Controls on depositional processes on the Australian Northwest Shelf: the Oligocene to recent carbonate succession analyzed on 2D/3D seismic reflection and borehole data
Aachen (2017) [Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (x, 125 Seiten) : Illustrationen, Diagramme, Karten
The three individual studies presented in this thesis focus on the controls of depositional processes on the Australian Northwest Shelf (NWS), an ocean facing tropical carbonate ramp. Main study areas are the Browse and Northern Carnarvon Basins. The core of each analysis is an interpretation of 2D and/or 3D seismic-reflection data, supported - where available - by borehole data. A total of 47,577 km 2D and 15,400 km² 3D seismic-reflection data in addition to over 50 industrial wells were available for analysis. The resulting seismic- and well-based interpretations are discussed against a background of sedimentological and oceanographic data of the respective study area, allowing comprehensive analyses of the controls on depositional processes and environmental changes over time. In the Browse Basin, numerous carbonate platforms are preserved in the subsurface. New insights are provided by several 2D and 3D seismic-reflection surveys calibrated by well data into the lateral extent and morphology of successive platform and reef sequences, including the internal architecture and, where drilled, their lithology and age. The oldest carbonate build-ups imaged are of Oligocene age (34.03 – 27.8 Ma) and may be interpreted to represent either a reef-rimmed platform or a giant bryozoan reef-mound complex. No further build-ups can be observed between this period and the late Burdigalian, when continuous tropical reef growth in the area began. The various reef-rimmed carbonate platforms consecutively coalesced, forming an extensive barrier reef as the shelf-margin prograded further towards the north-west. In the period between the mid-Langhian to early Tortonian, an elongate (along-margin) barrier reef system formed in the Browse Basin, extending for over 500 km throughout the available data. This system most likely continued to the southwest extending across the Roebuck into the Northern Carnarvon Basin. Following the early Tortonian, reefs can be observed to form as less connected, smaller build-ups on top or landward of the major middle Miocene barrier-reef system in the south of the study area, whereas reef fronts in the north during this time tend to be positioned further seaward. Around the Miocene-Pliocene boundary, most of the observed carbonate build-ups appear to have drowned.The development of relatively stationary, thick aggrading platforms in the northern Browse Basin is interpreted as a reaction to high rates of subsidence, contrasted by the generally thinner reef-rimmed platforms observed in the southern Browse Basin where rates of subsidence are lower. As a consequence, eustatic changes in the south had a larger impact on the local carbonate platforms, forcing more frequent migration and resulting in thinner, more widespread platforms. The tropical reef-rimmed carbonate platforms of the late Burdigalian to early Tortonian nevertheless display, despite these general differences in lateral distribution and architecture, a remarkable resemblance along the approximately 500 km imaged on the available data. The observed termination of reef-growth in the study area occurring around 6 Ma coincides with and may be caused by the onset of current-driven drift sedimentation, possibly a consequence of changing oceanographic conditions through the tectonic closure of the SE Asian Gateway during the collision of northern Australia with the Banda Arc.To further investigate the impact of the local current regime on the depositional environment of the NWS, the 3D geometry of sedimentary features recorded on the modern sea floor and the shallow subsurface of the northern Browse Basin were analysed on an extensive (<12,500 km²) industrial 3D seismic-reflection survey. Large scale seismic-morphological features observed on the modern seafloor include submarine terrace escarpments, fault-scarps, incised channels and restricted areas of seismic distortion interpreted as mass wasting deposits. Extensive fields of sediment waves with heights of up to 10 m occur in water depths below 250 m, generally located at the foot of submerged terraces, along the scarps of modern faults and along the shelf break, while NW-SE-oriented furrows and ridges occupy the more planar regions of the outer shelf. Both types of bedforms require flow velocities between 0.3 and 1.5 m/s to form, which may be induced by oceanic currents, gravity currents or internal waves. For the region under study, the most likely source of bottom currents of this magnitude are internal waves, an interpretation that is reached through a discussion of oceanographic background data and modelling results. Besides the documentation of 3D seismic-geomorphological features on the modern sea floor, similar bedforms could also be mapped in the subsurface down to depths of 500 ms two-way-time, demonstrating the potential for the preservation of such bedforms in the sedimentary record and indicating a comparatively constant local current regime.Continuing the investigation of the influence exerted by the local current regime on the depositional environment, especially in respect to the impact of internal waves, the modern seafloor in the Northern Carnarvon Basin (imaged on 49,717 km² of 3D seismic-reflection data) was also analyzed for large scale sedimentary bedforms. The site was chosen to complement the previous study due to the availability of comprehensive geophysical, sedimentary and oceanographic data. In water depths between 55-130 m, where internal waves, tides and storms may induce currents on the seafloor, two fields of large scale sediment waves were observed. The first is located on a comparatively narrow section of shelf northwest of the Montebello Islands and most likely formed as a result of increased bottom currents induced by tidal flows diverted around the islands. The second is located within a local seafloor depression containing poorly sorted sediments, north of the Serruier and Bessieres Islands. The observed sediment waves could have either formed from the coarser sediment fraction that was reworked by cyclone-induced bottom currents into sand waves, or alternatively from the finer sediment fraction that was shaped by less energetic along-slope oriented currents into mudwaves within the topographic depression. The local carbonate sediments are partially composed of ooids and peloids, grains that formed in shallow water during the initial stages of the post glacial sea-level rise. Today these stranded grains are no longer in equilibrium with the depositional environment and the bedforms in which they are incorporated. The observation indicates that for fossil examples from comparable high-energy ramp systems, the possibility that grains and bedforms may be out of equilibrium should always be taken in account when interpreting the palaeo-depositional environment.