1. 1/6/13 GEOL342 - Sedimentation and Stratigraphy
42: Sedimentation and Stratigraphy
2011
Geophysical stratigraphy:
Due to the lack of surface outcrop in many areas geophysical methods of correlation have been developed that
exploit the methods of physics to mapg the physical properties of rocks, including:
Density
Permeability
Porosity
Character of pore fluids
Variations in those properties with depth reveal the presence of different rock types and are used to create
vertical and lateral sections of rocks that can't directly be examined. There are two general approaches:
Well logs: that record information provided by probes that are placed down boreholes
Seismic studies: in which physical features of subsurface rocks are approximated based on seismic
wave propagation
Well logging
Direct sampling: Not all well information is remote. Bentonite muds are continually circulated through the drill
pipe as a coolant and lubricant for the drill bit. Rock chips are brought to surface with the mud, captured,
identified and logged, creating a direct lithologic record.
Aditional information comes from devices lowered into the borehole:
Caliper: measures the width of the drill hole. This indicates the presence of mudrocks, which are prone
to caving and sagging, hence constricting the borehole slightly.
Sonde: A probe lowered into hole to measure various electrical and physical properties of the rocks.
Gas detectors and gas chromatographs: measure gases in the well.
Gamma-ray log: measures natural radioactivity of the strata
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2. 1/6/13 GEOL342 - Sedimentation and Stratigraphy
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3. What the sonde records:
Spontaneous potential (SP) log: measures
difference in electrical potential between an
electrode on the sonde and one at the surface.
An electrical potential exists between the
natural pore fluids of the rock and the drilling
mud that invades those pores. Therefore, SP
logs are a measure of permeability:
Shales and limestone nearly
impermeable and have a 0 reading
Negative deflection for sandstones (high
permeability) or fluids with higher
conductivity than the drilling mud (like
saltwater)
Positive deflection for fluids with lower
conductivity than mud (like freshwater)
Resistivity (R) log: measures resistivity of
fluids in the surrounding rock to an applied
electrical current. Resistivity indicates amount
of fluid in the pore spaces, therefore R logs are
measures of porosity. Resistivity increases
with decreasing pore space.
High resistivity: dense rocks with no pores (quartzite, limestone.), rocks with non-conducting fluid in
their pores (like petroleum)
Low resistivity: rocks with significant porosity (sandstone), rocks with conducting fluid in their pores
(like salt water), rocks containing significant amounts of water in their crystal structure (clay-rich rocks).
Examples of Spontaneous Potential logs:
Fluvial deposit with point bar sequence and overbank mud shows fining-upward sequence.

4. 1/6/13 GEOL342 - Sedimentation and Stratigraphy
Deltaic deposit grading into shoreline Coarsening upward
sequence with thick sands.
Deltaic deposit grading into distributary channel and
interdistributary bay Coarsening upward sequence.
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5. 1/6/13 GEOL342 - Sedimentation and Stratigraphy
Deltaic deposit grading into delta plain Coarsening
upward sequence with thick sands.
Regressive shoreline Coarsening upward
sequence.
Dipmeter: measures resistivity in four directions. By this means, it locates contacts and identifies their dip
direction, allowing identification of folds, faults, and other structure.
Gamma-ray log: measures natural radioactivity of the rock. Most gamma radiation comes from decay of
40K. Therefore, the gamma-ray log is sensitive to rocks high in K-bearing minerals (feldspars, micas,
clays) including:
shales
feldspathic and lithic sandstones
In contrast, limestones and quartz rich sandstones produce low gamma ray values.
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6. 1/6/13 GEOL342 - Sedimentation and Stratigraphy
Well-logging techniques all have one significant drawback: They require you to drill a borehole. There is a less
expensive alternative:
Seismic stratigraphy
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7. 1/6/13 GEOL342 - Sedimentation and Stratigraphy
Shots, pulses of sound are generated:
by explosives or a mechanical thumper
on Vibraseis trucks on land
by a shipboard air gun at sea.
Those waves that are propagated nearly straight
downward can be reflected off subsurface
interfaces of materials of different densities, such
as contacts between rock units.
Travel time is recorded by an array of
geophones on land or hydrophones at sea.
Reflections from each shot are recorded
as individual seismic profiles by the
geophones.
