Lecture 8a: Stratigraphy, Paleomagnetism
• Questions
– How is stratigraphy related to analysis of sedimentary
environments?
– What happens when sea-level varies?
– How do variations in the terrestrial magnetic field get
recorded in rocks and used by geologists to reconstruct
history?
• Reading
– Grotzinger & Jordan, chapters 8 (again) and 14
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Principles of Stratigraphy (revisited)
• Recall the fundamental principles of stratigraphy:
original horizontality, superposition, cross-cuttting
• A more detailed study brings up three major themes:
– Uniformitarianism: the interpretation of ancient deposits by analogy to
modern, observable environments
– Cyclicity: climate, sea-level, annual, tidal variations, etc., all generate
repeating cycles of sedimentation
– Hierarchy: basic stratigraphic principles apply across a wide range of
space and time scales
• Definitions of stratigraphic elements: Rock units are organized
into a hierarchy of classifications
Name
Group
Formation
Member (Lens, Tongue)
Bed or Flow
Typical thickness
> 1000 m
100-1000 m
10-100 m
1-10 m
Lateral Continuity
Continent-wide
1000 km
100 km
10 km
There are also supergroups and subgroups, used when original group
definitions later prove inadequate to describe important associations.
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Stratigraphy:definitions
• The boundaries between rock units can be conformable
or unconformable.
– Conformable describes continuous deposition with no major
breaks in time or erosional episodes. This definition is scaledependent – just how long or large a gap is an unconformity
depends on the size duration of the units being divided.
• A vertical succession of strata represents progressive
passage of time, either continuously at the scale of
observation (conformable) or discontinuously
(unconformable).
• A lateral succession of strata represents changing
environments of deposition at the time of sedimentation
or diagenesis.
– Each recognizable environment in a lateral succession is
called a facies.
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Stratigraphy:definitions
• Unconformities are usually divided into four types:
• Angular unconformity is used when layers below are clearly tilted or folded
and then eroded before deposition continues on the eroded surface
• Disconformity is used when
beds above and below are
parallel but a well-developed
erosional surface can be
recognized, by irregular
incision, soil development, or
basal gravel deposits on top.
• Paraconformity is used for
obscure unconformities where
correlation with time markers
elsewhere indicates missing
strata, even though no
evidence of a gap is present
locally.
• Nonconformity is used for
deposition of bedded strata on
unbedded (usually igneous or
metamorphic) basement.
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Stratigraphy:definitions
• Any package of sedimentary strata bounded above and
below by an unconformity (of any kind) is a sequence.
– Traditional stratigraphy uses formations as the fundamental
units of the rock record, withinterpretation of sedimentary
environments as the essential product of stratigraphic studies.
– Sequence stratigraphy makes sequences the fundamental
units of the rock record and emphasizes periods of deposition
and nondeposition (episodes of rising and falling sea level?)
as essential information. Sequence stratigraphy grew out of
seismic stratigraphy; unconformities are easily distinguished
in seismic records, but lithology is often unknown.
• Sedimentary accumulation (hence the boundaries of
sequences) is controlled by changes in base level, the
elevation to which sediments will accumulate if the local land
surface is too low, or erode is the local land surface is too high.
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Stratigraphy: Base Level
On land, base level is set by the equilibrium profile of
river systems.
In marginal marine settings, base level is often the same
as sea level
In the deep sea there is no base level and sedimentation
is controlled only by sediment supply.
Changes in base level allow the sedimentary record to
preserve evidence of geological events:
• Relative sea level change is the most important determinant of
changes in base level.
• Local tectonic uplift or subsidence changes base level and leads to
erosion or accumulation.
• Changes in water supply or sediment load affect the equilibrium
profile of a river and therefore the base level downstream.
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Stratigraphy: Base Level
• On land, base level is set by the equilibrium longitudinal profile
of river systems, which evolve to a characteristic shape:
The parameters of the curve
for each river are
different. Changes in
these parameters will
cause the river to
aggrade or incise to
reach a new base level.
Parameters include the
elevation of the
headwaters, which may
change by uplift or
erosion; the elevation of
the mouth, which may
change up uplift or sealevel change; the
sediment supply, the
water discharge, the
type of rock being cut.
