Sandstone Petrography

Description of sandstones in thin section In this week's lab, you will learn the basics of the description and interpretation of sandstones in thin section. For each sample, you should describe the following.

Grain size Using your grain size scale (shown below) as a measuring tool, report the average or typical grain size of quartz and feldspar. If a range of grain sizes are present, you should also report the range.

Grain size can be expressed with diameter in microns, phi scale (negative base-2 log), or by the descriptive Wentworth scale terms(e.g., very coarse sand, medium sand). We will use the Wentworth terms, for which vc is very coarse sand, c is coarse sand, m is medium sand, f is fine sand, and vf is very fine sand. The U and L that follow these indicate the upper and lower ranges for each class, and it is optional to report this.

Grain sizes finer than this scale correspond to silt and clay, and grain sizes coarser than this indicate granules (2–4 mm), pebbles (4–64 mm), cobbles (64–256 mm), and boulders (> 256 mm).

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Sandstone Petrography

Sorting Describe the sorting of grains using the scale below. Sorting should be based on roughly spherical grains of the same density, such as quartz and feldspar.

Roundness Describe the roundness of quartz using the Powers roundness scale (below). Note that sphericity describes the overall shape of the grain and roundness/angularity describe the smoothness of the surface of the grain. Focus just on roundness.

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Sandstone Petrography

Grain types Name the different types of minerals present as grains in the sample. The most abundant grains in sandstones are quartz, alkali feldspar, plagioclase, muscovite, biotite, amphibole, and chert. Other grains may also be present. For help with identifying individual grains, go to the siliciclastic petrography gallery at http://strata.uga.edu/4500/labs/silicipetrography/. Sandstone thin-sections may be stained to help with identification of feldspars; if so, alkali feldspars will be stained yellow and plagioclase will be stained red.

For each grain type, estimate its abundance with the charts below (from Terry and Chilingar 1955 and Folk et al. 1970). You will need the percentage of quartz, total feldspar, and total lithics (all other grains, including chert) to classify the sandstone, so calculate and report these percentages. These three percentages should sum to 100%. Although you do not need to report the percentages of individual types of feldspar and lithic grains, you should note which types are predominant and which are rare.

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Sandstone Petrography

Matrix Sandstones may contain matrix, that is silt-sized and clay-sized grains, and you should estimate the percentage of the rock that is composed of matrix. In most sandstones, this will be low (<10%).

Determine whether the sample is grain supported or matrix (silt and clay) supported. The one-grain-diameter rule is the easiest way to do this. In a grain-supported rock, most grains will lie within one grain diameter of other grains. In a matrix-supported rock, most grains will separated from one another by more than one grain diameter, with the space between grains filled with matrix.

Matrix-supported sandstones are much less common than those that are grain-supported, owing to the difficulty of depositing sand, silt, and clay simultaneously.

Cementation Identify the mineralogy of any cements holding the grains together. Common cements include quartz, chert, calcite, hematite, and goethite. For help in identifying cement, see http://strata.uga.edu/4500/labs/silicipetrography/.

If there is more than one type of cement, be sure to report which are dominant and which are minor.

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Sandstone Petrography

Naming the rock Once you have described each specimen, you should be able to apply a descriptive name to the sample with the Dott (1964, modified by Pettijohn et al. 1987) classification shown below. The first step is to determine whether the sample is grain-supported (and therefore an arenite) or matrix-supported (and therefore a wacke). Remember that matrix is silt and clay-sized material.

Arenites and wackes are subdivided based on a Q-F-L ternary diagram, where Q corresponds to percent quartz, F corresponds to percent feldspar (both alkali and plagioclase), and L refers to lithics (rock fragments, volcanic fragments, micas, amphiboles, chert, etc.).

The simplest way to rapidly classify an arenite is to determine whether there are more feldspar grains or lithic grains; this will tell you whether to use the left side or the right side of the ternary diagram. Once you know this, use the percentage of quartz to determine how far up or down on the triangle you are: quartz arenites have 95% or more quartz, subarkoses and sublitharenites have 75–95% quartz, and arkosic arenites and litharenites have less than 75% quartz.

This approach also applies to wackes. If the rock is greater than 95% quartz, it is a quartz wacke. It is a feldspathic graywacke if feldspars dominate and a lithic graywacke if lithics dominate.

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Sandstone Petrography

Maturity and provenance For each specimen, you will make two basic interpretations from your observations.

1) Textural maturity reflects the degree to which the sand has been mechanically processed. As a sand becomes more mature, clay is winnowed away (lowering the percent matrix), grain size becomes better sorted, and grains become more rounded. Based on these three observations and the figure below from Folk (1951), determine whether the sample is texturally immature, submature, mature, or supermature. Be sure to support your interpretation of textural maturity with your evidence (e.g., amount of clay, sorting of grains, rounding of grains).

2) Provenance reflects the geologic setting of the source area of the sand. For example, uplift of granitic basement rock would favor a sandstone rich in quartz and feldspar. Alternatively, uplift of a volcanic arc would produce a sandstone with abundant lithic fragments.

