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(a) The cement (but not the grains) in the sandstone reacts when in contact with dilute HCI. The cement consists of (b) The sandstone, overall, has a reddish color. The cement contains. (c) The cement is very strong, and when scratched with a steel needle it has the same hardness as the grains. The cement consists of ,

(a) The cement (but not the grains) in the sandstone reacts when in contact with dilute HCI. The cement consists of (b) The sandstone, overall, has a reddish color. The cement contains. (c) The cement is very strong, and when scratched with a steel needle it has the same hardness as the grains. The cement consists of ,

Applications and Investigations in Earth Science (9th Edition)
Applications and Investigations in Earth Science (9th Edition)
9th Edition
ISBN:9780134746241
Author:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa
Publisher:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa
Chapter1: The Study Of Minerals
Section: Chapter Questions
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minerals is the dominant component of the cement depends on the source of the groundwater and on the chemical environment in which precipitation takes place. In some cases, the cement that holds clasts together provides a clue to the deposi- tional environment. For example, the formation of hematite requires oxygen, so sedi- mentary rocks cemented by hematite, which therefore have a reddish color, come from environments where water contained dissolved oxygen. In contrast, sedimentary rocks with cement containing pyrite must have formed in anoxic (oxygen-free) environ- ments, because if oxygen had been present, the pyrite would have dissolved. Reddish sedimentary rocks, or redbeds, generally indicate deposition in terrestrial environments (e.g., rivers, alluvial fans). You can practice cement identification in Exercise 6.8. EXERCISE 6.8 Identifying Cements Name: Section: Course: Date: How can you tell what minerals occur in a cement? Simply remember the basic physical properties of the minerals (e.g., hardness, the ability to react with HCI, color). With these properties in mind, answer the following questions concerning sandstone, a type of sedimentary rock consisting of cemented-together grains of quartz sand. (a) The cement (but not the grains) in the sandstone reacts when in contact with dilute HCI. The cement consists of (b) The sandstone, overall, has a reddish color. The cement contains (c) The cement is very strong, and when scratched with a steel needle it has the same hardness as the grains. The cement consists of 6.6 Sedimentary Structures: Clues to Ancient Environments 6.6.1 Beds and Stratification Gravity causes all sediment to settle to the floor of the basin in which it was depos- ited. Over time, layers of sediment, called beds, accumulate. Beds are the most basic sedimentary structure, each bed representing a single depositional event. The dif- ferent colors, grain sizes, and types of sediment from each event distinguish one bed from another (FIG. 6.9). Beds range from a millimeter to several meters thick, depending on the depositional process involved. Many beds are fairly homogeneous, with uniform color, mineralogy, and texture, and their surfaces (called bedding planes) are smooth. But some contain internal variations or have distinct features on their surfaces. These other sedimentary struc- tures provide important information about the rock's history. Next, we look at a few examples of sedimentary structures and the history they record. 6.6.2 Sedimentary Structures 6.6.2a Graded Beds Graded beds are layers in which the grain size decreases progressively from the bottom to the top (FIG. 6.10). These beds form during a 6.6 SEDIMENTARY STRUCTURES: CLUES TO ANCIENT ENVIRONMENTS 161
Transcribed Image Text:minerals is the dominant component of the cement depends on the source of the groundwater and on the chemical environment in which precipitation takes place. In some cases, the cement that holds clasts together provides a clue to the deposi- tional environment. For example, the formation of hematite requires oxygen, so sedi- mentary rocks cemented by hematite, which therefore have a reddish color, come from environments where water contained dissolved oxygen. In contrast, sedimentary rocks with cement containing pyrite must have formed in anoxic (oxygen-free) environ- ments, because if oxygen had been present, the pyrite would have dissolved. Reddish sedimentary rocks, or redbeds, generally indicate deposition in terrestrial environments (e.g., rivers, alluvial fans). You can practice cement identification in Exercise 6.8. EXERCISE 6.8 Identifying Cements Name: Section: Course: Date: How can you tell what minerals occur in a cement? Simply remember the basic physical properties of the minerals (e.g., hardness, the ability to react with HCI, color). With these properties in mind, answer the following questions concerning sandstone, a type of sedimentary rock consisting of cemented-together grains of quartz sand. (a) The cement (but not the grains) in the sandstone reacts when in contact with dilute HCI. The cement consists of (b) The sandstone, overall, has a reddish color. The cement contains (c) The cement is very strong, and when scratched with a steel needle it has the same hardness as the grains. The cement consists of 6.6 Sedimentary Structures: Clues to Ancient Environments 6.6.1 Beds and Stratification Gravity causes all sediment to settle to the floor of the basin in which it was depos- ited. Over time, layers of sediment, called beds, accumulate. Beds are the most basic sedimentary structure, each bed representing a single depositional event. The dif- ferent colors, grain sizes, and types of sediment from each event distinguish one bed from another (FIG. 6.9). Beds range from a millimeter to several meters thick, depending on the depositional process involved. Many beds are fairly homogeneous, with uniform color, mineralogy, and texture, and their surfaces (called bedding planes) are smooth. But some contain internal variations or have distinct features on their surfaces. These other sedimentary struc- tures provide important information about the rock's history. Next, we look at a few examples of sedimentary structures and the history they record. 6.6.2 Sedimentary Structures 6.6.2a Graded Beds Graded beds are layers in which the grain size decreases progressively from the bottom to the top (FIG. 6.10). These beds form during a 6.6 SEDIMENTARY STRUCTURES: CLUES TO ANCIENT ENVIRONMENTS 161
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