Lesson 8: Mudrocks Flashcards
define mudrocks in terms of composition
they have to be at least 50% silt or finer particles
describe mudrocks in terms of abundance
- most abundant among sedimentary rocks
- compromise 50% of stratigraphic record
mudstones vs siltstones vs shales
mudstones are massive, siltstones are more silt than clay, shales have fissility (they can easily split along planes)
what is the difference between conglomerates and mudstones in terms of erosional process, what is the implication?
unlike conglomerates with various agents for reworking, mudstone particles actually have little to no modification/reworking during erosion and transportation.
reason: they are mostly transported as suspended particles
effect: cannot be rounded by collision = they are mostly angular; maintained their crystal shape
3 main processes that affect the microfabric of mudrocks
- physiochemical processes
- burial diagenesis
- bioorganic processes
3 specific processes under physiochemical processes
- electrochemical
- thermochemical
- interface dynamics
2 specific processes under burial diagenesis
- mass gravity mechanisms
- cementation
3 specific processes under bioorganic processes
- biomechanical
- biophysical
- biochemical
refers to the bonding among minerals, the electromagnetic bonds
electrochemical processes
refers to how temperature affects the interaction between minerals and their ions
thermochemical processes
refers to how the overall environment affects mudstone formation
interface dynamics
refers to how overburden affects the formation of mudstones (compresses them)
mass gravity mechanisms
process mainly involving cementing minerals
cementation
differentiate burial diagenesis vs bioorganic proesses
burial diagenesis is more on lithification while bioorganic processes are more on biological aspects
an example of this process would be bioturbation (burrowing animals)
biomechanical processes
refers to when organic particles create new minerals
biochemical processes
specific example of biochemical processes affecting microfabric
when organic matter produces methane gas/natural gas which affects the overall make up of mudstones
property where splitting between roughly planar and parallel surfaces can occur (has parting)
fissility
what does fissility among shales favor?
favors the abundance of peptizing agents
explain peptizing agents
agents that orient mineral assemblages of rock
example of peptizing agent
organic matter (needs anoxic environment)
2 requirements for peptizing agents in the fissility of shales (high chance for shales to form)
- organic matter are the main peptizing agents
- anoxic environments favor the preservation of organic matter
why is it that organic matter needs to be preserved in anoxic environments?
because aerobic environments oxidizes the organic matter which will cause it to decay.
thus, no oxygen = no decay = preserved organic matter = fissility
enumerate entire mineralogy of lutites
silicate minerals:
- quartz
- feldspar
- zeolite
clay minerals:
- kaolinite
- smectite-illite-muscovite
- chlorite, corrensite, vermiculite
- sepiolite and attapulgite
oxides/hydroxides
- Fe-O
- gibbsite
carbonates:
- calcite
- dolomite
- siderite and ankerite
sulfites and sulfates
mineral composition of most coarse grained lutites (silty/siltstones)
silicate minerals:
- quartz
- feldspar
- zeolite
pyroclastic materials that have undergone low temperature alteration under seawater
zeolite
mineral composition of most fine grained lutites
clay minerals:
- kaolinite
- smectite-illite-muscovite
- chlorite, corrensite, vermiculite
- sepiolite and attapulgite
oxides/hydroxides
- gibbsite
mineralogy of coatings
Fe-O
example of coatings
Hematite -most common in shales
goethite or limonite -may be more common in modern muds
weathering and oxidation products of iron minerals and gives color/pigmentation to the lutites
coatings
mineral composition of concretions/cement in lutites
carbonates:
- calcite
- dolomite
- siderite and ankerite
what is gibbsite
an aluminum hydroxide clay
what do sulfides indicate
- reducing environment
- post-deposition
what do sulfates indicate
hypersaline environment
example of sulfate in lutites
gypsum
example of evaporites in lutites
halite, gypsum, potassium salts or sulfosalts
other constituents found in lutites
apatite, volcanic glass, heavy minerals
accumulated organic matter (phosphate oxide mineral)
apatite, ex. guano
occurs along zeolite
volcanic glass
rare occurrence in lutites
heavy minerals
describe organic substances in lutites
discrete and structured
examples of organic substances in lutites
vitrine and kerogen in oil shales
particles from wood/plant materials
vitrine
altered remains of microorganisms
kerogen
how to classify lutites (bases)
there is no formal classification because there are a lot of ways to classify it:
- color
- abundance of minerals/organics
- particle size (mostly used)
- fabric/microstructures
how to classify lutites based on color
red shale -associated with iron oxidation
black shale -a lot of organic matter within shale
green shale -anoxic environment with iron minerals
white siltstone -eroded away pyroclastic materials
example of how the abundance of minerals/organics can be used to classify lutites
- oil shale: bc a lot of oil, petroleum products preserved within shale)
- argillaceous shale: because it is mainly composed of clay minerals (mud-sized doesn’t mean composed of clay/muddy minerals)
example of how fabric/microstructures can be used to classify lutites
- fossiliferous: if there are fossils
- on the basis of fissility (see chart)
- bedded siltstone: if the siltstone is bedded:”)
why is it that lutites are the most common sedimentary rocks?
- most are of marine origin
- occurs in all ages: meaning at all times, there is always going to be a deep, basin-like area with gentle currents to make it possible for lutites to form, regardless if they would be preserved or not
what is reflected in sedimentary records about lutites
- high abundance of fine-grained material
- erosional and efficiency of transport agents
4 transport agents of lutites
- wind
- bottom currents
- suspension transport mechanisms
- turbidity currents
expound about the transportation agents for lutites
if transportation agent is sufficient, it can carry sediments to portions where it can be preserved (deposited then eventually lithified)
example of wind as a transport agent
sand from the sahara desert gets carried by wind to the amazon rain forest. thus the desert is feeding the forest
discuss how bottom currents can be transport agents
if there are fine particles with bottom currents, it can carry the particles away from deposition area
discuss how suspension transport mechanisms can be transport agents
finer sediments are usually transported through suspension because of their size
discuss how turbidity currents can be transport agents
good transportation mechanism for silt and finer particles - bouma sequence, fining up
3 different environments of lutites
fluvial, coastal and marine, eolian
3 specific depositional areas under fluvial for lutites
- meandering rivers and anastomosing streams
- deltas
- estuary
4 specific depositional areas under coastal and marine for lutites
- lagoons
- deep waters
- shelf and slope
- tidal flats
example of sediments that make up lutites from eolian environments
loess deposits
describe setting in meandering rivers and anastomosing streams
- near final base level
- water velocity becomes slow
- calm river causes deposition in floodplain
describe deltas and estuary
- transition between rivers and seas
- low energy environment
- if silt/mud is still suspended by flowing water, coming to contact with standing water will suddenly decrease velocity and cause desposition
describe setting in lagoons
- enclosed areas from the sea
- no deposition for a time in a year, but sediments from the sea eventually get deposited here
T or F: lagoons are good hosts of mudstones (sulfate rich shales)
true
Lutites found in deep waters and shelf slope
turbidites and deep marine shales
describe tidal flat settings
- platform areas that reveal if high or low tide
- gentle environment
- enough to form shales, thin layers or intercalation because of episodic high and low tides
describe loess deposits
structureless (no internal structure within it)
