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Lecture 8a: Stratigraphy, Paleomagnetism

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Sequence stratigraphy grew out of seismic stratigraphy; unconformities are easily distinguished in seismic records, but lithology is often unknown. – PowerPoint PPT presentation

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Title: Lecture 8a: Stratigraphy, Paleomagnetism


1
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

2
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.
3
Stratigraphydefinitions
  • 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 scale-dependent 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.

4
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.

5
Stratigraphydefinitions
  • 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.

6
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.

7
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 sea-level change the
sediment supply, the water discharge, the type of
rock being cut.
8
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
9
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.

10
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

11
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.

12
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.

13
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).
14
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.
15
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.

16
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?).

17
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.

18
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.

19
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.

20
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.

21
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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