Extensional Subsidence, Inversion And Volumetric Contraction In The Bass Basin Of Australia: A Seismic Study
Das, Pradipta Kumar
Doctor of Philosophy, 2001
University of Adelaide
The Bass Basin, a Mesozoic-Cenozoic interior rift basin, has evolved through two rifting episodes, the Otway rifting and Tasman Sea rifting. Previous workers in the basin have variously interpreted the structural history and tectonic evolution of the basin with some recent views expressing the separate identity of two basins, the Durroon Basin and Bass Basin, corresponding to the southeastern and northwestern area of the previously known Bass Basin. The present study, which is mainly seismic based, was aimed to differentiate the influence of two rifting episodes on the basin and to characterise in detail, the structural style and subsidence history of the Durroon area and Bass area with an aim to ascertain the possible reasons why the two areas behaved differently in response to crustal extension. An additional objective of the study was to follow up the initial identification in 2D seismic data of a completely new type of fault system, a layer-bound polygonal fault system, by mapping the three dimensional characteristics of the fault structure using 3D seismic data and then delineate its possible extent in the basin from 2D data. In addition to the above, an attempt has been made to identify new possible direct hydrocarbon indicators in the basin while discussing the general hydrocarbon potential of the basin.
About 6000 line km of 2D seismic data and biostratigraphic and log information from 29 wells have been utilised to study the different fault patterns and the subsidence history in the basin. The Durroon and Bass areas have been considered separately because of variable density in the data set corresponding to the two areas. The interpretation was carried out using the seismic sequence stratigraphic approach following a modified version of the model proposed by Prosser (1993). While dividing the whole stratigraphy into its pre-rift, syn-rift and post-rift components, eight sequence boundaries were identified and correlated throughout the seismic data set. The extensional subsidence history in the two areas were studied in tandem, tracing the equivalent chronostratigraphic units in both areas. Although the structural style is distinctly different, it is suggested that the Durroon area and Bass area be referred as two separate sub-basins of the Bass Basin instead of current proposal for two separate basins as per the recent literature. It is newly proposed to use Durroon Sub-basin and Burnie Sub-basin respectively for the two areas.
The major erosional unconformity seen on seismic data near the Durroon-1 well within the Early Cenomanian succession has been interpreted from the overall seismic reflection geometry and internal seismic facies characteristics both above and below it, to be the rift-onset unconformity. This unconformity marks the onset of the Tasman Sea rifting phase (dated 96 Ma, A. distocarinatus zone). This major tectonic event may have been synchronous with the initiation of sea-floor spreading in the Southern Ocean.
The structural style in the Durroon Sub-basin is characterised by large, domino-style, tilted basement fault blocks. Fault throws are in the order of 4 - 5 Km and occur along rotational normal faults. Three major fault blocks, the Bark, Anderson and Boobyalla, have undergone extremely large tectonic subsidence related to extensional normal faulting. This is in complete contrast to the structural style in the adjacent Burnie Sub-basin to the northwest. Although two opposing major half-graben bounding-faults have accommodated initial extension with a large fault displacement (2-3 km) in this area, the predominance of sedimentary loading and syn-sedimentary faulting marks the structural style there with a large number of both planar and listric faults. The granitic nature of the basement in the Durroon Sub-basin as opposed to basement composed of metasediments in the Burnie Sub-basin is thought to have played an important role in controlling the way the two areas behaved in response to crustal extension.
The structural style in the Durroon Sub-basin has been shaped through three major extensional phases that are reflected in different fault trends. The initial response to extensional forces was the development of discrete fault segments across the area. The present day fault pattern subsequently developed through progressive propagation and linkages of the initial fault segments. The obliquity of Tasman rifting had a profound influence on the structural development. The offshore extension of the NE-trending Arthur Lineament probably acted as a buttress to limit the tensional stresses, resulting in the Late Cretaceous history in the Burnie and Durroon Sub-basins being very different. However, from the Early Tertiary, the two sub-basins became linked. Interestingly, the equivalent area corresponding to the Arthur lineament has been proposed as a transfer zone dividing the two sub-basins (Pelican-Squid transfer zone of Setiawan, 2000). It is suggested that perhaps, during the Otway rifting history, the area acted as a transfer zone that was also the limiting zone to the effect of tensional stresses during Tasman Sea rifting stage. However, all the earlier studies including this one suffer from the very poor data coverage in the intervening area between the two sub-basins corresponding to the proposed Pelican-Squid transfer zone (Setiawan, 2000; Das and Lemon, 1999) along the Arthur Lineament.
A simple shear-pure shear model of extension has been invoked to explain the major westerly-dipping normal faults seen in the Moore Basin west of the Lord Howe Rise (Stagg et al., 1999) and the ENE-dipping basin-bounding faults in the Durroon Sub-basin. These two areas perhaps formed initially as conjugate pair in the southeastern Australian Continent during the rift valley development prior to opening of Tasman Sea. Otway rifting had already created major weakness in the crust in the area and superimposition of later Tasman Sea rift tectonics resulted in an apparently clean, sharp split in the continental crust seen in the vicinity of the shelf and slope area of the SE Australian margin.
