Paleoenvironmental and paleohydrochemical conditions of dolomite formation within a saline wetland in arid northwest Australia. (1st April 2018)

• Record Type: Journal Article
• Authors: Mather, Caroline C.; Skrzypek, Grzegorz; Dogramaci, Shawan; Grierson, Pauline F.
• Is Part Of: Quaternary science reviews. Volume 185(2018)
• Page Start: 172
• Page End: 188
• Publication Date: 2018-04-01
• Subjects: Dolocrete -- Pleistocene -- Paleolimnology -- Northwest Australia -- Pilbara -- Inorganic geochemistry -- Sediment mineralogy -- Stable isotopes -- Paleohydrology; Geology, Stratigraphic -- Quaternary -- Periodicals; Stratigraphie -- Quaternaire -- Périodiques; 551.79
• DOI: 10.1016/j.quascirev.2018.02.007
• Languages: English
• ISSNs: 0277-3791

Abstract

Abstract: Groundwater dolocrete occurring within the Fortescue Marsh, a large inland wetland in the Pilbara region of northwest Australia, has been investigated to provide paleoenvironmental and paleohydrological records and further the understanding of low temperature dolomite formation in terrestrial settings over the Quaternary Period. Two major phases of groundwater dolocrete formation are apparent from the presence of two distinct units of dolocrete, based on differences in depth, δ 18 O values and mineral composition. Group 1 (G1) occurs at depth 20–65 m b.g.l. (below ground level) and contains stoichiometric dolomite with δ 18 O values of −4.02–0.71‰. Group 2 (G2) is shallower (0–23 m b.g.l.), occurring close to the current groundwater level, and contains Ca-rich dolomite ± secondary calcite with a comparatively lower range of δ 18 O values (−7.74 and −6.03‰). Modelled δ 18 O values of paleogroundwater from which older G1 dolomite precipitated indicated highly saline source water, which had similar stable oxygen isotope compositions to relatively old brine groundwater within the Marsh, developed under a different hydroclimatic regime. The higher δ 18 O values suggest highly evaporitic conditions occurred at the Marsh, which may have been a playa lake to saline mud flat environment. In contrast, G2 dolomite precipitated from comparatively fresher water, and modelled δ 18 O values suggested formation from mixing between inflowing fresher groundwater with saline-brine groundwater within the Marsh. The δ 18 O values of the calcite indicates formation from brackish to saline groundwater, which suggests this process may be associated with coeval gypsum dissolution. In contrast to the modern hydrology of the Marsh, which is surface water dependent and driven by a flood and drought regime, past conditions conducive to dolomite precipitation suggest a groundwater dependent system, where shallow groundwaters were influenced by intensive evaporation. Highlights: Two geochemically distinct dolocrete units were identified within the Fortescue Marsh. The deeper unit formed from highly saline and evaporated groundwater. The shallower unit formed by mixing at the interface of fresh and saline groundwater. Both units precipitated within the phreatic zone, most likely as primary dolomite. Isotope geochemistry of carbonates provides an environment and hydrological proxy.