Exmouth Plateau Research Articles

Multi‐Elemental Statistical Features of Early Paleogene Sediments From the Mid‐Latitude Eastern Indian Ocean

AbstractThe early Paleogene, including the Paleocene and Eocene, is characterized by Warmhouse and Hothouse climate states with superimposed transient warming events known as hyperthermals. While these paleoenvironmental changes are well‐documented in the Pacific and Atlantic Oceans, records of such changes in the Indian Ocean are limited. Here, we present a new data set of bulk chemical composition and stable isotopic ratios from the late Paleocene to middle Eocene sediments on the Exmouth Plateau in the mid‐latitude eastern Indian Ocean. The bulk δ13C and δ18O suggest a warming period called the Early Eocene Climatic Optimum (EECO) and cooling toward the middle Eocene in a long‐term perspective. In the short‐term, we identified at least six hyperthermals (Paleocene–Eocene Thermal Maximum, H2, I1, J, ETM3, and L) in the studied sections. By applying independent component (IC) analyses (ICA) to the bulk chemical composition data, we identified six ICs corresponding to sediment source materials and post‐depositional processes. The ICA result infers an increase in detrital materials or a decrease in carbonate rain flux, and an increased population of higher‐order consumers in the oceanic ecosystem during both long‐term (EECO) and short‐term (hyperthermal) warmings around the Exmouth Plateau. Furthermore, ICs representing diagenetic processes and post‐depositional remobilization of elements showed excursions across the hyperthermal horizons, indicating that changes in the redox state of pore or bottom water are associated with hyperthermals. This study provides critical insights into the paleoceanography of the Indian Ocean, highlighting the response of marine environments to both long‐term and short‐term climatic events during the early Paleogene.

Population dynamics and palaeoenvironmental inferences of Turonian planktonic foraminiferal assemblages from SE Tanzania and ODP Site 762 (Exmouth Plateau, eastern Indian Ocean)

The Tanzania Drilling Program (TDP) recovered hemipelagic sediments from land-based boreholes that yielded extraordinarily well-preserved, diverse Turonian foraminifera. Reliable oxygen and carbon isotopic analyses through most of the Turonian Stage and biostratigraphic data were documented. This study compares Turonian foraminiferal population dynamics and associated geochemical proxy records among the TDP boreholes with correlative records from Ocean Drilling Program Site 762 (Exmouth Plateau, eastern Indian Ocean). The two regions were separated by ∼12° latitude and ∼5000 km of ocean, with the Indian continental plate located between. Taxonomic turnover is similar and is nearly simultaneous in both regions across the transition from the Helvetoglobotruncana helvetica Zone to the Falsotruncana maslakovae Zone (∼93.0 Ma). Changes include overlapping extinctions of all helvetoglobotruncanids, most dicarinellids and one species of Praeglobotruncana and subsequent first appearances of marginotruncanid and falsotruncanid species. Increased abundance of biserial planktonic foraminifera is recorded at multiple TDP sites and at Site 762 across the contact of the Fa. maslakovae and Huberella huberi zones c. 0.5 Myr after the Hv. helvetica / Fa. maslakovae transition. Geochemical records at the two sites do not indicate associated palaeoenvironmental changes in surface water conditions that would explain coordinated changes in species composition on opposites sides of the Indian Ocean.

The magma plumbing system of the Northern Carnarvon Basin, offshore Australia: multiscale controls on basin-wide magma emplacement, and implications for petroleum exploration

