Publication Name: Eastern Australian Basins Symposium 2001
Authors: J.T. Keetley, K.C. Hill and C. Nguyen
Date Published: November 2001
Number of Pages: 31
Reference Type: Magazine Article
Abstract:
Early Devonian turbidites in the Lachlan Fold Belt were deformed in the Middle Devonian, resulting in superbly exposed fold and thrust structures at Cape Liptrap in Victoria, which have been cited as a possible analoguefor oil-bearing structures in the Papuan Fold Belt. Where sand: shale was 1:2 - 1:4, the Liptrap section experienced 10 - 20% layer parallel shortening with stacked, symmetrical, intra-sandstone fault-bend folds and relatively uniform shortening and thickening of mudstone horizons. This was followed by 30 - 40% shortening by buckle folding of the whole section with position of the short limb controlled by the thrust thickened sandstone, observed on scales of 1- 10m. Where sand: shale was -1:1, the section deformed as 2 - 20m chevron folds which persisted
vertically for 1.5 to 2 times the wavelength of the fold before changing form. In shale-dominated sections, minimal fold and thrust deformation was observed. In the sandstone units of the folds a-c extension joints and shear joints are common. The extension joints, in particular, occur at outcrop and 100+ m scale and some have been reactivated as sinistral and dextral transtensional faults, possibly during Early Cretaceous rifting in Bass Strait with around 10m displacement. The Cape Liptrap structures are similar to those generated in physical centrifuge models, but caution is necessary in applying them to the Papuan Fold Belt, particularly due to the different mechanical stratigraphy and pre-existing extensional faults. The Liptrap structures are good analogues for the internal geometry of the fold and thrust structures that form in the Papuan Fold Belt and may be used to determine the likely geometry of the lateral faults that compartmentalise these structures. Ultimately it is hoped that finite
element modelling of deformation of the well known Liptrap mechanical stratigraphy will give confidence to similar modelling in PNG.