3. Wacke (Graywacke)
• Physically hard.
• Dark grey in colour,
• Enigmatic (ambiguous) rocks.
• Texturally, an immature
sandstone.
• Composed of sand mixed
with mud. Wikipedia
• Graywackes are associated with graded bedding.
4. Wacke (Graywacke)
• It has a dark colour, indicating its deposition in
the low-oxygen environment of the deep sea.
• Contains more than 15% clay.
• Coarse-grained sandstone that consists of
poorly sorted angular to subangular grains of
quartz and feldspar.
• Clasts of monocrystalline quartz are often the
most abundant framework component (25%
to 50%), although the proportion fluctuates.
5. Wacke (Graywacke)
• Grains of chert, mudrock, limestone, polycrystalline
quartz, and volcanic rocks are also quite common.
• Clasts of detrital muscovite, biotite, and chert occur
in accessory amounts.
• SiO2 content of wackes ranges from 50% to 70%,
reflecting the moderate amount of quartz and
feldspar.
http://weiminhan.files.wordpress.com/2010/03/2010-03-17_161215.jpg?w=390&h=229
7. Wacke (Graywacke)
• Grain size distribution is bimodal with a
primary peak for framework grains and a
secondary peak for matrix.
• Many wackes were deposited by waning
turbidity currents.
• They routinely display graded bedding, sole
markings, and the systenatic upward changes
in sedimetary structures and grain size
characteristic of turbidities.
9. Wacke (Graywacke)
• Deep water abyssal and bathyal body fossils,
pelagic fauna and flora, and retransported
shallow-waterorganic remains are all found
within wacke sandstone sequences.
• Some wackes were deposited within submarine
fan complexes, but there are also example of
more distal deposits settled out of more diffuse
density flows that spread out and dispersed
across broad, flat abyssal plain surfaces.
10. Wacke (Graywacke)
• Wackes are the dominant sandstone of the
Archean.
• It contains quartz, Like other sandstones, but
it also has more delicate minerals and small
fragments of rock (lihtics).
• Its grain are not well rounded. But this hand
specimen is in fact a graywacke which refers
to a specific origin as well as a wacke
composition and texture.
11. Wacke (Graywacke)
• The origin of greywacke was a
problem. Gravel, sand and
mud should not be laid down
together, because river
outflow drops heavier grains
first.
• Now geologists think
greywacke is formed by
submarine avalanches or
strong turbidity currents.
Wikipedia
12. Formation of Greywacke
sandstone
• Sandstone is a type of sedimentary rock which is
made from particles (sediment) that have been
glued together. In this case, the sand is mixed
with mud and clay and squeezed so that all the
water disappears and the particles are pushed
close together.
• In summary, the formation of greywacke rock is a
result of the rock cycle processes of erosion,
transport of eroded material by rivers, deposition
onto the sea floor and then pushing up by plate
tectonic movement.
13. Where is graywacke sandstone deposited?
• Graywacke sandstone is a sedimentary rock that is made
up mostly of sand-size grains that were rapidly deposited
very near the source rock from which they were
weathered.
• Graywacke is deposited in deep ocean water near volcanic
mountain ranges, where underwater landslides and density
currents called turbidites quickly transport sediment short
distances into a subduction zone or ocean trench.
• This type of sandstone contains fewer grains made of
quartz and more made of feldspars, volcanic rock
fragments, as well as silt and clay than most sandstone.
• It is therefore also known as “dirty sandstone.”
• The volcanic rock fragments give graywacke a greenish-
gray color.
14. What makes the beds in graywacke?
• Graywacke sandstone deposits display flat-lying beds,
each composed of sedimentary particles of different
sizes.
• The sandstone beds can be from inches to many feet
thick and are often separated by thin, dark shale beds.
• Each sandstone bed was formed during a single
turbidite or submarine landslide event and was
deposited over a short period of time from hours to
days.
• The thin shale beds formed between turbidite events,
when mud particles slowly settled to the sea floor, and
may represent thousands of years.
15. What makes the beds in graywacke?
• Turbidites display graded bedding, that is, the
grain size decreases upwards in the bed.
• During a turbidite event, the larger and
heavier grains settle out first.
• As the energy in the landslide event
decreases, finer and finer particl es settle out
to the sea floor.
16. Are there fossils in graywacke?
• Graywacke sandstone occasionally contains fossil
mollusks which sometime scan be used to tell when
the rock was deposited.
• Sandstone deposits of the Franciscan Complex
contain clams and ammonites from the Jurassic and
Cretaceous periods.
• These provide ages for when the Franciscan oceanic
rocks got close enough to North America for
continental graywacke sediments to be deposited
onto them.
17. Are there fossils in graywacke?
• The shale layers between the graywacke beds may
contain microfossils that also can be used to date the
rocks and to determine the depth of water in which
they were deposited. Sometimes trace fossils areal
so visible in graywacke.
• Trace fossils are the marks and tracks of animal s
that burrowed, fed, crawled and lived in the
sediments. Trace fossils can provide information on
how deep the water was and how much oxygen was
present when the sediments were deposited.
18. Graywacke sandstone exposed at Baker Beach in the Presidio displays
small holes, called tafoni weathering, typical of this type of rock.
19. Wacke (Graywacke)
• As a rule greywackes are not fossiliferous, but organic
remains may be common in thefiner beds associated with
them.
• Their component particles are usually not very roundedor
polished, and the rocks have often been considerably
indurated by recrystallization,such as the introduction of
interstitial silica.
• In some districts the greywackes are cleaved, but they
show phenomena of this kind much less perfectly than the
slates.
• Some varieties include feldspathic greywacke, which is rich
in feldspar, and lithic greywacke, which is rich in tiny rock
fragments.
