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Fig. 15.16 Bar graphs of siltstone laminae thicknesses for
rhythmically bedded siltstone/mudstone at the top of the
Randfontein Formation, Johannesburg Subgroup, Witwatersrand
Supergroup; ( a ) complete data set; ( b ) data set with inferred
diurnal subordinate laminae removed. Note rhythmic thic-
kening and thinning of laminae best expressed after removal of
subordinates and interpreted as possible neap-spring-neap
cycles
15.3.6 Weeli Wolli Iron Formation, Australia
appearance. So-called microbands (Trendall 1973 ) are
typically 0.05 mm or less thick, and only the cyclic
stripes are readily discernible (Fig. 15.22a ). Locally,
silicification has prevented diagenetic compaction
allowing mineral couplets to be identified (Fig. 15.22b ).
Counts carried out by Williams ( 1989 ) on thin sections
of silicified pods indicate as many as 28-30 couplets
per microband. Cycles containing fewer couplets usu-
ally show evidence of amalgamation of hematite lami-
nae; counts for such cycles probably underestimate the
true cycle period (Williams 2000 ).
The Weeli Wolli Formation in the Hamersley Basin of
Western Australia is one of a number of iron-formation-
dominated stratigraphic intervals of the Hamersley
Group (Fig. 15.21 ). The age of the Weeli Wolli
Formation is well-constrained at ca. 2.5 billion years
(Trendall et al. 1990 ; Pidgeon and Horwitz 1991 ).
Cyclicity in the iron-formation is expressed as regular
variations in thickness of chert-rich and hematite-rich
components, giving the facies a characteristic striped
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