Abstract The Northern Carnarvon Basin (NCB) located on Australia's North West Shelf hosts an extensive (∼40 000 km 2 ) intrusive igneous complex related to Mesozoic rifting and break-up. Using an extensive suite of modern 3D seismic reflection surveys, we have mapped this intrusive system across the NCB. We identify three predominant intrusion morphologies: stacked sheets of large interconnected sill intrusions (up to ∼170 km long) and smaller (8–30 km long) isolated, strata concordant intrusions, which often interact with normal faults emplaced into deltaic sedimentary rocks, and variably sized (10–40 km long) saucer-shaped intrusions emplaced into marine shales, spread across seven zones (geographically constrained groups of intrusions of a specific morphology). We consider the zones’ margin-parallel orientation, suggesting control by subcrustal extensional processes during rifting, and the variation in intrusion morphology between these zones, suggesting a dominant control by host rock mechanical properties. We integrate previous work with our observations, constraining emplacement to between the Kimmeridgian and Valanginian, coinciding with key phases of margin evolution. Finally, we assess the impact of this intrusive complex on local petroleum systems. There is likely little to no adverse impact on source rock maturation or reservoir contamination by CO 2 , but there is a spatial dissociation between the location of groups of intrusions and the gas fields, particularly in the Exmouth Plateau. This suggests that migrating hydrocarbons may be blocked, baffled and/or redirected by emplaced igneous rocks.

The spatio-temporal variation of exhumation and its impacts on the resource distribution in the Northern Carnarvon Basin, Australia

Deformation and rock fracture during exhumation can enhance rock porosity and permeability, thereby improving fluid migration through reservoirs and destroying seal integrity in caprocks. Therefore, analysing the spatio-temporal distribution of exhumation in connection to fluid migration models can be crucial for energy resource exploration and storage. Geological and geophysical evidence suggests that parts of the Northern Carnarvon Basin (NCB) were exhumed. However, the spatio-temporal distribution of this exhumation and its impacts on the region's resource distribution is yet to be established. We analysed porosity, sonic logs and paleotemperature data from 210 boreholes using compaction and vitrinite reflectance analysis techniques to constrain the spatio-temporal distribution of exhumation and its correlation to the region's resource distribution. We discovered that multiple exhumation phases occurred since the Late Triassic. The highest exhumation occurred in the Mid Jurassic, where approximately 1000 m of sediment thickness was eroded from the western and southwestern parts of the Exmouth Plateau. Exhumation at this time spatially correlates with magmatic intrusions in the NCB, and paleotemperature profiles indicate that sediments in the exhumed regions were partly heated by increased paleo-basal heat flow, suggesting that magmatic bodies through underplating and crustal intrusions were the primary causes of this exhumation. Another significant exhumation phase occurred in the Early Cretaceous, removing ∼800 m of sediments from the Exmouth, Barrow and Dampier Sub-basins. Depth-dependent extension events and magmatic intrusions possibly caused exhumation at this time. Significant exhumation ceased in the Early Cretaceous, and our study has established that the exhumation episodes in the NCB were part of large-scale regional events that affected the Australian Northwest Shelf. Combining results from different estimation techniques increased the spatio-temporal coverage of the analysis and improved the precision and accuracy of exhumation results.

Syn‐rift tectono‐stratigraphic development of the Thebe‐0 fault system, Exmouth Plateau, offshore NW Australia: The role of fault‐scarp degradation

