Browse Basin Research Articles

Emplacement and landscape controls on ancient submarine volcanism: 3D seismic geomorphological analysis of Cretaceous volcanic activity in the Browse Basin, Australian North West Shelf

Ancient submarine volcanic systems preserved within sedimentary basins provide opportunities to investigate the time and spatial distributions of intrusive and extrusive rocks, though existing work has largely focused on the geometrical features of submarine volcanic products, with limited interrogation of their interactions with sedimentary processes. This study investigates a buried submarine volcanic system within the Browse Basin, Australian North West Shelf. Through the integration of three-dimensional seismic reflection survey with borehole data, our study reveals intricate geometric features of a submarine volcano and lava channels, and their relationship to an associated intrusive network of Early Cretaceous age that has been preserved beneath ∼3.5 km of sedimentary overburden. In planform, the volcano has a diameter >4.5 km with a preserved height reaching 650 m. Meandering lava flow channels extend ∼20 km southward of the volcanic edifice, forming lava lobes with pressure ridges at the flow termini. Abundant sheet intrusions, identified within underlying Jurassic and older strata, indicate a magma transport from the north. The localised accumulation of sills led to variations in paleo-seafloor topography, influencing the direction of lava flows and post-volcanic sedimentation patterns. Our findings have a broad range of implications encompassing submarine volcanism and their impacts on basin dynamics.

2023 Development and production review

Geopolitical uncertainty remained high in 2023 with Israeli actions in the Gaza strip, the continuing war in Ukraine and tension between China and several neighbouring countries. However, global inflationary pressures, surprisingly, eased during the year, and the prospect of a global recession faded from where it had been in 2022. In Australia continued activism around climate change issues, has impacted project developments and sentiment in the industry remained subdued. This paper looks back at the oil and gas production and development activities in Australia in 2023 providing historical context and trying to understand future implications for the industry. Liquefied natural gas (LNG) production in Australia remained close to record high levels although, not without problems. Both the Scarborough/Pluto 2 LNG project and the Barossa project continued to have legal and approval hurdles resulting in suspension of some execution activities. Elsewhere other projects continued with their plans, including the Kipper field in the Gippsland Basin, the Crux project in the Browse Basin and the Walyering project in the Perth Basin which was brought online in September 2023. The domestic markets also continued to have unique issues with major concerns about gas supply in the East, stalled developments and failing supply in the Northern Territory, and a renewed ban on any exports (including to other states) of gas from Western Australia.

Structural framework of the Caswell Sub-basin, North West Shelf, Australia

The Caswell Sub-basin, situated within the Browse Basin in the North West Shelf constitutes one of Australia’s primary hydrocarbon producing regions, with notable gas-condensate producing fields including Ichthys and Prelude. Jurassic syn-rift sandstones are extensively distributed across the basin and serve as one of the major reservoirs. However, reservoir sequences are typically intensely faulted and exhibit heterogeneity in thickness and lithofacies, with some areas experiencing localised erosion on uplifted fault blocks (e.g. Northern Caswell Sub-basin). Hence, understanding the nature of the 3D fault patterns and their growth history is crucial for evaluating the reservoir characteristics for field development and additional exploration activities. This study, therefore, aims to evaluate the structural framework and the tectonic evolution of the Caswell Sub-basin. Detailed structural interpretation of Paleozoic and younger sequences was conducted using multiple 3D seismic datasets extending over Ichthys, Prelude, Lasseter, and Crown fields. Our study focuses mainly on Mesozoic faulting patterns and kinematics evaluated from interpreted structural maps, thickness changes of each stratigraphic interval and fault throw profiles of major bounding faults. The extensional phase during the Early–Middle Jurassic triggered the development of NE–SW trending faults and the deepening of the Brewster Graben. While, from the Late Jurassic to the Early Cretaceous, the development of E–W trending faults in the north of the Crown and Lasseter fields indicate a shift in the regional stress regime. We highlight the importance of evaluating the structural linkage from basement to cover sequences to achieve a comprehensive understanding of reservoirs and associated petroleum systems.

