Geology 109L Lab 1A: Sedimentary Particles and Flow Velocity

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1 Geology 109L Lab 1A: Sedimentary Particles and Flow Velocity Goal: The purpose of this lab is to introduce you to sedimentary particles (the most common constituents that make up sedimentary rocks) and how they reflect depositional processes. Both the particles and the rocks they compose contain abundant information about the source of the sediment, transport processes, and the environment in which they were deposited. This information can be used to understand everything from the tectonic setting to why a specific fossil is present or absent. Special Note: The boundaries between geological subfields are not sharp, and particularly in the case of identifying the sources of sediment, think about using all of your knowledge from mineralogy, petrology, and tectonics to help you interpret the rocks. One of the things you will find in these labs is that information from other classes can be very useful and you can apply the knowledge you gain in this lab to your other classes! Sedimentary Particle Composition and Origin: The large variety of sedimentary particles can tell you about the source of the sediment and the depositional environment. The sediment source can, in turn, tell you about the tectonic environment, transport processes, and resident or upstream flora and fauna. There are two basic types of sedimentary particles. 1) Particles derived by the breaking down of previously existing rocks. These are often called "terrigenous" because they originated on land and are transported to the site of deposition. Particles are typically composed of siliciclastic minerals such as quartz, feldspar, heavy minerals and rock fragments. Volcaniclastic particles, such as pumice fragments, volcanogenic feldspars, volcanic rock fragments and glass shards, are both igneous and sedimentary. Carbonate clasts and shells can also be terrigenous, if they are eroded from an older rock. However, they are more often considered part of the next category. Transported particles contain abundant (or sometimes sparse) information about uplift and erosion (tectonics), transport processes, and climate. 2) Minerals and organic matter that formed in place by chemical and biochemical processes. These autochthonous sediments form within the depositional environment and provide information mostly on conditions specific to where they were deposited. They include organisms, such as corals, moluscs, some algae, diatoms, and radiolaria that precipitate CaCO 3 or SiO 2 shells or skeletons. When they die, the remains become sedimentary particles that can be lithified into a limestone or chert. In environments where these organisms are abundant, the sediments are composed mostly of their shells. Evaporites and organic-rich rocks are also autochthonous. The process of evaporation concentrates ions in water causing minerals such as gypsum and halite (rock salt) to precipitate. In swamps and bogs, plant debris can build up and form coal during burial, whereas in anoxic basins, the remains of pelagic organisms accumulate resulting in deposition of organic-rich shales. All of these deposits contain important information on the chemistry of the depositional environment and the organisms living there. Texture Sediment texture is defined as the size, sorting, shape and arrangement (fabric) of the grains that make up a sedimentary rock. 1. Size: Particles can range in size from house-size boulders to microscopic clays. (There won't be any boulders in lab, but there are some interesting ones in Puta Creek below Monticello Dam west of Winters.) The grain size can tell us about the speed and turbulence of the flow that deposited the particles. Mud, being composed of very small particles, is deposited in low energy environments or in pockets of low energy between larger particles where it is protected from being washed away. To transport large grains, a higher energy flow is required. It takes far more energy to move a boulder or cobble down a river than it does to move silt and clay; this intuitively obvious statement is Lab 1A, Page 1