AbstractThe syn‐rift architecture of extensional basins records deposition from and interactions between footwall‐, hangingwall‐, and axially‐derived systems. However, the exact controls on their relative contributions and the overall variable depositional architecture, and how their sediment volume varies through time, remains understudied. We undertook a quantitative approach to determine temporal and spatial changes in the contribution of fault‐scarp degradation to the syn‐rift tectono‐stratigraphic development of the Thebe‐0 fault system on the Exmouth Plateau (NW Shelf, offshore Australia), using high‐quality 3D seismic reflection and boreholes data. The magnitude of footwall erosion was measured in terms of vertical (VE) and headward (HE) erosion by calculating the volume of eroded material along the footwall scarp. A detailed seismic‐stratigraphic and facies analysis allowed us to constrain the architectural variability of the hangingwall depositional systems and the types of resulting deposits (i.e., fault‐controlled base‐of‐scarp, settling from suspension, and hangingwall‐derived). After addressing the syn‐rift tectono‐stratigraphic framework, we suggest that periods of significant erosion along the Thebe‐0 fault scarp are related to the accumulation of fault‐controlled base‐of‐scarp deposits characterised by comprising a lower wedge with chaotic to low‐continuity reflections. Footwall‐derived deposits characterised by an upward decrease in stratigraphic dip are interpreted as related to periods of reduced fault activity and sustained sediment delivery sourced from the footwall scarp and systems beyond it (e.g., antecedent systems). We then analysed the tectono‐stratigraphic framework and the volumetric comparison between material eroded from the fault‐scarp and accumulated in the basin, aiming to estimate the contribution of fault‐scarp degradation to the hangingwall syn‐rift fill. Our results suggest periods of enhanced fault activity control fault‐scarp degradation variability through time, and we agree with that described by previous researchers—fault throw variability along‐strike regulates the variability in the magnitude of erosion. However, we propose that fault‐scarp degradation timing and its spatial variability are also influenced by the interaction and linkage with adjacent normal faults and by sea level variations. Lastly, we determine broader similarities and differences with a system located in the same fault array (i.e., Thebe‐2 fault system), aiming to give insights into the tectono‐stratigraphic evolution of a broader area and the spatial variability in fault‐scarp degradation.

Constraints on the expanse of Greater India in the Early Cretaceous from radiolarians

Defining the original size of Greater India is a critical parameter for reconstructing the kinematics and timing of the India–Asia collision. Early Cretaceous paleontology-based paleoceanographic and paleogeographic reconstruction for the northern coast of East Gondwanaland provides independent constraint for the extent of Greater India. In this study, newly discovered Early Cretaceous radiolarian faunas from the Tethys Himalaya were objectively compared with non-Tethyan austral faunas from the Argo Abyssal Plain (AAP) and the typical Tethyan fauna from Italy using the Hayashi's Quantification Method Type III. We found that (1) The occurrence of austral cold-water radiolarian species at the western coast of Australia and northern coast of Greater India is attributable to northward intrusion of circumantarctic cold water via a seaway newly opened during the breakup of East Gondwanaland from ∼136 Ma; (2) Gyangze was under shallow warm-water conditions with typical Tethyan radiolarian fauna (Cluster 3) until the late Valanginian when cold-water species (Cluster 1) arrived from the south. A water mass boundary shift from Tethyan species dominant to austral cold-water species dominant in Gyangze indicates its lower paleolatitude than Ocean Drilling Program (ODP) Leg 123, Site 765 in the AAP; (3) The same austral cold-water radiolarian-bearing intervals from the late Berriasian to Valanginian indicate that Kangma in southern Tibet was proximal to ODP Site 765; (4) Different radiolarian faunas in Italy–Zhongba, Kangma, and the AAP reveal complex water conditions under different degrees of effect by Tethyan warm water and austral cold water at different water depths during the Barremian to Aptian. This radiolarian biogeography is highly consistent with the paleomagnetic reconstruction and indicates that eastern Greater India extended to the Exmouth Plateau (>2400 km) during the Early Cretaceous.

Using shallow hydroacoustic data to image seafloor mass transport deposits on the North West Shelf of Australia: links to neotectonics

Abstract Mass transport deposits have long been known on the Exmouth Plateau, offshore NW Australia, identified in 2D and 3D industry seismic lines. The expedition SO257 in 2017 collected 30 high-resolution, shallow seismic lines along targeted transects on the northern Australian margin. Many of these imaged mass transport deposits, with the top 700–800 m of the section captured in detail not available with industry seismic data. We present nine new high-resolution seismic lines from three separate areas of the North West Shelf. Slides in the Roebuck Basin show complex anastomosing ductile extensional mechanisms, with multiple slip surfaces and no headscarps or adjacent faults. Slides on the Exmouth Plateau have fault control, with surface fault offsets of up to 300 m, indicating seismicity as a likely triggering mechanism. Slumps along the western margin of Western Australia are more limited in extent, associated with surface notches, with indications of previous activity at depth. All areas show a repeated history of mass transport deposits. The area of the active landslide province offshore of NW Australia is far larger than the individual slides recognized on the Exmouth Plateau.

