Abstract

The Broughton Sandstone, a 500-foot thick horizontal sequence of Permian conglomerates, sandstones, and siltstones, interstratified with contemporaneous submarine volcanics (latites) occurs at Kiama, N.S.W. The sequence contains authigenic mineral assemblages characteristic of the laumontite zone of the zeolite facies. Albite, calcite, chlorite, quartz, clay minerals, and laumontite are the common authigenic minerals in the sediments, whereas in addition, prehnite, pumpellyite, epidote, and sphene occur in the latites. The clean-washed sediments are largely composed of volcanic rock fragments and detrital plagioclase with minor amounts of ferromagnesian minerals and quartz. The composition, sedimentary structures, and faunal assemblage indicate that the sediments were derived from a volcanic provenance, and deposited in a shallow-water marine, possibly near-shore environment. Physicochemical parameters of the early diagenetic environment appear to have significantly influenced the nature of the final authigenic minerals. Connate water and the breakdown of detrital constituents, especially calcic plagioclase, provided the chemical elements for the formation of the authigenic minerals. The low-grade 'burial' metamorphism, induced by increasing temperatures in response to increasing load pressures, alone, could not have produced the observed mineral assemblages, since the rocks have been buried to a maximum depth of only about 3000 feet.

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