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cherts Flashcards

1
Q

general group name used for siliceous sedimentary rocks

A

chert

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2
Q

SiO2 minerals that are dominant in the composition of cherts

A

quartz, chalcedony, opal

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3
Q

impurities in cherts

A

clay minerals, hematite, calcite, dolomite, and organic matter

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4
Q

where does chert commonly occur?

A

pelagic, lower than CCD

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5
Q

the water depth at which the rate of supply of calcium carbonate from the surface is equal to the rate of dissolution

A

Carbonate Compensation Depth

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6
Q

what does chert host?

A

petroleum, phosphorites, uranium, manganese

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7
Q

what is OPAL-CT?

A

low temperature cristobalite disordered by interlayered tridymite lattices

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8
Q

what is OPAL-A?

A

metastable amorphous silica of biogenic origin and converts into Opal-CT then Quartz overtime

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9
Q

arrange the following from the least stable to most stable:

Quartz, Opal-A, Opal-CT

A

Opal-A
Opal-CT
Quartz

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10
Q

textural classification of quartz that form chert:

A

microquartz
megaquartz
chalcedonic quartz

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11
Q

minor element composition:

A

volcanic:
Al, Ca, Na, K, Mg, Ti

hydrogenous:
Fe, Mn, Ni, Cu

hydrothermal:
Fe, Mn, Ba

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12
Q

siliceous organisms in chert:

A

diatoms
radiolarian
silicoflagellates
spongy spicule

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13
Q

principal kinds of cherts:

A

Flint
Jasper
Novaculite
Porcellanite
Siliceous Sinter

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14
Q

chert that occurs as nodules

A

flint

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15
Q

red-colored chert

A

jasper

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16
Q

why is jasper red?

A

it has trace amounts of hematite

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17
Q

interbedded chert and hematite (BIF)

A

jaspilite

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18
Q

very dense, fine-grained, even-textured chert

A

novaculite

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19
Q

fine-grained siliceous rocks with a texture and fracture resembling unglazed porcelain

A

percellanite

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20
Q

porous, low-density, light-colored siliceous rock associated with hot springs and geysers and is used as an explroation marker

A

siliceous sinter

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21
Q

porous, low-density, light-colored siliceous rock associated with hot springs and geysers and is used as an exploration marker

A

siliceous sinter

22
Q

bedded cherts are also referred to as:

A

ribbon cherts

23
Q

bedded cherts show:

A

crossbedding, laminations, ripple marks, associated with siliceous ooze

24
Q

types of bedded cherts

A
  1. diatomaceous deposits
  2. radiolarian deposits
  3. siliceous spicule deposits
  4. bedded cherts with few or no skeletal remains
25
Q

are light-colored, soft, friable siliceous rocks composed chiefly of the opaline crustules of diatoms

A

diatomites

26
Q

consists of beds and lenses of diatomite that have well-developed silica cement or groundmass

A

diatomaceous chert

27
Q

diatomaceous deposits are composed of:

A

diatomites and diatomaceous cherts

28
Q

diatomites occur in:

marine
lacustrine
all of the above
none of the above

A

all of the above (both marine and lacustrine)

29
Q

why are diatoms only preserved in opal-A and rare in quartz cherts?

A

because when diatomaceous deposits are converted to quartz cherts during diagenesis, the diatoms do not survive.

30
Q

radiolarian deposits are composed of:

A

radiolarite and radiolarian chert

31
Q

is comparatively harder, fine-grained, chert-like quivalent of radiolarian ooze (indurated radiolarian ooze)

A

radiolarite

32
Q

well-bedded radiolarite that has well-developed siliceous cement or groundmass

A

radiolarian chert

33
Q

why do radiolarians survive diagenesis more than diatoms?

A

radiolarians are more robust and contains less surface area than diatoms

34
Q

siliceous rock composed primarily of siliceous spicules of invertebrate organisms, particularly sponges

A

spicularite/spiculite

35
Q

spicularite is associated with:

A

glauconitic sandstones, black shales, dolomite, argillaceous limestones, and phosphorites

36
Q

siliceous spicule deposits are deposited mainly in:

A

shallow waters

37
Q

what causes bedded chert with few or no skeletal remains?

A

inadequate examinations or severe diagenesis

38
Q

nodular chert have what color

A

tan to black

39
Q

nodular chert are rare in what type of rocks?

A

sandstone and mudstone

40
Q

nodular cherts are associated with

A

shelf-type carbonate rocks concentrated parallel to bedding

41
Q

solubility of silica/chert

A

at 25 degrees celsius:

11 ppm for quartz
116 ppm for amorphous silica

42
Q

silicic concentration in seawater

A

13 ppm

43
Q

silica in seawater might come from:

A
  1. alteration of oceanic basalts and detrital silica sediments owing to interaction of seawater with hot volcanic rocks
  2. escape from silica-enriched pore waters of pelagic sediments
44
Q

silica concentration in modern ocean:

A

1ppm

45
Q

explain the precipitation of microquartz/chalcedony in undersaturated oceans

A

In undersaturated oceans, microquartz/chalcedony are unable to precipitate solely by inorganic processes. Precipitation of microquartz in undersaturated oceans are driven by silica secreting organisms (diatoms, radiolarians, silicoflagellates) building silica skeletons

46
Q

this is the bulk of silica in the modern ocean

A

diatoms

47
Q

ages of distribution silica-secreting organisms

A

radiolarians - cambrian/ordovician to holocen

diatoms - jurassic to holocene

silicoflagellates - cretaeceous to holocene

48
Q

explain the biogenic removal of silica

A

Silica secreting organisms remove silica saturated in the ocean to build silica skeletons, after death, these skeletons will then start to dissolve into solution. Some of the silica will form siliceous ooze while other dissolve and enter into pore spaces, thereby enriching the silica content of pore waters.

49
Q

explain the sequence of formation of quartz cherts

A

The formation of quartz chert starts with dissolution of Biogenic Opal-A in pore water and precipitation of this into intermediate metastable Opal-CT (porcellanite), which eventually dissolves and yields high silica solution which then precipitates again to form microquartz. Finally burial diagenesis transforms microquartz to megaquartz.

50
Q

microquartz are formed through what process?

A

solution-reprecipitation

51
Q

how does temperature and sedimentation affect the evolution of opal-a to microquartz?

A

high temperature and sedimentation yields to faster rate

52
Q

this serves as nucleus for the crystallization of opal-CT

A

magnesium hydroxide compounds

sp (7 decks)

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