Predicting missing sonic logs with seismic constraint

Compressional and shear sonic transit time logs (DT and DTS, respectively) provide important petrophysical and geomechanical information for subsurface characterization. However, they are often not acquired in all wells because of cost limitations or borehole problems. We develop a method to estimate the DT and DTS simultaneously from other commonly acquired well logs such as gamma ray, density, and neutron porosity. Our method consists of two consecutive models to predict the sonic logs and the seismic traces at the well locations. The model predicting the seismic traces adds a spatial constraint to the model predicting the sonic logs. Our method also quantifies the uncertainties of the prediction, which come from the uncertainties of neural network parameters and input data. We train the network on four wells from the Poseidon data set located on the Australian shelf in the Browse Basin. We test the network on the other two wells from the Browse Basin. The test results indicate better predictions of sonic logs when we add the seismic constraint.

Petrophysical characteristics of igneous rocks in the Outboard Browse Basin, North West Shelf of Australia: implications for predicting igneous sequences prior to exploration drilling

Abstract The Browse Basin is one of Australia's major hydrocarbon provinces, where significant discoveries have been made in recent decades including the Ichthys and Prelude fields, which accounted for ∼15% of the cumulative Australian liquified natural gas (LNG) production in 2019–20. This rift basin hosts extensive Mesozoic intrusive and extrusive igneous rocks, having been identified from both well and seismic data, and which are recognized as one of the key challenges for exploration and production activities in this region. Their impact on petroleum exploration is demonstrated by the number of wells which encountered unpredicted or thicker than expected igneous rock units both within and adjacent to target sections. This study therefore aims to document the reasons of such unexpectedness, and to develop capability to predict the occurrence of igneous rock units prior to drilling in the Browse Basin and other rift settings that contain igneous rocks. Multiple case studies of uncommercial exploration wells are developed by integrating petrophysical and seismic reflection data, focusing in particular along the outboard part of the basin where igneous rocks are most prevalent. Our study highlights the importance of understanding petrophysical, spatial and chemical heterogeneities of igneous rocks in basins to explain their emplacement and distribution, and thereby predict their occurrence prior to exploration and development activities.

Machine-Learning Techniques Classify, Quantify Cuttings Lithology

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 22867, “Automatic Lithology Classification of Cuttings With Deep Learning,” by Takashi Nanjo, Akira Ebitani, and Kazuaki Ishikawa, Japan Organization for Metals and Energy Security, et al. The paper has not been peer reviewed. Copyright 2023 International Petroleum Technology Conference. Reproduced by permission. _ Wellsite geologists spend approximately 70% of their time on cuttings descriptions. In addition, two or three wellsite geologists generally are assigned to a drilling campaign, to be replaced at the end of a shift. Machine-learning (ML) and artificial-intelligence (AI) techniques have the potential to solve these issues because of their advantages in prediction speed, objectivity, and consistency. The authors’ aim is to automate the task of cuttings descriptions with these techniques. A trained model for cuttings description has the potential to realize quantitative, high-speed cuttings description. Methods and Materials Six wells were selected for this study (Poseidon-1, Ichthys2A ST-1, Ichthys2A ST-2, Dinichthys north1, Ichthys north1, and Ichthys north1 ST1). The wells were drilled in the Browse Basin through various lithologies. The authors focused on four lithologies (carbonate, sandstone, mudstone, and volcanic) and collected the cuttings from these lithologies. The total number of cuttings was 160 (21 carbonate, 58 sandstone, 52 mudstone, and 29 volcanic). The cuttings were used in their original dry condition. Each cutting was transported to a Petri plate in a random and sparse layout using tweezers. A stereomicroscope and a digital camera were prepared. Magnification was 6.3X, and picture size was 1,600 pixels in width and 1,200 pixels in height. Pictures were taken randomly, with approximately 20 photographs for each sample. The annotation was conducted by geologists using annotation software. Five label classes were used (sandstone, mudstone, carbonate, volcanic, and background). The total number of annotation data was 1,978. The annotated data were split as training label data, validation label data, and test label data. The pictures were also split as training data, validation data, and test data. The picture and label data were paired. In this study, 1,496 pictures and label data were used as training data sets, 320 pictures and label data were used as validation data, and 162 pictures and label data were used as test data. Four architectures of semantic segmentation were created (PSPNet, Unet, FPN, and Linknet), including two different networks (ResNet152 and EfficientNetB7), and training models were created. The total number of trained models was eight. As a result, the combination of PSPNet and EfficientNetB7 showed the best performance. The combination of PSPNet and EfficientNetB7 was chosen as the architecture of this project. Hyperparameters were chosen in the best practice case. Mean intersection over union (IOU) was used as the model evaluation index.

