Geological Map Of Godrevy by rcvqFJ


									    Investigating Sediments

• Sediment Sieving
• Calculating Ca
  CO3 Content of
  Sediment Samples
• Point Counts
• Measuring a,b and
  c axes of Clasts
Sieving of Sediment Samples
              After returning from the field with
              sediment samples in clearly
              labelled bags
              Open up the bags and pour each
              sample carefully into a suitable
              plastic or cardboard container
              Spread the sediment sample out
              into a thin layer which completely
              covers the base of the container
              Place the containers in a location
              where they will dry out completely
              and not be disturbed. A sunny
              window sill is ideal.
              Alternatively pour the sample into a
              foil container and leave in the oven
              overnight at a temperature of 140°C
 Cleaning the Sieve Stack
Take a stack of sieves
from - 4 to + 4 phi and
arrange in descending
mesh order from
coarsest to finest with
the collecting pan at the
Take each sieve in turn
and brush vigorously
with the special brush
to remove any particles
trapped from previous
Weighing the Empty Sieves

             Weigh each sieve empty
             in turn plus the collecting
             pan and make a note of
             the results on A4 paper.
             Re-assemble the sieve
             stack in order, coarsest
             (16mm) at the top to
             finest (0.063mm) at the
             bottom with the collecting
             pan underneath
    Preparing to Sieve the Samples
Check that the sieves fit
snugly together, any possible
gaps should be sellotaped, to
avoid loss of sediment
during shaking

Place a suitable container on
top of an electronic balance,
preferably with an accuracy of
0.1g or better

Use the tare facility to zero the
display, then carefully add
material by hand until 1000.0g
appears on the digital display
Sieving the sediment samples
           Carefully pour the 1kg (1000g)
           sample into the top sieve and
           seal with the purpose-made lid
           Shake the sieve stack for a
           minimum of five minutes using an
           up and down action interspersed
           with a circular motion.
           It might be worth involving other
           students here to save on elbow
           grease! Ideally the sample should
           be shaken for at least 15 minutes
Re-weighing After Shaking

               Take off the lid from the
               sieve stack and carefully
               dismantle the sieves in
               size order
               Re-weigh each sieve and
               the collection pan with
               its contents on the
               electronic balance
               Record the sieve weight
               plus sediment value on
               A4 paper
 Calculating the % Calcium Carbonate
content of a beach or dune sand sample

                       Place a suitable beaker
                       on an electronic scale
                       that measures to at least
                       the accuracy of 0.1g
                       Press the tare function to
                       re-set the scale readout
                       to zero
                       Gradually add the sand
                       sample by sprinkling it
                       carefully until 5.0g is
                       recorded by the display
Remove the beaker from the
scales and gradually pour in
a few centimetres of 0.5
molar hydrochloric acid
The calcium carbonate
content of the sand will
effervesce violently and
release carbon dioxide
Carefully swirl the contents
of the beaker until the
reaction runs to completion
ie stops fizzing
Continue adding acid and stir
with a plastic or glass rod to
ensure that all grains enter
into reactions
When the sand/acid mixture
stops effervescing all the
calcium carbonate in the
sample will have been
Weigh a sheet of filter paper on
the electronic balance, then
make a funnel to filter the
sand/acid mixture
Rinse out sediment that sticks
to the side of the beaker and
pour into the filter paper cone
When all the liquid has
passed through the filter
paper, open it out fully
and place in a suitable
container on top of
absorbent paper
Place the container in a
location where it will be
able to dry out fully and
not be disturbed
A sunny window sill site
or on top of a radiator
would probably be ideal
Re-weigh the filter paper
and filtrate when fully dry
Subtract the weight of the
filter paper recorded
Express the weight loss
from the original 5.0g
sample of sediment as a
This gives the calcium
carbonate content as
percentage by weight loss
Point Count Of 100 Grains
      This simple technique allows the
      mineral composition of a sand
      sample to be determined as
      percentage constituents
      Take an A4 sheet of plain paper
      and draw a thin black line on it
      approximately 10cm long. Rub a
      Pritt Stick (or similar) along the line
      Carefully sprinkle a sand sample
      along the line so that the grains are
      clearly separate from each other
      Place the paper with the sand grains
      under a binocular microscope
Bring the sand grains into
focus using the binocular
microscope with the black
line in the centre of the field
of vision
Using a tally sheet or
proforma, to record your
observations attempt to
identify 100 consecutive
grains that lie closest to, or
are actually on the black line
Experiment with different
magnifications x10, x25 and
x40 depending on the size of
the constituent grains
   A Simple Guide To Grain Identification
Quartz: grey, white, colourless, glassy, conchoidal fracture,
rounded or subrounded grains
Feldspars: pink/flesh or creamy white colour, blocky or
rectangular shape due to 2 cleavages meeting at 90°
Calcite: pinkish, yellowish, brown, white, may show
ornament/ribbing if derived from shell fragments
Micas: black or silver in colour, pearly lustre, present as
thin flakes due to perfect basal cleavage
Tourmaline: greenish black in colour, often present as thin
needle-like slivers
Metallic ore grains: metallic lustre, black, silvery or grey in
colour, often appear like bits of ‘screwed up silver paper’
Other: anything that does not fit in with the above
An example from a student’s notebook to show the recording of
 point counts for a beach and dune sand (2 samples of each)
When the point count of 100
grains has been completed,
the composition as
percentage constituents can
be established
This information is easily
plotted as a pie chart
It is interesting to compare
results within the teaching
group to see how much
variation is evident
Remember just 100 grains
from a sample of probably
many billions is unlikely to
be very representative!
Measuring Particle Size and Shape
                         Sediment samples with
                         larger clasts allow
                         measurement of their a,
                         b and c axes
                         The a axis is the longest
                         direction that can be
                         measured on any clast
                         B axes are the maximum
                         distances at right angles
       c axis            to the a axis on the clast

   a axis                C axes are the shortest
                b axis   and are usually the ones
                         on which the clasts
                         would naturally lie, ie
                         are frequently vertical
 Measuring Particle Size and Shape
• The a, b and c axial values can be measured
  by eye using a ruler, but much more
  accurately using external calipers
• The Excel sediment spreadsheet calculates
  values for sphericity and flatness once the
  axial lengths have been entered into the
  appropriate cells
• The spreadsheet also classifies clasts as
  spheres, rods, blades or discs and plots
  them automatically onto a Zinng diagram
  using b/a and c/b ratios as co-ordinates
Enter the Data into the Excel Spreadsheet

                           Enter the data into
                           the spreadsheet:
                           Sieve weight ‘empty’
                           Sieve weight ‘full’
                           a, b and c axial
                           measurements of at
                           least 10 clasts from
                           any one sample
                           The spreadsheet will
                           calculate all the
                           values needed for
                           the investigation
                                                         Periglacial Head

                                                80% of material
Bi modal distribution                          coarser than 1mm
                        Positively skewed
                        distribution with a
                        higher proportion of
                        coarser material

                                                                    Very poorly sorted as the line
                                                                    slopes gently from left to right.
                                                                    All sieves and pan contained
                                                                    some material

                Periglacial Head Deposit

           Sphericity values quite low to
             moderate from 0.4 to 0.6

               Flatness values moderate to
                      high 283 to 475

               Most of the clasts, 9 out of
                 10 classify as blades
               produced by freeze-thaw?
The End
            I just absolutely
          adore doing Geology

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