Geomorphic and hydrological controls on groundwater dolocrete formation in the semi‐arid Hamersley Basin, northwest Australia

Abstract

Groundwater dolocretes may exert an important geomorphic control on landscape evolution within sub‐humid to arid regions. However, the geomorphic and hydrogeological settings of dolocrete remain poorly described. The hydrochemical conditions of dolomite precipitation in groundwater environments are also not well known. Classic models of dolocrete formation explain dolomite precipitation from highly evolved groundwaters at the terminus of major drainage but do not explain dolocrete distributed in regionally elevated landscapes, upgradient of major drainage. This study investigated the mineralogy, micromorphology and stable carbon and oxygen isotope compositions of three dolocrete profiles within a regionally elevated sub‐basin of the Hamersley Ranges in the Pilbara region of northwest Australia. We sought to establish the environmental and hydrochemical conditions and present a model for dolocrete formation. We found that dolocrete formed within zones of emerging groundwater under saline‐evaporitic conditions within internally draining sub‐basins, most likely during the Late Miocene and Pliocene. Saline‐evaporitic conditions were indicated by: (i) the mineralogy, dominated by dolomite, palygorskite and smectite; (ii) desiccation features and the presence of phreatic and vadose cements, indicative of a shallow fluctuating water table, and; (iii) dolomite δ18O values (median = –5.88 ‰). Dolomite precipitation was promoted by evaporation and carbon dioxide degassing from shallow magnesium (Mg)‐rich groundwater. These factors appear to have been the major drivers of dolocrete development without a requirement for significant down‐dip hydrochemical modification. Primary dolomite precipitation was possible due to the presence of microbial extracellular polymeric substances (EPS). EPS provided negatively charged nucleation sites, which bound Mg2+, overcoming kinetic effects. High microbial activity within groundwater systems suggest these processes may be important for dolocrete formation worldwide and that groundwater dolocretes may be more pervasive in landscapes than currently recognized. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

Modern dolomite formation caused by seasonal cycling of oxygenic phototrophs and anoxygenic phototrophs in a hypersaline sabkha

“…In both cases, dolocrete formation is suggested to be stimulated by more saline evaporitic paleoenvironmental conditions, possibly playa lakes or saline mudflats, as evidenced by δ 18 O values. Interestingly, the most recent study observed extensive fossilized EPS and organic filaments in more shallow sections of the same cores 74 indicating a possible presence of microbial mats at the dolomite forming locations, even though no information about microbial composition exists. The paleoenvironment in which this occurred looks remarkably resembles the Khor Al-Adaid sabkha.…”

“…Another very recent study on the same post-glacial dolostone cap rocks discovered enrichment of newly identified biomarker 25,28-bisnorgammacerane (25,28-BNG) which can be formed as a result of a transition of microbial mat dominated by cyanobacteria to one dominated by heterotrophs 72 . Further paleo-evidence for the proposed mechanism facilitating low-t dolomite formation comes from studies that examined dolocretes were formed within the Hammersley Basin during the quaternary age (37 to > 45 ka) and Late Miocene and Pliocene (5.3–3.5 ma) 73 , 74 . In both cases, dolocrete formation is suggested to be stimulated by more saline evaporitic paleoenvironmental conditions, possibly playa lakes or saline mudflats, as evidenced by δ 18 O values.…”

Unravelling calcrete environmental controls in volcanic islands, Gran Canaria Island, Spain

“…The presence of these dolomite-rich horizons near the coast, and the isotopic values of the dolomite, heavier than those of calcite suggest a source of water isotopically heavier than evolved meteoric water, such as seawater, similar to the described in the middle Devonian of Alberta (Williams and Krause, 1998), which also could supply the magnesium. The setting of formation of these dolomite-rich calcretes is different of those commonly described as a result of the evolution of groundwater to reach evaporitic conditions in terminal basin settings (Colson and Cojan, 1996;Jutras et al, 2011;Jutras, 2017) or as a result of the emerging groundwater also under evaporitic conditions (Mather et al, 2019). The scarce spheroidal dolomite found mostly cementing voids of the volcanic fragments is probably the first dolomite phase formed, as in Mirador del Rio profile in Lanzarote, where the initial weathering of the basalts could produce locally enriched Mg-solutions favoring dolomite formation (Capo et al, 2000;Whipkey et al, 2002;Whipkey and Hayob, 2008) as a cement (Alonso-Zarza et al, 2016).…”

Controls on Interactions Between Surface Water, Groundwater, and Riverine Vegetation Along Intermittent Rivers and Ephemeral Streams in Arid Regions

“…The quasi-hypothetical model used in this study was inspired by lowland IRES encountered in arid northwest Australia (Cook & Dogramaci, 2019;Dogramaci et al, 2012Dogramaci et al, , 2015Mather et al, 2019;Tugwell-Wootton et al, 2020) and was conceptualized as a symmetrical V-shaped river-aquifer cross-section with an IRES at its center, flanked by a floodplain vegetated with riverine vegetation and underlain by a relatively conductive aquifer ( Figure 1). Owing to the symmetry of the cross-section, half-space could be simulated, and the computational burden significantly reduced.…”