Structural Evolution of the Thirlmere and Mount Tomah Monoclines: Southern Sydney Basin NSW

Abstract

The Southern Sydney Basin is a geological region of sub-horizontal conforming strata, including significant coal measures that have been mined for over 100 years. This apparently simple ‘layer-cake’ geology has overlooked many complexities associated with intrusions and a variety of geological structures. This over-simplification of geology has contributed to uncertainty in groundwater model outcomes, and impacts to surface hydrology and groundwater systems that have occurred at some sites. The aim of this study was to identify and characterize geological complexities within the Southern Sydney Basin with a particular focus on near surface groundwater and wetlands which could be sensitive to these direct or indirect disturbances. An initial desk top review of existing drill-hole data, outcrop maps and typical spatial data was undertaken to highlight areas of possible structural inconsistency and areas with a high probability of faults or other structures such as monoclines. This data was then used in combination with field based geological mapping and high quality digital terrain modelling to assist the current development of a series of kinematic (geologically restored) cross-sections. This has enabled the preliminary modelling of fault propagation folds associated with the inversion of growth faults, which will be important in the development of a framework to better identify and define the geometry of aquitards, associated with the Thirlmere Lakes and groundwater surface expression dependent ecosystems (swamps) over the Southern Sydney Basin. This greater understanding of the features around the Thirlmere Lakes area will lead to the development of a structural evolution model that further explains the incision of Blue Gum Creek and the development of Thirlmere Lakes within an entrenched meander.