The role of surface processes in basin inversion and breakup unconformity

At divergent plate boundaries, extensional tectonics lead to subsidence, continental rifting and the formation of continental margins. Yet, within this extensional context, transient compressional structures (stress inversion) and phases of uplift (depth inversion) are frequently recorded with no corresponding change in plate motion. Changes in gravitational potential energy during the rifting process have been invoked as a possible source of compressional stresses, but their magnitude, timing and relationship with depth inversions remain unclear. Using high-resolution 2D numerical experiments of the full rifting process, we track the dynamic interplay between the far-field tectonic forces, loading and unloading of the surface via surface processes, and gravitational body forces. Our results show that rift basins tend to localize compressive stresses, they record transient phases of compressional stresses up to 30 MPa and experience a profound depth inversion, 2 km in magnitude, when sediment supply ceases, providing a novel explanation for the breakup unconformity, a well-documented phase of regional uplift typically associated to continental breakup.

Uncertainties in breakup markers along the Iberia-Newfoundland margins illustrated by new seismic data

Plate tectonic modellers often rely on the identification of break-up markers to reconstruct the early stages of continental separation. Along the Iberian-Newfoundland margin, so-called break-up markers include interpretations of old magnetic anomalies from the M-series, as well as the J-anomaly. These have been used as the basis for plate tectonic reconstructions on the belief that these anomalies pinpoint the location of first oceanic lithosphere. However, uncertainties in the location and interpretation of break-up markers, as well as the difficulty in dating them precisely, has led to plate models that differ in their depiction of the separation of Iberia and Newfoundland. We use newly available seismic data from the Southern Newfoundland Basin (SNB) to assess the suitability of commonly used break-up markers along the Newfoundland margin for plate kinematic reconstructions. Our data shows that basement associated with the younger M-Series magnetic anomalies is comprised of exhumed mantle and magmatic additions, and most likely represents transitional domains and not true oceanic lithosphere. Because rifting propagated northward, we argue that M-series anomaly identifications further north, although in a region not imaged by our seismic, are also unlikely to be diagnostic of true oceanic crust beneath the SNB. Similarly, our data also allows us to show that the high amplitude of the J Anomaly is associated to a zone of exhumed mantle punctuated by significant volcanic additions, and at times characterised by interbedded volcanics and sediments. Magmatic activity in the SNB at a time coinciding with M4 (128 Ma), and the presence of SDR packages onlapping onto a basement fault suggest that, at this time, plate divergence was still being accommodated by tectonic faulting. We illustrate the differences in the relative positions of Iberia and Newfoundland across published plate reconstructions and discuss how these are a direct consequence of the uncertainties introduced into the modelling procedure by the use of extended continental margin data (dubious magnetic anomaly identifications, breakup unconformity interpretations). We conclude that a different approach is needed for constraining plate kinematics of the Iberian plate pre M0 times.

A study on tectonic and sedimentary development in the rifted northern continental margin of the South China Sea near Taiwan

A series of Cenozoic rifted basins developed in the northern margin of the South China Sea (SCS). Tainan Basin is one of these rifted basins near Taiwan, lying in the outer margin. We have used reflection seismic data in the deepwater areas and boreholes drilled in the shelf of the Tainan Basin to understand the tectonic and sedimentary development in the northern SCS margin near Taiwan. Four key stratal surfaces (i.e., the base of the Pleistocene Series, the base of the Pliocene Series, the 17 Ma maximum flooding surface [MFS], and a breakup unconformity of approximately 30 Ma in age) and seven seismic facies (i.e., continuous- and parallel-layer seismic facies, wavy seismic facies, chaotic seismic facies, U-shaped canyon-cut seismic facies, imbricated-layer seismic facies, high-amplitude reflector package seismic facies, and extrusive volcanism seismic facies) are recognized from seismic data with ages constrained by borehole stratigraphy drilled in the shelf. We have established a model for Cenozoic tectonic and sedimentary development in the rifted northern margin of the SCS near Taiwan. The occurrence of Paleogene fault-bounded grabens/half-grabens topped by a breakup unconformity and draped by postrift sediments indicates that these deepwater rifted basins developed on the continental crust, attesting that a thinned continental crust underlies the deepwater study area, rather than oceanic crust as reported in some literature. Postbreakup extrusive volcanic bodies, of early Miocene age, were buried by thick deepwater sediments. Fairly continuous stratal surfaces of 17 Ma MFS reveal that volcanic activities ceased to be active since middle Miocene. A series of channel cut-and-fills is observed in late Miocene, Pliocene, and Pleistocene strata beneath and to the south of the modern Formosa Canyon. Two distinct fields of deepwater sediment waves developed since middle Pleistocene are found lying to the west of modern deformation front/Manila Trench and to the north and south of the Formosa Canyon, respectively.

