Page 182 - Laboratory Manual in Physical Geology

Quartz sand (sediment)
Quartz sand (sediment)
Quartz sand (sediment)

CEMENTATION 2

A. Sandstone cemented with white quartz or calcite.

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B. Sandstone cemented with reddish hematite.

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C. Sandstone cemented with yellow to brown limonite.

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Void space
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FIGURE 6.5 Cementation of quartz sand to form Calcite crystal
sandstone. Quartz and iron oxides (limonite, hematite) are the cement
most common cements that help hold together quartz sandstone.

Calcite ( FIGURE 6.6 ) can also cement together sandstones. Shells

Compaction ( FIGURE 6.4 ) and fusion of quartz sand grains (like C
pushing together two balls of clay) may accompany cementation Photomicrograph ( 26.6)
in deeply buried layers of sandstone. Original sample width is 1.23 mm
FIGURE 6.6 Formation of the biochemical (bioclastic) limestone. A. Shell gravel and blades of the sea grass Thalassia have
accumulated on a modern beach of Crane Key, Florida. Note pen (12 cm long) for scale. B. Sample of gravel like that shown in part A, but it is
somewhat older and has been cemented together with calcite to form limestone (coquina). C. Photomicrograph of a thin section of the sample
shown in B. Note that the rock is very porous and that it is cemented with microscopic calcite crystals that have essentially glued the shells
together.

Sedimentary Processes, Rocks, and Environments ■ 161