Provenance, diagenesis, tectonic setting, and geochemistry of Hawkesbury Sandstone (Middle Triassic), southern Sydney Basin, Australia

Authors: SAMIR MAHMOUD ZAID, Fahad Al Gahtani

Abstract: The Hawkesbury Sandstone is an important groundwater reservoir in the southern part of the Sydney Basin, Australia. However, its diagenesis and provenance and its impact in reservoir quality are virtually unknown. The present study aims to reconstruct the parent rock assemblages of the Hawkesbury Sandstone, their tectonic provenance, and the physiographic conditions under which these sediments were deposited. Samples from the EAW 18a and EDEN 115 field representing the Middle Triassic Hawkesbury Sandstone were studied using a combination of petrographic, mineralogical, and geochemical techniques. The Hawkesbury Sandstone is yellowish brown in color, siliceous, and partly calcareous; it originated as sands were deposited in fluvial channels. Texturally, Hawkesbury Sandstone is medium- to coarse-grained, mature, and moderately well sorted. Scarcity of feldspars indicates that the rock is extensively recycled from a distant source. Hawkesbury Sandstone has an average framework composition of Q_{92.07}F_{0.31}R_{7.62}, and 95.9% of the quartz grains are monocrystalline. The Hawkesbury Sandstone is mostly quartz arenites with subordinate sublithic arenites, and bulk-rock geochemistry supports the petrographic results. Petrographic and geochemical data of the sandstones indicate that they were derived from craton interior to quartzose recycled sedimentary rocks and deposited in a passive continental margin of a syn-rift basin. The cratonic Lachlan Orogen is the main source of Hawkesbury Sandstone. The chemical index of alteration, plagioclase index of alteration, and chemical index of weathering values (3.41-87.03) of the Hawkesbury Sandstone indicate low-moderate to high weathering, either of the original source or during transport before deposition, and may reflect low-relief and humid climatic conditions in the source area. Diagenetic features include compaction: kaolinite, silica, mixed-layer clays, siderite, illite, and ankerite cementation with minor iron-oxide, dolomite, chlorite, and calcite cements. Silica dissolution, grain replacement, and carbonate dissolution greatly enhance the petrophysical properties of many sandstone samples.

Keywords: Provenance, Hawkesbury Sandstone, Sydney Basin, Australia

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