Using palaeogeographic reconstructions to understand lithological variability within the Early Cretaceous Gage Sandstone and South Perth Shale in the Vlaming Sub-Basin, offshore southern Perth Basin

The Early Cretaceous Gage Sandstone and South Perth Shale formations are a prospective reservoir-seal pair in the Vlaming Sub-basin. Plays include post-breakup pinch-outs in the Gage Sandstone with the South Perth Shale forming top seal. The Gage reservoir has porosities of 23–30% and permeabilities of 200–1,800 mD. It was deposited in palaeotopographic lows of the Valanginian breakup unconformity and is the lowstand component of the thick deltaic South Perth (SP) Supersequence. To characterise the reservoir-seal pair, a detailed sequence stratigraphic analysis was conducted by integrating 2D seismic interpretation, well log analysis and new biostratigraphic data. Palaeogeographic reconstructions for the SP Supersequence were derived from mapping higher-order prograding packages and establishing changes in sea level and sediment supply. Higher resolution Gage reservoir reconstructions were based on seismic facies mapping. The Gage reservoir forms part of a sand-rich submarine fan system and ranges from canyon confined inner fan deposits to middle fan deposits on a basin plain. Directions of sediment supply are complex, with major sediment contributions from a northern and southern canyon adjacent to the Badaminna Fault Zone. The characteristics of the SP Supersequence differ markedly between the northern and southern parts of the sub-basin due to variations in palaeotopography and sediment supply. Palaeogeographic reconstructions reveal a series of regressions and transgressions leading to infilling of the palaeo-depression. Palaeogeographic reconstructions for the SP Supersequence portray a complex early post-rift depositional history in the central Vlaming Sub-basin. The developed approach is applicable for detailed studies of other sedimentary basins.

Detrital zircon geochronologic tests of the SE Siberia-SW Laurentia paleocontinental connection

Strikingly similar Late Mesoproterozoic stratigraphic sequences and correlative U-Pb detrital-zircon ages may indicate that the Sette Daban region of southeastern Siberia and the Death Valley region of southwestern North America were formerly contiguous parts of a Grenville foreland basin. The Siberian section contains large numbers of detrital zircons that correlate with Grenville, Granite-Rhyolite, and Yavapai basement provinces of North America. The sections in both Siberia and Death Valley exhibit west-directed thrust faults that may represent remnants of a Grenville foreland thrust belt. North American detrital-zircon components do not occur in Siberian samples above a ~600 Ma breakup unconformity, suggesting that rifting and continental separation blocked transfer of clastic sediment between the cratons by 600 Ma. Faunal similarities suggest, however, that the two cratons remained within the breeding ranges of Early Cambrian trilobites and archeocyathans.

