Gazzi-Dickinson Method
Encyclopedia
The Gazzi-Dickinson method is a point-counting technique used in geology to statistically measure the components of a sedimentary rock
, chiefly sandstone
. The main focus (and most controversial) part of the technique is counting all sand-sized
components as separate grains
, regardless of what they are connected to. Gazzi-Dickinson point counting is used in the creation of ternary diagrams
, such as QFL diagrams
.
from a sedimentary rock and a slide advance mechanism that will randomly select points on the slide with a petrographic microscope
. A minimum of 300 representative points, and preferably 500 points, should be used to perform the count. On each randomly selected point that lands on a sand grain, the operator needs to decided the make up of the area chosen, whether it is a mineral grain that is sand sized (larger than 62.5 micrometres) or a finer-grained fragment of another rock type, called a lithic fragment
([.g. a sand-sized piece of shale). These are then converted to percentages and used for compositional comparisons in provenance
studies. Typically, only framework (non-matrix
) grains are counted, or non-framework are counted and then excluded from percentages when using things like 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.
in the 1970s established the method and the use of the method to use the composition of sandstones to infer tectonic processes. This was in contrast to ideas presented by sedimentary geologists at Indiana University
at the time, who used the more traditional "QFR" or "rock fragment
" method of Robert Folk (1974) (which later grew into the Folk classification scheme), in which all grains that are connected are considered rock fragments, and the individual components are disregarded.
The best way to explain the differences in these two schools of thought is with an example: A sand rich in grus
, or a granitic sand, when point counted with these two methods would have drastically different results. A QFR-style count would be rich in rock fragments, when a Gazzi-Dickinson point count would show the sand rich in quartz
and feldspar
. Proponents of the Indiana School would say that information is lost by not counting rock fragments. Proponents of Gazzi-Dickinson point counting would say that small changes in erosion
al transport would change the composition of the sand. In general, more geologists use the Gazzi-Dickinson method to determine the composition of sandstones today than other petrographic methods.
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
, chiefly sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
. The main focus (and most controversial) part of the technique is counting all sand-sized
Particle size (grain size)
Particle size, also called grain size, refers to the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which is the size of a single crystal inside the...
components as separate grains
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, regardless of what they are connected to. Gazzi-Dickinson point counting is used in the creation of ternary diagrams
Ternary plot
A ternary plot, ternary graph, triangle plot, simplex plot, or de Finetti diagram is a barycentric plot on three variables which sum to a constant. It graphically depicts the ratios of the three variables as positions in an equilateral triangle...
, such as QFL diagrams
QFL diagrams
A QFL diagram or QFL triangle is a type of ternary diagram that shows compositional data from sandstones and modern sands, point counted using the Gazzi-Dickinson method...
.
Technique
To perform a point count using the Gazzi-Dickinson method, you need a randomly selected thin sectionThin section
In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral, soil, pottery, bones, or even metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a...
from a sedimentary rock and a slide advance mechanism that will randomly select points on the slide with a petrographic microscope
Petrographic microscope
A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks...
. A minimum of 300 representative points, and preferably 500 points, should be used to perform the count. On each randomly selected point that lands on a sand grain, the operator needs to decided the make up of the area chosen, whether it is a mineral grain that is sand sized (larger than 62.5 micrometres) or a finer-grained fragment of another rock type, called a lithic fragment
Lithic fragment (geology)
Lithic fragments, or lithics, are pieces of other rocks that have been eroded down to sand size and now are sand grains in a sedimentary rock. They were first described and named by Bill Dickinson in 1970. Lithic fragments can be derived from sedimentary, igneous or metamorphic rocks)...
([.g. a sand-sized piece of shale). These are then converted to percentages and used for compositional comparisons in provenance
Provenance
Provenance, from the French provenir, "to come from", refers to the chronology of the ownership or location of an historical object. The term was originally mostly used for works of art, but is now used in similar senses in a wide range of fields, including science and computing...
studies. Typically, only framework (non-matrix
Matrix (geology)
The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...
) grains are counted, or non-framework are counted and then excluded from percentages when using things like QFL triangles. This can create problems with pseudomatrix
Pseudomatrix
Pseudomatrix, as defined by Bill Dickinson in 1970, is the term for lithic fragments that have been deformed to become a traditional sandstone matrix. This is formed when a lithic-rich sandstone is compacted...
, 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 and Bill Dickinson, starting in 1970. Dickinson and his students (most notably Ray Ingersoll, Steven Graham, and Chris Suzcek) at Stanford UniversityStanford University
The Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university on an campus located near Palo Alto, California. It is situated in the northwestern Santa Clara Valley on the San Francisco Peninsula, approximately northwest of San...
in the 1970s established the method and the use of the method to use the composition of sandstones to infer tectonic processes. This was in contrast to ideas presented by sedimentary geologists at Indiana University
Indiana University
Indiana University is a multi-campus public university system in the state of Indiana, United States. Indiana University has a combined student body of more than 100,000 students, including approximately 42,000 students enrolled at the Indiana University Bloomington campus and approximately 37,000...
at the time, who used the more traditional "QFR" or "rock fragment
Rock fragment
A rock fragment, in sedimentary geology, is a sand-sized particle or sand grain that is made up of multiple grains that are connected on the grain scale. These can include grains which are sand-sized themselves , or finer-grained materials...
" method of Robert Folk (1974) (which later grew into the Folk classification scheme), in which all grains that are connected are considered rock fragments, and the individual components are disregarded.
The best way to explain the differences in these two schools of thought is with an example: A sand rich in grus
Grus (geology)
Grus is an accumulation of angular, coarse-grained fragments resulting from the granular disintegration of crystalline rocks generally in an arid or semiarid region. Grus sand, when cemented into a sandstone, will form an arkose....
, or a granitic sand, when point counted with these two methods would have drastically different results. A QFR-style count would be rich in rock fragments, when a Gazzi-Dickinson point count would show the sand rich in quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
and feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
. Proponents of the Indiana School would say that information is lost by not counting rock fragments. Proponents of Gazzi-Dickinson point counting would say that small changes in erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
al transport would change the composition of the sand. In general, more geologists use the Gazzi-Dickinson method to determine the composition of sandstones today than other petrographic methods.