Application of a Pre-Trained CNN Model for Fault Interpretation in the Structurally Complex Browse Basin, Australia

Fault detection is an important step in subsurface interpretation and reservoir characterization from 3D seismic images. Due to the numerous and complex fault structures in seismic images, manual seismic interpretation is time-consuming and requires intensive work. We applied a pre-trained CNN model to predict faults from the 3D seismic volume of the Poseidon field in the Browse Basin, Australia. This field is highly structured with complex normal faulting throughout the targeted Plover Formations. Our motivation for this work is to compare machine-learning-based fault prediction to user-interpreted fault identification supported by seismic variance attributes. We found reasonably satisfactory results using CNN with an improved fault probability volume that outperforms variance technology. Therefore, we propose that this workflow could reduce time and be able to predict faults quite accurately in most structurally complex areas.

Estimation of Rock Mechanical Parameters Using Well Log Data in the Well Poseidon 1, Lower Cretaceous, Browse Basin, Northwest Shelf, Australia

This work implements a well-logging approach to evaluate the rock Elastic Parameters, basic Rock Mechanics, and in-situ Stresses in an interval of the Well Poseidon 1 located in the offshore Browse Basin in the Timur Sea of Australia. For this aim, there were used several correlations for the Young’s Modulus, Uniaxial Compressive Strength; one formula for Poisson’s Ratio and one for Angle of Internal Friction, and the Poroelastic Model with tectonic strains for the horizontal stresses with calibrations from logs. All these estimations were compared to a homologous section with triaxial test laboratory data and with previous studies. The results reached indicates that the chosen correlations can replicate under the adequate considerations the values of the rock mechanics and they give also good estimations for in-situ stresses in the interval studied. Moreover, these can be used to model the Young’s Modulus, Uniaxial Compressive Strength, Internal Friction Angle with fair precision. This methodology can be used in order to create geomechanical models by estimating the basic parameters needed, these models can be applied to Improved Oil Recovery, Enhanced Oil Recovery, Carbon Capture and Storage, and Carbon Capture Utilization and Storage projects.

Petrophysical properties of igneous rocks in continental rift basins – a case study from the Mesozoic sequence of the Browse Basin, North West Shelf of Australia

Capturing the ubiquitous distribution and properties of igneous rocks is necessary to succeed in hydrocarbon exploration, field development and underground gas storage when dealing with magma-rich rift margins. In recent years, numerous researches have revealed detailed morphologies of intrusive and extrusive rock units embedded within sedimentary basins owing to advances in 3D seismic data. Outcrop studies have also provided deeper understandings of their occurrence and geometries. However, there has been comparatively little focus on the rock physics of igneous bodies and their relationships to seismic expression. To bridge this gap between the outcrop and the seismic information, we undertook a well-log-based petrophysical study of igneous rocks using subsurface dataset from the Browse Basin located in the Australian North West Shelf. In this contribution, we describe a classification of volcanic facies (e.g. lava flows, volcaniclastics, intrusive sills) based on well log motifs and textures apparent in borehole image data. Statistics of the petrophysical properties of each volcanic facies are also analysed in order to examine their correlation to seismic patterns. Our study implies the importance of detailed characterisation of igneous petrophysical properties of igneous rock units where concentrated within a sedimentary basin, which will help reconcile interpretations of seismic data.

