The techniques and interpretation of the proportions of the framework grains, have been relatively controversial in the mid-1980s [E.g. Discussion and Reply of Ingersoll et al. (1984)]

Provenance to most sedimentologists dates back to the work of Pettijohn et al. (1972, Sand and Sandstone). This interpretation considers that provenance includes all aspects of a source area:

• source rocks

• climate

• relief

The signal from the source rocks are going to be greatly mitigated by the last two factors.

The manner in which you count the grains has a significant effect on the interpretation of the point counts - competing "schools" [Indiana vs. Arizona] of thought developed with significant numbers of arguments.

Another of the classic papers that considers the way in which you count grains is that of Ingersoll et al. (1985): The effect of grain size on detrital modes: A test of the Gazzi-Dickinson point-counting method, Jour. Sed. Pet., 54, 103-116.

General approaches to point counting are primarily related to contrast in emphasis

• P. Gazzi (Italian) and Bill Dickinson independently came up with a point counting scheme that is meant to minimize the grain size variability - Gazzi-Dickinson approach

• the "traditional" method that separates grain sizes (considered more in line with climatic influences

TEST OF TWO APPROACHES USING HOLOCENE SANDS

Grain parameters (a bit different than Dickinson and Suczek):

Data collected from 6 unconsolidated Holocene sands of mixed provenance from arroyos in New Mexico

• prior to sieving, each sample was split with all fragment size fractions

• other fraction separated into size fractions 4-3Ø [very fine sand=0.0625-0.125 mm], 3-2Ø [fine sand=0.125-0.25mm], 2-1Ø [medium sand=0.25-0.50 mm], 1-0Ø [coarse sand=0.50-1.0 mm], 0-(-1)Ø [very coarse sand=1-2 mm]

• sand fractions poured in epoxy mold and sectioned

• stained for plagioclase and K-feldspar

• each operator counted 300 points [L. van der Plas and A. C. Tobi (1965). A chart for judging the reliability of point counting results. American Journal of Science, 263, 87-90.]

Distinctions between the "traditional" and Gazzi-Dickinson approaches

• Point counting stage used with a grid spacing for both.

• Gazzi-Dickinson approach assumes that any sand-sized (>0.0625 mm) mineral, whether as a separate monomineralic grain or as part of lithic grain is counted as a crystal. If the lithic fragments have grains smaller than the sand-sized they are counted as lithic grains. Advantages:

  • modal composition does not change with physical disaggregation

  • counting poorly-sorted sands or sandstones much easier and quicker

• The "traditional" approach would count the lithic grain with sand-sized crystals as lithic grains.

• Generally results in higher % of Q, F, M and D values, but lower L

• There are generally similar results between the two approaches with fine-grained sediments

• Lithic fragments counted as such if there is a single "other" mineral vs. the Indiana 10% cutoff.

Results of Gazzi-Dickinson vs. Traditional

Point counts of unsieved fractions:

size fractions for GD more clustered in 78, 85, 103 and 110

Better clustering, and probably a better source rock indicator.

GD generally shows much less variation with grain size relative to T

Major Critiques of GD method (sed wars)

Suttner and Basu point out that:

• sacrifice petrographic information at the expense of rapid data collection - unwarranted loss of information

• similar consistent results can be attained by use of specific grain size populations

• ultimate goal of sandstone petrology is paleogeology, not general tectonic setting

• proper counting of inclusions

• classification of chert fragments as anything but a sedimentary rock seems incorrect and should not be lumped with Qm

• improper definition of rock fragment, as they define them

Ingersoll et al. response:

• increased awareness of problems

• use of GD should be based on goals, and not used for paleoclimatology

• use chert term a bit loosely

Decker and Helmold point out that:

• does not affect grain size bias

• loss of genetic information in counting lithic fragment as crystal

• QFL plots give misleading view of sandstone composition and maturity - under-represents lithic fragments

• Recommendation: examine medium to coarse sands and record provenance-diagnostic features for all lithic clasts in the sand range.

Ingersoll et al. response:

• grain size information is transformed

• they want to incorporate all provenance information

• dislikes the tone and the discussion of results