Download Sedimentary rocks are derived from sediment and chemical

Sedimentary rocks are derived from sediment and chemical precipitates that result
from weathering and erosion processes. Sediment and chemical precipitates
accumulate in sedimentary basins, such as lakes or oceans, and are converted to
sedimentary rock over time.
Sediment and chemical precipitates can accumulate in a variety of sedimentary
environments.
The type of sediment or chemical precipitate can tell you a lot about the
environment of deposition. For example think about where coal or limestone
forms? What information can you infer from these rocks.
Sediment is converted to sedimentary rock following three processes. 1. Accumulation
of sediment in a sedimentary basin, such as this deltaic environment. 2. Burial and
compaction of sediment as it accumulates. 3. Cementation of the sediment by
cementing agents dissolved in groundwater..
Cementing Agents
1. Silica, 2. Fe-oxide
3. Calcite, 4. Clays
5. Tar (organics)
How would you identify a
specific cementing agent in
rock.
What is the predominant
cementing agent of the
sandstone shown in the upper
image (note it does not react
with HCl acid and is clear)?
What is the predominant
cementing agent of the
sandstone shown in the lower
image?
Sediment may composed of various grain sizes that range from the smallest clays to
boulder-sized clasts. The grain size of sediment provides information regarding the
energy regime of transport. What can you infer about the stream’s energy regime during
certain times of the year based on the size of the cobbles in the above image?
This conglomerate formed from sediment deposited in a river channel. The clast
lithology provides information regarding the eroded source rock and imbrication (“piggybacking”) of clasts tells you the direction of paleo-stream flow.
Note that this sedimentary rock classification chart is slightly different than the
chart we provided in lab. It adds a “bioclastic” subdivision.
Clastic sedimentary rocks are comprised of cemented “pieces” of weathered
rock that was deposited into a sedimentary basing. Clastic sedimentary rocks
are subdivided based on grain size. The conglomerate, shown above, may
have grain sizes that range between gravels (diameter > 2 mm) and boulders.
Sedimentary breccia has grain-sizes
similar to conglomerate, except the
clasts are angular and have not been
subjected significant transport and
erosion.
Can you think of a natural environment
of deposition where large clasts can be
transported without significant erosion or
abrasion.
What does the degree of rounding or
angularity tell you about distance of
transport?
Sandstone is clastic sedimentary rock comprised of cemented, sand-sized
(0.5 – 2 mm) clasts. The sandstone pictured above is predominantly quartz
clasts cemented with silica. How could you determine the cementing agent of
given sandstone rock?
What cementing agent holds the sand grains together in the above sandstone?
What is the diagnostic property that helps you identify the cementing agent?
Mineralogically mature sandstones are predominantly composed of quartz.
Immature sandstones (arkosic), such as the sample shown above, will
contain minerals, such as feldspar and micas, which over time will weather
and be removed from the sediment. What does the maturity of a sandstone
tell you about distance of transport?
Siltstones form in low energy environments, such as lake of marine basins. They
are often laminated (deposited in thin depositional layers).
Shale or mudstone is deposited in the
lowest energy environments and is
typical of marine settings distal from
the continental margin (i.e., terrerstrial
sediment source).
Note the fine-grained texture. The clay
minerals (< 0.002 mm) are too small to
see without magnification of a
microscope.
Chemical sedimentary rocks precipitate directly from an aqueous solution.
Halite (common table salt) is produced an evaporite deposit in the desert lake
basin in southeastern California.
Large deposits of salt can accumulate in restricted marine basins, where water
flow is restricted and evaporation high. The lighter salt can migrate upward into
the denser rock forming bulbus domes or diapirs. Salt dome form important
stratigraphic traps for Gulf Coast oil.
Most limestones are biogenic and derived from calcite formed from biological
processes, such as this fossiliferous limestone shown above.
Inorganic limestone precipitates in warm waters because the solubility of calcite
(calcium carbonate) is directly proportional to CO2 content in water. Warm water
typically holds less CO2 than cold water, which causes the solubility of calcite to
decrease.