The distribution and morphology of Late Jurassic to Early Cretaceous igneous intrusions in the Northern Carnarvon Basin

The Northern Carnarvon Basin (NCB) contains extensive networks of igneous intrusions emplaced during Late Jurassic and Early Cretaceous rifting that led to the breakup of the Greater India from Australia. We present the first basin-wide study of the distribution and morphology of these igneous intrusions through the interpretation of regionally extensive 3D and closely spaced 2D seismic data across the Exmouth Plateau and Exmouth Sub-basin. We observe three dominant intrusion morphologies: (1) Saucer-shaped intrusions up to ~20 × 40 km, but commonly much smaller, present in Jurassic strata of the southern Exmouth Plateau and central Exmouth Sub-basin; (2) Large, stacked, strata parallel, sheet intrusions, often >100 km in length, dominant in Triassic strata in the Exmouth Plateau and southern Exmouth Sub-basin; and (3) Variably sized, predominantly strata parallel and occasionally fault hosted intrusions (ranging in dimension from ~5 × 8 to ~35 × 65 km) present in Jurassic rocks in the Exmouth Sub-basin, and uppermost Triassic rocks in the Exmouth Plateau. We suggest that the morphologies of intrusions in the NCB are predominantly controlled the mechanical properties of their host rocks.

Nature and preservation of Late Jurassic breakup-related volcanism in the Carnarvon Basin, North West Shelf, Australia

The North West Australian Margin, which formed as Greater India rifted from Australia during the Jurassic to Early Cretaceous, is recognised as an archetypal magma-rich rifted margin, with records of extensive igneous activity in the Exmouth Plateau and Exmouth Sub-Basin (ESB) of the Northern Carnarvon Basin (NCB). Pre-breakup magmatism is manifested by a large ∼400 × 150 km intrusive sill complex, emplaced into Triassic and Jurassic strata in the Late Jurassic and Early Cretaceous. An apparent lack of extrusive igneous rocks has caused previous works to describe the region as a large intrusive igneous province. Here, we describe two recently identified Upper Jurassic volcanic centres: the Pyrenees Volcano in the eastern ESB (first reported here), and the Toro Volcanic Complex (TVC), in the western ESB. Although offset by Early Cretaceous normal faulting, the edifice of the Pyrenees Volcano and associated lava flows are well preserved beneath a protective carapace of Upper Jurassic strata below the angular Intra-Hauterivian Unconformity on the Novara Arch. In contrast, a significant proportion of the TVC was peneplaned beneath an intra-Valanginian (Early Cretaceous) unconformity following breakup-related uplift. As Upper Triassic to Lower Cretaceous strata appear to have been eroded over the Ningaloo Arch in the southern ESB, we postulate that Late Jurassic extrusive volcanism may have been more spatially extensive, prior to erosion associated with Early Cretaceous exhumation in the southern NCB. Hence our findings suggest that the NCB was potentially host to significantly more extrusive volcanism than has been preserved within basin fill. Our findings also have broader implications for the conditions required to preserve extrusive igneous material in sedimentary basins within large igneous provinces that have undergone complex histories of rift-related vertical motion.

Diagenetic priming of submarine landslides in ooze-rich substrates

Abstract Oozes are the most widespread deep-sea sediment in the global ocean, but very little is known about how changes in their physical properties during burial impact slope stability and related geohazards. We used three-dimensional seismic reflection, geochemical, and petrophysical data acquired both within and adjacent to 13 large (in total ~6330 km2) submarine slides on the Exmouth Plateau, North West Shelf, Australia, to investigate how the pre-slide physical properties of oozes control slope failure and emplacement processes. Our integrated data set allows potential slide surfaces to be detected within ooze successions, a crucial advance for improved submarine geohazard assessment. Moreover, we demonstrate that the interplay of tectonics, ocean current activity, and silica diagenesis can prime multiple slides on very low-gradient slopes in tropical oceanic basins. Therefore, the diagenetic state of silica-rich sediments should be considered in future studies to improve slope stability assessments.

