Describing siliciclastic rocks in hand sample Many aspects of siliciclastic rocks are best described in thin section, yet you will often be faced with the necessity of describing them in hand sample. That is the purpose of this week's lab. In addition to working with sandstones, you will add conglomerates and mudrocks to your repertoire this week. For all types of siliciclastic rocks, the first thing to observe is the overall grain size. If most of the rock is made up of grains greater than 2 mm in diameter (the maximum for sand), the rock is classified as a conglomerate. If most of the rock contains grains less than 2 mm in diameter, but visible with a hand lens (therefore greater than about 1/16 mm), the rock is a sandstone. If you are unable to recognize individual grains in most of the rock, the rock is a mudrock of some sort. Once you have determined whether the rock is a conglomerate, sandstone, or mudrock, you will need to describe other characteristics of the rock. GEOL 4500 1
Conglomerates There is relatively little agreement among sedimentologists on how to classify conglomerates, but there is broad agreement on how to describe them. A description of a conglomerate should include the following: 1) Report the grain size of the clasts (grains larger than sand, that is, larger than 2 mm) in millimeters or centimeters. Give the typical size of most clasts as well as the range of sizes. 2) Describe the sorting, using the sorting charts we used in the sandstone petrography lab last week. Remember that sorting is based on a log scale. 3) State whether the rock is grain-supported or matrix-supported. In conglomerates, matrix refers to sand, silt, and clay (note that this is different than for sandstones, where matrix is only silt and clay). In a grain-supported rock, most of the clasts will be in contact with one another, although those points of contact may not be visible on a given surface. In a matrix-supported rock, clasts will be surrounded entirely by matrix. The one-grain-diameter rule that you learned for sandstones also works for conglomerates. 4) Report the composition of the clasts. Common clasts include quartzite, chert, granite, schist, and volcanics, but clasts of sandstone, limestone, and almost any type of rock are possible. Clasts of bone or wood may also be present. The classification we use will emphasize the percentage of ultrastable clasts, that is, those composed of quartzite, chert, and vein quartz. 5) Determine whether the clasts are likely derived from outside the basin (extraclasts) or inside the sedimentary basin (intraclasts). Extraclasts include fragments of obviously older sedimentary rock, as well as fragments of igneous and metamorphic rocks. Intraclasts are most commonly made from recently lithified sediments, such as much chips. 6) Describe the matrix, particularly whether it is graded, laminated, or bedded, or whether it is structureless. Name the sample with the Boggs classification (shown below). For this classification, you only need to know whether the rock is clast-supported or matrix-supported, and the percentage of ultrastable (quartzite, chert, vein quartz) clasts. GEOL 4500 2
Siliciclastic Hand Samples Sandstones Last week, we described sandstones in thin section, and this week we will do this in hand sample. The procedure is essentially the same, but with a few complications because we cannot see the detail that we could with a microscope. Identifying grains in hand sample takes practice and you should not expect to be able to identify all grains, particularly in finer-grained sandstones. The principal difficulty will be learning to distinguish quartz, feldspar, and lithic grains without the aid of a thin section. Recall that quartz can be distinguished by its glassy luster, its translucency in small grains, and its tendency to conchoidal fracture. Feldspars tend to be more opaque and often display cleavage planes that reflect the light like tiny mirrors. All grains that are not quartz or feldspar should be regarded as lithic fragments, and you should be able to identify most of the common grains (e.g., muscovite, biotite, chert, amphibole). A hand-sample description of a sandstone should be the same as last week: grain size, sorting, roundness, grain types and percentages, matrix, grain-support vs. matrix-support, and cementation. In hand sample, you should also evaluate how well cemented the sample is, from well-cemented to poorly cemented, based on how well grains stay attached to the sample. From this, name the rock with the Folk (1980) classification, the same that we used last week. GEOL 4500 3
Mudrocks A mudrock description should include the following: 1) Describe the relative percentage of clay and silt, based on its grittiness against your teeth. This percentage will admittedly be crude, but you trying to distinguish between rocks that are primarily silt (highly gritty on the teeth), an even mix of silt and clay (somewhat gritty), or primarily clay (rather smooth against the teeth). This approach takes practice and you should double-check all the mudstones when you are finished. 2) Describe the stratification, focusing on whether the sample is bedded (layering is thicker than 10 mm) or laminated (layering thinner than 10 mm). Describe whether it is well-laminated, splitting into thin parallel sheets, or crudely laminated and breaking irregularly. 3) Describe the color, and note fossils, burrows, or other unusual features. From your description, use the Potter, Maynard, and Pryor (1980) classification below to name the rock. All of our samples will be indurated (no metamorphosed rocks and no non-indurated sediment), so you will use only those two rows of the classification. GEOL 4500 4
What to turn in Type your sample number, rock name, and a good one-paragraph description for each of the samples, following the same format as last week s lab. You do not need to include the maturity and provenance interpretations for the sandstones that we did last week. This lab is due at the beginning of the next lab period. References Boggs, S., Jr., 1995. Principles of Sedimentology and Stratigraphy. Prentice Hall: New Jersey, 726 p. Folk, R.L., 1980. Petrology of Sedimentary Rocks. Hemphill Publishing Company, Austin. 182 pp. Pettijohn, F.J., P.E. Potter, and R. Siever, 1987. Sand and sandstone, 2nd ed. Springer-Verlag, New York, 553 p. Potter, P.E., J.B. Maynard, and W.A. Pryor, 1980. Sedimentology of shale. Springer-Verlag, Berlin, 306 p. GEOL 4500 5