Gazzi-Dickinson method
The Gazzi-Dickinson method is a
Technique
To perform a point count using the Gazzi-Dickinson method, a randomly selected thin section from a sedimentary rock is needed, with a slide advance mechanism that will randomly select points on the slide with a petrographic microscope. A minimum of 300 representative points (preferably 500 points) should be used to perform the count. On each randomly selected point that lands on a sand grain, the operator must determine the make-up of the area chosen, i.e. whether it is a mineral grain that is sand sized (larger than 62.5 micrometers) or a finer-grained fragment of another rock type, called a lithic fragment (e.g. a sand-sized piece of shale). These counts are then converted to percentages and used for compositional comparisons in provenance studies. Typically, only framework (non-matrix) grains are counted, or non-framework grains are counted and then excluded from percentages when using descriptive devices such as QFL triangles. This can create problems with pseudomatrix, which are lithic grains that have been deformed and thus blend in with (or have become) matrix.
History
The Gazzi-Dickinson method came out of separate work by P. Gazzi in 1966
The best way to explain the differences in these two schools of thought is with an example: A sand rich in
See also
References
- ^ Gazzi, P., 1966, "Le Arenarie del Flysch Sopracretaceo dell'Appennino Modenese: Correlazioni con il Flysch di Monghidoro". Mineralogica e Petrografica Acta 12:69 97.
- ^ Dickinson, W.R., 1970, 2Interpreting detrital modes of graywacke and arkose": Journal of Sedimentary Petrology, v. 40, p. 695–707.
- ^ Dickinson, W.R., 1985, "Interpreting provenance relation from detrital modes of sandstones", in Zuffa, G.G. (ed.), Provenance of Arenites: NATO ASI Series, C 148, D. Reidel Publishing Company, Dordrecht, 333–363.
- ^ Dickinson, W.R., Suczek, C.A., 1979, "Plate tectonics and sandstone compositions": American Association of Petroleum Geologist, 63, 2164–2182.
- ^ Graham, S.A., Dickinson, W.R., and Ingersoll, R.V., 1975, "Himalayan-Bengal Model for Flysch Dispersal in Appalachian-Ouachita system", Geological Society of America Bulletin, vol. 86, pp. 273–286.
- ^ Ingersoll, R.V., Bulard, T.F., Ford, R.L., Grimn, J.P., Pickle, J.P., Sares, S.W., 1984, "The effect of grain size on detrital modes: a test of the Gazzi-Dickinson Point Counting method": Journal of Sedimentary Petrology, 54, 103-116.
- ^ Folk, R.L., 1974, Petrology of Sedimentary Rocks: Austin, Texas, Hemphill Press, second edition, 182 p.