Development and production review 2022

The Petroleum Exploration Society of Australia (PESA) Year in Review Development and Production review looks back at the oil and gas production and development activities in Australia in 2022, with a look at both the historical context of these activities, and the future implications. With the invasion of Ukraine, increasing inflationary pressures, supply chain problems, the prospect of a global recession looming, the impact of COVID-19 still lingering, and increased activism around climate change issues, sentiment was subdued for most of the year, yet there were still several highlights and events. Liquefied natural gas (LNG) production again hit record high levels although, as expected, Australia lost its position as the top LNG exporting country in the world to the USA. The development of the Scarborough/Pluto 2 LNG project continued, as did the Barossa project, although the latter hit legal and approval hurdles resulting in suspension of drilling activities (which are not expected to impact the overall schedule). Final investment decisions were taken on the further development of the Kipper field in the Gippsland Basin, the Crux project in the Browse Basin and the Walyering project in the Perth Basin. The domestic markets continue to have unique issues with major concerns about gas supply in the East co-existing with stalled developments and falling supply in the Northern Territory in stark contrast to the West, where there is significant development and take-over activity.

Sanding evaluation and real-time sand management decisions in high-rate gas wells, Ichthys Field, Browse Basin Australia

This paper demonstrates the workflow of an integrated petrophysical, geomechanical sanding assessment and cement bond log evaluation to define a simple cut-off for reliable sand production assessments and well completion decisions. This workflow provides valuable insights for timely and fit-for-purpose sand management strategies particularly at early field life where the commonly used sanding prediction methodologies cannot be calibrated with field data because of the lack of sanding observations. The successful application of this methodology is shown in the Ichthys gas field, Browse Basin, with high rate and long high-angle producers with subsea wellheads and a long pipeline to an onshore plant. Advanced geomechanical analysis combing analytical and numerical sanding methodologies are used to define a single property cut-off for well-life sand-free production. The availability of such a simple cut-off has assisted the well construction engineers to follow a quick, cost-effective, simplified but thorough approach for sand production assessment and completion selection as opposed to being empirical, or too time consuming. The field first came on stream in July 2018. The workflow has been used in development wells and its validity and sand control decisions are confirmed, as to date all wells are producing sand-free. A similar workflow is being developed for the secondary target at some 400–500 m deeper for the subsequent phases of field development.

Stepping off the edge: the geological framework of deepwater northern Australia

The northern part of deepwater Western Australia and the adjacent Indonesian territory is a vast, underexplored region which includes an area of Triassic geology atypical of the region. Seismic data highlights a substantial elongate Triassic depocentre mappable from the northern margin of the Browse Basin northwards to the Timor Trough. This is informally named the Outer Ashmore Trough. The Outer Ashmore Trough is bounded by an inboard hinge-zone. The axis of the trough extending beneath the Ashmore Reef-1 and North Hibernia-1 wells. The western flank (only partially mappable) is a steeply dipping hinge-zone bounded by an outer high. The trough is mappable northwards, until it is incorporated into the imbricate wedge of the Timor Trough. The trough is floored by thickened Early Triassic shales of the Mount Goodwyn Formation (coeval to the Bedout petroleum system in the vicinity of the Dorado Field). This shale-prone marine interval is succeeded by a thick shallow marine clastic interval incorporated into seaward-dipping gravity-driven faults with an interpreted decollement in the Early Triassic shales. The Norian was dominated by a broad carbonate platform not observed elsewhere on the north-western Australian margin. This interval shares similarities with the outcropping Triassic in Timor Leste. A likely interpretation is that the Outer Ashmore Trough succeeds hyperextended Late Permian rifting. The trough and its adjacent margins offer prospectivity of a potentially significant scale.

Implementation of the Poisson Impedance Inversion to Improve Hydrocarbon Reservoir Characterisation in the Poseidon Field, Browse Basin, Australia

