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ABSTRACT: First in Situ Study of Deep-Sea Polymetallic Nodules and Their Environment Conducted with the French Submersible Nautile

G. Pautot, R. Le Suave, P. Cochonat

During the Nixonaute cruise of Ifremer (October-November, 1988), we made the first direct observations of polymetallic nodule fields and of their surrounding structural and sedimentary environment and took in situ measurements of physical properties of the seafloor.

Sixteen dives were performed in the Central Pacific Ocean at approximately 5000 m water depth between Clarion and Clipperton fracture zones. Three representative areas were studied: one with typical, elongated abyssal hills, the second one near seamounts, and the third one an abnormally flat area suitable for future tests of mining systems.

The nodule distribution and the variability of their morphology and size are clearly related to the seafloor physiography and to the thickness of the Plio-Quaternary siliceous pelagic clays. In particular the boundaries of the nodule fields were studied.

In situ measurements of cohesion were made by a vane shear device operated from the submersible. Within the upper acoustically transparent unit, different geotechnical facies exist that do not correspond to any apparent change in lithology.

The geological observations on the seafloor, and particularly along cliffs, showed a well-defined stratigraphic sequence that we sampled. The sequence from the base upwards is basalt, metalliferous sediments, massive beds of chalk (Oligocene to upper Miocene), stiff Fe/Mn-impregnated clay marking a Mio-Pliocene hiatus and Plio-Quaternary siliceous pelagic clays.

The dissolution processes are very active not only on the vertical sides of the calcareous outcrops creating detrital clays but also along fissures and diaclases. This leads to a very instable environment with the triggering of very large sheet-like portions of cliffs. The deep-sea landscape in the area of these escarpments is reminiscent of subaerial plateaus with karst topography.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990