Chalk, such as the rock that comprises the “white cliffs” of Dover, England
forms from the accumulation of microscopic calcareous marine organisms on
the ocean floor.
Oolitic limestone forms from oscillation (by wave action) of sand grains or shell
fragments and the precipitation of concentric rings of calcite around the
nucleating particle. Wave agitation causes the release of dissoved CO2 gas
from ocean water, which reduces the solubility of calcite (it precipitates). The
oolites are cemented together by calcite.
Why does limestone tend to
precipitation in shallow
water and chert in deep
ocean water?
The calcium carbonate compensation depth (CCD) represents the abrupt
transition of increased dissolved CO2 in ocean waters, which increases the
solubility of CaCO3. Limestone will not precipitate below this depth and silica
precipitation (formation of chert) dominates.
Chert is composed of almost pure
silica (SiO2) that tends to precipitate
in deep ocean basins below the
CCD.
Chert, like obsidian and quartz,
fractures conchoidally, which makes
it an ideal material to construct
arrowheads and axe blades
Coal forms from terrestrial organics
that accumulate in sedimentary
basins where organic production
exceeds organic decomposition.
As organics accumulate peat form
and is later converted to lignite and
then bituminous coal with additional
compaction and pressure.
Anthracite coal is produced under
the highest heat and pressure
conditions.
Why does sediment become stratified
into depositional layers?
Depositional layers signify the end of
one depositional event and start of
another.
They can signal change in the sediment
itself, such as the Triassic lake beds of
the Painted Desert, AZ), or a shift in
energy, such as waxing and waning
flood of a river.
Cross-beds consist of sedimentary
layers deposited at an angle
compared to the underlying beds.
They form when sediment is
deposited by a moving current such
as a river or stream (shown on the
left images) or wind.
Cross-beds form when sediment is
deposited by a moving current such
as a river or stream or wind (such as
the migrating dunes).
Sand grains saltate (bounce) along the stossface and then avalanche down the steep, lee
face, where wind velocities are reduced.
The Navajo Formation in Arizona and Utah is comprised of sandstone that formed
from migrating dune fields.
Can you determine the paleo-wind direction based on the cross-bedding shown in the
two images?
Mudcracks will form when shrink-swell
clays dry out during a period of
dessication, such as an ephemeral lake.
Deltas form when rivers flow into a
still body of water (i.e., a lake or
marine embayment) and deposit
sediment as the stream’s energy is
reduced. Deltas are subdivided
into three distinct layers: 1. Topset,
2. Foreset beds, and 3. Bottomset
beds.
This post-glacial delta formed when melt water streams flowed from the Cascades
into a proglacial lake dammed behind by the Puget Ice Sheet. Can you see the
contact between foreset and topset beds?
Ripples can from from currents (wind and water), such as those shown on the top
right image or from wave oscillation, such as those shown on the image on the
bottom right image. Which of these ripples patterns is comprised of asymmetric
ripples and why do think asymmetric ripples form versus symmetric ripples?
Graded beds form because large
sediment grains settle faster in a
water column than smaller
sediment grain.
In an event such as a submarine
landslide, a turbidity current may
be created with a chaotic mixture
of sediment suspended in the
water. It will settle out onto the
bottom of the ocean floor and be
graded with the largest grains on
the bottom of the “turbidite layer.”
Multiple turbidity currents can result in turbidite sequences that contain many
units of graded beds.
This turbidite sequence was deposited in a marine setting similar to the Puget
Sound, which occupied the Central Valley of California. The turbidite sequence has
been deformed by compressive stresses and compised the Vaca Mountains lying
between the Central and Napa Valleys.
Another important attribute of sedimentary rocks is the presence of fossils, most
commonly found in fine-grained clastic sedimentary rocks or biogenic sedimentary
rocks. Why would fine-grained sedimentary rocks most likely preserve fossils?
Think about energy regime and presence or absence of oxygen.