Like the better known Dales Gorge Member of the Brockman Iron Formation, lower in the Hamersley Group, the Weeli Wolli Formation contains banded iron-formation, in which layers (mesobands) of chert with regular internal laminae (microbands) alternate with mesobands of dark iron-rich material known as chert-matrix. These mesobands of chert-matrix are not internally homogeneous, as is usual in the Dales Gorge Member, but have a regular striping macroscopically defined by slight and grada-tional color variations at intervals of about 2 mm. Some of the chert mesobands also have an internal striping, defined by groups of texturally distinct microbands, at intervals of about 10 mm. In spite of the difference of scale, the striping within chert and chert-matrix mesobands is demonstrably equivalent at oblique mesoband margins. Microband counts within three chert mesobands from drill core yield, for 23 stripes, a mean of 23.3 microbands per stripe, with a range of 21 to 28 and an accuracy of about ±0.3. If previously published genetic hypotheses for Hamersley Group iron-formations are accepted, each microband represents one year and there was thus a strong 23.3-year environmental cyclicity during deposition of the Weeli Wolli Formation, about 2,000 m.y. ago. Although this interval is close to the present double sunspot, or Hale, cycle there is no trace of any subdivision of the Weeli Wolli Formation period and the correspondence may therefore be fortuitous. This type of evidence appears to have great potential strength for determining the secular stability of the solar system. © 1973 Society of Economic Geologists, Inc.