2 actually a very important key to interpreting sedimentary rocks. We will use semi-quantitative versions of this concept extensively in class. The grain size of clastic rocks is related to a number of factors in addition to flow speed: 1) The size of the minerals in the source area influences the final grain-size distribution. For example, an argillite-rich (shale) source area will not yield coarse sand grains unlike a granitic source area. Thus, no matter what the flow speed, sand will not be transported or deposited if it does not exist in the source area. Similarly, if clays are not present, shales can not accumulate. 2) The fracturing, dissolution, and abrasion of grains during transport act to decrease their size. The longer grains are actively transported, the smaller they become. The rate of decrease depends on the original size of the grain, the energy of the transporting system, the length of time in the system, and the composition of the grain itself. As a general rule, the greater the distance from the source area, the smaller and better rounded the grain will be. 3) The flow conditions under which the sediment is deposited influence the grain size. A high mountain stream deposits coarser material (boulders and cobbles with areas of gravel and sand) than the lower Sacramento River (mostly sand and mud). This is because turbulence and flow velocities are vastly different, and these two variables are the most important controls on the maximum grain size that the streams can move. Grain sizes are commonly reported using the Wentworth Scale (p. 12, textbook), which is a scale based on particle diameter in mm. The phi (Ø ) scale quantifies the Wentworth Scale by taking the negative logarithm base 2 of the grain size in millimeters. The phi scale is useful for statistical analyses of grain size and grain sorting. In lab, you can use either the words or the phi value for reporting grain size. 2. Sorting: Sorting refers to the size range of grains in the sediment or rock. A well sorted sediment is one that consists of a single sized sediment (for example, most grains are fine sand). A poorly sorted sediment is one that contains a broad range of grain sizes (for example, both boulders and find sand, as well as cobbles, gravel, etc.). Sorting is a good qualitative measure of the consistency of flow during deposition. Well sorted sediments are the result of consistent flows that are barely strong enough to transport the largest particles and are plenty strong enough to transport smaller particles. All the small particles get washed down flow. A well-sorted beach sand is a good example: the waves are strong enough to transport the larger grains and they sort out smaller particles and wash them off shore. Different beaches have different grain sizes due substantially to wave size (which correlates to water velocity on the beach) and sediment source. A poorly sorted sediment is often the result of very rapid deposition, i.e. it was unceremoniously and rapidly dumped into place without time for flows to sort the sediment by grain size. Flood deposits and land slides are two good examples. (Of course, if the source of sediment consists only of one grain size, all deposits will be well sorted.) Poorly sorted sediments are also deposited in low energy, fine-grained environments where large, shell secreting organisms live. Here, currents are too slow to wash away the shells or to wash in any outside grains larger than mud. Thus, large grains remain mixed in with the fine grains that the currents can just barely transport. The determination of sorting in a sediment can be approached qualitatively (which is usually good enough for most sedimentologists and stratigraphers) or statistically. Statistical studies of sorting usually involve sieving unconsolidated sediments to determine the abundance of each grain size. They are usually restricted to studies of process sedimentology where the researchers are trying to understand the details of particular types of flow. In studies of lithified rocks, the sediments can not be easily sieved and statistical grain size distributions require extensive microscope work. In this class, as in many sedimentological studies, the qualitative approach is sufficient and most practical. To estimate sorting, identify the finest and coarsest abundant grains in the sample. If the size range is all within one size class on the Wentworth scale, it is well sorted (or very well sorted); Lab 1A, Page 2

