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Northern Virginia Community College Alexandria, Virginia Dr. Victor P. Zabielski

September 2012

GOL 105: Sedimentary Rock Identification

Objective: Sedimentary rocks are the most abundat type of rock on the surface of the Earth. They are composed of sediments that have either accumulated or formed at the surface of the Earth and hold the key to understanding much of the surface history of the earth, including the history of life on this planet. This lab is designed to familiarize you with some of the most common types of sedimentary rocks and teach you how to distinguish each type by examining its chemical composition and the physical characteristics if the its component particles.

Introduction Sedimentary rocks can be divided into two distinct categories: Clastic/Detrital and Chemical/Authigenic. The primary difference between the two concerns the origin of the particles that make up the rock. Clastic/detrital rocks are composed of particles (sediment) that were transported (either by wind, water or ice) some distance from the location where they were originally created and then subsequently cemented together. These types of rocks will exhibit a Clastic Texture where individual mineral crystals or sediment grains will be observed with cement between the particles The particles that make-up chemical/authigenic rocks were precipitated in situ (in place) or nearly in situ and have not been transported any significant distance from the location of their formation. These rocks generally contain minerals crystals the have "grown' together rather than being cemented, although some may be cemented. These types of rocks may exhibit a Crystalline Texture where individual mineral crystals will be observed in a tightly packed orientation, interlocking like pieces of a puzzle, with little or no cement binding the mineral crystals together.

Clastic Texture

Crystalline Texture

Clastic/Detrital All clastic rocks are composed of cemented particles (clasts) that have all been subjected to moderate to intense physical and chemical weathering. For clastic rocks, the character of the particles comprising the rock (clasts) will help to determine the exact classification of the rock. The four most imporant particle properties are 1) particle size, 2) particle composition, 3) particle shape and 4) particle sorting. Particle Size Sediment particles are officially classified by the size of the diameter of the particles, but can generally be classified into ranges from coarse to very fine. The general scheme is as follows (which follows Table 5.1 in your textbook, pg. 135 or Part 6A in Laboratory 6 in your lab manual): Size general (diameter in mm) name classification rock >256 boulder coarse (gravel) conglomerate/breccia 64 mm-256 mm cobbles coarse (gravel) conglomerate/breccia 2 mm-64 mm pebbles coarse (gravel) conglomerate/breccia 1/16 mm-2 mm sand medium sandstone/arkose 1/256 mm-1/16 mm silt fine siltstone <1/256 mm clay very fine shale Generally speaking, the size of a particle is controlled by the energy of the medium moving it and the distance the particle has been transported from its original source. A high energy environment can deposit and move very particles whereas a low energy environment will move and deposit finer particles. Similarly the closer you are to the source of the material, the larger the particles that will be deposited. Particle Composition Although the size of the particles is the primary diagnostic property, the composition and/or the shape of the particles can also be used to further distinguish some rocks. Although composition can be vary greatly, most sedimentary rocks are composed of only a few different types of minerals, namely quartz, clay, feldpars, calcite and mafics. For instance, both sandstone and arkose are composed of sand-sized particles, but arkose contains a large percentage of feldspars, whereas most sandstones are composed of mostly quartz grains. In this case arkose is a type of sandstone, but provides one more level of description, thus identifying it as a feldspathic sandstone. The composition of the sediment particles will be primarily controlled by the composition of the rocks from which it is derived. Additionally, the movement and transport of the particles will preferentially weather and erode the softer minerals. Thus the more a sediment has been subjected to weathering, the greater the percentage of durable minerals will be present. This is referred to as the "maturity" of the sediment. A more mature sediment will have a greater percentage of quartz, since quartz is one of the most durable common minerals.

A very immature sediment will have a greater percentage of mafic minerals, most of which are less durable than quartz Particle Shape The shape of a particle can also be linked to the maturity of the sediment. The shape of particles can range from well-rounded to angular (see Figure 6.1, page 113 in your lab manual). The greater the maturity of a sediment, the more rounded its constituent particles. A sediment containing very angular grains, obviously has not been transported very far or for very long and is thus an immature sediment. Both a conglomerate and a breccia are composed of coarse particles, but the particles in a conglomerate will be rounded (smooth) and the particles in a breccia will be very angular. Typical weathering (physical and chemical) and transport in a river will produce well-rounded particles and thus a conglomerate. The grinding together of two pieces of land along a fault with minimal subsequent transport of particles will result in very angular particles, and thus a breccia. Sometimes breccia can be created by explosive eruptions, such as the volcanic breccia that you looked at in the previous lab. Particle Sorting The sorting of particles in a sediment refers to the range of particle sizes that are present. A well sorted sediment will contain particles of only one size. A poorly sorted sediment will contain particles of varying sizes. The sorting of a sediment is primarily controlled by the method of transport or erosion. Wind will create the best sorted sediments, thus sand dunes will be composed of well sorted sediment, usually in the fine sand to sand size range. Movement of sediment by water will generally produce a well sorted sediment, however this may not be as well sorted as the wind sorted sample in the previous example. Water transported sediments can range in size from cobbles to clay. Transport by ice, such as in glaciers, commonly produce the most poorly sorted sediments with particle sizes ranging from clay to boulder all deposited simulatneously.

