MOHIT KUMAR

Wacke ( Graywacke)

Wacke (Graywacke)• Physically hard.• Dark grey in colour, • Enigmatic (ambiguous) rocks.• Texturally, an immature sandstone.• Composed of sand mixed with mud.• Graywackes are associated with graded

bedding.

Wikipedia

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.

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

http://weiminhan.files.wordpress.com/2010/03/2010-03-17_161215.jpg?w=390&h=229

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.

http://earthsci.org/mineral/rockmin/sed/silsed2.gif

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.

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.

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

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.

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.

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.

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.

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.

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.

Graywacke sandstone exposed at Baker Beach in the Presidio displays small holes, called tafoni weathering, typical of this type of rock.

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.

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