The prediction process of lithology and fluid content is the most important part of reservoir characterization. One of the methods used in this process is simultaneous seismic inversion. In the Poseidon field, Browse Basin, Australia, the parameters generated through simultaneous seismic inversion cannot accurately characterize the reservoir because of the overlapping impedance values between hydrocarbon sand, water sand, and shale, which cause a high level of ambiguity in the interpretation. The Poisson impedance (PI) inversion provides a solution to this problem by rotating the impedance a few degrees through coefficient c. Coefficient c is obtained through the target correlation coefficient analysis by finding the optimum correlation coefficient between the PI and the target log, namely, gamma rays, effective porosity, and resistivity. The results show that the PI gives better outcomes in separating hydrocarbon-saturated reservoir zones. Based on the results of the lithology impedance–gamma rays, the ϕI-effective porosity cross-plot, and the fluid impedance-water saturation (Sw) cross-plot, with optimum correlations of 0.74, 0.91, and 0.82, respectively. The lithology of hydrocarbon-saturated porous sand is at values of LI ≤ 2800 (m/s)(g*cc), ϕI ≤ 5500 (m/s)(g*cc) and FI ≤ 4000 (m/s)(g*cc). The presence of low values for LI, ϕI and FI correlates accurately with the presence of hydrocarbons in the well. Each value of c is then applied to the seismic data. The results show that this method can determine the distribution of gas-saturated porous sand on the seismic inversion section in the northeast–southwest direction.

Adaptive boosting of random forest algorithm for automatic petrophysical interpretation of well logs

The power of Machine Learning is demonstrated for automatic interpretation of well logs and determining reservoir properties for volume of shale, porosity, and water saturation respectively for tight clastic sequences. Random Forest algorithms are reputed for their efficiency as they belong to a class of algorithms called ensemble methods, which are traditionally seen as weak learners, but can be transformed into strong performers and they promise to deliver highly accurate results. The study area is located offshore Australia in the Poseidon and Crown fields situated in the Browse Basin, which are gas fields in tight complex clastic reservoirs. There are 5 wells used in this study with one well manually interpreted which is subsequently used in developing a machine learning model which predicts the output for the other 4 wells. The basic open hole logs namely Natural gamma ray, Resistivity, Neutron Porosity, Bulk Density, P-wave and S-wave sonic travel-time, are used in interpretation. One of the wells has a missing S-wave travel-time log which was also predicted by developing a Random Forest Machine Learning model. The results indicate a very robust improvement in performance when Random Forest algorithm was combined with Adaptive Boosting when interpreting the well logs. The training accuracy using Random Forest alone was 98.21%, but testing was 77.62% which suggested over-fitting by the Random Forest model. The Adaptive Boosting of the Random Forest algorithm resulted in the overall training accuracy of 99.40% and an overall testing accuracy of 97.03%, indicating a drastic improvement in performance. S-wave travel-time log was predicted by preparing a training set consisting of Natural gamma ray, Resistivity, Neutron Porosity, Bulk Density, and P-wave travel-time logs for the 4 wells using Random Forest which gave a training accuracy of 99.79% and a testing accuracy of 98.54%. Machine learning algorithms can be successfully applied for interpreting well log data in complex sedimentary environment and their performance can be drastically improved using Adaptive Boosting.

Concurrent 14. Presentation for: New imaging results and improved interpretation of Late Triassic depositional features within the Browse Basin, North West Shelf of Australia

Presented on Wednesday 18 May: Session 14 The Maxima 3D is located approximately 300 km from the north-west coast of Australia, within the Brecknock–Scott Reef anticlinal trend, at the margin of the Scott Plateau and Caswell Sub-basin. The survey was acquired with eight overlapping swaths in and around the Scott Reef, which is a large carbonate build-up on the present-day seafloor. The area within and around the reef trend represents a significant challenge for imaging, involving complex survey acquisition, very shallow to deep water depths and strong velocity distortion associated with the present-day carbonate build-up. In light of this, CGG undertook a reprocessing test of a swath of the Maxima 3D, to apply the latest processing techniques and demonstrate the potential imaging uplift in a very challenging area. The enhanced data swath has led to a step change in interpretation capabilities for the data, resulting in the identification of what is inferred to be a Late Triassic, isolated reef build-up, on a structural high that developed contemporaneous with early basin formation. These Late Triassic reefal build-ups have been identified by CGG within a number of large complexes, through a comprehensive review of the Triassic prospectivity potential, and represent an emerging play type for the North West Shelf. To access the presentation click the link on the right. To read the full paper click here

New imaging results and improved interpretation of Late Triassic depositional features within the Browse Basin, North West Shelf of Australia