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Stratigraphy: Base Level
A knickpoint (resistant bed or
lake) where the form of the
river is interrupted leads to a
nested set of river profiles.
The placing of an artificial knickpoint in a
river by building a dam has curious
consequences, both upstream and
downstream.
A waterfall must retreat because it is steeper
than the equilibrium gradient for the
reach of the river below the falls.
A sudden drop in base-level leads to the
formation of river terraces
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Stratigraphy: Relative Sea Level
• Relative sea level is the depth of water relative to the
local land surface.
– Relative sea level can change due to local vertical tectonic motions or due
to eustatic sea level variations (i.e. global changes in the volume of ocean
water or of the ocean basins).
• In both sequence and traditional stratigraphy, the critical events
that determine the locations of environments and unconformities
are transgressions and regressions.
– A transgression is a landward shift in the coastline, and hence
a landward shift in all marginal marine environments. A
regression is a seaward shift in the coastline.
– A drop in relative sea level always causes a regression. A transgression
hence requires rising relative sea level. However, rising sea-level can
result in transgression, stationary shorelines, or regression depending on
sediment supply.
– This asymmetry results because sediment flux from land is always positive,
and because transgression during sea-level fall would create unstable, oversteepened long-valley profiles.
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Stratigraphy: Relative
Sea Level
• rising sea-level can
result in transgression,
stationary shorelines,
or regression
depending on sediment
supply.
• Whether transgression or
regression occurs controls the
preservation potential and
vertical succession of
environments like barrier islands
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Stratigraphy: Walther’s Law
• We are now ready to state the third fundamental tenet of
traditional stratigraphy, lateral continuity, which is
expressed by Walther’s Law:
– In a conformable vertical succession, only those facies that can
be observed laterally adjacent to one another can be
superimposed vertically
– That is, if the lateral shifting of sedimentary environments is
controlled by continuous changes in base-level, each point
accumulating sediments vertically passes through all
intermediate environments continuously.
– Thus, e.g., deep-sea sediments directly overlying a terrestrial
flood-plain facies demands an unconformity in between.
• Consider again the vertical succession of beach facies, which maps the
lateral succession of beach facies onto a single point as the beach
progrades outwards during a regressive relative sea-level rise.
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Stratigraphy: Transgression and Regression
• In vertical succession,
transgression is
recognized by
progression from inland
towards deep water
sediments moving up
section; regression, if
preserved, is recognized
by progressively
shallower water facies
moving towards
continental settings as
you go up section.
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Stratigraphy: Transgression and Regression
On a regional-continental
scale, transgression is
recognized by lateral
migration of
environments with time,
from the coast towards
the interior, and
regression by migration
of environments towards
the coast.
The ideal sequence consists of a transgressive clastic formation, a
carbonate formation deposited when essentially the whole continent
was flooded, and a regressive clastic formation (less often preserved
after erosion).
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Sequence Stratigraphy
On a continental scale in
North America, there
are recognized six
major transgressions
and regressions,
bounded by five major
regional
unconformities. These
sequences were named
in North America by
Sloss (1963), but they
correlate fairly well
with patterns seen on
other continents. They
are therefore interpreted
as major changes in
eustatic sea level, not as
continental-scale uplift
and subsidence.
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Sequence Stratigraphy
• Superimposed on the major Sloss sequences are secondorder cycles of transgression and regression usually called
Vail curves, and superimposed on these are third-order
cycles that are correlated with individual reflectors in
seismic sections of marine strata. Tracing and correlating
these sequences is the main project of sequence
stratigraphy.
• Repeated transgressions and regressions, presumably
related to cyclic rises and falls of sea level, lead to cyclic
sedimentation episodes in sedimentary basins. In
particular, the Pennsylvanian strata of the eastern U.S.
show at least 50 distinct cylcothems consisting of the
triplet of deposits: marine-fluvial-coal. Each is a
regression, probably caused by withdrawal of water from
the oceans during a glacial advance.
