Lec 7 - Mudrocks

Mudrocks comprise of how much percent of sedimentary rocks

45%-55%

Main constituents of mudrocks

clay and quartz

what does the color of a mudrock tell you?

indicates whether the depositional environment is anoxic/oxygen-deficient or oxidizing

Describe fissility of mudrocks

• fissile — shale

• papery — varves

• block — mudstone

Sedimentary structures in mudrocks

laminated, bedded, slumped, bioturbated, massive

Fissility

tendency to break into sheets

Lamination

caused by variation in grain size and/or changes in composition. Deposited in relatively short periods of time by turbidity currents OR develops over months or years

siltstones

similar texture to those found in fien sandstones

why are some mudrocks massive?

• sediment-water flows have a high viscosity at time of deposition

• bioturbation

• mass sediment movement

• dewatering

• soil processes and root growth

color of mudrocks is a affected by?

mineralogy and geochemistry.

• organic matter content

• pyrite content

• oxidation state of Fe

Dark gray to black mudrocks

increasing organic matter and pyrite

Red and purple mudrocks

presence of ferric oxic, hematite occuring chiefly as grain coatings and intergrowths with clay particles

green mudrock

made with ferrous iron w/in the lattices of illite and chlorite. No hematite, organic matter or iron sulfides. Green color may also develop from red mudrocks through the reduction of hematite by migrating pore waters.

olive and yellow mudrock

mixing of green minerals and organic matter

color mottling in different shades of gray mudrock

due to bioturbation

yellow/reds/browns mudrocks

can be pedogenic processes, espeically in lacustine and floodplain muds

what are clays

hydrous aluminous silicates with sheet and layered structures similar to mica (phyllosilicates)

how are clays classified

• based on number and arrangement of tetrahedral and octahedral sheets in their basic structure

• interlayer cations

• substitution in the tetrahedral and octahedral sheets

• presence of water

what are the building blocks of clay minerals

octahedral and tetrahedral layers

desccribe an aluminum octahedra

6 oxygen linked with aluminum that can be replaced or substituted by Fe or Mg

describe silica tetrahedra

3 oxygen atoms shared with adjacent tetrahedra, linked together to form a hexagonal network

what is a 1:1 phyllosilicate structure

simplest phyllosilicates result from bonding one tetrahedral layer to one octahedral layer.

name a clay with a phyllosilicate 1:1 structure

Kaolinite, dickite, nacrite, halloysite

Describe the Phyllosilicate structure 2:1

1 octahedral layer sandwhiched between two tetrahedral layers.

What happens to Montomorillonite in good drainage conditions

Mg will be leached and Kaolinite will form instead of montmorillonite

how do Smectites form

result from weathering and alteration of basic rocks

Nontronite results from

alteration of basaltic glass

Montmorillonite and beidellite result from

the alteration of volcanic ash to give bentonite clay deposits

how are other smectites formed? (Nontronite, biedellite, and saponite)

variable modes of substitution in the octahedral and tetrahedral layers

What is Illite?

most common clay mineral in sedimentary rocks. Has a 3 layer structure but Al3+ replaces Si4+ in the tetrahedral layer. This results in a deficit of charge which is balanced by K ions in interlayer positions.

describe Chlorite

has brucite in between layers, substitution by Fe2+ occurs which imparts the green color

How are mixed layers formed

interlayering of common clays

how does feldspar occur in clays

generally present in low concentrations due to low physical and chemical stability. May be preferentially preserved in muddy sediments because of low permeability.

are muscovite and biotite common in clays?

muscovite is common, biotite less common

How does calcite occur in mud

calcite occurs as skeletal debris or diagenetic calcite, dolomite, or siderite

Is organic matter common in mudrocks?

Yes, particularly black shales

what are the 3 origins of clay minerals

inheritance, neoformation, and transformation

inheritance

detrital and stable in their present location

neoformation

formed in situ and either have been precipitated from solution or formed from amorphous silicate material

transformation

clays are modified by ion exchange or cation rearrangement

where does clay mineral formation take place

• weathering and soil environment

• depositional environment

• during diagenesis and into low-grade metamorphism

where does Kaolinite occur (>20%)

acid tropical soils where leaching is intensive. Dominant in low-latitude areas particularly off major rivers draining regions of tropical weathering

Montmorillonite >50%

Product of intermediate leaching and weathering. Common in temperate soils with good drainage and neutral pH, in poorly drained soils and in arid-zone soils which are highly alkaline.

Where does Illite (>40%) occur

where degree of leaching is limited. More common in ocean-floor muds of higher latitudes. Typically reaches the ocean via rivers and wind transport

where does chlorite occur (>20%)

intermediate leaching in temperate acid soils; also in soils in arid region where chemical processes are minimal

Montmorillonite are related to what systems?

MOR systems and volcanic oceanic islands.

Compaction

expels water and reduces thickness of sediment by a factor of up to 10. Further compaction through water loss requires temperatures approaching 100^oC at depths of 2-4km.

how do changes in clay mineralogy take place during diagenesis

rise in temperature and increase in burial depth.

differential compaction

sand and clay compact differently leading to complex outcrop patterns.