The Maxima 3D is located approximately 300 km from the north-west coast of Australia, within the Brecknock–Scott Reef anticlinal trend, at the margin of the Scott Plateau and Caswell Sub-basin. The survey was acquired with eight overlapping swaths in and around the Scott Reef, which is a large carbonate build-up on the present-day seafloor. The area within and around the reef trend represents a significant challenge for imaging, involving complex survey acquisition, very shallow to deep water depths and strong velocity distortion associated with the present-day carbonate build-up. In light of this, CGG undertook a reprocessing test of a swath of the Maxima 3D, to apply the latest processing techniques and demonstrate the potential imaging uplift in a very challenging area. The enhanced data swath has led to a step change in interpretation capabilities for the data, resulting in the identification of what is inferred to be a Late Triassic, isolated reef build-up, on a structural high that developed contemporaneous with early basin formation. These Late Triassic reefal build-ups have been identified by CGG within a number of large complexes, through a comprehensive review of the Triassic prospectivity potential, and represent an emerging play type for the North West Shelf.

3-D seismic chronostratigraphy of reefs and drifts in the Browse Basin, NW Australia

Abstract In its subsurface, the Browse Basin on the Australian Northwest Shelf accommodates one of the largest Miocene–Pliocene carbonate platform provinces on Earth. This study presents a comprehensive three-dimensional (3-D) documentation and investigation of the Neogene Browse Basin carbonates based on the analysis of 15 3-D seismic reflection volumes covering ca. 33,000 km2 and 22 industry boreholes. Eight basin-wide seismic marker horizons tied to Sr isotope and biostratigraphic data provide a robust 3-D chronostratigraphic framework that (1) newly documents the occurrence of Miocene to recent carbonate systems in vast inboard basin areas; (2) presents the 3-D distribution and dynamic development of carbonate platforms through time constrained by absolute ages; and (3) supports the detailed 3-D interpretation of Miocene to recent depositional processes and key stratigraphic controls. Around 18.6 Ma, the extent of all Browse Basin carbonate platforms was ca. 8600 km2, rising to >10,000 km2 between 15.6 Ma and 12 Ma. By ca. 9.7 Ma, the total platform extent had decreased to ca. 5700 km2, waning to <2800 km2 in the Pliocene. The observed reef demise is less abrupt than previously thought and comprises two steps, with the first in the late Miocene on the outer shelf and the second in the Pliocene on the middle and inner shelf. The Miocene outer-shelf platform demise coincided with (1) strong subsidence that outpaced sedimentation, (2) an increase in NW–SE-oriented tidal current activity, (3) the development of NNE–SSW-oriented bottom currents, and (4) the onset of drift sedimentation. The Pliocene demise of middle-shelf reefs coincided with (1) an increase of clastic sediment input from land and (2) stalling of the Indonesian Throughflow around 3.7 Ma. The basin-wide, 3-D seismic chronostratigraphic analysis presented supports a re-evaluation of the key controls for Miocene and Pliocene reef growth and decay on the NW Shelf and highlights the complexity of interacting global, regional, and local processes and peculiarities in carbonate platform development.

Combined Effect of Pore Water Overpressure, Far-Field Stresses, and Strength Parameters in Wellbore Stability

Studying the stability of wellbores drilled to access reservoirs can be a challenging issue in overpressured basins because accurate estimation of the far-field stresses, overpressure, and rock strength parameters can be difficult to achieve. Unexpected overpressures can induce tensile stresses around the wellbore, and the selection of an appropriate strength criterion and rock properties play an important role in determining the limit mud pressures. This study focuses on the stability analysis of wellbores by considering the mutual interaction of far-field stresses, fluid overpressure, and strength parameters of isotropic rock. We performed sensitivity analyses with the Mohr–Coulomb and Hoek–Brown criteria in two overpressured fields (North Sea basin and Browse basin) to highlight the influence of the uncertainties related to the rock strength. We defined an effective stress path (ESP) failure line to analyze the failure limit condition in the tension and compression zone. The analysis results indicated that the Hoek–Brown criterion better describes rock failure conditions, especially in the tension zone. Furthermore, we suggested using two different frictional components of strength for the tension and compression zone because it is a conservative approach, particularly at high overpressures. The mud pressures obtained from the uniaxial radial/tangential (HF) conventional analysis give different failure limits with respect to the ESP approach suggested in this study. These differences are low, and the mud weight margin can be low. In addition, we carried out numerical simulations with FLAC to investigate the extent of failure. The results indicated that the mud weight margin between the onset of local failure and borehole collapse is very low at high overpressures. Finally, the geomechanical analysis of wellbore stability in overpressured basins indicated the need for improving the accuracy in determining the strength parameters of the rock.