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Causes of sea-level change
• Relative sea level can change due to local or regional tectonics,
which cause vertical motions (uplift and subsidence). Global sea
level can only change by altering either the volume of sea water or
the volume of the ocean basins themselves.
• On time scales of 103–105 years, glaciation can quickly tie up and
release enough water to change global sea level by ~200 m. But
Sloss cycles have time scales of 108 years and amplitudes of 1000 m!
• Changes in the global configuration of continents and the working of
plate tectonics can affect global sea level by changing the volume of
the oceans:
– when continents are assembled into supercontinents, the area of shallow
shelves is greatly decreased and the mean age of the ocean crust is a
maximum, because there are few small oceans and one big one. This should
lead to a big fall in sea level (Permian through Jurassic regression?).
– when continents rift, a new, shallow ocean is created at the expense
somewhere of an old, deep ocean. Sea level should rise.
– an increase in spreading rate of the global ridge system leads with time to
increase in the volume of water displaced by the mid-ocean ridges and a sealevel rise (cause of Cretaceous transgression?).
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Paleomagnetism
• As we have already discussed, the Earth’s magnetic
field varies with time and records of the paleomagnetic
field are preserved in rocks. Let’s look in more detail.
• Magnetization of rocks:
– At high temperatures, all materials are paramagnetic, meaning
their magnetization is proportional to the applied field, and
zero in the absence of an applied field
– Materials with unpaired electron spins can undergo a phase
transition to ferromagnetic behavior at a temperature called
the Curie Point.
Material
Fe
Magnetite (Fe3O4)
Hematite (Fe2O3)
Curie Point (°C)
770
578
675
Specific Magnetization (A m2/kg)
227
93
0.5
– A magnetic mineral crystallized above the Curie point and
then cooled through it acquires a thermal remanent
magnetism (TRM) in the same direction as and with intensity
proportional to the applied field.
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Paleomagnetism
– If a magnetic mineral is formed by chemical alteration or
metamorphism at temperatures below its Curie Point, it
acquires a chemical remanent magnetism. If a given rock
cooled at one time with some magnetic minerals and was
altered later to grow new magnetic minerals, the TRM and
CRM may point in different directions.
• They can be separately measured by progressive demagnetization of a
sample with increasing temperature.
– If magnetic particles are eroded from a source, transported,
and deposited in a new rock under appropriate conditions, all
below the Curie Point, they will have a preferred orientation
governed by the magnetic field at the time of sedimentation, a
depositional remanent magnetism. This will typically be
~1000 times weaker than the magnetic moment in a lava
where each little dipole is perfectly aligned, but it is
measurable.
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Paleomagnetism
• Measurement of the vector remanent magnetic field in a
rock sample gives the declination and inclination of the
field at the time and location of acquisition.
• If the terrestrial magnetic field was a simple dipole at
the time of acquisition, this measurement gives a virtual
magnetic pole:
– The declination gives the orientation of the great circle on
which the pole lies, and the inclination gives the magnetic
latitude of the sample.
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Paleomagnetism
• A measured virtual
magnetic pole
reveals several facts:
– Magnetic polarity at
time of
magnetization,
assuming you know
which hemisphere
the sample was in
and have some rough
idea of horizontal
– Intensity of the field
at the time of
magnetization, if you
correct for the
susceptibility of the
particular sample.
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Paleomagnetism
– The apparent latitude of the sample at the time of
magnetization. If it does not match the present latitude, you
can infer that the sample has moved north or south.
• There are terranes on the west coast of North America whose
magnetic inclinations imply motions of thousands of kilometers.
• You get no information on longitude, which is a limitation in the
reconstruction of positions of continents in the past; this is
particularly serious before the Mesozoic, when there are no marine
magnetic anomalies to go by.
– Tectonic rotations about a vertical axis show up through
anomalies in the measured declination.
– A sequence of virtual magnetic poles from a series of rocks
of different ages attached to one stable continent defines an
apparent polar wander (APW) path.
• “Apparent” because it is not clear without a fixed frame of reference
whether it is the continent or the pole that has wandered.
• However, the difference between APW paths for two different
continents gives an accurate measurement of the relative motion
between the two continents.
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