Information from each geophone in the
array is correlated, processed to remove
noise, and summed up across the array,
yielding a vertical line in which reflectors
as shown as wave-shaped deviations. This
is a one-dimensional plot of reflectors
beneath the shot
The array and thumper are then moved
slightly along a transect and a new seismic
shot made, which yields a separate trace.
Ultimately individual traces are displayed
together as seismic profiles,
approximating two dimensional images of
reflectors below the transect, each vertical line of which represents one shot.
More ambitious seismic techniques involve the deployment of two-dimensional geophone arrays to develop
three-dimensional seismic profiles.
Definitions:
Reflector: boundary that creates a seismic reflection
Reflection: acoustic waves created by sounds bouncing off of a reflector
Impedance: physical rock property of sound propagating through rock. A function of average sound
velocity and rock density
Impedance contrast: physical boundary within rocks producing a reflection
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Seismic stratigraphy can be
used for both deep and
shallow structural analysis.
Layered reflectors appear as
distinct horizons, whereas
structureless units or those of
uniform density show
random reflections. (E.G.:
the contrast between marine
sediments and a rising salt
structure - right.) But what,
exactly are these reflectors?
Simply, they are density
contrasts. These may be
caused by:
contacts between rock units
interface of different pore fluids (E.G.: petroleum and water)
unconformities
diagenetic features
The traces of seismic reflections have numerous aspects that can be measured:
amplitude
duration (2-way travel time)
area, etc.
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9. 1/6/13 GEOL342 - Sedimentation and Stratigraphy
This is pleasingly quantifiable, however a large element of inference
goes into the interpretation of seismic profiles, because:
Seismic profiles are NOT cross sections because the vertical
scale is two-way travel time, not thickness.
Reflector horizons needn't be lithologic boundaries. Layers with
high concentrations of chert nodules make nice reflectors, for
instance.
The resolution of seismic stratigraphy is low. A single seismic
pulse on a seismic profile may be up to 150 m. thick. (right)
As with so much else in stratigraphy, the ability to amass large quantities of information compensates for the
uncertainty inherent in the information. In this case, patterns that are likely to be connected to stratigraphy can be
observed at great depths in unexposed rock on land or beneath the sea, into which no well has been bored.
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10. 1/6/13 GEOL342 - Sedimentation and Stratigraphy
Of course, if well-log or
outcrop information is also
available, seismic reflectors
can reliably be connected to
known lithologies. By this
means we have learned that
marine sediments tend to
contrast strongly with
continental ones. (right)
The presence of petroleum
can be revealed by
anomalous horizontal
reflectors indicating the
interface of petroleum and
water, or by a brightening of
the profile caused by the
presence of gas.
Cornell University, through the Consortium on Continental Reflection Profiling (COCORP) has used seismic
methods to profile major orogenies. Among the interesting results: Whereas the traditional view was that the
Piedmont and Blue Ridge had deep crustal roots, it develops that they are underlain by extensive thrust faults
and have actually been thrust onto Paleozoic sediments.
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11. GEOL342 - Sedimentation and Stratigraphy
Seismic sequences: The geometry of
unconformities that truncate beds is
sufficiently distinctive to be identifiable in
seismic profiles, allowing identification of
seismic sequences - unconformity
bounded "packages" whose presence is
revealed by seismic reflections. Indeed,
the development of sequence stratigraphy
has gone hand-in-glove with that of
seismic stratigraphy, because only
seismic methods can identify sequence
boundaries on a large scale.
When connected to lithologic
information, these can be correlated with
age to identify sea level cycles. (right)
For many, the hope has been that these
would be caused by global eustatic sea
level change, enabling their use in global
sequence stratigraphy. In 1977, Vail,
Mitchum, and Thompson published a
summation of first and second order sea
level curves based on major
unconformities.
Second-order cycles appeared to be
markedly asymmetric, because of the
depositional asymmetry of transgression
and regressions in which transgressions are erosional, but sediment can continue to aggrade up during
regressions. Identifying onlap and offlap of sediments onto continents, enabled Haq and colleagues to develop
an adjusted curve showing sea level over the last 200 my.
As your text makes clear, the reliability and usefulness of the Vail curve is controversial.
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