Sandstone Petrography

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Claude Bishop
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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.

1 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 grain size, sorting, roundness, grain types, matrix, and cementation as detailed below. In addition, you should classify (name) the rock and interpret its provenance.. Grain size Using a grain size scale, 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 as well as the average. We will use the Wentworth terms for grain size, where 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 ( mm), and boulders (> 256 mm). Note that the grain size scale also shows two other ways of reporting grain size, although we will not use these. The first is grain size in microns (shown on the left); recall that there are 1000 microns in a millimeter. The second is the phi scale, which is the negative base-2 log of grain size in microns; this is useful for thinking about grain size as a doubling scale. GEOL

Sorting Describe the sorting of grains using the scale below of Jerram (2001). Sorting should be based on roughly spherical grains of the same density, such as quartz and feldspar.

2 Sorting Describe the sorting of grains using the scale below of Jerram (2001). 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 refers to how oblong the grain is, whereas roundness is the smoothness of the grain. GEOL

3 Grain types Name the different types grains in the sample. The most abundant grains in sandstones are quartz, feldspar (potassium feldspar and plagioclase), and rock fragments (including chert). Accessory grains like muscovite, biotite, amphibole, and zircon may also be present. For help with identifying individual grains, refer to the siliciclastic petrography gallery at silicipetrography/. Sandstone thin-sections may be stained to help with identification of feldspars; if so, potassium feldspars will be stained yellow or brown, and plagioclase will be stained pink. Estimate the percentage of quartz, total feldspar, and total lithics (rock fragments, including chert) using the charts below (from Terry and Chilingar, 1955 and Folk et al., 1970). These percentages, which should sum to 100%, will be used to classify the sandstone. If you have different types of feldspar or lithic (rock) fragments, you should report their percentages. You do not need to report the percentages of accessory minerals like muscovite, biotite, zircon, etc. GEOL

4 Matrix Sandstones may contain matrix (silt and clay), and you should estimate the percentage of the rock that is composed of matrix. In most sandstones, this will be low (<10%), owing to the difficulty of simultaneously depositing clay, silt, and sand. Cementation Identify the mineralogy of any cements holding the grains together. The most common cements are quartz and calcite, but other cements include hematite, limonite, goethite, clays, chert, and chalcedony. For help in identifying cement, see If there is more than one type of cement, be sure to report which cements are dominant and which are minor. Naming the rock Once you have described each specimen, you should be able to apply a descriptive name to the sample with the Folk (1980) classification, shown on the next page. The classification starts with the main ternary diagram, with the top pole being quartz, both monocrystalline and polycrystalline, but not including chert. The bottom left pole represents all of the feldspars (plagioclase and potassium feldspars) plus fragments of granite or gneiss. The bottom right pole consists of all other rock fragments, including chert (a sedimentary rock), limestone, sandstone, shale, volcanics, slate, schist, etc. A rock name is determined from the percentages of these three components. Note that matrix (silt and clay sized particles) and cements are not included in these percentages. Neither are accessory minerals such as micas, heavy minerals (zircon, tourmaline, apatite, amphibole, etc.), or opaque grains (pyrite, hematite, magnetite, etc.). From this point, one approach is to follow the smaller triangles to apply more specific names to the various types of litharenite and arkose. Another approach, one that we will follow, is to modify the rock name on the main triangle with adjectives that convey the type of rock fragment (for example, a chert litharenite or a volcanic sublitharenite) or the dominant feldspar (for example, a potassium feldspar arkose or a plagioclase lithic arkose). Taking this approach conveys just as much information, while minimizing the number of rock names that must be memorized. Rock names should also be modified with adjectives that describe their textural maturity, which reflects the amount of matrix (clay and silt), sorting, and angularity. For example, a rock could be described as a supermature quartz arenite or a submature chert litharenite. GEOL

5 Provenance For each specimen, you will want to interpret its tectonic setting (provenance) from your observations. 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 volcanic rock fragments. Provenance falls into three basic categories: continental block (dark gray), magmatic arc (white), and recycled orogen (stripes). GEOL