3 if the size range covers 4 size classes on the Wentworth scale, it is poorly sorted. Note that the size range within one category is much larger for large particles. Thus, a conglomerate composed of cobbles ranging in size from 50 to 60 mm in diameter covers only 1 size class and is well sorted, whereas a sandstone with grains ranging from 0.1 to 1 mm covers 3 size classes and is only moderately sorted. The following table give the number of classes versus sorting name. You can also use the figures in the textbook, p. 20, to get an intuitive feeling for what well sorted versus poorly sorted sediments look like. Standard Number of size Sorting Deviation of φ classes Present < Very Well Sorted Well Sorted Moderately Well Sorted Moderately Sorted Poorly Sorted > Very Poorly Sorted 3. Shape (p. 21, textbook): The shape of sedimentary grains can be described in terms of sphericity (spherical shape) or roundness (the smoothness of the grain edges). Sphericity refers to how close the particle approaches the shape of a sphere versus a rod or a plate. Roundness (or angularity) is how rounded (or jagged) the edges of the particle is. You can have a well-rounded particle with low sphericity (think of a great skipping stone that is flat and very smooth), as well as have a very angular particle with high sphericity. Sphericity reflects the inherent physical properties of a mineral grain and is often determined by the shape of the mineral prior to weathering, the shape of the organism in the case of skeletal grains, or zones of weakness in the rock for lithic (rock) fragments. Minerals which display a conchoidal fracture (quartz) will eventually show a high degree of sphericity, whereas a mica grain with basal cleavage will never look like a ball bearing because it breaks much more easily into flat plates. The sphericity of particles can be described as spheroidal, meaning it is close to being equidimensional; platy, meaning disk-like; or prismatic, meaning rod-like. Use these three terms to answer the questions in lab. Roundness is determined by the abrasion of the particles during transport and varies with the transportation distance and energy. The longer a grain is actively transported, the rounder the edges become because small protrusions get broken off. Faster flows round grains more quickly because collisions between grains are more forceful. Soft minerals round more quickly than hard minerals. Often, sand sized soft minerals are broken into microscopic particles. Textural Maturity: The sorting and shape of grains, as well as their composition, can be summarized into a description of the textural maturity of a sediment or rock. An immature sediment/rock will contain poorly sorted, angular grains and will often include soft and soluble minerals. It is called immature because its characteristics were not changed substantially by transport processes. A texturally mature sediment/rock consists of well rounded grains that are well sorted. Soft and soluble minerals are rare. High textural maturity generally is a sign of a long transport history that changed the grain characteristics and sorted them by size. Textural maturity is an important feature to note when interpreting sedimentary sequences, because differences reflect variations in source rock, depositional environment, transportation processes, climate, geography, and a host of other conditions. 4. Fabric: Fabric is the spatial arrangement and orientation of the grains in a sedimentary rock. (Because this lab is focused on sediments before they are lithified, fabric will not be analyzed in this lab. However, you will need to understand it for later labs.) Some fabric elements, such as the orientation of non-spherical grains, can indicate paleocurrent directions. For example, elongated Lab 1A, Page 3

4 pebbles, gravel or cobbles tend to stack up (imbricate) in a position that is the most hydrodynamically stable. This would be with the flat faces gently dipping up-current. Another useful fabric is grain packing which refers to the spacing or density of grains in the rock. Well packed grains are in contact with each other and the rock is said to be grain supported. In contrast, a matrix supported rock contains grains floating in a matrix of finer grained sediment where the grains are not in contact with each other. Grain-supported rocks tend to be deposited when sediments are either well sorted or the proportion of large grains is high relative to small grains. Matrix-supported rocks form from sediments that are poorly sorted, were deposited in quiet water environments with autochthonous shells supported by fine-grained sediment, or from flows with very high concentrations of fine grained sediment like mud flows. Thus, grain packing has complex origins, but when used with measures of sorting, analysis of sedimentary structures, and characterization of depositional environments, it is very useful. Many more fabric elements are also visible in thin sections, but we will not be looking at those in this lab. Composition The composition of sediments and sedimentary rocks provides both descriptive and genetic information. The resistance of certain minerals to physical and chemical weathering and the abrasive effects of transport makes their abundance within a sediment an important indicator of the intensity and duration of the processes that led to deposition. For example, if soft, unstable minerals are present, weathering and abrasion were not substantial during transport. In addition, the composition can reveal much about the provenance (source terrain) and can aid in the interpretation of regional tectonics. For example, abundant clasts of andesitic volcanic fragments within a sand matrix may suggest that the sediment was deposited in proximity to a volcanic arc (depending on other sedimentary characteristics), which implies deposition within a convergent margin tectonic regime. Compositional information is commonly so important that in many sedimentary rock classification schemes, compositional characteristics are used as modifiers to the rock name. For example, a quartz dominated sandstone would be called a quartz arenite whereas a feldspar-rich sandstone would be called a feldspathic arenite. As you read through the following review of some common particle compositions, think about possible origins of the different minerals. Quartz: Quartz is the most common sedimentary particle. Quartz is very hard (7 on Mohs scale), has no cleavage, and is chemically stable. Thus it is only slightly abraded during transport, and it does not dissolve significantly. A sediment that starts out with diverse sand compositions will eventually end up being dominated by quartz with sufficient transport and weathering, because quartz is both very abundant and very resistant to weathering and fracturing. Quartz can be well rounded or angular, depending on how long it has been in the transportation cycle. Feldspar: Feldspars are a group of important rock-forming minerals. Plagioclase (CaAl 2 Si 2 O 8 ) weathers quickly, is gray or creamy to white colored, has two directions of cleavage and striations, which are only visible on large crystals. Volcanogenic feldspar is usually plagioclase and tends to be soft, euhedral (you can see the crystal shape), and chalky in texture and color. Potassium feldspar (KAlSi 2 O 8 ) is pink to cream in color, has two directions of cleavage but has no striations. Sodium feldspar is similar looking to K-spar. Na- and K-spar are chemically more stable than plagioclase, but they tend to alter to clay minerals as they weather. Feldspars have a hardness of 6 on Mohs scale. Thus, they abrade more rapidly than quartz during transport. Mica: Micas are soft, and most sedimentary micas are either biotite (dark) or muscovite (light). They are platy and shiny, and weather quickly to clays. Micas abrade rapidly during transport. Rock fragments: As you can guess, pre-existing rock fragments can look like almost anything, from black chunks of shale to pieces of granite, schist, volcanic rock, or limestone. During transport, they commonly break up into component mineral grains. Lab 1A, Page 4

