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Mudrocks
Introduction
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Mudrks mostly silt & clay
Sometimes called argillites
Make up 65% of sed rks
Difficulties studying
mudrocks
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Recessive
F. grained
Clay alteration
Hard to get to modern
analog
• Mineral i.d. difficult (qtz
vs. felds)
• Sed structure not common
as in sandstone
• Thus problem w/ strat.
column
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Organic rich mudrocks -economically imp.
Thin section of mudrock.
Hard to distinguish grains
Recessive Mudstone
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Overturned
Mississippian Lisburne
Formation (resistant
carbonate) in
depositional contact
with overturned
Permian Echooka
Formation (recessive
mudstone), on the
south face of Atigun
gorge, Alaska. (photo:
Alan Carroll)
More Recessive Mudstone
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Contact between lower, light
brown sandstone and dark brown
silty mudstone within Imperial
Formation on a tributary to the
Arctic Red River, Northwest
Territories.
www.nwtgeoscience.ca
photo shows the character of
bedding at a scale of a few
meters. The thicker sand beds
are typically a little coarsergrained and tend to be more
resistant and stick out of the cliff.
The finer-grained material is
commonly in thinner beds and
more recessive.
clasticdetritus.com/.../
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Clays most abundant
• Kaolinites [Al2Si2O5(OH)4]
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formed in warm moist climates where
Ca, Na, and K ions leached and
removed by weathering.
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kaolinite clays indicates a source in a
humid tropical climate.
• Smectites  Are expanding clays.
 Expand by taking in water between
layers.
 Montmorillinite(½Ca,Na)0.7(Al,Fe,Mg)4Si,Al)8O20(OH)4.n
H2O is a good example.
 Form from weathering of Fe -Mg rich
ign & meta rocks in temperate climates
 Most abundant clays in modern
sediment.
• Illites - K1-1.5Al4Si7-6.5Al1-1.5O20(OH)4
 Formed by weathering of feldspars in
temperate climates and by alteration of
smectite clays during diagenesis.
 Have structure similar to muscovite.
• Mixed layer clays
 Interlayering between smectites like
layers and illite like layers in same
crystal
 Common in modern sediment.
• More illite w/time
 i. 80% clay minerals in Paleozoic rks is
illite
 ii. Reasons:
 increased volcanism; increased plant
life,., climatic changes, diagenetic
processes
Mudrock compositions
http://soils.missouri.edu/tutorial/page8.asp
Mudstone Composition Continued
Qtz
• Mostly silt-size, angular
 Feldspars
• Low concentrations
 Other
• Muscovite, calcite (skeletal &
diagenetic), pyrite, glauconite,
hematite, etc.
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Classification
Grain Size
Description
>2/3 silt
Abundant silt sized grains Silt-shale
visible with a hand lens
Siltstone
>1/3, <2/3 silt
Feels gritty when chewed
Mudshale
Mudstone
>2/3 clay
Feels smooth when
chewed
Clayshale
Claystone
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Fissile
Rock
Nonfissile
Rock
Depends on grain size & if rk fissile or not
Fissile rock tends to break along sheet-like planes
nearly parallel to bedding planes
Fissility caused by clay minerals deposited with
sheet structures parallel to depositional surface.
Texture
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Grain Shape
• Clays and
quartz usually
angular
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Not much
rounding
because grains
small & carried
in suspension
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Thin section; cross polarized.
Scale: each tick mark = 1 mm
geohistory.valdosta.edu
Texture Continued
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Fissility—Depends on
• Abundance of clay-more
clay more fissile
• Orientation of clays
 Clay grains adhere to
one another
 Adhesion of grains
called flocculation
• Also depends on
salinity & organic
matter=more =
more flocculation
 Bioturbation
• Destroys orientation
of clays
 Diagenesis
• Aligns grains
perpendicular to
max stress direction
 Get slaty
cleavage and
foliation in
metamorphic
rocks
geology.uprm.edu
Structureless Mudstone
geology.about.com
Describing Mudrocks
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Fissility--part parallel to
bedding
Bioturbation-massiveness?
Flocculation inhibits fissility
Laminations
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Lamination vs bed?
• 1 cm
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Origin of lamina
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a. productivity variation
b. grain size
c.composition
d. biochemical
No laminations = massive
(bioturbation/redeposition)
Laminations due to textural differences Sandlaminated dark grey mudstone from unit
MMa, Tom ore deposit, Paleozoic, Northern
Canada gsc.nrcan.gc.ca/.../ sedex/tom/index_e.php
Laminated Phospatic Mudstone,
Monterey Fm, Mussel Roc
Cross laminated mudrock, Brazil
Describing
Mudrocks
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Concretions
• Nodular or
stratiform
• Some Form
immediately after
deposition;
Evidence?
Cannonball Concretions, New Zealand
More Concretions, North Dakota
Describing Mudrocks
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Colors
• Gray to black, generally > 1% o.m.
• Conditions favorable for o.m. preservation
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Little oxygen
Rapid sedimentation
Low temperatures of water
Low permeability
Oxygen present, o.m. goes to water & carbon dioxide
• 3. Red, brown, yellow, green--iron present
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Reflect oxidation state of Fe
Oxidizing conditions the most Fe = Fe+3
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Give rock red, brown, orange colors
Hematite (Fe2O3) = red color
Iron hydroxide [FeO(OH)] (geothite) = brown color
Limonite gives sediment yellow color
Lack of iron then green (illite, chlorite, & biotite)
Use color for descriptive purposes
Color of Mudrocks:
Green-oygenated environment
Black-Organic-rich, low oxygen
Depositional
Environments
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A. Major mudrock types
• Residual--weathering & soil
formation on pre-existing rock
 i. Preservation potential?
