EARTHQUAKE ACCELEROGRAMS, OOLONG, NSW 67

i >' IV

v

i ~ IV

"' UJ

~ Ill ;;; z UJ ,._ z

•

30

Observed intenstlies

Calculated intenstlies

100

R ( km)

30 June 1977

'-, 4 July 1977

'- -......_ -......_

-......_ -......_

-......_ . -......_ '- -......_

""'-......_• -......_

-......_-......_ . -......_

-......_

-......_~ e Observed intensities '-..

-- Calculated intenstltes

100

300

'

300

R (km) 155 / 89 -2~A

Figure 7. Observed and calculated intensities-Bowning earthquakes. Plot of intensities showing the relation between cal­culated and observed radii of isoseismals.

Acknowledgement The figures were drawn by S. Royle.

References DENHAM, D., & SMALL, G. R., 1971-Strong-motion data

centre. Bureau of Mineral Resources, Geology & Geophysics, Canberra. Bulletin of the N ew Zealand Societey for Earthquake Engineering, 4, 1.

ESTEVA, R. L., & RosENBLUETH, E., 1964-Expectios de temblores distancias moderadas y grandes. Sociedad Mexicana de lngenieria Seismica, 2, 1-18.

EvERINGHAM, I. B. & SMITH, R. S., 1979-lmplications of fault plane solutions for Australian earthquakes on 4 July 1977, 6 May 1978 and 25 November 1978. BMR Journal of Australian Geology & Geophysics, 4, 297-301.

McEWIN, A. J. , UNDERWOOD, R., & DENHAM, D., 1976-Earthquake Risk in Australia. BMR Journal of Aus­tralian Geology & Geophysics, 1, 15-21.

STANDARDS ASSOCIATION OF AUSTRALIA-AS2121-The SAA Earthquake Code.

BMR Journal of Australian Geology & Geophysics, 5, 1980, 67-68

Manganese nodules from the Tasman Sea off Sydney

N. F. Exon, D. Moreton 1, G. Hicks 1

Two manganese nodules were recently recovered by IlMAS Kimbla from a water depth of 4300 m, in a northeast-trending trough about 250 nautical miles southeast of Sydney (155°35'E. 36 °15'S). They were associated with greenish gray calcareous mud laid down below the lysocline, but above the carbonate compensation depth. The nodules are subspherical and about 10 em in diameter, and have a high clay content, a low Mn:Fe ratio, and low contents of Ni (0.25%), Cu (0.17%) and Co (0.06%).

During a short geological cruise in May 1979 by HMAS Kimbla, to whose captain and crew we are most grateful, two manganese nodules were recovered from the deep sea about 250 nautical miles southeast of Sydney (Ex on, 1979) . Sampling was carried out using two Benthos free-fall grabs at each station ; they sample a bottom area of about 40 x 40 em, and trap material

1. Scho-()) of Earth Sciences, Monash University, Clayton, Vic. 3160.

in a net with 6 mm mesh. Station 5, at which the nodules were recovered, was at 155o35'E, 36°15'S, 40 miles northwest of Gascoyne Seamount, and the esti­mated water depth was 4300 m. The samplers re­covered 1-2 kg of greenish gray calcareous mud, and two large manganese nodules.

The mud is 17 percent sand, 4 percent coarse silt (40-63 microns) and 79 percent fine silt and clay, and contains 28.6 percent CaC03 by weight. The coarser fractions are dominated by mainly whole, but corroded,

68 N. F. EXON & OTHERS

planktonic foraminifera, with some calcareous ben­thonic foraminifera, and siliceous sponge spicules and nets. Fresh angular quartz and feldspar make up about 25 percent of the silt fraction. Other minor components are brown glass, volcanic rock fragments, biotite, man­ganese micronodules, radiolaria and echinoid spines. The fine fraction consists mainly of clay minerals.

The sediments were obtained from a trough trending northeastward, and were deposited below the lysocline but above the carbonate compensation depth. The fresh angular silt-sized quartz and feldspar in the sedi­ments suggest derivation from acid or intermediate volcanics, perhaps by turbidity currents stemming from the Lord Howe Rise or Gascoyne Seamount.