Stratigraphic record of continental breakup, offshore NW Australia—Discussion

AbstractReeve et al. (2022) address the stratigraphic record of continental breakup by focusing on a set of stratigraphic unconformities from a proximal sector of the NW Australian continental margin, inboard from the Exmouth Plateau. They suggest that such unconformities can potentially document a well‐defined three‐stage process: end of the syn‐rift phase, formation of a wide continent‐ocean transition zone (COTZ) and generation of ‘true’ Penrose‐type oceanic crust. We counterargue that continental breakup is a protracted event that can only be understood via seismic‐ and chronostratigraphic correlations of strata, and their composing sequences, across and along rifted margins. Tying proximal stratigraphic unconformities to magnetic anomalies outboard from the study area in Reeve et al. (2022) is open to question. In parallel, we suggest that age resolutions of ca. 1 Ma are not achievable using the micropaleontological data presented in Reeve et al. (2022), with an important reworking of microfossil assemblages potentially occurring during the erosional process forming local and regional unconformities. Our discussion addresses these points in more detail.

Caracterización geoquímica y geomecánica de la Formación Mungaroo, offshore del noroeste de Australia

The geochemical characterization of the Mungaroo Formation rocks shows the presence of kerogen type II/III and III which characterizes the Exmouth Plateau sub-basin as a gas prone basin. Using 13 burial histories constructed from well data, we identified three tendencies: One with high sedimentation rates between the Middle Triassic and Upper Triassic (69.3-95.3 m/Ma), another one with high sedimentation rates between the Lower and Upper Cretaceous (67-158 m/Ma), and the last one with low sedimentation rates (18-40 m/Ma) during the Upper Cretaceous until present time. All these trends defined active generation zones (or gas kitchens) between 2,000 and 4,400 km2. High sedimentation rates during the Cretaceous and Triassic times were key to the burial history of the Mungaroo Formation because they allowed these rocks to reach the required depths to transform its organic matter. In contrast, in the area with low sedimentation rates, radiogenic heat flow was the trigger for the transformation of the organic matter. The generation/expulsion of hydrocarbons from these shales occurs since 100 My, consequently explaining the large gas accumulation found in the sub-basin. Currently, the Mungaroo formation is in the gas generation window at depths of 4,500 to 5,500 m below sea level. The shales of this formation show TOC% values higher than 2% reaching the gas generation zone (Ro% >1.3) and suggesting its potential as an unconventional gas reservoir. However, geomechanical features such as low fragility, under pressure, and thickness, condemns its unconventional potential.

Concurrent 14. Presentation for: Major geological surfaces of the Exmouth Plateau from the new Broadband PreSDM reprocessed 3D seismic data with full waveform inversion

Presented on Wednesday 18 May: Session 14 The Barrow Sub-basin and Exmouth Plateau, of Australia’s North West Shelf, contain Australia’s largest gas reserves, underpinning the three major LNG facilities, including Gorgon, Wheatstone and Pluto. In addition, there remains significant near-field exploration potential such as the Sasanof prospect. While there is full 3D seismic coverage in the basin, the existing 3D surveys are of different vintages covering only parts of the basin with diverse acquisition and processing parameters. Multi-Client Resources has produced an extensive 3D seismic reprocessing project (BEX-MC3D) incorporating 23 legacy 3D surveys to provide a seamless 25 570 km2 3D dataset. The BEX-MC3D was processed from field tape through DUG Technology’s advanced Broadband PreSDM processing sequence with Full Waveform Inversion (FWI) imaging. The BEX-MC3D provides one continuous superlative 3D dataset covering almost the entire Barrow Sub-basin and the Exmouth Plateau. Major geological surfaces corresponding to key tectonic events from overburden to reservoirs have been interpreted from the BEX-MC3D Priority area in the current study. These high-fidelity imaged surfaces form a basin-wide framework for: (1) rapid exploration lead and prospect identification through the creation of trap and reservoir visualisations; (2) improved reservoir connectivity analysis and modelling of producing and discovered fields to support better reservoir management and field development decisions; and (3) improved understanding of basin-burial history for reservoir quality and geo-mechanical property prediction. This paper will present and discuss major geological surfaces in the context of future exploration lead/prospect identification, reservoir characterisation for field development and management, and improving overall understanding of the basin history. To access the presentation click the link on the right. To read the full paper click here

