The new concept of "structure layer unit" of polymetallic nodule is proposed based upon a great number of internal structure studies of nodules and their time-space relation to associated sediments. The growth and nongrowth periods ofthe polymetallic nodule could be comparable respectively with depositional stages and sedimentary hiatuses of the sediments in the researching area. The whole formation process of the polymetallic nodule should take place in a certain environment that explained to be kineticstatic- progressive development. The condition for preservation of the polymetallic nodules depends on several factors such as biogenetic agency, intensity of bottom current, size of the nodules, static pressure from the overlay, sedimentation rate, tectonic environment and geochemical factor control. The polymetallic nodule nowadays occurred on the surface of sea bottom could be regarded as "survivors," because they will undergo to be dissolved away completely, if once deep buried by sediments.


The polymetallic nodule is a very useful mineral resource formed in certain environment and occurred over the marine surficial sediments. Earlier investigation results indicate the scdimcntatioil rales of associated sediments are more rapid than that of the nodules. But why did the nodules not be buried by sediments? Researchers have been interested in this question for a long time. Another question is whether we can find relevant mineral beds of the polymetallic nodules in the marine of the ancient continents on the basis of the rule "present is the key of the past," since the polymetallic nodule is a geologic product over the long geologic history and distributed in world oceans. Considering the data obtained from CP and CC blocks in Central Pacific Ocean during the three cruises ofHY4-861, 871 and 881, the present paper considers that formation and preservation of polymetallic nodule are two aspects of the same question.

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