Stratigraphy of Tertiary deposits, paleooceanology, the formation of manganese and phosphorite nodules

Micropaleontological investigations in two survey areas show that Tertiary outcrops of different ages are distributed on the bottom surface or under a thin layer of Quaternary sediments. The continuous sediment sequence from the Late Eocene to the Early Miocene (40-17 million years) and the regional hiatus between 17 and 1 million years were revealed. Age intervals of radiolarian complexes in nodules are Tertiary mainly Oligocene. Abundant manganese nodules occur on the bottom mainly on the surface of Quaternary sediments covering the Tertiary deposits of various ages. The statistical probability of the nodule formation age is mainly Oligocene. Consequently, the ages of the manganese nodules and sediments are different, usually older nodules occur on younger deposits. Most of the micropaleontologically characterized Quaternary sediments are 0.7-0.9 million years old. Micropaleontological investigations were carried out also in a block of the dense ancient miopelagic clay covered by the manganese crust. In contrast to nodules, the crust could be formed only after formation of an erosion surface, on which it accrued. The distribution of microfossil complexes in different samples taken from the block shows that the dense miopelagic clay is of the Late Oligocene age. Early and Middle Quaternary and Recent sediments penetrated it through cracks and burrows. Obviously, the manganese crust was formed in the Quaternary time. Micropaleontological complexes of Middle and Late Miocene, and Pliocene are present neither in the ancient clay, nor in the manganese crust, nor in Quaternary deposits containing mixed complexes of different ages. During that time the present erosion surface of the Late Oligocene clay was covered by younger deposits of the Late Oligocene, Miocene and Pliocene, which were eroded and washed away in the Quaternary. Probably the erosion begun about 0.9-0.7 million years ago at the beginning of “Glacial Pleistocene” when the ocean circulation, particularly near-bottom currents became more active. The assumption was stated (Barash, Kruglikova, 1999, 2000), that the erosion of Tertiary deposits by near-bottom currents could be intensified by an effect of strong earthquakes in tectonically active zones which can be affective within several thousands of kilometers. In watersaturated nonconsolidated medium of the ocean bottom the surface seismic Love and Rayleigh waves should be propagated which can cause a vibration effect. The seismic vibration effect on the surface sediment layer must disintegrate and stir up sediments, which are then carried away by the bottom current. Large-size components of the sediment including manganese nodules cannot be carried out by the current and form residual deposits. The same vibration effect causes ancient nodules to float up onto the surface of the Quaternary sediments. This assumption was considered from the point of view of geophysical mechanisms (Kuzin, Barash, 2001, 2002a, 2002b). We proposed the new approach, which is based on mechanical influence on sediments of Rayleigh waves, generating by strongest (М7.5) earthquakes within the nearest Central American seismic active region. The proposed approach is based on data concerning of oscillations, which were excited by Rayleigh waves of some catastrophic earthquakes (for example, Lisbon, 1775, Assam, 1950, and Alaskan, 1964). These oscillations are observed at epicentral distances from 2000-4000 to 8000 km. The considered mechanism is realistic and allows to use quantitative characteristics of Rayleigh wave amplitudes for the explanation of mechanical influence on sediments. The study of Rayleigh wave amplitudes was carried out with using of 200 records of earthquakes with M=6.0-8.2 at distances 560-9200 km for Petropavlovsk-Kamchatsky and North-Kurilsk seismic stations. For the analysis there were used data for various seismoactive regions of the Pacific Ocean. The close seismotectonic and seismic analogy between Kurile-Kamchatka and Central American segments of the Pacific belt allows us to apply the data recorded in the first region for study of same phenomena in the second one. Then for distances between investigated areas and the seismoactive Central American region of 3000-5400 km we have amplitudes of Rayleigh wave about 0.5 mm for single earthquake. The earthquakes with M = 7.5 – 8.5 can recure 20 times per 1000 years and capable to cause the “seismological erosion” of sediments up