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Chapter 5 Conglomerate and Sandstone

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Chapter 5 Conglomerate and Sandstone Chapter 5 Conglomerate and sandstone Conglomeratic rock is also known as rudite. Skim through the section entitled “Composition”, but concentrate on the information about textural characteristics and classification of conglomerates. Make sure that you understand the characteristics and depositional environments of the 5 types of conglomerate listed across the bottom of Figure 5.5. There is a lot of information about thin section microscopy of sandstones. We’ll be doing some of this, but don’t try to understand it just by reading about it. You will soon figure out what to look for with practice. It’s very important to become familiar with the Udden-Wentworth scale (Figure 5.13) for classifying clastic rocks. You will come across lots of references to Ф (phi) units, so you need to understand how they relate to grain diameter. The relationship is as follows: – Ф D= 2 (where D=diameter in mm) (The minus sign is critical. For example if Ф = –2 , D = 22 = 4, so the diameter is 4 mm. Conversely, if Ф = 2, D = 2-2 = 1/(22) = ¼, or 0.25 mm) The minus sign is used so that Ф values for the more common (ie. small) grain sizes will be positive, rather than negative. Ф (phi) -10 -9 -8 -7 -6 -5 -4 -3 -2 mm 1024 512 256 128 64 32 16 8 4 boulder (>256 mm) cobble (>64 mm) pebble (>4 mm) Ф (phi) -1 0 1 2 3 4 mm (dec.) 2 1 0.5 0.25 0.125 0.063 mm (fract.) 2 1 1/2 1/4 1/8 1/16 sand (> 0.063 mm) Ф (phi) 5 6 7 8 9 10 mm (dec.) 0.031 0.016 0.008 0.004 0.002 0.001 mm (fract.) 1/32 1/64 1/128 1/256 1/512 1/1024 silt (> 0.004 mm) clay (no lower limit) You should read through the information on the use of statistical techniques to study grain size distribution, but don’t worry if its not totally clear. We will do some of this in the lab. Classification of sandy sediments and sandstones is very important to understanding their history. We will talk about chemical and mechanical Malaspina University-College – GEOL 201 – Sedimentary Geology – May 2004 maturity, which is what Figure 5.19A is all about. The important things to remember are as follows: Rock fragment content decreases with maturity Feldspar content decreases with maturity Chemical maturity Quartz content increases with maturity Matrix (clay & silt) content decreases with maturity Degree of sorting increases with maturity Mechanical maturity Degree of rounding increases with maturity Figure 5.20 is a depiction of the most commonly used classification scheme for sandstones, so make sure that you understand what it means. The “Percent matrix” axis is a measure of how much silt and clay sized material the sandstone has, where an arenite ranges from 0 to 15% and a wacke ranges from 15 up to 75%. (A rock with more than 75% fine material is considered to be a mudrock.) The triangular part of the diagram allows us to specify the proportions of quartz, feldspars and rock fragments that make up the framework grains (as opposed to the matrix and cement). In case you’re not familiar with ternary (ie. triangular) diagrams, here is a generic example with a few points plotted (the numbers are the proportions of A, B and C in per cent): Malaspina University-College – GEOL 201 – Sedimentary Geology – May 2004 And the following is an example of the sandstone diagram, with the corners labelled: Quartz, Feldspar and Rock fragments, and the names of the arenite fields: The last part of the chapter discusses the characteristics and genetic environments (weathering, transportation and deposition) of the various types of sandstone. Chapter 6 Mudrocks Mudrocks (a.k.a. lutites) include all siliciclastic sediments with at least 75% silt and/or clay sized fragments. They are the most abundant of the sedimentary rocks, but as pointed out, they are not very well understood, partly because they are not well exposed. We can see this clearly around Nanaimo. Most of the rocky hills, ridges and cliffs around us are made up of sandstone and conglomerate, while the soft and boggy valleys are underlain by mudrocks. Many of the Gulf Islands are almost entirely composed of sandstone, while mudrocks are typically hidden beneath the intervening straits and bays. Malaspina University-College – GEOL 201 – Sedimentary Geology – May 2004 If you have done Geology 312 (and still remember any of it!) you will already have some understanding of the mineralogy of clays, and their significance to a wide range of geological issues. When minerals and rocks get broken down into very small pieces virtually the only two types of minerals that remain chemically stable are quartz and the clay minerals. Fine silt is typically a mixture of quartz and clay minerals, but clay- sized material (<0.004 mm) is almost entirely made up of