Page 38 - Petrology of Sedimentary Rocks

8. Replace the two creased papers as before and now hold the sieve upside down and
pound it sharply on the paper, STRIKING THE TABLE EVENLY WITH THE RIM
(otherwise you will dent the screen). Add the sand thus jarred loose to the
balance pan. Make sure that all grains end up inside the balance pan by repeating
this process. Weigh the sample in the balance pan to 0.01 gm. (if you have less
than 1.0 gral:?s in any sieve fraction, it should be weighed on the chemical balance
to 0.001).

9. Examine each sieve fraction (after it is weighed) under the binocular microscope
and estimate the percent of aggregates in each fraction. THIS IS ABSOLUTELY
NECESSARY FOR VALID WORK! The best way to do this is by spreading grains
on a micropaleo grid and counting 100 of them. This takes only a very little time.
Start estimating at the coarsest sizes and work down until NO MORE aggregates
appear. If any fraction over 25% aggregates, it should be re-disaggregated and
run through the screens again. Record the percentage of aggregates in each
size--these must be subtracted from the sample weight as shown in the sample
computation.

IO. Store the sample in corked vials, paper envelopes, or folded paper packets. Label
and finer
each fraction as to both coarser -- limits, and sample number.
DATA FORM

Held on: Raw % Aggre- Corrected Cumulative Cumulative Ind iv.
Mesh Mm 4 Weight gates Weight Weight Pet. Pet.
I8 1.0 0.0 3.5 gm 20 2.8 2.8 9.1 9.1
35 0.5 1.0 10.7 IO 9.6 12.4 40.3 31.2
60 0.25 2.0 8.3 5 7.9 20.3 65.9 25.6
5.6 25.9 84.0 18. I
0.19
3.0
2.9
1;: 0.12 2.5 5.6 - - 2.9 28.8 93.5 9.5
230 0.06 4.0 I .3 1.3 30. I 97.7 4.2
Pan - 0.7 0.7 30.8 100.0 2.3
To get cumulative percent, divide each figure by the last number in the Cumulative
Weight column. Do not obtain cumulative percent by adding UP individual percentages
(last column) as “rounding-off” error will distort the cumulative percentages.

This analysis illustrates another trick: if you are sieving at a coarse screen interval, it
is wise to insert an additional screen at about the modal diameter (the most abundant
grade size). This makes curve plotting much easier.

- IF the sample contains any gravel (Material coarser than 2.0 mm):
To get a representative sample of the gravel fraction, it is necessary to have a
much larger sample, say 0.5 to 2 or even more kilograms. In this case, place the total
sample on the 2 mm. screen and sieve by hand. Weigh the total amount of gravel
retained, and the total amount of sand passing through. This enables you to obtain the
percentage of gravel in the sample.

Sieve the entire amount of gravel by hand through as coarse screens as are
avai lab le. For pebbles too large, a set of wire squares of proper diameter is
constructed, and the pebbles passed through endwise and counted. The number of
pebbles unable to pass through each screen is then weighed. Screen and weigh all the
pebbles in this manner.

32