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

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 water recharge from the southwest and bacterial activity in mudrocks. In the southwest part of the basin repeated meteoric water recharge and aerobic bacterial activity created oxygenated and mildly acidic pore waters and led to the formation of kaolin (I). Along the present coastal area and in the far northeast part of the basin a lack of meteoric water recharge combined with continued bacterial activity removed all oxygen and raised the pH value, producing anoxic and neutral to mildly alkaline pore waters and led to the formation of chlorite in litharenites with abundant volcanic rock fragments. All authigenic carbonate was formed by direct precipitation from pore water. The carbonate type was determined by relative cation concentration and Eh condition in the pore water. The amount of Ca-bearing carbonate is related to the detrital mineralogy of the sandstone, with high abundance of volcanic rock fragments favouring formation of large amounts of Ca-bearing carbonates. Later dissolution of authigenic carbonate and labile detrital grains was largely caused by the migration of escaping compactional water containing CO2 which was probably sourced from the maturation of organic matter in mudrocks. Kaolin (II) was formed by precipitation from pore waters and replacement of volcanic lithic, feldspar and mica grains as a result of reaction of these grains with migrating pore waters. Authigenic quartz overgrowth developed by overlap and/or emergence of initial micro-quartz crystal outgrowths. Precipitation of illite resulted from the build-up of K+ in the pore waters.