Grain Size Clastic in Sedimentary Rocks

Physical and chemical processes break up solid rock into smaller grains (particles). •Some grains, like rock pebbles, quartz grains, and mica flakes are basically fragments of the original rock. •Others, like clays, are chemical weathering products that grew in soils as other minerals weathered away. For example, as feldspars weather away they transform into tiny clay particles plus dissolved potassium, sodium, and calcium ions that wash away to the sea. Grains are transported by wind, water, ice, mudflows, and so on to places of deposition. Places of deposition could be desert sand dunes, deep ocean floors, river channels, river flood plains, or lakes, to name a few. All thin section images in this presentation are © Kurt Hollocher, 2004. These images may be freely used and copied for educational purposes. 1 The diagram on the left shows a simplified grain size chart for loose clastic sediment (e.g., sand, silt), and their respective sedimentary rocks (e.g., sandstone, siltstone). The pictures represent some of the places where one can find sediment of the grain sizes to the left. The top photo is of desert pavement. Mudflows and flash floods bring materials of all grain sizes to the desert floor. Wind then blows the fine- grained materials away, leaving pebbles and larger grains behind. The second photo is of a sand dune. The sand was blown from some sediment supply area and deposited here. Coarser-grained particles remained behind in the supply area, whereas silt and clay-size particles were blown away. The third photo is of a flood of the Mississippi River. The slow flow of the river through this housing tract permits mostly silt to transported and deposited. The bottom photo is of shale deposited on the floor of a sea. The weak currents permitted fine-grained clay to slowly settle and accumulate over many thousands of years. 2 Most clastic sediments have been sorted according to grain size. That is to say, the sediments have only a narrow range of grain sizes making up most of the deposit. The reason for this is that flowing water and air can carry particles only below a particular size. Larger particles won’t move and are left behind. At any particular flow speed, only the larger particles will be deposited and the finer ones will be carried away. The end result is that clastic sedimentary deposits typically have a narrow size range for most of the grains. 3 This is a set of photos of clastic rocks in thin section, and magnifications of 40x and 200x. The coarse sandstone has sand grains typically 1-2 mm across. The grains touch one another and make up most of the rock. Smaller grains were deposited in the quiet waters in the spaces between the grains. The fine sandstone has grains typically 0.2-0.5 mm across. Again, these grains touch one another and finer-grained material fills the space between the larger grains. Notice that many of the grains are rounded, indicative of long transport or exposure to a rough environment like a beach were the corners were worn off. The siltstone has grains in the size range 0.01-0.1 mm. Though some are in the sand range, most are smaller and in the silt range. The grungy- looking material in between the clear quartz grains is partly clay that has formed fthiitbkdffldifrom the in situ breakdown of feldspar grains. The shale is made almost entirely out of clay, though a few grains of quartz occur up to 0.02 mm across. Even at 200x most of the clay grains are not resolvable. The horizontal white lines in the left shale photo are cracks that formed while the rock was being cut to make the thin section. 4 This is a coarse sandstone because most of the volume of the rock is made up of sand grains between 1 and 2 mm, in the upper part of the sand size range. The grains touch one another, showing that there wasn’t a lot of fine-grained material to get in the way. Fine grained particles did fill in the gaps between the larger sand grains. Most of the grains seen here are of quartz, though any kind of mineral in this size range would be sand also. 5 This fine-grained sandstone is made up mostly of rounded quartz grains 0.2 to 0.5 mm across. Finer-grained material fills in the spaces between the sand grains. Rounded grains of quartz like this usually mean the individual grains were transported great distances in rivers and streams, or the grains spent time in a high- energy environment such as a beach. Rounding of angular grains is caused by the corners breaking off as they hit one another. 6 Siltstone is made up mostly of grains in the silt size range. Most of the grains in this rock are 0.01-0.1 mm across. At this scale it is difficult to actually see the smaller grains. 7 In shale the grains are mostly clay, and almost all are smaller than the thin section is thick, 0.03 mm. Although a few grains of silt-size quartz are visible (white specks) and possibly some charcoal (black specks), the clay is too fine-grained to see clearly even at high magnification. In particular, the grains are so small that ten to hundreds may be stacked up in the thickness of the thin section. The horizontal white lines are cracks that formed in the rock while the thin section was being made. 8 200x view of the coarse sandstone. The brown material is mostly iron hydroxide that precipitated from groundwater. It is one of the materials that cements this rock together. 9 200x view of the fine-grained sandstone. The reddish-brown layers are hematite, which originally helped cement this rock together. Clear quartz cement overgrows the hematite in some places. 10 200x view of the siltstone. Much of the grungy material is feldspar and mica that have weathered to clay between the time of deposition and now. This siltstone is mostly cemented by the clay that grew in place from feldspar weathering. 11 200x view of the shale. Notice that even at five times higher magnification, the shale does not look much different than at low magnification, because the clay minerals are still too small to see. 12 This is a small mudflow. Mudflows are viscous masses of loose material and water that flow downhill. They can have as much as about half sediment suspended in water, to as little as about 10% sediment. The important point is that in mudflows it is difficult for sediment particles to be separated according to grain size, which wind and water can do easily if it has <1% sediment in it. 13 The main point of showing this thin section is to see that the large grains are not touching each other. They are separated by, and so were deposited with, great masses of smaller grains of all sizes. The same is true for medium-size grains, small grains, and so on down to the clay size range. This sediment could not have been deposited by a steadily flowing stream. Other clastic sediments that tend to be poorly sorted according to grain size include glacial till and landslides. 14 Full size 40x view of the mudflow deposit showing the range of grain sizes, and showing that the larger grains are separated by smaller grains at all size scales. 15 Full size 200x view of the mudflow deposit. This shows large grains, small grains, and tiny grains <0.01 mm across. Still smaller grains are present but are too small to clearly see. The brown material is hematite that helps cement this rock together. 16 From moment to moment, or place to place, water or wind currents will vary with depth, eddies, flow speed, and so on. This causes different grain sizes to be transported and deposited in different places at different times, even in the same stream or the same stretch of desert. The patterns of grain size variation can help tell things about channel size and shape, flood plains, oxbow lakes, and so on. It is all part of the rock record that we read to extract an interesting story about the Earth’s history. 17.

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