Major geological surfaces of the Exmouth Plateau from the new Broadband PreSDM reprocessed 3D seismic data with full waveform inversion

The Barrow Sub-basin and Exmouth Plateau, of Australia’s North West Shelf, contain Australia’s largest gas reserves, underpinning the three major LNG facilities, including Gorgon, Wheatstone and Pluto. In addition, there remains significant near-field exploration potential such as the Sasanof prospect. While there is full 3D seismic coverage in the basin, the existing 3D surveys are of different vintages covering only parts of the basin with diverse acquisition and processing parameters. Multi-Client Resources has produced an extensive 3D seismic reprocessing project (BEX-MC3D) incorporating 23 legacy 3D surveys to provide a seamless 25 570 km2 3D dataset. The BEX-MC3D was processed from field tape through DUG Technology’s advanced Broadband PreSDM processing sequence with Full Waveform Inversion (FWI) imaging. The BEX-MC3D provides one continuous superlative 3D dataset covering almost the entire Barrow Sub-basin and the Exmouth Plateau. Major geological surfaces corresponding to key tectonic events from overburden to reservoirs have been interpreted from the BEX-MC3D Priority area in the current study. These high-fidelity imaged surfaces form a basin-wide framework for: (1) rapid exploration lead and prospect identification through the creation of trap and reservoir visualisations; (2) improved reservoir connectivity analysis and modelling of producing and discovered fields to support better reservoir management and field development decisions; and (3) improved understanding of basin-burial history for reservoir quality and geo-mechanical property prediction. This paper will present and discuss major geological surfaces in the context of future exploration lead/prospect identification, reservoir characterisation for field development and management, and improving overall understanding of the basin history.

Three-dimensional seismic analysis of a polygonal fault system (PFS) in the Northern Carnarvon Basin, Australia: Implications for fluid flow migration and gas hydrate system

Polygonal fault system (PFS) is confined to fine-grained strata and have significant potential for hydrocarbon plumbing systems in sedimentary basins from deeper structures to shallow-depth gas hydrate systems. Based on a three-dimensional (3D) seismic reflection dataset from the Exmouth Plateau, Northern Carnarvon Basin, NW Shelf Australia, two different tiers of PFS (PFS–I and PFS-II) are characterized in section as well as in the plan views. The PFS-I is consisted of Early Jurassic-Early Cretaceous shallow marine limestone and marl, while the PFS-II is composed of Late Cretaceous to Paleogene bathyal facies with mudstone, calcareous mudstone, marl and silty mudstone. The polygonal faults of PFS-I show relatively low amplitude reflections, which are difficult to identify in seismic amplitude sections. PFS-I can be effectively depicted via coherency sections and time slices, with a good relationship between the low coherent black anomalies and the polygonal faults. In contrast, the polygonal faults of PFS-II are characterized by high-medium amplitude and low continuity seismic reflections, which can be easily identified in the seismic profiles and coherency time slices. The polygonal faults are represented by small throws (5–15 m) and multidirectional strikes. In addition, a series of gas chimneys, pockmarks, and shallow-depth high-amplitude anomalies of the gas hydrate system have also been identified above the deeper Triassic tectonic faults and uplifts. The polygonal faults might increase the connectivity and permeability of the seal and fine-grained layers, resulting in a certain local reduction of seal integrity and an increase in the possibility of fluid leakage. The typical seismic characteristics of PFS and the shallow depth high-amplitude anomalies of the gas hydrate system indicate a significant relationship among the deep-depth Triassic thermogenic gas, the fluid flow migration, and the development of the generation of gas hydrate system.

