Buried lava flows crossing the Great Divide in north Queensland: discovery using magnetic methods, and implications for hydrology

Abstract

Rapid rises in saline groundwater in part of the Cattle Creek catchment, a sub-catchment of the Mitchell River within the Mareeba Dimbulah Water Supply Scheme irrigation area in north Queensland, led us to search for underground structures that might be influencing groundwater flow. Aeromagnetic survey data suggested the presence of two basaltic lava flows buried in alluvium. Each flow is several kilometres long and one crosses the current Great Divide, the major regional drainage divide. Ground magnetic surveys, geological field investigations and a drilling program confirmed the existence of the flows. The larger flow, in the southern part of the catchment, runs approximately parallel to the Walsh River, is several hundred metres wide and ∼10 m thick, and is buried under ∼7 m of alluvium. The position of the flows shows that the Great Divide in this region has migrated west since the eruption of the basaltic lava at 1.79 Ma. In this region, it is possible that groundwater flow follows the trend of the subsurface strata, crossing the current surface drainage divide. Groundwater recharge in Cattle Creek may therefore be influenced by management of irrigation water in the adjacent catchments.