TITLE INTRODUCTION DETRITAL ROCKS CHEMICAL ROCKS BIOCHEMICAL ROCKS ENVIRONMENTAL CLUES CREDITS
BIOCHEMICAL LIMESTONE
Limestones are rich in calcium carbonate. In a biochemical limestone the calcium carbonate is locked into the fossil shells and skeletons of marine organisms. (In a chemical limestone, the calcium carbonate is in the form of calcite mineral crystals.) As marine plants and animals die in the ocean, scavengers, waves or currents may break them apart, resulting in biochemical sediment comprised mainly of broken fragments of the original skeletons and shells. Below are several images of so called skeletal limestones, the names of which are based on the sizes of the skeletal grains, or on the main type of fossil present within the skeletal limestone.
row A Image 1 shows a skeletal limestone (technically a calcirudite based on the large sizes of a majority of the skeletal fragments). The most common fragments are of trilobites and bryozoans. Image 2 is a gastropod limestone (or gastropod calcirudite). Image 3 is a view of an oyster limestone (or oyster calcirudite) exposed along the beach at Del Mar, California (image by Dr. Rick Behl). Image 4 shows a brachiopod limestone (or brachiopod calcirudite). Image 5 shows a much finer-grained limestone called micrite. Most micrites are composed of microscopic, calcareous plant and animal fragments. Click on an image to enlarge it.
row A
image 1 image 2 image 3 image 4 image 5
row B Images 1, 2 and 3 were taken using a petrographic microscope. Each shows a thin-section view of a skeletal limestone where the rock sample was cut, ground and glued to a glass slide so that light would pass through showing excellent details of the fossils and surrounding matrix. These thumbnail images are approximately 4X actual size, and all are from Ordovician-aged sedimentary rocks. Image 1 is an ostracod valve. Image 2 shows part of a brachiopod valve and a conodont (amber). Image 3 shows another brachiopod fragment (long) and some echinoderm parts. The last two images show limestone in natural settings. Image 4 shows Jill, Bruce and Paul just prior to descending through the limestone layers that comprise the upper part of Grand Canyon. Image 5 finds geology student Zach ripping into a limestone outcrop, searching for fossils.
row B 
image 1 image 2 image 3 image 4 image 5
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