99-Cu-8 This is a fine grained sandstone from one of the geology field camp project areas near Cuba, New Mexico. Observe in hand sample the primary sedimentary structure, ripple lamination. Can you determine facing direction with this sample? Use your hand lens and look at the darker laminations. These are very clay rich laminations relative the the lighter or quartz rich areas. Now look at the thin section under crossed polarized light (CPL) and your lowest magnification. Note that there are large areas that look to be pores but they can not be such. Think grain size and gravity here. These are an artifact of the thin sectioning process. The dark clay rich areas you observed in hand sample correspond to these areas. They result form the much softer clay rich zones being ground down much faster that the remainder of the rock and are thus lost in the thin sectioning process. Some things that you see in a thin section can be the result of the manufacturing of the thin section. Switch to a higher power and remain in CPL. Note the grain types: mostly monocrystalline quartz but also some feldspars (especially plagioclase) and there are a number of white mica grains which are all lined up in the same orientation (we say they have a preferred orientation). Note the calcite cement that surrounds the grains.
99-Jc-6 This conglomerate is from the Jack's Cabin geology field camp project area near Gunnison, Colorado. The sediment accumulated during the Pennsylvanian as the result of erosion of the Ancestral Rocky Mountains. The source area is near at hand and providing debris from the erosion of the lower Paleozoic section. You do not need a hand lens to see that it is polymictic. Use your hand lens and look both at the rough surface and the cut surface; look particularly at the character of the cement. Now go to the thin section and go to low magnification, cross polarized light (CPL). Check out the big grains first and not that they are rock fragments.....sedimentary rock fragments.....chert rock fragments. Fine examples of chert, look them over thoroughly. They are some other rock fragments here also. Now look at what is between the rock fragments. Two items; in some areas there is a finer grained sand and silt while in others there is chalcedony cement. Garb the hand sample and your hand lens and now look again at what is between the grains.
00-MO-4 This sandstone sample is from the Saint Peter Sandstone of Thayer, Missouri. It is what we used to call an orthoquartzite. The coloration is all secondary deposits of limonite. Note that it is easy to abrade grains form the sample; we say that it is slightly friable. If you look at the sawed face you can see that it is rather porous. Examine the thin section under plain polarized light, low magnification, and with the substage diaphragm closed down about 75%. Note the relative apparent topography between the positive mineral grains and the lower lying pore spaces. This is a rather good method of spotting pore space in sandstone thin sections and here these can be seen quite easily. Open the diaphragm back up and switch to CPL and observe the retardance colors; how thick is the thin section. Come up in magnification one objective. Look at for a dust rim on the quartz grains, many of them have such and these are rather good examples. Note that the vast majority of grains are monocrystalline quartz but also there are a few rock fragments and less than 1% feldspar grains. Under Folk's 1980 classification this would be a quartzarenite.
03-SP-01 This is a pebble that I picked up near Crested Butte, Colorado. Technically this is a rock fragment. You can spend some time looking at it with your hand lens but don't as there is nothing to see. Garb the thin section and lay it down on a piece of white paper. Note the textural nature of this; sort of a non-descript-swirlly pattern. The is the result of bioturbation. Not exactly a true trace fossil but worthy of note any way because it says the sediment experienced some sort animal activity. It is always a good practice to lay your thin section on a white paper and see what you can see; sometimes nothing and other times you may learn something. You can also learn a lot by making your own thin sections. Look at the thin section under cross polarized light (CPL) low magnification. How is the thickness? Go up one objective. There are quartz grains, some feldspar, micas, a few rock fragment, some brown amorphous areas which are kerogen and a clay matrix. The matrix accounts for more than 15% of the sample but less than 75% so by Dott this is some sort of wacke so you have a wacky rock fragment, sorry, it is Friday, it is late, and I am hungry.
03-SP-04 This is a sandstone from Cuzco, Peru. Note the color banding on the hand sample. This banding is called Liesegang banding, a secondary feature of this rock. Look at the thin section under your lowest power and in plain polarized light (PPL) and try to observe the nature of the banding. Once you have discovered the banding in thin section then locate one of the darker bands and start to go up in magnification but still remain in PPL. See if you can discover the mineral that is responsible for the darker bands. Next look at any portion of the slide under cross polarized light (CPL) and observe the matrix between the grains. This is a real fine example of a clay matrix. Note also that the predominant grain type here is monocrystaline quartz but there are also present some rock fragments. This has no observable porosity.