5 Oolites: These are little balls of CaCO 3 that have been rolled around in a high energy carbonateprecipitating environment. They look kind of like fish eggs (or jawbreakers!) with a concentric internal structure. They are common on beautiful tropical beaches, and you will see them in the Bahamian lab. Peloids: Peloids (or pellets) are either fecal pellets or some other sand-size spherical or ellipsoidal particle that displays no internal structure. This is a catch-all term for "we are not sure what they are" but they are composed of carbonate. Skeletal Particles: Most of the skeletal particles you will see in sedimentary rocks in lab will be from marine organisms, but they can also be derived from fresh water or terrestrial organisms. Some examples of marine skeletal particles include: bivalve shells, bryozoans, gastropods, echinoderm spines, and coral fragments. Identifying the organisms can be challenging, but provides interesting information on depositional environments and paleobiology. Lab 1A, Page 5

6 Geology 109L, lab 1: Sedimentary Particles Name: TA: Lab Assignment Part 1: Characterization of Sands In this exercise, you will examine a variety of unconsolidated sediments collected on the Broome Town Beach, Western Australia (BR7-00 samples), and from Putah Creek, California (numbers only). Please take care not to spill or mix up the samples. For each sample, identify the three most abundant grain compositions, and characterize each in terms of size, sphericity and rounding. Then characterize the sorting of the entire sample. Use dilute HCl to test for calcite and aragonite. Sample: BR Sample: BR Lab 1A, Page 6

7 Sample: BR Sample: BR Lab 1A, Page 7

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10 Sample: 1 Sample: 2 Sample: 3 Lab 1A, Page 10

11 Sample: 4 Sample: 5 Part 2: Interpretation of Your Results Read the web page for the lab ( and use the photographs to place the samples into context in your mind. Then answer the following questions. (These questions might be hard for you to answer exactly the first day of class. I am looking for qualitative answers with your best reasoning. Chapters 2-4 have more information.) 1. How does the grain size of the Broome samples change from off-shore to near the beach? What does this trend suggest in terms of relative wave energy? 2. For Broome samples, how do the grain size changes correlate with the sedimentary structures observed where each sample was collected? (Use the information on the web page.) Lab 1A, Page 11

12 3. How does the sorting of the grains change from off-shore to near Broome beach? What does this tell you about variations in flow speed? 4. For Broome samples, how are the shells different between off-shore and near shore samples? Which shells are from organisms that lived in the environment where the sample was collected, and which are more likely to have been transported into the environment where they were collected? (Use the information on the web page.) 5. Based on sorting in Putah Creek samples 1-4, would you characterize flow in Putah Creek as steady and constant or highly variable? For Putah Creek sample 5, the sorting is very different from the other 4 samples. What does this suggest about the flow that deposited sample 5 relative to the other samples? 6. For Putah Creek samples, would you characterize the samples as mature or immature? Why? 7. The sediment source for all Putah Creek samples is the same. Why does sample 5 have a different dominant composition? Lab 1A, Page 12