Beachrock is the consolidated deposit which results from lithification by calcium carbonate of sediments in the intertidal zones of tropical and subtropical coasts. Ages of Japanese beachrocks, mostly due to the radio carbon dating of cemented skeletal fragments of some organisms, range from 6500 B. P. to near the present. Rapid cementation is suggested by artifacts incorporated in them : It was reported that beachrocks observed on Pacific atolls in 1951 contained materials used during military action seven years earlier. We made a paleomagnetic study of beachrocks from Heron Island, located at the south end of the Great Barrier Reef, Australia and from Okinawa, Kume-jima and Miyako-jima, Japan.
We obtained the following results :
(1) The beachrocks treated have NRM intensity ranging from 10^<-7> to 10^<1-6> Am^2/kg, which allows precise measurements of their magnetization by a commercial magnetometer.
(2) NRM directions of the beachrocks are more or less scattered in every site. It is no
ted, however, that they point upward at Heron Island, while downward at Japanese islands corresponding to the geomagnetic field directions is the Southern and Northern Hemisphere. This means that rocks of similar genesis, e. g. silcrete and calcrete, have a capability to record past geomagnetic field reversals.
(3) Closely focused NRM directions were observed at 2 sites of Shimajiri, Kume-jima. These rocks have comparatively high intensities of the order of 10^<-6> Am2/kg possibly due to weathered basalt fragments incorporated in them. Magnetic cleaning gave the closest grouping of remanence directions for Shimajiri (A) at 5mT with Im=56.85ﾟ, k=275, A^<95>=4.62ﾟ, N=5 and for Shimajiri (B) at 1.25mT with Im=37.71ﾟ, Dm=0.87ﾟ, k=517, A^<95>=2.27ﾟ, N=9. Under the a. f. demagnetization up to 50mT, four test samples of Shimajiri (B) held their initially focused remanece direction unchanged with smooth decay of their intensity to about one third of the initial values. The mean remanence direction of Shimajiri (B) well agrees with the present field direction (inclination=37ﾟ, declination=-3ﾟ) , but that of Shimajiri (A) clearly deflects toward a deeper inclination. Although ^<14>C ages are unavailable for these rocks at present, (A) is certainly older than (B) because the latter is located near the present sea level, but the former occupies a higher position.
We conclude that some beachrocks, whose best could be those occurred on volcanic islands, have a power to detect Holocene geomagnetic secular Variation in the oceanic region where no data has been available, and that paleomagnetic studies of silcrete and calcrete enable to extract geomagnetic polarity record in their remanent magnetizations. We consider that beachrocks acquired their net magnetization through reorientations of the constituent magnetic grains which were advanced along the ambient geomagnetic field during the tidal action twice a day.
2ー3南北両半球のビ-チロックがそれぞれ上下方向の残留磁気を示したことから,類似した成因をもつSilcreteやCalcreteのようなChemical sedimentsが,測定可能な残留磁気を有し,かつ,過去の地磁気逆転を記録していることは確実であり,Chemical sedimentsの古地磁気という新しい研究分野が開拓できると考えている。