6 The continental block category refers to uplift of granitic basement, which supplies quartz and abundant feldspar, but relatively few lithic grains. As such sources become progressively more weathered, sandstones derived from them become more enriched in quartz. The magmatic arc category refers to sandstones that are derived from volcanic 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 already be relatively rich in quartz. These source areas may initially supply abundant sedimentary and metamorphic rock fragments, but with greater weathering over time, these sands become progressively more quartz-rich. Two of the most widely used provenance diagrams of Dickinson (1985) are presented below, and they differ primarily in where chert is plotted. The ternary diagram on the left has total quartz (quartz, including chert), all feldspars, and all lithics (rock fragments) as its three corners. The one on the right includes chert as a lithic grain, which makes sense, given that chert is a sedimentary rock. This leaves only monocrystalline quartz at the top, with the feldspar corner unchanged. GEOL

7 Samples In this lab, you will work with the following samples. Note that ZZ on the slides often looks like 22. PS2: Cenozoic. Pacific coast, California. ZZ-73: Catskill Group, Devonian. Port Matilda, Pennsylvania. f1: Horizon and locality unknown FOU: Fountain Formation, Pennsylvanian. Cañon City, Colorado. ZZ-16: Horizon and locality unknown ZZ-123: Catskill Group, Devonian. Port Matilda, Pennsylvania. ZZ-D: Dakota Sandstone, Cretaceous. Cañon City, Colorado. ZZ-112: Cretaceous. Drumheller, Alberta : Horizon and locality unknown RC: Ralston Creek Formation, Jurassic. Cañon City, Colorado. M: Morrison Formation, Jurassic. Cañon City, Colorado. GC: Glencairn Formation, Cretaceous. Cañon City, Colorado. ZZ-107: Harding Sandstone, Ordovician. Cañon City, Colorado. 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 Folk classification. Use maturity and dominant grain type as adjectives, and if some other feature is particularly noteworthy, such as pervasive hematite cement, add that as another adjective. Paragraph 1: Rock description, including grain size, size sorting, quartz roundness, grain types and their percentages, the percent matrix in the rock and types of cement, as well as any other features that you observe (like burrows, grading, lamination, etc.). Your description should be complete, but should emphasize those aspects of the rock that are most distinctive. Paragraph 2: Interpretation of the provenance of the sandstone and the evidence supporting it. Here is an example of a writeup that you should use as a model: GEOL

8 ZZ-478. Submature volcanic litharenite ZZ-478 is a poorly sorted, coarse-grained, lithic arenite. Most grains are volcanic rock fragments (60%), with abundant plagioclase (20%) and lesser potassium feldspar (10%), and minor angular quartz (10%). Biotite and muscovite are present as accessory minerals. The sandstone lacks matrix. The sample is well-cemented, primarily with quartz cement, and lesser chlorite cement. Planar lamination is present, and it is defined by changes in grain size and quartz abundance. The provenance of the sample is transitional arc, based on the abundance of volcanic lithic grains and feldspar grains. 2. Turn in a single Q-F-L ternary diagram (included below) with all samples plotted on it. Label each sample on the plot by its ZZ number. 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., Interpreting provenance relations from detrital modes of sandstones. In: Provenance of Arenites, G.G. Zuffa, ed., Reidel, Dordrecht, p Folk, R.L., P.B. Andrews, and D.W. Lewis, Detrital sedimentary rock classification and nomenclature for use in New Zealand. New Zealand Journal of Geology and Geophysics 13: Folk, R.L., Petrology of Sedimentary Rocks. Hemphill Publishing Company, Austin. 182 pp. Jerram, D.A., Visual comparators for degree of grain-size sorting in 2-D and 3-D. Computers in Geosciences 27. Terry, R.D. and G.V. Chilingar, Summary of Concerning some additional aids in studying sedimentary formations by M.S. Shvetsov. Journal of Sedimentary Petrography 25: GEOL

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Sandstone Thin-Section Lab

Sandstone Thin-Section Lab Overview In this week's lab, you will learn the basics of the description and interpretation of sandstones in thin section. For each of the ten samples listed below, you should describe the following: