Chris Elders

The origin of basin-scale syn-extensional synclines on the southern margin of the Northern Carnarvon Basin

Fault geometry exerts a dominant structural control on the deformation of hanging wall sequences during extension and contraction. Numerical, kinematic and sandbox modelling studies have demonstrated that characteristic anticline–syncline pairs are produced during the extension of ramp-flat faults. These features are commonly recognized in smaller fault-scale structures, but remain underappreciated in larger, basin-scale settings. The Lewis Trough, situated within the Northern Carnarvon Basin, Western Australia, is a basin-scale, largely unfaulted syncline with an associated anticline along its western flank, rather than the fault-related graben typical of the region. We present kinematic models demonstrating that a SE-dipping, ramp-flat fault geometry can produce relative highs and lows in Jurassic strata as well as honouring the asymmetrical onlap pattern within the Lewis Trough. This study indicates that the Lewis Trough formed during the Early Jurassic, a period typically associated with high rates of extension and not during the Late Triassic Fitzroy Compression Event. This study also highlights the importance of the Locker Shale in partitioning deformation of the Permian and Mesozoic fault systems and as a diffuse zone that variably partitions displacement between stacked Permian and Mesozoic fault systems.

The Mesozoic structural evolution of the Gorgon Platform, North Carnarvon Basin, Australia

ABSTRACT The Gorgon Platform is located on the southeastern edge of the Exmouth Plateau in the North Carnarvon Basin, North West Shelf, Australia. A structural analysis using three-dimensional (3D) seismic data has revealed four major sets of extensional faults, namely, (1) the Exmouth Plateau extensional fault system, (2) the basin bounding fault system (Exmouth Plateau–Gorgon Platform Boundary Fault), (3) an intra-rift fault system in the graben between the Exmouth Plateau and the Gorgon Platform and (4) an intra-rift fault system within the graben between the Exmouth Plateau and the Exmouth Sub-basin. Fault throw-length analyses imply that the initial fault segments, which formed the Exmouth Plateau–Gorgon Platform Boundary Fault (EG Boundary Fault), were subsequently connected vertically and laterally by both soft- and hard-linked structures. These major extensional fault systems were controlled by three different extensional events during the Early and Middle Jurassic, Late Jurassic and Early Cretaceous, and illustrate the strong role of structural inheritance in determining fault orientation and linkage. The Lower and Middle Jurassic and Upper Jurassic to Lower Cretaceous syn-kinematic sequences are separated by unconformities.

Fault linkage and reactivation on the northern margin of the Dampier sub-basin

The north-west margin of the Dampier sub-basin is characterised by a strongly segmented fault pattern. NE trending faults define the edge of the Rankin Platform, and separate it from the Kendrew Trough. However a secondary set of NNE trending faults define smaller scale graben on the edge of the Rankin Platform that preserve Lower and Middle Jurassic sediments. This strongly suggests oblique reactivation of an inherited NE trending basement fabric under WNW oriented extension during Middle Jurassic extension.

Oblique reactivation of inherited fabrics in rift basins: applications to the Northern Carnarvon Basin

Rift basins are typically developed on heterogeneous continental crust. Inherited basement fabrics exert a fundamental control rift basin geometry, and on the geometry of individual faults. Many rift basins are also the result of multiple rift episodes and early formed structures will exert further control on the way in which faults evolve in subsequent rift events. Inherited fabrics and fault reactivation are often invoked to explain rift orientation and segmentation, often with little independent evidence for their existence. However analogue models of orthogonal and oblique rifts show that predictable fault patterns result from the partitioning of stress between pre-existing structures and superimposed extension directions. The Northern Carnarvon Basin provides an ideal laboratory in which to test these models. High resolution 3D seismic data allows detailed imaging of fault patterns developed during separate Lower-Middle Jurassic and Lower Cretaceous rift events. Fault patterns clearly reveal the influence of older structures, most likely related to Carboniferous and Permian rifting, enabling contemporaneous stress patterns to be revealed.

Evolution of detached listric fault systems in the Ceduna Delta, Bight Basin: Insights from 3D seismic data

Deformation of the Cretaceous Ceduna Delta system is dominated by gravitationally driven listric extensional faults. They were initiated as strongly listric faults during deposition of the Cenomanian White Pointer deltaic sequence, coincident with the final stages of rifting and break up between Australia and Antarctica. The faults were progressively reactivated during deposition of the post break up Santonian to Maastrichtian Hammerhead deltaic sequence, propagating upwards as relatively planar sequences associated with narrow zones of downward converging secondary faults. Individual faults segments maintain a characteristic curved geometry in map view which link together to form relatively long continuous NW-SE trending faults which rotate to a NNW orientation in the west of the study area (towards the break of slope at the edge of the delta top). Previously unrecognised N-S trending faults that are confined to the lower part of the sequence control some of the segmentation of the NW-SE trending faults. Understanding the evolution of these fault systems will help to better define the risks associated with Cretaceous plays in this highly prospective frontier petroleum province.