Distribution of diagenetic alterations in fluvial channel and floodplain deposits in the Triassic Narrabeen Group, southern Sydney basin, Australia

Abstract

In the Narrabeen Group, floodplain shale and siltstone are abundant in the Wombarra Claystone, Stanwell Park Claystone and Bald Hill Claystone whereas the Newport Formation is a fine-grained lagoonal deposit. Mediumand coarse-grained fluvial channel sandstone is common in the Coalcliff, Scarborough and Bulgo. Sandstone composition includes mostly lithic grains with minor quartz, feldspar, mica and heavy minerals; litharenites and sublitharenites are common and quartzarenites are rare. The diagenetic alterations include compaction, quartz overgrowths, authigenic clay minerals, carbonate cement and authigenic feldspar. Quartz overgrowths are present in the channel, floodplain, lagoonal and shallow marine facies but are more common in the channel facies. Also, the channell facies are characterised by pore-filling carbonate cement, grain-coating carbonate cement and carbonate replacement of grains. Carbonate cement, pore-filling clays and grain-coating clays occur in all the channel, floodplain, lagoonal and shallow marine facies. Fluvial channel and floodplain facies contain feldspar overgrowths which disappear in the lagoonal and shallow marine facies. Dissolution of unstable grains is observed in the channel, floodplain and lagoonal facies but is more abundant in the channel facies in the Narrabeen Group. Mechanical and chemical compaction is conspicuous in the channel facies but also occurs in the floodplain and lagoonal facies.