a variety of clay minerals. It is important, therefore, to have some understanding of the various clay minerals, what makes them different and under what conditions they form. You might find the following classification scheme useful: Name Properties The three main textural types of mudrock: siltstone mudrock with over 68% of the particles larger than clay size (feels gritty) mudstone mudrock with 35 to 68% of the particles larger than clay size claystone mudrock with at least 66% clay-sized particles (feels slick) Various other categories: shale any mudrock that has fissility (tendency to split into layers) black any mudrock that has sufficient organic material to make it black in “shale” colour ferruginous any mudrock that has sufficient oxidized iron to make it red in “shale” colour bentonite bentonite rich rock (typically forms from volcanic ash that has accumulated under water) calclutite calcareous mudrock Don’t worry about the section entitled “Proterozoic mudrock facies”, or Figures 6.11 and 6.12. Chapter 7 Diagenesis Any changes that take place within sedimentary rocks from the time of the accumulation of sediments up until the onset of metamorphism (if there is any) are referred to as diagenesis. These processes include the following: • compaction • bioturbation (as described in Ch. 4 under sedimentary structures) • soft-sediment deformation (as described under sedimentary structures) • cementation and de-cementation • alteration of clay minerals • alteration of organic matter Malaspina University-College – GEOL 201 – Sedimentary Geology – May 2004 Make sure that you understand what these various processes are, why they happen and how it can affect the rocks. The most complex diagenetic changes are related to the alteration of clay minerals, and many of these processes are not very well understood. Some of the transformations are summarized below. increasing temperature Æ smectite Æ mixed-layer (interlayers of smectite and illite) Æ illite kaolinite Æ illite (if K is present) or Æ chlorite (if Mg is more abundant) illite Æ muscovite Chapters 8 to 13 include descriptions of a variety of different depositional environments or facies models. As noted in the introduction to Chapter 8, these models are deliberate idealizations of the environments and the rocks that form within them. While models are very useful for understanding what we see in the rocks, they are never perfect and there are always many exceptions. We need to be careful not to let the models constrain and colour what we observe. There are many, many depositional environments, and I’m not expecting you to understand all of the features of all of them. We will be concentrating on a few important ones. Chapter 8 Terrestrial sedimentary environments The term “terrestrial” refers to any sedimentary environment on land as opposed to within the oceans. It includes wind-blown (aeolian) deposits, alluvial fans (and talus deposits), and all river and lake deposits. Prothero and Schwab did not include glacial deposits in the 1st ed. and don’t give them a lot of room in the 2nd, so we’ll cover those a bit in here – and we’ll look at lots out in the field. Alluvial fan deposits are quite common in desert areas where there is active normal faulting producing rising fault blocks. A good example is the Basin and Range area of the southwestern US. A braided fluvial system exists wherever a river has more sediment than it can carry. This happens wherever there is rapid erosion, because of recent uplift, or glaciation (eg. in many mountainous parts of BC) or volcanism Malaspina University-College – GEOL 201 – Sedimentary Geology – May 2004 (eg. at Mt. St. Helens where there is abundant pyroclastic debris from the 1980 eruption) – or some combination of these factors. Make sure that you understand the differences between longitudinal bars (L-bars) and transverse bars (T-bars). The former tend to be rich in gravel, while the latter are finer (sandy), and are typically cross-bedded (see Appendix A of the lab manual). Mature rivers flowing across broad flood plains tend to meander. These systems produce a variety of deposits. Cross-bedded point-bar sands are common, but a number of other deposit types are likely to be present, as summarized under “Diagnostic features of meandering fluvial systems”. The flood plains of meandering rivers are good environments for the accumulation of coal. Make sure that you understand the material on lacustrine and aeolian environments, particularly the summaries of “Diagnostic features …” at the end of each section. Several types of sedimentary deposits are formed under glacial conditions. The ice itself creates a unique type of deposit known as glacial till, which is a mixture of fragments ranging in size from very fine clay to very large boulders.
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