• Detrital--erosion,
transportation & deposition
• Weathering & alteration of
volcanic deposites
B. Residual
• Calcretes (caliche)--common
where evap>precip
C. Detrital
• Marine/non-marine
• Distinguishing features:
 Fossils, bioturbation to
laminated
 Deposition below active
wave base
 May pass shoreward to
sandstones
 May be organic rich
 Local example is Monterey
Fm.
Residual Soil
http://blass.com.au/definitions/resid
ual%20soil
Raymond Wiggers
Dropstone in laminated mudstone,
Brazil
Mudcracks in red-brown mudstone, Watahomigi Formation.
Red from hematite. Courtesy USGS
Depositional Environments
Continued
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Non-marine
• Common in river floodplains, assoc. w/s.s.
• Lacustrine environments--varved
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Glacial lakes = coarse = spring melting, winter= fines
Non-glacial lakes--opposite- why?
Volcaniclastic derived mudrocks
• Volcanic material alters to clay
• If alteration is to montmorillonite then
mudrock known as bentonite
• How identify volcaniclastic origin of mudrock?
Marine Sediments
Most ocean floor covered by marine sediments
• Sediment thickness is thinnest at mid-ocean ridge and thickest
at continental margins
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Sediment Accumulation Rates Cm/1000yrs
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Continental Margin
• Shelf• Slope
• Fraser River Delta
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15-40
20
700,000
Deep Sea
• Coccolith Ooze
• Clays
0.2-3.0
0.03-0.8
Types of Ocean Sediments
• Terrigenous – “rock-derived
•Biogenous – “life-derived”
• Hydrogenous – “water-derived”
• Cosmogenous – “cosmic-derived”
Lithogenous Sediments
• Derived from the weathering of rocks – continents or volcanic
islands
• Transported by rivers, glaciers or wind
• Most deposited on continental margins
• Covers about 45% of ocean floor
 Composed mostly of quartz sand and
clay
Lithogenous Sediments - Deltas
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Lithogenous
sediments added
to marine
environment by
deltas
Delta common
features
Pelagic and Neritic Defined
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Pelagic sediments deposited in deep ocean away
from shelf processes influences
• Usually clays, unless turbidites – other gravity
flows, ice rafting
Neritic sediments deposited in shallow water over
shelves.
Pelagic sediments in abyssal plains most red
clays
Growing anthropogenic contribution –factory
dust, plastic (PCBs), time markers
Lithogenous
Sediment
Examples
Mississippi River
Sahara Desert
Mt. Pinatubo
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Red Clays
– Terrigenous from rivers, dust, and volcanic
ash
– Transported to deep ocean by winds and
surface currents
– Common in deep oceans, clays most
common
– Accumulates 2 mm (1/8”) every 1,000 years
Red Clays--Pacific
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Lacks calcium
carbonate
material
Note siliceous
materials—
Diatoms &
sponge spicules
Paula Worstell
Sediment Distribution
• Calcareous and Siliceous Oozes
Biogenous Sediment
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Biogenic ooze – greater than 30% biogenous
sediment
• Composed mostly of hard skeletal parts of once-living
organisms
• Two main compositions of hard parts:
1. Calcium Carbonate (CaCO3)
a)Coccolithophore (phytoplankton)
b)Foraminifera (zooplankton)
c)Pteropod--molluscs
2. Silica (SiO2)
a) Diatoms (phytoplankton)
b) Radiolarian (zooplankton)
• Distribution depends on chemistry, ocean productivity
Biogenous –Foraminifera
Calcareous Examples
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Composed of CaCO3
Foraminifera
www.noc.soton.ac.uk
• Widespread in relatively
shallow areas
Coccolithophore
Biogenous – Siliceous Examples
Radiolarians
Diatoms
• Composed of SiO2
• Base of food chain
• Like forams Benthic ones
better survive
Sediment Distribution –
Calcareous/Siliceous
Biogenous – Siliceous Ooze
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Covers 15% of ocean floor
• Distribution - areas of high productivity (zones of upwelling)
• Dissolve more slowly than calcareous particles
•Seawater undersaturated wrt silica, siliceous particles should dissolve
•Surface waters more depleted
•Bottom waters colder, most dissolution on seafloor
• Diatoms common at higher latitudes
• Radiolarians common at equatorial regions
Siliceous Oozes
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How do planktonic organisms
get to bottom?
Lightweight, drift
Biopackaging—marine snow,
feacal pellets
Biogenous – Calcareous oozes
Cover greater than 50% of ocean floor
• Distribution controlled by dissolution processes
• Calcium Carbonate Compensation Depth (CCD) – the depth at which
the rate of accumulation of calcareous sediments equals the rate of
dissolution
• Cold bottom waters undersaturated with respect to CaCO3
– slightly acidic ( CO2)
– readily dissolves CaCO3
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Lysocline = depth at which
dissolution of carbonate
material begins
Most dissolution takes
place on seafloor, only
pass short distance
through corrosive zone
Depth of CCD depends on
degree of undersaturation,
productiviy, & flux
faculty.uaeu.ac.ae/
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Paleoclimatology/Productivity
• Diatomaceous Rocks
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Monterey, Sisquoc Fm
Increased Miocene Oceanic Productivity
Miocene sealevel changes
• Phosphatic Rocks
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o.m. content 4-30
high productivity
low oxygen levels in oceans
• Oxygen Isotopes & Mudrocks
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O2 isotopes in shells in deep marine rocks
Construct isotope curves
Show changes in ocean temp.
Tie to sea level curve
• Carbon Isotopes & Mudrocks
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Reflect changes in productivity, continental runoff, ocean
circulation, atmospheric
gsc.nrcan.gc.ca/.../ sedex/tom/index_e.php
Laminated Monterey Formation