Lecture 8a: Stratigraphy, Paleomagnetism

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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 et al. 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

There are also supergroups and subgroups, used
when original group definitions later prove
inadequate to describe important associations.
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Stratigraphydefinitions

• The boundaries between rock units can be
  conformable or unconformable.
• Conformable is meant to describe continuous
  deposition with no major breaks in time or
  erosional episodes. This definition is somewhat
  scale-dependent just how long or large a gap is
  an unconformity depends on the size and time
  significance 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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Stratigraphydefinitions

• 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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Stratigraphydefinitions

• Any package of sedimentary strata bounded above
  and below by an unconformity (of any kind) is a
  sequence.
• Traditional sedimentology and stratigraphy judge
  formations to be the fundamental units of the
  rock record, and interpretation of sedimentary
  environments to be the essential product of
  stratigraphic studies.
• Sequence stratigraphy makes sequences the
  fundamental units of the rock record, and hence
  emphasizes periods of deposition and
  nondeposition (closely related to episodes of
  rising and falling sea level) as the 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, and depend on various parameters.
Changes in these parameters will cause the river
to aggrade or incise to reach a new equilibrium
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 sea-level 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, over-steepened 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 Walthers Law

• We are now ready to state the third fundamental
  tenet of traditional stratigraphy, lateral
  continuity, which is expressed by Walthers 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
  second-order 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 103105 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 sea-level rise (cause of Cretaceous
  transgression?).

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Paleomagnetism

• As we have already discussed, the Earths
  magnetic field varies with time and records of
  the paleomagnetic field are preserved in rocks.
  Lets 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.

• 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.