Timing and characteristics of fractures along the Eastern Otway coastline, Great Ocean Road, Victoria

Abstract

Folded reservoir rocks (Eumeralla Formation) within the Otway Basin are of significant exploration interest for their geothermal and tight gas reservoir potential. Brittle deformation within the Eumeralla Formation (ca 113–100 Ma), along the Otway coastline of Victoria, reflects a protracted history of extension, contraction, erosion and uplift, since Australian–Antarctic separation in the Early Cretaceous. This paper investigates the relative timing of heterogeneously distributed fracture populations within both folded and unfolded areas along the Otway coastline. Data collection occurred across two stratigraphic units spanning the Early Cretaceous (Eumeralla Formation) to the late Cenozoic (Demons Bluff Group), in order to measure fracture geometries and determine the relative timing of fracture formation.

An unmanned aerial vehicle has been used to facilitate the systematic and inexpensive acquisition of high-resolution orthophotographs along coastal platforms, in order to complement traditional field mapping of fracture populations. A NE–SW-orientated fracture set is observed exclusively within the Eumeralla Formation (St George fold hinge), while a NW–SE-orientated fracture set pre-dates hinge-parallel fractures. Significant fracture formation has been linked to a period of mid-Cretaceous uplift within the eastern Otway Basin (ca 95 Ma). The Eumeralla Formation and Demons Bluff Group also host a NNW–SSE-orientated fracture set that is inferred to largely post-date mid-Cretaceous folding. Sinistral displacement of fold hinges within the Eumeralla Formation, combined with thrust faulting within Cenozoic sediments, is interpreted to have occurred during an episode of late Miocene to early Pliocene shortening. This paper provides new insights into the history of brittle deformation in the eastern Otway Basin.

Keywords:

• Otway Basin
• brittle deformation
• UAV
• fracture formation
• Eumeralla Formation
• Eastern View Formation
• Demons Bluff Group
• Torquay Group

Acknowledgements

I would like to acknowledge the assistance received from the Geological Society of Australia (GSA) as part of the Endowment Fund that helped to support this research. Additionally, thank you to Terry Smith of GSA (Victoria) for assisting with high-resolution photography of the Soapy Rocks outcrop.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was funded by the Geological Society of Australia through Endowment Fund.