Regional stratigraphic correlation transects across the Great Artesian, Lake Eyre basins and Upper Darling Floodplain region (part 2) ‒ Infilling data and knowledge gaps

This report presents a stratigraphic review of some key boreholes across the Jurassic-Cretaceous Eromanga, Surat and Carpentaria basins that form the groundwater Great Artesian Basin (GAB), as well as across the overlying Cenozoic Lake Eyre Basin (LEB), completed during the National Groundwater Systems (NGS) Project. The NGS Project is part of Exploring for the Future (EFTF)—an eight year, $225 million Australian Government funded geoscience data and information acquisition program focused on better understanding the potential mineral, energy and groundwater resources across Australia. The study presented here builds on previous work (Norton & Rollet, 2022a) undertaken as part of the ‘Assessing the Status of Groundwater in the Great Artesian Basin’ Project, commissioned by the Australian Government through the National Water Infrastructure Fund – Expansion. Although not intended to be a major re-interpretation of existing data, this stratigraphy review updates stratigraphic picks where necessary to obtain a consistent interpretation across the study area, based on the refined geological and hydrostratigraphical framework developed through this project. Problems and inconsistencies in the input data or current interpretations have been highlighted to suggest where further studies or investigations may be useful. This study includes Phase 2 of the National Groundwater Systems Project, which was undertaken by Catherine Jane Norton in collaboration with Geoscience Australia; and compiled, processed and correlated a variety of borehole log data to review the stratigraphy and improve the understanding of distribution and characteristics of Jurassic and Cretaceous sediments across the Eromanga and Surat basins and overlying LEB. To complement the previous 322 key boreholes compiled in Phase 1 (Norton & Rollet, 2022) additional stratigraphic correlations have been made between geological units of similar age (constrained using palynological data) from 706 key boreholes along 35 regional transects across the GAB and from 406 key boreholes along 20 regional transects across the central LEB. Also included in this study is Phase 3 in-fill work of four additional transects, extending the study further south in New South Wales, to tie in to the Cenozoic of the Murray Basin. This later phase 3 of the project also included a review and quality control of approximately 2,572 central LEB boreholes, and the addition of 278 boreholes in the GAB in southern Queensland and New South Wales. Phase 3 also expanded on the results used for mapping regional sand/shale ratios that began in the previous phase (Evans et al., 2020; Norton & Rollet, 2022a). Normalised Gamma Ray (GR) calculations have now been made for 1,778 LEB boreholes and 676 GAB boreholes spanning the entire sequence from the surface, through the Cenozoic and down to the base Jurassic unconformity. The previous phase, mentioned above, concentrated on either just the LEB or the GAB intervals from Cadna-owie Formation to base Jurassic. An additional 17 transects in the LEB and 27 transects in the GAB were created to visualise the lithological variation. The distribution of generalised sand/shale ratios are used to estimate the thickness of sand and shale in different formations, with implications for formation porosity and the hydraulic properties of aquifers and aquitards. This study fills data gaps identified in the previous study (Norton & Rollet, 2022) and refines the regional distribution of lithological heterogeneity in each hydrogeological unit, contributing to an improved understanding of connectivity within and between aquifers. The datasets compiled and examined in this study are in Appendix A. Attempts were made to standardise lithostratigraphic units, which are currently described using varying nomenclature, to produce a single chronostratigraphic chart across the entirety of the GAB and LEB basins. The main stratigraphic correlation infill in the GAB and LEB regions focused on: • The transition between the Eromanga and Surat basins in New South Wales and the tie-in to existing transects in Queensland and South Australia, • The Eromanga Basin in South Australia and Queensland and the tie-in to Phase 1 transects, • The central Eromanga Basin and Frome Embayment areas, extending the GAB units to the overlying Lake Eyre Basin stratigraphy to better assess potential connectivity between these basins, • The transition between the Lake Eyre and Murray Basins in the Upper Darling Floodplain (UDF) area in New South Wales and the tie-in to Phase 1 transects in New South Wales. This report and associated data package provide a data compilation on 706 and 278 key boreholes in the Surat and Eromanga basins respectively, to assist in updating the geological framework for the GAB and LEB. Recommendations are provided for further studies to continue refining the understanding of the stratigraphy in the Great Artesian and Lake Eyre basins.