Petrology and diagenesis of Narrabeen group sandstones, Sydney Basin, New South Wales∗

Abstract

The petrology of fluvial sandstones from the Narrabeen Group was investigated to document sandstone composition and to assess factors controlling sandstone diagenesis. The sandstones are dominated by detrital quartz and lithic grains. The average composition varies in ascending stratigraphic order from R (rock fragments) > Q (quartz) to Q > R. Major diagenetic events in order of occurrence were: (i) formation of hematite, chlorite and mixed‐layer illite/smectite, and first generation of kaolin (I); (ii) precipitation of calcite, siderite and ankerite; (iii) dissolution of labile detrital grains and carbonate cement, and formation of second generation of kaolin (II); (iv) quartz overgrowth and precipitation of illite; and (v) precipitation of calcite and ankerite. These events were controlled both by detrital mineralogy and pore‐water chemistry. Kaolin (I) and chlorite are mutually exclusive. At shallow burial depth different pore‐water chemistry prevailed in different parts of the basin through meteoric w...