Geohydrology of the Tamala Limestone Formation in thePerth Region: Origin and role of secondary porosity

[摘要] The Tamala Limestone formation is the world’s most extensive eolianite deposit, extending from Cape Range on the central coast of Western Australia to Albany on the south coast. In the Perth region, the Pleistocene to Holocene carbonate eolianites of theTamala Limestone extend up to 10 kilometres (km) inland of the modern coast and up to 40 km or more offshore in their submarine extent. During the past 500,000 years eustatic sea level has fluctuated by more than 100 metres (m) on five occasions, varying from approximately 120 m below present sea level to approximately 10 m above present sea level. This has caused cyclic inundation of Tamala Limestone by the sea and associated fluctuation of the inshore watertable within the formation. Contemporary sea level is at an interglacial high stand and the present-day watertable is estimated to be within 2–6 m of the palaeo-watertable maximum.Although surface exposures of Tamala Limestone are extensively investigated by geologists, the present-day sub-aerial zone has not been subjected to prolonged saturated conditions and does not provide suitable evidence for developing conceptual models for the aquifer pore-system and geohydrology below the modern watertable where the formation has undergone different diagenesis. The Eogenetic Karst model developed for carbonate eolianite aquifers of Bahamas and Bermuda is considered to provide the best conceptual model for pore-system development and geohydrology of the Tamala Limestone Formation in the Perth region.According to the equivalent porous medium model of Vacher and Mylroie (2002) the Tamala Limestone can be classified as an eogenetic karst of early- to mid-development. Eogenetic karst develops through meteoric diagenesis at the location of sedimentdeposition and is characterised by dual porosity that consists of many connected channels within a matrix of interparticle porosity. According to this theory, diffuse solutional attack of eolianites with large primary porosity leads to a diffuse-flow aquifer rather than one dominated by conduit flow. Evidence considered in this study connecting the Tamala Limestone to the eogenetic karst model includes the geohydrological similarities between Tamala Limestone, LucayanLimestone (Bahamas) and carbonate eolianites of Bermuda; plus evidence of a predominantly diffusive flow matrix based on sub-surface visual observations, full-depth coring, geophysical surveys, and analysis of aquifer tidal propagation. Assessing risk associated with Managed Aquifer Recharge (MAR) into Tamala Limestone depends fundamentally on the adopted geohydrological conceptual model. The eogenetic karst model implies a dispersive flow paradigm, except in areas where cavern development and large-scale conduit flow is prevalent. Tamala Limestone has large tovery large transmissivity owing to a well-developed dual-pore system and there is potential for recharge water to move rapidly away from infiltration and injection sites under forced hydraulic gradients induced by those operations.There is sufficient evidence in this study to suggest that conventional single-medium models are inadequate for the purpose of assessing MAR into Tamala Limestone. The dual-medium approach provides a plausible explanation of observed tidal propagation at coastal sites and this approach has proven to be a better alternative for explaining observed mass transport in groundwater at field sites where the aquifer contains smallscalepreferential flow pathways. This approach is not implemented in most popular groundwater simulation software and will require the development specialised groundwater modelling capability or acquisition of specialised services.

[发布日期] 2012-04-02 [发布机构] CSIRO

[学科分类] 地球科学(综合)