Chemical/Authigenic Authigenic rocks are all created through chemical reactions and have been subjected to little if any weathering or erosion. Because their particles were all created via chemical reactions, the primary diagnostic property is going to be the chemistry of the rock (as opposed to the size of the particles as was seen in clastic rocks). Some authigenic rocks are produced with the aid of living organisms (plants or animals) and we refer to these as organic authigenic rocks. Most limestones fall into this category. Other authigenic rocks are produced solely by chemical processe (such as evaporation of seawater). These are referred to as inorganic authigenic rocks. The most common authigenic rock is limestone, which can come in several different varieties (fossiliferrous limestone, oolitic limestone, coquina, micrite, chalk, travertine, etc). The single thing all these rocks have in common is their chemistry. All are made of calcite. A simple drop of hydrochloric acid on any of these rocks will provide an easy way to identify them. In most authigenic rocks, it will be difficult to identify any single particles, unless the rock contains fossils.

Part I: Divide all the rocks in your sample box into two categories 1) clastic and 2) authigenic. Remember, most limestones are authigenic rocks, and there is a very easy test to find a limestone. Use the explanations given above to further help you divide the rocks into each of the two categories. Part II: Now further classify each rock as to its unique name. Use the provided pages to keep track of your notes for each rock and what characteristics you feel are the most important for identifying each rock sample. Below is a list of some of the terms that can be used to describe each rock: Composition: quartz, calcite, feldspar, clay, fossils, halite, gypsum, mafics, shells, coal (organics), cryptocrystalline quartz. Particle Size: see previous table Roundness: very angular, angular, round, very round Sorting: poorly sorted to well sorted Note: The particle size, roundness, and sorting will be most applicable for clastic rocks and may not be relevant for chemical rocks. Use the two Tables at the end of this lab to assist you in identifying the rocks Definitions Ooilitic ­ composed of ooids (small rounded particles composed of calite formed inorganically in shallow tropical waters) Arkose ­ a sandstone composed of quartz and abundant feldspar. Graywacke ­ a sandstone composed of both mafic and felsic particles Coquina ­ a rock composed of cemented shell fragments

Sedimentary Rock Identification Chart

Composition (for clastic rocks only) Sorting Particle Size (for clastic rocks only) Roundness (for clastic rocks only)

Sample Number

Clastic / Chemical

Rock Name

Primary Properties

Sedimentary Rock Identification Chart

Composition Particle Size (for clastic rocks only) Roundness (for clastic rocks only) Sorting (for clastic rocks only)

Sample Number

Clastic / Chemical

Rock Name

Primary Properties

Sedimentary Rock Identification Chart

Composition Particle Size (for clastic rocks only) Roundness (for clastic rocks only) Sorting (for clastic rocks only)

Sample Number

Clastic / Chemical

Rock Name

Primary Properties

Sedimentary Rock Identification Chart

Composition Particle Size (for clastic rocks only) Roundness (for clastic rocks only) Sorting (for clastic rocks only)

Sample Number

Clastic / Chemical

Rock Name

Primary Properties

Clastic Rocks

Size gravel or pebble Composition Shape rounded angular quartz feldspars and quartz abundant mafics quartz / clay calcite clay mud sand Rock Name conglomerate breccia quartz sandstone arkose greywacke siltstone go to chemical chart shale

silt

Chemical Rocks

Composition Crystalline / Clastic Crystalline Particle constituents parallel or concentric layers mud (crystalline) ooids fossils shell fragments mud (soft) gypsum organic matter halite quartz Rock Name travertine

Calcitic

micrite ooilite fossiliferrous limestone coquina chalk gypsum bituminous coal rock salt chert

Non-calcitic

Crystalline

Clastic

Sedimentary Rock Identification Key S-100 S-102 S-107 S-110 S-116 S-118 S-120 S-150 S-152 S-180 S-200 S-250 S-258 S-260 S-263 S-270 S-295 Limestone Breccia Conglomerate Graywacke Oolitic Limestone Fossiliferrous Limestone Coquina (shell hash) Siltstone Quartz Sandstone (white and red) Arkose Shale (black) Bituminous Coal Micrite (calcareous mudstone) Travertine/Stalactite (Fig. 6.8b and 6.8c, pg 134) Chert/Flint Chalk Gypsum Rock Salt (Fig. 6.7a, pg 134)

Refer to Figure 6.3 (page 131) in AGI Lab Manual

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