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News and information
Partner list posted Gas up vans before tomorrow morning
Leave some space in your notebook
NIU students with yellow GPS come see me withyour unit
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Practical field procedures
Estimate total thickness of section Graphical or trigonometric
Determine level of detail for your description Depends on purpose
Determine section starting and ending points Walk entire section if time permits
Determine measurement method
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Organizing your notebook
Purpose for day What is the level of detail you are aiming for?
Geographic location
Method used
Starting and ending point descriptions
Scale for graphical column
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Notebook layout
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Denver, CO strat column
Geologic ages along left Split columns for extra thick
sections
Weathering profile
Capture lithologic variabilitywithin units
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Phosphoria Fm.
Graphical representation ofthickness
Text description ofimportant lithological
characteristics
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Measuring stratigraphic sections
Why measure sections? Acquire fundamental geologic information
Needed for basic characterization of an area
Assist in mapping
Facilitate correlations Establish relative ages
Facies and sedimentologic analyses
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Where do we measure sections?
Is there a need? Basic geological characterization, mapping,
correlations, facies analysis
Good exposure
Easy access Others can duplicate or examine your work
Structurally simple Low potential for tectonic thickness variations
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Variables in section measurement
True thickness (T) Perpendicular distance from top to bottom
Slope distance (s orw) Distance from top to bottom measured along the surface
Surface slope (s) Bedding dip (d)
Traverse orientation (a)
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Sources of error
Surface slope 2-5 degrees?
Bedding dip 2-5 degrees?
Surface distance 1-2%?
Traverse orientation 2-5 degrees?
Strive for < 10% total thickness error
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Simplest case of section measurement
Horizontal landsurface
Traverse normal tostrike
Constant dip
t= wsin d
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General case of section measurement
Seven possibilities
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Measurement techniques
Compass and Tape Must know: s, d, s, a
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Measurement techniques
Hewett (1920) method
T
E
No need to measure
surface slope angle or
distance!
T= Ecos d
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Hewett method procedure
Determine eyeheight Measure strike and dip of section to be
measured
Design traverse If normal to strike
Set inclinometer to measured dip
If oblique to strike
Set inclinometer to apparent dip in the direction a
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Hewett method procedure
For the measured dip or apparent dip,calculate how much section thickness is equalto one eyeheight
T= Ecos d
Stand at base of section Site to new position along traverse
Move to newly cited position
Record T of stratigraphic thickness
Repeat the above steps until reaching the topof the section
Estimate thickness of last fraction of section
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Mertie method
Allows for traverses that are not normal tobedding strike
T= s (sin dcos s sina) +/- s(sin scos d)
Use (+) when slope and bedding dip in opposite directions
Use (-) when slope and bedding dip in the same direction When a= 90, this equation reduces to one of the
general cases previously discussed