Provenance falls into three basic categories: continental block (dark gray), magmatic arc (white), and recycled orogen (stripes).

The continental block category refers to uplift of granitic basement, which supplies quartz and abundant feldspar, but relatively few lithic grains. As such sources evolve, sandstones derived from them become progressively more enriched in quartz.

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Sandstone Petrography

The magmatic arc category refers to sandstones that are sourced from volcanic island arcs and volcanic continental arcs found along subduction zones. Young arcs that have been undissected by erosion supply abundant lithic grains, especially volcanic grains and chert. As these arcs become progressively dissected by streams, the roots of these volcanic chains are exposed, producing fewer volcanic grains and greater amounts of feldspar and quartz.

The recycled orogen category refers to the uplift of previous generations of sedimentary and metamorphic rocks in fold and thrust belts. Because these sediments have already gone through at least one cycle of weathering, they tend to start off as relatively rich in quartz. These source areas may initially supply abundant sedimentary and metamorphic rock fragments, but over time, sands sourced from such orogens become progressively more quartz-rich.

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Sandstone Petrography

Specimens In this lab, you have ten samples to describe and interpret.

ZZ-72: Navajo Sandstone, Jurassic. Zion National Park, Utah.

ZZ-74: Juniata Formation, Ordovician. Potters Mills, Pennsylvania.

ZZ-76: Catskill Group, Devonian. Port Matilda, Pennsylvania.

ZZ-79: Horseshoe Canyon Formation, Cretaceous. Drumheller, Alberta.

ZZ-108: Castlegate Member, Cretaceous. Tuscher Canyon, Utah.

ZZ-110: Cutler Group, Permian. Moab, Utah.

ZZ-134: Chuckanut Formation, Eocene. Bellingham, Washington.

ZZ-209: Horizon and locality unknown.

ZZ-280: Horizon and locality unknown.

ZZ-320: Flathead Sandstone, Cambrian. Bighorn Mountains, Wyoming.

What to turn in 1. Turn in a writeup of your descriptions and interpretations of each sample. Each sample should follow the following format:

ZZ-123. Rock name, according to the Dott classification. If necessary, add a descriptive adjective to the rock name to capture some particularly distinctive feature, such as hematitic litharenite.

Paragraph 1: Rock description, including grain size, size sorting, quartz roundness, grain types and their percentages, the percent matrix in the rock, grain vs. matrix support, and types of cement, as well as any other features that you observe. Your description should be complete, but should emphasize those aspects of the rock that are most distinctive.

Paragraph 2: Rock interpretation, including one sentence on the textural maturity and the evidence for it, and another sentence on provenance and the evidence supporting it.

Here is an example of a writeup that you should use as a model:

ZZ-478. Lithic arenite.

ZZ-478 is a poorly sorted, coarse-grained, lithic arenite. Most grains are chert and volcanic fragments (60%), with abundant plagioclase and lesser alkali feldspar (30%), and minor angular

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quartz (10%). The texture is grain-supported, with no matrix. The sample is well-cemented, primarily with hematite, but also with minor quartz cement. Planar lamination is present, and it is defined by changes in grain size and quartz abundance.

The sample is texturally submature, based on the poor sorting, angular quartz grains, and the lack of clay.

The provenance of the sample is transitional arc, based on the abundance of lithic grains and feldspar grains.

2. Turn in a single Q-F-L ternary diagram with all ten samples plotted on it. Label each sample on the plot. A ternary diagram is included below.

Like every lab, this lab should be typed and should follow all the normal conventions of good writing, including topic sentences, correct grammar, spelling, etc, which will also be required of all subsequent writeups in this course, although it may not be stated explicitly for each lab. This lab is due at the beginning of the next lab period.

Do not use any reference materials for this lab, unless approved by Dr. Holland or the teaching assistants.

References Dickinson, W.R., 1985. Interpreting provenance relations from detrital modes of sandstones. In: Provenance of Arenites, G.G. Zuffa, ed., Reidel, Dordrecht, p. 333-361.

Folk, R.L., 1951. Stages of textural maturity in sedimentary rocks. Journal of Sedimentary Petrology 21:127-130.

Folk, R.L., P.B. Andrews, and D.W. Lewis, 1970. Detrital sedimentary rock classification and nomenclature for use in New Zealand. New Zealand Journal of Geology and Geophysics 13:937-968.

Jerram, D.A., 2001. Visual comparators for degree of grain-size sorting in 2-D and 3-D. Computers in Geosciences 27.

Pettijohn, F.J., P.E. Potter, and R. Siever, 1987. Sand and sandstone, 2nd ed. Springer-Verlag, New York, 553 p.

Terry, R.D. and G.V. Chilingar, 1955. Summary of “Concerning some additional aids in studying sedimentary formations” by M.S. Shvetsov. Journal of Sedimentary Petrography 25:229-234.

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