Sandstone Thin-Section Lab

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1 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: Texture Using your grain size scale (shown at right) as a measuring tool, what is the average grain size of quartz and feldspar? If a range of grain sizes are present, report the average grain size as well as the range. Express your measurement using the descriptive terms from the Wentworth scale in the leftmost column, where vf is very fine sand, f is fine sand, m is medium sand, c is coarse sand, and vc is very coarse sand. The U and L that follow can be used to indicate the upper and lower parts of each range, as in upper fine sand (fu) to lower medium sand (ml). Grain size can also be expressed on the phi scale, based on the negative base-2 log of grain size. Although the phi scale is commonly used among soil scientists and environmental geologists, we will not use it in this course. Using the scale shown at the right, what is the sorting of the grains? Like grain-size, base your estimate of sorting on the roughly spherical grains of the same density, such as quartz and feldspar. Jerram, 2001 Sedimentary Geology

2 Using the Powers roundness scale shown at right, what is the roundness of the grains? Base your estimate of roundness on a consistent standard, such as quartz and feldspar. Pettijohn et al., 1987 Grain types What minerals are present as grains in the sample? Using the visual estimation charts below, what is the percentage of each grain type in the sample? The percentages of your different grain types should sum to 100%. For help with identifying individual grains, go to the siliciclastic petrography gallery at Terry and Chilingar, 1955 and Folk et al., 1970 Sedimentary Geology 2

3 Matrix What is the percent matrix within each sample? Is the sample grain supported or matrix supported? The one grain diameter rule is the most useful way of telling whether a rock is grain or matrix supported. For 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. Realize that it is generally not possible to have a cement-supported rock. Realize also that sandstones with a large proportion of matrix are much less common than those with very little matrix, owing to the difficulty of depositing sand, silt, and clay simultaneously. Cementation What is the mineralogy of any cements holding the grains together? Realize that there may be more than one cement present. What is the dominant cement in the rock? Naming the rock Once you have described each specimen, you should be able to apply a descriptive name to the sample with the Pettijohn and Potter classification shown below. The first step in this classification is based on whether the sample is grain supported (arenites - leftmost/frontmost ternary diagram) or matrix supported (wackes - middle ternary diagram). Final determination of the rock name is then based on the relative percentages of the grain types, expressed in terms of percent quartz, percent feldspar, and percent rock fragments (also called lithics). For this classification, percent feldspar includes all types of feldspars. Rock fragments or lithics includes all grains that are not quartz or felspar, such as amphiboles, micas, and chert. Pettijohn et al., 1987 Sedimentary Geology 3

4 For each specimen, you should also make some basic interpretations. First, interpret the textural maturity of the sample based on its percent matrix (clay), its sorting, and its roundness (see figure at right). Folk, 1951 Second, interpret the provenance, that is, the likely tectonic setting from which the sand was derived, based on the relative percentages of grain types. For this class, focus on the left ternary diagram, which plots total Quartz (Qt), versus total Feldspar (F), versus total Lithics (L), which is all other grain types combined. Dickinson, 1985 Sedimentary Geology 4

5 You have ten samples to work with in this lab: ZZ-204: Morrison Formation, Jurassic, Cañon City, Colorado. ZZ-205: Glencairne Sandstone, Cretaceous, Cañon City, Colorado. ZZ-206: Ralston Creek Formation, Lower Jurassic, Cañon City, Colorado. ZZ-208: Fountain Formation, Pennsylvanian, Cañon City, Colorado. ZZ-73: Catskill Group, Devonian, central Pennsylvania. ZZ-210: Dakota Sandstone, Cretaceous, Cañon City, Colorado. ZZ-123: Catskill Group, Devonian, Port Matilda, Pennsylvania. ZZ-107: Harding Sandstone, Middle Ordovician, Cañon City, Colorado. ZZ-160: Wise Formation, Pennsylvania, Tacoma, Virginia. ZZ-112: unknown unit, Cretaceous, Drumheller, Alberta. Since most of the samples come from the Cañon City area of Colorado, this provides an opportunity to reconstruct broadly the tectonic history of this region. Based on your interpretations of maturity and provenance, how would you reconstruct the geologic history of this area? Be sure to consult a geologic time scale so that you are aware of the amount of elapsed geologic time between individual samples. Note that the ZZ-205 sample from the Glencairne Formation is perhaps less than a million years older than the ZZ-210 sample from the Dakota Formation. 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 Pettijohn classification presented on the handout. Paragraph 1: Rock description, including sorting and angularity, grain types and percentages, percent matrix, 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 textural maturity and provenance. 2. Turn in a single Q-F-L ternary diagram with all ten samples plotted on it. Label each sample on the plot. Use the ternary diagram handed out in class or the one shown below. 3. Finally, turn in a one paragraph summary of the geologic history of Cañon City based on the sandstone samples you have. The 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. Sedimentary Geology 5

6 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., Stages of textural maturity in sedimentary rocks. Journal of Sedimentary Petrology 21: 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: Jerram, D.A., 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, Sand and sandstone, 2nd ed. Springer-Verlag, New York, 553 p. 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: Sedimentary Geology 6

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