Description
EXERCISES
Sedimentary Structures
Exercises
PRE-LAB EXERCISES
1. Match the sedimentary structure with the environment in which it is most likely to
be found. Put the letter in the blank.
1.
Mudcracks
a. turbidity currents
2.
Stromatolites
b. dried up lake
3.
Symmetrical ripples
c. glacial lake
4.
Asymmetrical ripples
d. tidal flat
5.
Graded bedding
e. river
6.
Varves
f. wave-washed shoreline
2. Which of the sedimentary structures in this lab may be useful in determining
paleocurrent directions? (List four different structures.)
3. Which of the sedimentary structures in this lab may be useful in helping
determine the top from the bottom of a bed (up indicators)? (List four different
structures.)
Sedimentary Structures Exercises
129
LABORATORY 5
Sedimentary Structures
In this lab you will learn to recognize and identify sedimentary structures. Primary
sedimentary structures are those that form during (or shortly after) the sediment is
deposited. Some primary sedimentary structures are produced by water or wind that
moves the sediment. Other primary sedimentary structures, such as footprints, worm
trails, or mudcracks, form after deposition. Primary sedimentary structures can pro-
vide information about the environmental conditions under which the sediment was
deposited because certain structures form in quiet water under low-energy conditions,
whereas others form in moving water or high-energy conditions. Some form along
the shore, some form in the deep sea, and others form in the open air. Primary sedi-
mentary structures are important clues for the geologist attempting to interpret Earth
history.
Sedimentary structures can be grouped into two basic types. Inorganic sedimen-
tary structures are those formed by physical processes, like moving wind or water,
or drying. Organic or biogenic sedimentary structures are those formed through the
activities of animals or plants.
INORGANIC SEDIMENTARY STRUCTURES
Bedforms and Surface Markings
Bedforms and surface markings are sedimentary structures that form on the surface of
a bed of sediment. At the time of formation, the surface of a bed is equivalent to the sea
floor or the bottom of a lake or river. In a sequence of sedimentary rocks, bedforms and
surface markings are found on the bedding planes. Bedforms are produced as water
or wind moves across the sediment surface, and they include various types of ripples.
Surface markings include mudcracks and raindrop prints.
Ripples are undulations of the sediment surface produced as wind or water moves
across sand. Three types of ripples are covered in this lab: asymmetrical ripples, sym-
metrical ripples, and interference ripples. As ripples move under the influence of
moving water or wind, cross stratification (an internal bedding structure with inclined
layers) develops (see below). Ripples tend to form in sand-sized sediment.
Asymmetrical Ripples
Asymmetrical ripples form in unidirectional currents (such as in streams or rivers)
(Figure 5.1). Crests of asymmetrical ripples may be straight, sinuous (curvy), or
lingoid (lobelike or tongue-shaped), depending on water velocity. These ripples are
asymmetrical in cross-sectional profile, with a gentle slope on the upstream side and
a steep slope on the downstream side. Because of this unique geometry, asymmetrical
ripples in the rock record may be used to determine ancient current directions or paleo-
current directions. Note that cross stratification (an internal bedding structure with
inclined layers) forms as asymmetrical ripples slowly migrate downstream, and it can
also be used as a paleocurrent indicator.
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