Multi-attribute geomorphological characterization of the Cenozoic deep-water strata in the Exmouth Plateau, Northern Carnarvon Basin: a contribution to the understanding of deep-water mixed carbonate–siliciclastic environments

Abstract The stratigraphic architecture and relationships to the expression of deep-water architectural elements geomorphology are explored for Cenozoic deposits in the Exmouth Plateau Sub-basin in Northern Carnarvon Basin. We used a high-quality 3D seismic survey and an integrated seismic attribute analysis to provide a guide for seismic interpretation of the mixed carbonate–siliciclastic depositional systems focused on four aspects: morphology, lithology, strata thickness and reflector distortion. Three depositional elements are classified into several categories: mass transport deposits (MTDs), channel systems (channel–levee and braided-like) and lobes, whose deposition is closely related to eustatic changes, and can be explained in a sequence stratigraphic framework. The falling stage system tract is characterized by the initial development of MTDs deposited on the basin floor and small erosive channels on the upper slope. Lobes are deposited in lowstand system tracts while highstand system tracts are characterized by the deposition of MTDs and progradational segments. This mixed carbonate–siliciclastic example proved to be sensitive to other variables such as climate (Middle Miocene Climate Optimum) and plate tectonics (northward Australian Plate drift), which affect geomorphological aspects such as the dimensions and preservation of internal characteristics.

Can deepwater bottom currents generate clinothems? An example of a large, asymmetric mounded drift in Upper Jurassic to Lower Cretaceous sediments from northwestern Australia

Abstract Clinoforms and clinothems are ubiquitous in shallow marine and shelf margin environments, where they show remarkable seaward progradation trends. Consensus holds that these features do not form in deepwater settings. This study describes an example of a large, asymmetric mounded deposit formed in Upper Jurassic to Lower Cretaceous sediments along the Exmouth Plateau (offshore northwestern Australia). Although it formed in deepwater environments, the deposit has previously been interpreted to reflect either a deltaic or shelf margin system based on clinoform and clinothem geometries. We support that this deposit shares similarities with a delta drift that evolved into a large, mounded drift (~180 km in length, ~120 km in width, and up to ~1.7 km in sedimentary thickness) that exhibits two migration trends: one westward and the other northeastward. Three evolutionary phases are proposed: (1) an onset drift stage (ca. 146.5–143.5 Ma); (2) a growth drift stage (ca. 143.5–138.2 Ma); and (3) a burial stage (ca. 138.2 Ma), which marks the completion of the drift and a shift in depositional style. The drift asymmetry and clinoform orientations indicate the influence of a northward-flowing water mass with two main cores. Our analysis thus suggests that bottom currents can create complex deposits with geometries that resemble clinothems in deepwater environments.

Tectonic and stratigraphic evolution of the central Exmouth Plateau, NW Shelf of Australia

A detailed tectono-stratigraphic analysis of the Mesozoic through Present-day structures and sedimentary sequences has been carried-out on a very large merged 3-D seismic survey that covers the great part of the central Exmouth Plateau, northern Carnarvon Basin, NW Shelf of Australia. Four main megasequences bounded by regional unconformities have been delineated. They are; (1) Megasequence-I - Early-Late Triassic pre-extension, (2) Megasequence-II - Latest Triassic (Rhaetian)-earliest Late Jurassic (Oxfordian) syn-extension; (3) Megasequence-III - Late Jurassic-Early Cretaceous post-extension; and (4) Megasequence-IV - Early Cretaceous-Present-day passive margin strata. Three populations of extensional faults and a previously unrecognised compressional phase of folding have been identified. The main Mesozoic (Rhaetian-Oxfordian) WNW-directed extension produced domino-style planar extensional faults (Fault System 1) with two principle fault sets; (i) large N–S to NNE-SSW-trending, west-dipping faults and (ii) smaller-scale, largely antithetic east-dipping faults. The large faults are possibly reactivated pre-existing deeper and older faults (possibly related to listric growth faults of the large Lower-to Mid-Triassic Mungaroo delta) as well as completely new faults developed in the upper part of the Triassic Mungaroo Formation. Synclinal hangingwall basins of this system are strongly segmented and underfilled with little (<450 m) syn-extensional sedimentation.Fault System 1 was reactivated during the Early Cretaceous-Neogene post-extension stage and strongly influenced the development of the segmented NNE-SSW-trending Fault System 2 in the Cenozoic strata. Two stacked polygonal non-tectonic fault tiers (Fault System 3) separated by little deformed stratigraphy occur within the Berriasian to Pliocene post-extension sequences. In the early post-extension phase, Late Jurassic to Early Cretaceous NNE-SSW-directed minor contraction event produced broad WNW-ESE-striking folds in the Triassic-Early Cretaceous strata. This study documents the development of the extensional fault systems and proposes new 4-D tectono-stratigraphic evolutionary models for the central Exmouth Plateau. The results of this study have wider implications for the tectono-stratigraphy of the northern Carnarvon Basin and future exploration.