to 10 m/My but they are ineffective for manganese nodules movement, because of large dimensions (3-5 to 10 cm) of the nodules. For the explanation of nodules movement it was necessary to use another mechanism. For solution of this problem we have used for the first time the mechanism of near-bottom tsunami in open ocean, which are excited by the same way as Rayleigh wave by the strongest (M7.5) earthquakes of the Central American seismic zone. It is known from observations near the California and in the Alaskan Bay, that these tsunami are propagated in the near-bottom water layer (at 3000-5000 m ocean depths) with amplitudes of 0.8-3.0 cm and velocities up to 180 m/s (Filloux, 1982; Milburn, Bernard, 1990) (fig.2). Minerals of ocean. Int. Conf. 20-23 April. St.Petersburg. VNIIOkeangeologia. 2002. P.55-57. Mechanism of manganese nodules accumulation and their maintenance at the sediment surface (Clarion-Such perturbations of the near-bottom water layer are capable to cause both the erosion of sediments and the transference (rolling, overturning) of manganese nodules over the ocean bottom. As a result they can occur on the surface of sediment of various ages. That confirms our supposition about the mechanisms of “floating-up” of massive manganese nodules or their maintenance on the bottom surface. Thus, this assumption suggests one and the same reason for the peculiarities of the Clarion-Clipperton zone, that is, the regional stratigraphic hiatus, the formation of the residual nodule fields, and maintenance of ancient nodules at the surface of the Quaternary deposits. REFERENCES Barash, M.S., and S.B. Kruglikova. Age of radiolaria from ferromanganese nodules of the Clarion-Clipperton Province (the Pacific Ocean) and the problem of nodule unsinkability. Oceanology, 1995, V.34, N6, p.815-828. Barash, M.S., and S.B. Kruglikova. Age of manganese nodules of the Clarion-Clipperton Province and the problem of nodule maintenance at the sediment surface. PACON 99 Proceedings, Symp. on “Humanity and the World Ocean”, 2000, p. 220-230. Barash, M.S., S.B.Kruglikova, and V.V.Mukhina. Stratigraphic features of sediment formations of the Clarion-Clipperton Province (the east equatorial Pacific). Oceanology, 2000, V.40, N3, p.424-433. Filloux, J.H. Tsunami recorded on the open ocean floor. Geophys. Res. Lett. 1982. V.9. N.1. P. 25-28. Kuzin, I.P., and M.S. Barash. On new approach to solution of problem of “floating-up” of manganese nodules in near-equator part of the Pacific Ocean. 14th Int. School on Marine Geology, Abstracts, Moscow, Russian Ac. Sc., 2001, V.2, p. 286-287. Kuzin, I.P., and M.S. Barash. On influence of Rayleigh waves of strongest (М7.5) Central American earthquakes at sediment cover within the Clarion-Clipperton Province. Oceanology, 2002 (in press). Kuzin, I.P., and M.S. Barash. On possible mechanism of manganese nodules “floating-up” in the near-equatorial part of the Pacific Ocean. Oceanology, 2002 (in press). Milburn H.B., Bernard E.N. Deep ocean tsunami observations. Workshop on scientific uses of undersea cables. Jan.30-Febr.1, 1990. Honolulu. Hawaii.Washington, D.C. 1990. P.69-73.

The area studied is characterized by a regional stratigraphic gap from the early Miocene up to the Quaternary. Deposits from the late Eocene to the early Miocene were revealed at the bottom surface or under a thin sedimentary cover. Ferromanganese nodules, mostly of the Oligocene age, are deposited over the surface layers of the Tertiary or Quaternary sediments. A detailed micropaleontological study of a block of dense ancient clay coated with a ferromanganese crust was carried out. A study of the composition of the radiolarian and diatomaceous complexes found proved that the crust was formed in the Quaternary over an eroded surface of late Oligocene clay. In the Quaternary, the Neogene sediments were eroded and washed away by the near-bottom currents. It is likely that the erosion began 0.9-0.7 Ma at the beginning of the "Glacial Pleistocene." The erosion could be initiated by the loosening and resuspension of the surface sediments, resulting from the seismic action generated by strong earthquakes in the subduction zone of Central America. The same vibration maintained residual nodules at the seafloor surface. Thus, for the area studied, a common reason and a common Quaternary interval for the formation of the following features is supposed: the regional stratigraphic gap, the formation of the residual nodule fields, and the position of the ancient nodules over the surface of the Quaternary sediments.