Fractures of the Eumeralla Formation, Otway Ranges, Australia: timing and generation of fluid flow

Abstract

Over 261 naturally occurring fractures were recorded from 10 field locations in the Otway Ranges, Victoria, Australia. Fractures were sampled from the upper Jurassic –lower Cretaceous Eumeralla Formation, a volcanogenic sandstone. Eight fracture sets were recorded with defined orientations. Twenty-seven fracture samples from across the Otway Ranges were thin sectioned and analysed using an optical microscope and Scanning Electron Microscope (SEM). Host rock and fracture petrography were determined, including identification of host rock and fracture cement mineral compositions, along with fracture specific textures, such as calcite twinning, crack-seal textures, cataclastic deformation and cross-cutting cements. Siderite cements are observed to be present in all fracture sets and imply the presence of fluid flow during all periods of deformation, from lower Cretaceous extension to NW –SE Miocene compression. The addition of calcite cements in Fracture Sets One, Fracture Set Two, Fracture Set Four, Fracture Set Five and Fracture Set Seven indicate two periods of enhanced calcite and siderite fluid flow predominantly during times of NW - SE compression in the mid-Cretaceous and Miocene.