The manganese nodules are subspherical with a rather irregular and rough surface, and are about 10 em in diameter. They consist of numerous concentric shells, many less than 0.1 mm thick, of brown and black metal oxides, and pale yellowish clay, in roughly equal proportions. The shells enclose pumice fragments up to 1 em long, which are very altered to clay and silica. The cores of the nodules consist of a number of smaller nodules (1-2 em cp) formed around pumice fragments. A few radial cracks about a millimetre thick are filled with metal oxides.

The metal contents of the nodules (dried at 105'C) were determined at BMR using the atomic absorption method for Mn, Ni, Cu and Co, and a volumetric method for Fe. The results are compared, in Table 1, with results from crusts recovered at Galathea Station 574 (Ahrens & others, 1967) to the southeast (159'39'E, 39 ' 45'S), with results from 24 nodules in the Australian region between 135 ' E and 170'E (Noakes & Jones, 1976), and with average values from the northeast Pacific (Cronan, 1972).

This is only the second record of seafloor manganese nodules from the central Tasman Sea, the first being at an E ltanin station 500 km to the south. The low

Station / region Weight % (dry basis)

Ni+ Ni+ Fe Mn Ni Cu Co Cu Cu+Co

Kimbla 5B/ 1 16.0 6.7 0.29 0.21 0.07 0.50 0.57 Kimbla 5B / 2 15.6 5.0 0.22 0.14 0.05 0.36 0.41 Eastern Australia 14.7 8.3 0.18 0.10 0.09 0.28 0.37 Galathea 574 8.19 26.2 1.25 0.60 0.19 1.85 2.04 Galathea 574 7.33 27.3 1.30 0.60 0.18 1.90 2.08 Northeast Pacific 9.44 22.33 1.08 0.63 0.19 1.71 1.90

Table 1. Analyses of manganese nodules.

values of Mn, Ni, Cu and Co are comparable to those of nodules from southern waters off southeastern Australia. The Galathea analyses were carried out on encrustations, and are most unlikely to reflect accurately the bulk composition of the material.

References AHRENS, L. H., WILLIS, J. P., & 00STHUIZEN, C. 0., 1967

-Further observations on the composition of manganese nodules, with particular reference to some of the rarer elements. Geochimica et Cosmochimica Acta, 31, 2169-80.

CRONAN, D. S., 1972-Regional geochemistry of ferro­manganese nodules in the world ocean. In D. R. Horn (Editor), Papers from a conference on ferro-manganese deposits on the ocean floor. Office of the International Decade of Ocean Exploration, National Science Foundation, Washington, DC, 19-29.

ExoN, N. F., 1979-Deepsea sediments and manganese nodules from the southern Tasman Sea. Bureau of Mineral Resources Record 1979/62 (unpublished).

NOAKES, L. C., & JoNES, H. A., 1976-Mineral resources off-shore. In C. L. Knight (Editor), Economic Geology of Australia and Papua New Guinea: 1, Metals. Aus­tralasian Institute of Mining and Metallurgy, Melbourne, 1093-1104.

BMR Journal of Australian Geology & Geophysics, 5, 1980, 68-69

New microform publications

The reports whose titles and abstracts appear below have recently been issued as microfiche.

Report 191 Microform MF69

Officer Basin seismic, gravity, magnetic, and radiometric survey, Western Australia, 1972.

by P. L. Harrison & I. Zadoroznyj

ISBN 0 642 04014 1 67 pp., 45 pis., 1978. 4 fiche, $2.00

The Bureau of Mineral Resources made a seismic, gravity, magnetic and radiometric survey in the Western Australian part of the Officer Basin in 1972, on a northeast traverse between the Yilgarn and Musgrave Blocks. The survey comprised a series of depth probes using refraction and single to twelve-fold CDP reflection recording. The other geophysical measurements were taken at 1-km intervals mainly along the· seismic traverse.

A model of the geology and structure between the Yilgarn and Musgrave Blocks was obtained from the integrated

interpretation of the BMR and earlier surveys. The model shows that there are three main rock sequences: Phanerozoic sediments up to 1300 m thick, Proterozoic sed•iments up to 5700 m thick, and an unde-rlying layered sequence in the southwest, of unknown nature, but which probably consists of igneous and metamorphic rocks up to 12 000 m thick and may be distinct from basement of the Yilgarn Block. It is possible that the rocks comprise metasediments intruded by basic rocks.