Page 197 - Laboratory Manual in Physical Geology

A CTIVIT Y 6.4 Biochemical and Chemical Sediment and Rock

Name: ______________________________________ Course/Section: ______________________ Date: ___________

A. Seashells are grains of sediment made by the biochemical processes of organisms, so they are grains of biochemical sediment.
When you find a rock with a fossil seashell, then you have found evidence that the rock contains sediment deposited where
the sea animal lived (i.e., in the ocean, in a marine environment). Some limestone is entirely made of the seashells or broken
pieces of seashells. Obtain a seashell (e.g., hard clam shell) and draw it to the right of this paragraph. It may be easiest to trace
it, then fill in the outline with details of what the shell looks like inside or out. Next, place the shell into a plastic sandwich
bag and take the bag to the hammering station in your lab. Lightly tap the bag with the hammer to break up the shell into
pieces. Return to your table and view the broken pieces of shell with a hand lens.

1. The shell fragments that you just made are called clasts (from the Greek klastós , meaning “broken in pieces”). Geologists
commonly refer to several different kinds of clastic sediment. Circle the one that you just made.
■ pyroclastic sediment —volcanic bombs and/or volcanic rocks fragmented by volcanic eruption
■ bioclastic sediment —broken pieces of shells, plants, and/or other parts of organisms
■ siliciclastic sediment —broken pieces of silicate mineral crystals and/or rocks containing them

2. Compared to FIGURE 6.3 , what is the roundness of your clasts? _________________________________

3. What is the roundness of the clasts in this picture (× 1 scale)? __________________________________
Explain how and in what environment the shell clasts could have
attained their roundness.

4. Some limestone is made of shells that are calcareous (calcite or aragonite), like visible seashells, but they are microscopic
and cannot even be seen with a hand lens. Chalk is such a limestone. Some chalk used with modern blackboards is clay or
plaster-of-Paris, rather than real chalk. Obtain a piece of chalk from your lab room or instructor. Explain how dilute HCl
(hydrochloric acid) can be used to help you test your chalk and find out if it is real chalk or not. Then conduct your test
and report the results of your test.

5. Based on FIGURE 6.10 (page 165), how and where does chalk form?

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