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

with your unit

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

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

Notebook layout

Denver, CO strat column

Geologic ages along left Split columns for extra

thick sections Weathering profile Capture lithologic

variability within units

Phosphoria Fm.

Graphical representation of thickness

Text description of important lithological characteristics

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

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

Variables in section measurement

True thickness (T) Perpendicular distance from top to bottom

Slope distance (s or w) Distance from top to bottom measured along the

surface Surface slope () Bedding dip () Traverse orientation ()

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

Simplest case of section measurement

Horizontal land surface

Traverse normal to strike

Constant dip t = w sin

General case of section measurement

Seven possibilities

Measurement techniques

Compass and Tape Must know: , , s,

Measurement techniques

Hewett (1920) method

T

E

No need to measuresurface slope angle ordistance!

T = E cos

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

Hewett method procedure For the measured dip or apparent dip,

calculate how much section thickness is equal to one eyeheight

T = E cos 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 top of the section

Estimate thickness of last fraction of section

Mertie method

Allows for traverses that are not normal to bedding strike

T = s (sin cos sinssin cos Use (+) when slope and bedding dip in opposite

directions Use (-) when slope and bedding dip in the same

direction When = 90, this equation reduces to one of

the general cases previously discussed