The interpretation that the Durroon Formation (Smith, 1986) is confined to the Durroon area and has been deposited under the influence of the Tasman Rift and a later interpretation that the Durroon Formation is equivalent to the Golden Beach by Hill et al., (1995, Figure 3) suggest that the extensional stresses due to Tasman Sea rifting event did not extend beyond the Durroon area. The depositional style of the Durroon Formation, i.e., sedimentation during active rifting on tilted fault blocks, is certainly analogous to the Golden Beach Group in the Gippsland Basin. On the basis of the present study, however, it is believed that although the extensional stresses of Tasman Sea rift had greater impact on the Durroon area in terms of massive displacement along normal extensional faults, leading to greater amount of fault related extension, the seismic data in the Bass area suggest that the impact of Tasman Sea rift had expressed quite differently. Chronostratigraphically, an equivalent unit to that of Durroon Formation could be correlated in the Bass area also based on seismic reflection characteristics but remains subjective in the absence of actual drilling information in this area. However, it is safe to predict that eventhough the two units might have been deposited during the same time interval (Early Cenomanian-Early Campanian), they would be lithostratigraphically quite different.
Although most of the previous workers in the basin have concluded that the extension amount in the Durroon area is less than the Bass area, the results of the present study do not support that view. A preliminary estimate of the amount of extension by measuring the pre-faulted basement length and the post-faulted basement length estimates extension of 1.36 in the Durroon Sub-basin compared to 1.22 in the Burnie Sub-basin. Although inversion structures have earlier been identified from seismic data in the Burnie Sub-basin, this study has identified for the first time seismic evidence of inversion structures in the Durroon Sub-basin along some seismic lines. However, basin inversion has earlier been inferred from thermochronology data and erosion of strata on crests of tilted fault blocks in the Durroon area (Hill et al., 1995). It is suggested that the inversion inferred from erosion of strata on uplifted footwall blocks, as seen near Durroon-1 well, should be concluded with caution because it can be explained by simple extensional model of hanging wall subsidence and footwall uplift related to thermo-mechanical behaviour of crust and subsequent erosion of the emergent crests of the fault blocks.
A new type of layer-bound fault system was identified on 2D seismic data in most parts of the basin. Details of this system have been drawn from a 3D seismic data set above the Yolla Field. This fault system in map view is composed of almost randomly oriented, high density, minor extensional faults organised in a polygonal network. The component faults are typically 500m-1000m long and have throws ranging from 10m to 40m. The average fault spacing ranges from 60m to 500m. In seismic sections, there are at least three units or stratigraphic tiers observed within the deformed interval showing a range of variation in the reflection characteristics and fault pattern development. The polygonal network of fault system shows a near equal distribution of fault strike orientations suggesting an isotropic stress regime during deformation of each unit within the deformed interval.
The polygonal fault system deforms the very fine-grained Late Tertiary calcareous clay and marl-dominated succession of the basal Torquay Group throughout almost the entire offshore Bass Basin. The seismic expression of this pervasively-deformed unit suggests that there is no displacement transfer to the basement structures and the stratigraphy overlying and underlying this sequence is undisturbed, characterised by continuous reflection patterns.
The development of polygonal fault systems by three-dimensional volumetric contraction of muddy sediments during early burial was first reported in literature from the central North Sea Basin (Cartwright, 1996). Layer-parallel volumetric contraction measured in seismic sections from the North Sea has recently been attributed to a process called syneresis of colloidal smectitic gels during early compaction history of sediments (Dewhurst et al., 1999). Syneresis results from the spontaneous contraction of a sedimentary gel without evaporation of constituent pore fluid. This process is driven by inter-particle attractive forces in marine clays and is governed by the change of gel permeability and viscosity with progressive compaction. A similar process is attributed to the seismically-observed complex polygonal fault system in the Bass Basin.
Detection of direct reflection from flat fluid contacts unconformable with the surrounding rock reflections to indicate gas-filled reservoirs is now well established in the geophysical literature. Two new 'flat spot anomalies', one in the Durroon Sub-basin and the other in the Burnie Sub-basin have been identified. The 'flat spot anomaly' in the Burnie Sub-basin, near Koorkah-1 and Seal-1 corresponds to a possible gas-liquid contact within the reservoir sands of M. diversus zone of the early Tertiary Eastern View Coal Measures. The structure has been studied in good detail and shows an elongated anticline oriented NE-SW, with faulted margins to both the north and south. Traces of gas and insitu hydrocarbons in the sands at the M. diversus level in Seal-1 and traces of free oil in the Paleocene section in Koorkah-1 suggest the good prospectivity of the structure. The other 'flat spot anomaly' identified in the Durroon Sub-basin has not been detailed due to paucity of both seismic and nearby well data.
The Bass Basin holds very good potential for stratigraphic hydrocarbon play development. There are several possibilities in which the interdigitation of syn-rift sandstone-rich fluvio-deltaic reservoirs with source-rich deep water lacustrine facies in the footwall and hangingwall slopes of tilted basement fault blocks might hold good hydrocarbon trapping situation.