The Layered Geology of Australia web map service is a seamless national coverage of Australia’s surface and subsurface geology. Geology concealed under younger cover units are mapped by effectively removing the overlying stratigraphy (Liu et al., 2015). This dataset is a layered product and comprises five chronostratigraphic time slices: Cenozoic, Mesozoic, Paleozoic, Neoproterozoic, and Pre-Neoproterozoic. As an example, the Mesozoic time slice (or layer) shows Mesozoic age geology that would be present if all Cenozoic units were removed. The Pre-Neoproterozoic time slice shows what would be visible if all Neoproterozoic, Paleozoic, Mesozoic, and Cenozoic units were removed. The Cenozoic time slice layer for the national dataset was extracted from Raymond et al., 2012. Surface Geology of Australia, 1:1 000 000 scale, 2012 edition. Geoscience Australia, Canberra.

Groundwater resources have been investigated in Pliocene aquifers in the Lower Darling Valley (LDV) near Menindee, N.S.W. Multiple lines of evidence indicate that modern aquifer recharge is episodic and occurs by lateral leakage from incised channels during high-magnitude floods. Minimal modern recharge occurs away from the incised channels. The LDV Quaternary fluvial sequence consists of scroll-plain tracts of different ages incised into higher, older and more featureless floodplains, which were also originally deposited by lateral-migration fluvial phases. Optically-stimulated luminescence (OSL) and radiocarbon dating has been used to date periods of scroll-plain activity. The youngest, now inactive, scroll-plain phase associated with the modern Darling River, was active in the period 7-2 ka. A previous anabranch scroll-plain phase has Last Glacial Maximum dates around 17-22 ka. Older less distinct scroll-plain tracts are associated with the anabranch system (~30ka) and the Darling River tract (~45-50 ka). The oldest dates of 85 ka and >150 ka come from lateral-migration sediments, that lack visible scroll-plain traces and lie beneath the higher floodplain. This chronologic sequence suggests regular recurrence of ~5 ka lateral-migration episodes separated by ~10 ka periods of quiescence. The active lateral-migration phases, with deeply scoured channels, clean coarse channel and point-bar sands and probably higher stream-flow discharges are tacitly associated with enhanced palaeorecharge episodes. The Pliocene Calivil Formation in the Larloona Palaeochannel is located 4-10 km east of the modern Darling channel and is isolated from modern recharge. However it retains relatively low salinity groundwater, which has radiocarbon ages that range from 11.9 to 16.2 ka (calibrated to numeric ages). These groundwater ages correspond with a main episode of palaeorecharge relating to the lateral-migration phase in the nearby Talyawalka anabranch of the Darling River which was active between 17 and 22 ka. The slightly younger groundwater age reflects minor contributions of more recent waters since the main palaeorecharge event. In this way, palaeorecharge episodes can be linked to the interpreted depositional history that implicitly includes the role of neotectonics.

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An integrated, multi-scale hydrogeophysical, hydrogeochemical and hydrogeological systems approach has been used to map and assess shallow (<100m) aquitards in unconsolidated alluvial sediments beneath the Darling River floodplain. The study integrated data from an airborne electromagnetics (AEM) survey (over an area of 7,500 km2), with targeted ground electrical surveys, borehole lithological and geophysical data (induction, gamma and nuclear magnetic resonance (NMR)), hydrogeological and hydrogeochemical data obtained from a 100 borehole (7.5 km) drilling program. AEM mapping has confirmed the near-ubiquitous presence of a relatively thin (5-10m) lacustrine Blanchetown Clay aquitard overlying the primary Pliocene fluvial aquifer. Mapping has revealed variations in Blanchetown Clay extent and thickness, with a complex sub-surface distribution. Variations in the elevation of the top of the Blanchetown Clay (20-80m AHD) are attributed partly to neotectonics, including warping, discrete fault offsets, and regional tilting. The aquitard properties of the Blanchetown Clay are demonstrated by hydrograph responses in overlying and underlying aquifers, by wetting profiles observed in drillcore, core moisture data, NMR, induction and gamma logging, laboratory permeameter measurements on cores, and hydrogeochemical data. AEM and induction logs indicate, in some areas, a decrease in electrical conductivity at the centre of the clay rich aquitard. Core moisture data and NMR logs of total water collected through the clay aquitard can show reduced water contents in the aquitard centre, although below the water table. These data also indicate that the clay aquitard can be partially saturated both from the top and the bottom. NMR provides good relative measurements of water contents in the aquitards, however, an inter-echo spacing of 2.5 ms in the Javelin tool limited the amount of water detectable in smaller pore spaces, leading to an underestimate of the water content in muds. While further research is required into factors influencing NMR responses, analysis of NMR responses when integrated with AEM signatures, and other hydrogeological and hydrochemical data, has helped identify zones of aquitard leakage and 'seals' for potential managed aquifer recharge (MAR) sites.

One of the primary requirements of managing water resources sustainably is to derive an understanding of the water balance. Groundwater recharge is a key input of any water balance model, but is seldom measured. Single point measurements are often applied to a catchment or region; other times, recharge is assumed to be a percentage of annual rainfall. In the absence of sufficient direct measurements, indirect approaches for estimating recharge in data poor areas using surrogate measures have been developed. These approaches commonly use rainfall data for recharge estimation. This approach may be a valuable tool in some data-poor landscapes. However, in some floodplain landscapes, particularly in losing reaches of river systems, it is not able to take into account understanding of local surface-groundwater interactions associated with river or lake leakage, or flood-related recharge. Using these indirect approaches, estimates of recharge in the Darling Floodplain region are very low (0.1 - 7.3 mm/yr; mean 1.8 mm/yr), assuming that groundwater and surface water are effectively disconnected. In contrast, measurements from a recently established bore network (40 bores over 7,500 km2) recorded a 0.5 - 3.4m response in the shallow aquifer to the 2010/11 floods and a 0.2-4m response in the deeper semi-confined aquifer. This indicates a high degree of connectivity between surface water and groundwater during major flood events (via lateral bank processes and faults). Similarly, hydrochemical estimates of recharge also need to be treated with caution if used in isolation. For example, in the Ord and Keep River Floodplains, very low estimates of recharge (0.1 mm/yr) were obtained using soil chloride concentrations in cracking black soil floodplains. However, analysis of bore hydrographs and rainfall data showed that recharge in these settings varies significantly with changes in rainfall intensity and amount, with deep (2-3m) cracks in the soils enabling bypass flow and much higher recharge rates (>8 mm/yr) in wetter years. In summary, indirect methods for recharge estimation in floodplains can be misleading, and should only be applied after basic elements of the hydrogeological system, hydrostratigraphy, and floodplain hydrodynamics have been established, and climate records considered.

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Part of the Kalimantan 1:250 000 Geological/Geophysical mapping series prepared BMR in cooperation with Pusat Penelitian dan Pengembangan Geologi (Indonesia).

Part of the Kalimantan 1:250 000 Geological/Geophysical mapping series prepared BMR in cooperation with Pusat Penelitian dan Pengembangan Geologi (Indonesia).

No abstract available