Hyperextension and polyphase rifting: Impact on inversion tectonics and stratigraphic architecture of the North West Shelf, Australia

Restoration of interpreted 2D deep seismic lines across Australia's North West Shelf indicates that rifting style changed significantly during multiple phases of extension and continental breakup. Early phase rifting during the Neo-Proterozoic and Early Paleozoic, characterised by low-angle detachment faults, crustal-scale necking and hyperextension of the crust (Metamorphic Core Complex Mode), was localised over pre-existing Proterozoic orogenic belts formed adjacent to major cratonic blocks. Subsequent extensional events in the Late Paleozoic and Mesozoic reactivated these low-angle rift fault systems. However, a series of narrow rift basins (Narrow Rift Mode), bound by high-angle normal faults that cut into the upper mantle, also began forming due to changes in the rheological architecture of the lithosphere following previous extension, crustal thinning and post-rift cooling. Wide rift mode extension, wherein extensional deformation was distributed over broad areas, may have occurred prior to, or in conjunction with, the development of more localised narrow rifts (dual mode rifting) in areas such as the Northern Carnarvon and Browse basins. Extensional deformation progressively localised into the narrow rift basins, which in some cases, matured into seafloor spreading centres during breakup, typically outboard of the major low-angle detachment fault systems. The resulting general structural architecture evolution is consistent with several other hyperextended margins from around the world.The rift events were punctuated by periods of thermal sag, often in conjunction with pulses of compressional deformation associated with plate-scale tectonic events. Structural restoration of 2D regional seismic lines shows a marked difference in the way the different polyphase rift basin elements responded to shortening. Low-angle detachment faults were relatively well oriented for reactivation under compression, leading to the development of inversion anticlines and associated convergent unconformities towards the inboard basin margin. In contrast, high-angle normal fault systems associated with the pre-existing narrow rift basins were poorly oriented for reactivation during compression and tend to be characterised by minor wrench/flower structures, generally with relatively limited uplift and erosion on the basin margins. Regional in-plane compressional stresses also worked in combination with vertical loading due to thermal decay to downwarp the lithosphere, forming broad rapidly subsiding synclinal basins above axes of the pre-existing rift basins. The evolving structural architecture developed during the compressional events had significant impacts on the associated sequence stratigraphic architecture.

Calibrated Suspended Sediment Observations Beneath Large Amplitude Non‐Linear Internal Waves

AbstractWhile it has been recognized for some time that large amplitude non‐linear internal waves (NLIW) can mobilize and transport sediment, quantitative observations of this process are rare. Rarer still are accompanying estimates of suspended sediment mass concentration (SSC) during the passage of NLIW. Here, we present high resolution observations of NLIW and the SSC response within the bottom boundary layer. The observations were made in 250 m of water in a mildly sloping region of the Browse Basin on Australia’s Northwest Shelf. We compare two independent but direct calibration methods, and employ Bayesian methods to estimate the uncertainty in SSC. During a large NLIW event, the peak mean SSC estimate at 0.49 m above the sea bed was 161 mg L−1, with a maximum time‐rate‐of‐change of 0.14 mg L−1 s−1. The unsteady boundary layer forcing under NLIW resulted in a variable time‐height dependent relationship between bed stress and SSC with increasing height above the sea bed. Suspended sediment was restricted to the bottom mixing layer, with sharp vertical gradients of up to 40 mg L−1 m−1 observed at the edge of the layer. The observations presented here are intended to offer guidance to numerical sediment modelers about likely SSC under strong NLIW.