44-6145 Ward's Natural Science sold this as a 'graywacke' from Grafton, New York. Look at the thin section under low magnification, CPL. This is what was geologists originally considered to be a graywacke, before the term got miss used and overused. Come up to 10 power objective and look over the thin section in both CPL and PPL. The grains are mostly feldspar and rock fragments plus a moderate amount of monocrystalline quartz. Between the grains is a brownish mineral and a greenish mineral, both are chlorite. It would take a fair amount of skill to do a modal analysis on this one, something for you to be shooting for.
44-6148 This was sold by Ward's Natural Science as a 'bitumenous; shale from Utica, New York. Hold the thin section to a white sheet of paper and look at the laminations. Look at the thin section under low power PPL. The dark brown areas are organic material or kerogen; that is why the shale is called bitumenous. The is the source material for petroleum. Go up one objective and look at it under both CPL and PPL. There are some silt size grains of quartz so if you and done the in-the-mouth test for clay content it would have been gritty. Note the few mica grains and note the carbonate area which are probably the mineral dolomite. You would not do a modal analysis on something like this.
44-7353 This was sold by Ward's Natural Science as a 'carbonaceous' shale from the Llewellyn Formation (Pennsylvnian) of St. Clair, Pennsylvania. Note that observing it on a white sheet of paper does not reveal much. Look at it under low magnification both CPL and PPL, see much? Try a higher magnification, see anything yet? There is a little mica in there but mostly just black amorphous organic carbon hence it is described as being carbonaceous. Making thin sections of shale is difficult and time consuming because they are so soft and generally not very revealing once finished.
CY-30 This 'graywacke' comes from Greenland. The particular stratigraphic unit from which it was collected is dominated by 'graywacke' so the papers say and the particular bed from which it was taken was described in the field as being a 'graywacke.' Observe the hand sample with your hand lens and you will see that it is made of a medium sand size dark gray to black grains set in a light gray cement or matrix. Now look at the thin section under low magnification and plain polarized light (PPL). Note that the dominant grain is glauconite (green). Note that a good deal of the other grains are feldspar, especially plagioclase. By Robert Folk (1980) this is a glauconite arenite. By no published classification that I know of can it be called a graywacke!!!
SP 800 This siltstone is from the Pennsylvanian of southern Illinois. Lay the thin section on a piece of white paper and observe the sedimentary structure; go ahead use your hand lens on it also. Hand sample will not show these features so here making the thin section is worth it just to observe the sedimentary structure of the sample. Look at the thin section at at least 10X and PPL. The black areas are amorphous carbon, the brown is kerogen. Switch to CPL and verify that this thin section is very thin, maybe 15 microns. You have to have a real thin thin section to work with this type of material. Look at the yellowish elongate mineral grains, they are clay/mica. Note their orientation relative to the orientation of the stratification as exhibited by the elongation of the black areas. Their orientation is oblique to that of stratification so these must be authigenic clay mineral grains or actually crystals since they were not transported but grew in place in the sediment after deposition.
SP 803 This siltstone is from the Pennsylvanian of Kentucky. Lay the thin section on a white sheet of paper and note the sedimentary structure. Next hole it up to a light and use your hand lens on it. Note the graded bedding. Which way is up? Now put the thin section on the microscope and using low power and PPL observe the graded bedding, neat???? Note each bed grades in the same direction. This sample is from a distal turbidite. Grains are mostly quartz, some detrital mica and detrital clay and some authigenic clay (see SP 800). The opaque areas are amorphous carbon and much of the brown material is kerogen.
SP 811 This sandstone is from the Castle Rock Formation of Douglas County, Colorado. Observe the un cut portion of the hand sample with your hand lens and note the pink grains, they are potassium feldspar. Note the grayish glassy grains, they are quartz. Note that there are a lot of white grains and note that they exhibit cleavage; these are feldspars. Now observe the same on the cut portion of the sample and note here also the abundant porosity. In hand sample you can call this an arkose. Now go to the thin section and observe under the lowest power and under crossed polarized light (CPL). What is the thickness of the thin section? Look around the thin section and not that feldspar is the dominant grain type but there is also an abundance of quartz and several different types of rock fragments. Note next that the area between the grains is dark. Now switch to plain polarized light (PPL) and observe closely the area between the grains. Observe the brown areas here and switch between CPL and PPL. Note that this material is isotropic. Also note the pore space. Run up the magnification a bit higher and look at these. The cement here in this sample is opal. This should not be considered matrix.
1268
1282
99 Cu 5
99 Jc 4
A-12
CP 571 impregnated with blue epoxy
GP-3
PV-17
SP 205
SP 222
SP 224
SP 507
SP 814
Ta.25584
Tuc.95566