Laboratory Manual for Introductory Geology 4e

6.5.4 Cements in Clastic Rocks
The most common cements in clastic sedimentary rocks are quartz and calcite
because the ions needed to make these minerals can occur in relatively high concentrations
in the groundwater from which cements precipitate. Which of these
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 depositional
environment. For example, the formation of hematite requires oxygen, so sedimentary
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) environments,
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:
Course:
Section:
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 HCl, 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 HCl. 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 deposited.
Over time, layers of sediment, called beds, accumulate. Beds are the most basic
sedimentary structure, each bed representing a single depositional event. The different
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 structures
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
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