are
transported and
deposited
4
Products of weathering
Primary
Minerals
Residual
Minerals
Feldspar
Clay minerals
K+, Ca+2, Na+
Aluminum hydroxide
Fe-Mg minerals
Hematite &
Limonite
Mg+2
Quartz
Quartz
Silica
Primary
Minerals
Solids that
remain in soil
Ions that are carried
away in water
----------Detrital sediments------------------
Dissolved
Ions
Chemical & biochemical
sediments
Types of sediment
Detrital – mineral & rock fragments
Biochemical – shells made of
calcite (CaCO3) by organisms
that extract the ions from water
Chemical – halite (NaCl) crystals
that precipitate from water
Detrital sedimentary rocks- classified by
grain size differences
Conglomerate
Breccia
Sandstone
Shale
All these rocks have clastic textures – the rocks are composed of
particles (fragments) that are cemented together
Detrital sediments are sorted by the processes of
transportation and deposition
Well sorted
Poorly sorted
Sands and sandstones can be used to interpret the erosional
history of an area and the kinds of rocks that were eroded
Wellsorted
Poorly
sorted
“Mature” sediments have little feldspar. This means that the sediment was
derived from weathered rocks or that the feldspar decomposed during
transportation in water.
The same
diagram is
used to
interpret the
provenance
(source rocks)
of sediments
Placer deposit– accumulation of sand or gravel containing minerals that are
highly resistant to chemical attack and economically valuable - gold, platinum,
ilmenite, rutile, zircon, diamond, garnet, magnetite, corundum, monazite.
These and other chemically resistant minerals are widely used to indicate the
provenance of sediments, e.g. kyanite, sillimanite, staurolite
indicate that metamorphic rocks were eroded. Diamonds indicate kimberlite;
gold indicates hydrothermal veins, etc.
Chemical and biochemical sedimentary rocks
Travertine
Coquina
Limestones – composed of calcite
Chalk
Deep ocean sediments are mainly fine-grained biochemical
sediment called oozes, with some hydrogenous (chemical)
and terrigenous (wind or water borne) detrital sediment
Calcareous & siliceous ooze
Manganese nodules are
hydrogenous - they
precipitate from deep ocean
water when the Mn oxidizes:
Mn+2 (dissolved) + O2 + 2e- =
MnO2 (pyrolusite)
Banded Iron Formations – ancient (>2.5 billion year old) sedimentary deposits of ironbearing minerals. These indicate that Earth’s early atmosphere was too poor in oxygen to
oxidize iron at the site of weathering. Dissolved iron was transported in water:
4Fe+2 (dissolved) + 3O2 = 2Fe2O3 (hematite)
Phosphates form in shallow marine environments where dissolved PO4-3
is carried by upwelling of deep ocean water. These areas are biologically
productive - many fossils are found, especially bone material.
Phosphate Mining of the Bone Valley Formation in Florida
Evaporite minerals
Shallow basin with high rate of evaporation – Gulf of Mexico,
Persian Gulf, ancient Mediterranean Sea, Red Sea
Sequence is: calcite, gypsum, halite with increasing evaporation
Evaporites
Seawater evaporation:
1)
2)
3)
4)
5)
6)
Gypsum
Calcite
Gypsum
Anhydrite
Halite (95% evaporation)
Sylvite
Other rare minerals
Dolomite may form afterward by
reaction of Mg+2 with calcite
Continental waters:
Borax & borates
Nitrates
Halite
Salt
dome
Sulfur is
produced by
sulfate
reducing
bacteria
which
consume the
oxygen in
gypsum or
anhydrite