STRATIGRAPHY OF THE SOUTHERN SAHUL PLATFORM

This study of the southern Sahul Platform area in the Zone of Cooperation is based on the identification of depositional sequences, their distribution and relationship to structuring events in order to predict the locations of favourable combinations of source, seal and reservoir facies with increased confidence. A sequence stratigraphic approach integrating well logs, palynology and seismic data was used to identify and map significant seismic horizons such as the Aptian and Tithonian unconformities.Early to Middle Jurassic sediments were deposited in a broad, northeast-southwest oriented sag basin with a northeastward sediment transport direction. Depositional environments range from non-marine to marginal marine in the Plover Formation to the shallow marine sediments of the Elang Formation. The Elang Formation, comprising two depositional sequences, represents the last of the sediments deposited before the Breakup Unconformity. These formations comprise the dominant reservoir facies, containing a number of oil and gas discoveries. Porosity degradation occurs in Jurassic reservoirs below 3,360 m.The Callovian Breakup Unconformity resulted in the initiation of the narrow, confined depocentres of the Sahul Syncline, Malita Graben and a series of east-west troughs. The Sahul Platform and Londonderry High comprise the flanks of these depocentres but were originally located within the depocentre of the Early to Middle Jurassic sag basin. The Flamingo Syncline is a younger feature developed in the Albian.Late Jurassic and Early Cretaceous sediments are confined mainly to the Sahul Syncline and Malita Graben and are absent or represented by thin, condensed sections on the flanking highs. The condensed sections on horst blocks are a result of sediment bypass rather than considerable erosion. Reservoir facies of Tithonian-Berriasian age are interpreted to occur within east-west troughs constituting another reservoir section apart from the Bathonian-Callovian sediments. Wells distant from the Sahul Syncline and Malita Graben, have encountered hydrocarbons, indicating that the area contains mature source rocks, capable of charging traps away from the immediate vicinity of the depocentres.

Ella Bay Formation: Early Cambrian shelf differentiation in the Franklinian basin, central eastern Ellesmere Island, Arctic Canada

Strata of the Lower Cambrian (Atdabanian) Ella Bay Formation reflect progradation and exposure of a rimmed carbonate platform, subject to intermittent introduction of siliciclastic material from an inshore coastal sand belt or fluviodeltaic source. Initial rapid progradation of platformal carbonates was related to late rift subsidence. Stabilization of depositional sites of oolitic and stromatolitic platform marginal carbonates during middle and late Ella Bay times reflects the earliest phase of differential subsidence of the platform and deep-water basin within the Franklinian mobile belt. Carbonate strata landward of the rim accumulated in a protected shelf setting that was for the most part below effective storm wave base. Towards the end of Ella Bay times, the outer rim became emergent as a result of differential rotation along listric faults as the continental margin began to subside in response to early sea-floor spreading. Dissolution of carbonates along the outer rim during this phase led to the development of distinctive breccia-conglomerates as cave and karst fill. Carbonate production in the lagoon became highly restricted as siliciclastics derived from the inshore clastic belt flooded the area. The pronounced change in style between the Ella Bay Formation and overling clastics of the Ellesmere Group, which consists of a thick sequence of northwesterly prograding clastic wedges, may reflect a change to more rapid subsidence as the ocean began to spread. The local unconformity between the Ella Bay Formation and the Ellesmere Group is thus interpreted as a breakup unconformity.

THE NORTH WEST SHELF

The region of the North West Shelf dealt with in this paper is underlain by three of the four basins which make up the Westralian Superbasin. The Bonaparte Basin lies outside the scope of this paper; the other basins are the Browse Basin, the offshore Canning Basin, here named the Western Canning Basin, and the offshore Carnarvon Basin, here called the Northern Carnarvon Basin. Sediments belonging to ten depositional sequences (Pz5, Mzl to Mz5, and Czl to Cz4) are present in the basins, the oldest being of Late Carboniferous and Permian age (Pz5).Deposition commenced in rift (interior fracture) basins under fluvial/deltaic conditions in the Late Permian/Early Triassic (Mzl), when the North West Shelf was part of Gondwana. Continental breakup took place in the Middle Jurassic (breakup unconformity between Mz2 and Mz3), and marine conditions prevailed over the Westralian Superbasin thereafter, with deposition taking place in a marginal sag setting. Siliciclastic sediments gave place to carbonates in the Late Cretaceous (Mz5) as the Indian Ocean grew larger.Parts of the area have been under permit since 1946, and to date some 227 exploration wells have been drilled. The most intensive exploration has taken place in the Northern Carnarvon Basin (191 wells), followed by the Browse Basin (20 wells), and Western Canning Basin (16 wells). Thirty- four economic and potentially economic discoveries have been made. The main target reservoirs are Triassic, Jurassic and Cretaceous, and the regional seals are Triassic and Cretaceous. The fields are of two types: pre- breakup unconformity (mainly tilted horst blocks), and post- breakup unconformity (usually four- way dip closures). Of the five producing fields, the North Rankin Gas Field is a pre- breakup field, while the four oil fields (Barrow, Harriet, South Pepper and North Herald) are all post- breakup.