Chapter 6

Prepared by Iggy Isiorho forDr. IsiorhoSediments and Sedimentary Rocks

Index

On pg. 121

Sediment

Sediment – Loose, solid particles that can originate by (1) weathering and erosion of preexisting rocks, (2) chemical precipitation from solution, usually in water, and (3) secretion by organisms.

Fig. 6.1

Size Matters Gravel – Rounded particles coarser than 2 mm

in diameter.

Sand – Sediment composed of particles with a diameter.

Silt – Sediment composed of particles with a diameter of 1/256 to 1/16 mm.

Clay – Sediment composed of particles with diameter less than 1/256 mm. (Table 6.1)

Transportation Rounding – The grinding away of sharp

edges and corners of rock fragments during transportation.

Sorting – Process of selection and separation of sediment grains according to their grain size for grain shape or specific gravity.

Fig. 6.2

Deposition Deposition – The settling or coming to rest of

transported material.

Fig. 6.3 Fig. 6.4

One of the most important jobs of geologists studying sedimentary rocks is to try to determine the ancient environment of deposition of the sediment that formed the rock.

Lithification

Lithification is the general term for a group of processes that convert loose sediment into sedimentary rock. Most sedimentary rocks are lithified by a combination of compaction, which packs loose sediment grains tightly together, and cementation, in which the precipitation of cement around sediment grains binds them into a firm, coherent rock. Crystallization of minerals from solution, without passing through the loose-sediment stage, is another way that rocks may be lithified.

Fig. 6.6

Crystallization

Crystallization – Crystal development and growth.

Crystalline texture – An arrangement of interlocking crystals.

Fig. 6.7

Types of Sedimentary Rocks Clastic sedimentary rocks Chemical sedimentary rocks Organic sedimentary rocks

Clastic Rocks Sedimentary Breccia – A coarse-grained sedimentary

rock (grains coarser than 2 mm) formed by the cementation of angular rubble. (Fig. 6.8)

Conglomerate – A coarse-grained sedimentary rock (grains coarser than 2 mm) formed by the cementation of rounded gravel. (Fig. 6.9)

Sandstone – A medium-grained sedimentary rock (grains between 1/16 and 2 mm) formed by the cementation of sand grains. (Fig. 6.10)

Matrix – Fine-grained material found in the pore space between larger sediment grains. (Fig. 6.12)

Turbidity currents – A flowing mass of sediment laden water that is heavier than clear water and therefore flows downslope along the bottom of the sea or a lake. (Fig. 6.13)

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Fig. 6.8

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Fig. 6.9

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Fig. 6.10

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Fig. 6.12

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Fig. 6.13

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Clastic Rock II Rocks consisting of fine-grained silt and clay are called

shale, siltstone, claystone, and mudstone. Shale is a fine-grained sedimentary rock notable for its

splitting capability (called fissility). (Fig. 6.14) Fine-grained rocks such as shale typically undergo

pronounced compaction as they lithify. (Fig. 6.15) A rock consisting mostly of silt grains is called siltstone.

Somewhat coarser-grained than most shales siltstones lack the fissility and laminations of shale. Claystone is a rock composed predominately of clay-sized particles, but lacking the fissility of shale. Mudstone contains both silt and clay, having the same grain size and smooth feel of shale but lacking shale’s laminations and fissility.

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Fig. 6.14

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Fig. 6.15

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Chemical Sedimentary Rocks

Chemical sedimentary rocks have been precipitated from an aqueous environment. Chemical sedimentary rocks are either precipitated directly by inorganic processes or by the actions of organisms.

Limestone is a sedimentary rock composed mostly of calcite. Limestones are either precipitated by the actions of organisms or are precipitated directly as the result of inorganic processes. The two major types of limestone can be classified as either biochemical or inorganic limestone.

Fig. 6.18 Fig. 6.20 Fig. 6.22 Table 6.2 6.1

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Fig. 6.18

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Fig. 6.20

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Fig. 6.22

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Table 6.2

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6.1

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Organic Sedimentary Rocks Coal – A sedimentary rock formed

from the consolidation of plant material. It is rich in carbon, usually black, and burns readily.

Fig. 6.25

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Fig. 6.25

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Sedimentary Structures

Sedimentary Structures – A feature found within sedimentary rocks, usually formed during or shortly after deposition of the sediment and before lithification.

Bedding – An arrangement of layers or beds of rocks.

Original horizontality – The deposition of most water-laid sediment in horizontal or near horizontal layers that are essentially parallel to the earth’s surface. (Fig. 6.26)

Bedding plane – A nearly flat surface separating two beds of sedimentary rock.

Sedimentary Structures II Cross-bedding – An arrangement of relatively thin

layers of rock inclined at an angle to the more nearly horizontal bedding planes of the larger rock unit. (Fig. 6.27 Fig. 6.28)

Graded bed – A single bed with coarse grains at the bottom of the bed and progressively finer grains toward the top of the bed. (Fig. 6.29)

Mud cracks – Polygonal crack formed in a very fine-grained sediment as it dries. (Fig. 6.31)

Ripple marks – Any of the small ridges formed on sediment surfaces exposed to moving wind or water. The ridges form perpendicularly to the motion. (Fig. 6.32)

Fossils

Fossils – Traces of plants or animals preserved in rock.

Fig. 6.33 Fig. 6.34

A contact is the boundary surface between two different rock types or ages of rocks. In sedimentary rock formations, the contacts are usually bedding planes.

Fig. 6.35

Source Area

The source area of a sediment is the locality that eroded and provided the sediment. The most important things to determine about a source area are the type of rocks that were exposed in it and its location and distance from the site of eventual deposition. (Fig. 6.36)

Environment of Deposition

Fig. 6.38 shows the common environments in which sediments are deposited. Geologists study modern environments in great detail so that they can interpret ancient rocks.

Continental environments include alluvial fans (Fig. 6.39), river channels (Fig 6.40), flood plains, lakes, and dunes.

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Table 6.1

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Fig. 6.1

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Fig. 6.2

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Fig. 6.3

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Fig. 6.4

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Fig. 6.7

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Fig. 6.26

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Fig. 6.27

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Fig. 6.28

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Fig. 6.29

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Fig. 6.31

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Fig. 6.32

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Fig. 6.33

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Fig. 6.34

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Fig. 6.35

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Fig. 6.36

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Fig. 6.38

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Fig. 6.39

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Fig. 6.40

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