Publisher's Synopsis
Excerpt from California Journal of Mines and Geology, 1950, Vol. 46
On the sea floor most of the nodules are partly buried in sediment. The exposed upper surface can be identified by the thin dark coating of manganese oxide deposited during an interruption in phosphate deposi tion. Some nodules have a plano-convex shape with the convex side uppermost. The tops of many nodules also have numerous attached organ isms including solitary corals, bryozoans, serpulid worms, brachiopods, tunicates, and sponges. The buried bottom sides usually are barren or have only serpulid worm tubes.
Internal Structure. Fractured surfaces of the phosphorite vary in color from light brown when pure to almost black when high percentages of manganese oxide or organic matter are present. Although some of the nodules are massive and homogeneous, the large majority are layered. The layers are irregular, non-concentric, and range in thickness from a few millimeters to a few centimeters. Near its top each layer grades to darker phosphorite which ends in a glazed surface that may be covered by a thin black film of manganese Oxide. These films clearly represent time intervals during which the growth of the nodule was temporarily arrested. Above the glazed surface or the film the next layer begins with an abrupt change to lighter color. Some of the layers contain many frag ments of igneous, metamorphic, or sedimentary rocks including pieces of earlier phosphorite.
Examination of polished sections shows that in addition to the layers and the rock fragments most of the nodules contain numerous oolites composed of concentric layers of phosphorite (pl. 2a). Some of the oolites have clearly identifiable nuclei consisting of foraminiferal tests, glauconite, or clastic mineral grains. Other foraminiferal tests are directly cemented into the nodules. Although a few tests are composed of unaltered calcium carbonate, most are replaced by calcium phosphate. Invertebrate remains other than foraminiferal tests are present but not common; these include fragments of corals, bryozoans, and diatoms. Associated or incorporated vertebrate remains include sharks' teeth, a turtle plastron, whales' carbones, and other miscellaneous bones of fish and sea mammals.
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