Detailed Stratigraphic and Structural Analysis of the Early Cretaceous Gippsland Basin Outcrops, West Gippsland, Victoria, Australia

Abstract

The study tried to answer questions regarding the stratigraphy, structure and depositional environment of the Strzelecki Group based on its unique coastal outcrops near Wonthaggi township, west Gippsland. The outcrops were mapped and logged in detail, depositional system was interpreted while inferred major faults were determined from field observations and assisted by Google Earth and magnetic images. Unmanned Aerial Vehicle (UAV) imaging of the fracture dominated platforms resulted in high-resolution orthophoto generation. Due to the lack of obvious stratigraphic markers horizons, vitrinite reflectance and palynology were hired to determine fault displacements. The subsurface geology was mainly based on available access and bore reports from the Wonthaggi coalfield. The outcrops can be described as succession of fluvial channel sandstones deposited within a braided river system and flood-plain mudstones and coal. Inland, north of the Kongwak Fault, the succession is dominated by sand bodies of a high energy river system. To the south, although, sandstone beds are common, mudstones with thin coal beds are prominent and locally make up over half of the stratigraphy. Observations of the sharp contacts are evidences for high fluctuation in energy of environment. The types of plant material indicate a humid climate. Although no sediment section greater than 200 m thick is exposed in any one fault block, a total thickness of at about 1500 m is inferred to be exposed along the coastline. However, because of the amount of faulting and probable repeated sections, the true thickness of upper Strzelecki Group exposed in the coastal outcrops is only about 300 m. Structural analysis of the faults/fractures shows that they occur in three distinct azimuth based sets, pertaining to different phases of regional extension and compression. Four generations of faulting were determined based on fault geometries and crosscutting relationships. These are Early Cretaceous, E-W striking normal faults; Late Cretaceous, NW-SE striking normal faults; Tertiary, NNE-SSW striking reverse faults; and later Tertiary, reactivated NW striking reverse faults. Fault-joint relationships were determined to be syn-developmental as fracturing exhibiting Mode-I geometries related to extensional normal faulting and compressional reverse reactivation. An estimation of the amount of displacements shows the largest faults have displacements of up to 300 m and show both normal and reverse movement.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015