Evolution of normal fault displacement and length as continental lithosphere stretches

AbstractContinental rifting is accommodated by the development of normal fault networks. Fault growth patterns control their related seismic hazards, and the tectonostratigraphic evolution and resource and CO2 storage potential of rifts. Our understanding of fault evolution is largely derived by observing the final geometry and displacement (D)‐length (L) characteristics of active and inactive fault arrays, and by subsequently inferring their kinematics. We can rarely determine how these geometric properties change through time, and how the growth of individual fault arrays relate to the temporal evolution of their host networks. Here we use 3D seismic reflection and borehole data from the Exmouth Plateau, NW Shelf, Australia to determine the growth of rift‐related, crustal‐scale fault arrays and networks over geological timescales (&gt;106 Ma). The excellent‐quality seismic data allows us to reconstruct the entire Jurassic‐to‐Early Cretaceous fault network over a relatively large area (ca. 1,200 km2). We find that fault trace lengths were established early, within the first ca. 7.2 Myr of rifting, and that along‐strike migration of throw maxima towards the centre of individual fault arrays occurred after ca. 28.5 Myr of rifting. Faults located in stress shadows become inactive and appear under‐displaced relative to adjacent larger faults, onto which strain localises as rifting proceeds. This implies that the scatter frequently observed in D‐L plots can simply reflect fault growth and network maturity. We show that by studying complete rift‐related normal networks, rather than just individual fault arrays, we can better understand how faults grow and more generally how continental lithosphere deforms as it stretches.

Normal Fault Kinematics and the Role of Lateral Tip Retreat: An Example From Offshore NW Australia

AbstractUnderstanding how normal faults grow is key to determining the tectono‐stratigraphic evolution of rifts. According to recent studies, normal faults tend to grow in two temporally distinct stages: a lengthening stage, followed by a throw/displacement accumulation stage. However, this model is still debated and not widely supported by many additional studies. Relatively few studies have investigated what happens to a fault as it becomes inactive, that is, does it abruptly die, or does the at‐surface trace‐length progressively shorten by so‐called tip retreat? We, here, use a 3D seismic reflection data set from the Exmouth Plateau, offshore Australia, to develop a three‐stage fault growth model for seven normal faults of various sizes and to show how the throw‐length scaling relationship changes as a fault dies. We show that during the lengthening stage, which lasted &lt;30% of the faults’ lives, faults reached their near‐maximum lengths, yet accumulated only 10%–20% of their total throw. During the throw/displacement accumulation stage, which accounts for c. 30%–75% of the faults’ lives, throw continued to accumulate along the entire length of the faults. All of the studied faults also underwent a stage of lateral tip‐retreat (last c. 25% of the faults’ lives), where the active at‐surface trace‐length decreased by up to 25%. The results of our study may have broader implications for fault growth models, slip rate variability during fault growth, and the way in which faults die, in particular the role of lateral tip‐retreat.