Sag Basin Research Articles

Basin geometry and salt diapirs in the Flinders Ranges, South Australia: Insights gained from geologically-constrained modelling of potential field data

The Adelaide Basin in Australia is a complex of late Neoproterozoic to Early Cambrian rift and sag basins which was inverted during the Cambro–Ordovician Delamerian Orogeny. The deposition of evaporitic sediments during the earliest stage of basin development in the late Neoproterozoic (Willouran age) played a major role in the subsequent tectonic evolution of the basin. Previous studies have shown that early mobilization, vertical transport and withdrawal of the evaporites influenced the sedimentation during the late Neoproterozoic and Early Cambrian. The evaporites also influenced deformation during the inversion of the basin and the development of the Delamerian fold and thrust belt. However, the control exerted by basement structures in the deposition of the evaporitic beds and the role of these tectonic structures in the later inversion of the basin have been poorly constrained. In this work, we use a combination of published and original geological observations along with the interpretation of potential field datasets (total magnetic intensity and Bouguer anomaly data) to better constrain the geology of the basin at depth. We construct two and a half dimensional forward models of the potential field data along selected profiles across the Adelaide Basin. These models are constrained by the geology at the surface, drill hole data and measured petrophysical properties (specific gravity and magnetic susceptibility). We achieved the best fit between observed and modelled potential fields with a model favouring thick-skinned deformation, where the diapirs are not randomly distributed, but located directly above basement-penetrating normal faults. Furthermore, these normal faults were probably active during the sedimentation of the late Neoproterozoic and Early Cambrian sediments, and underwent partial or total inversion during the Cambro–Ordovician Delamerian Orogeny. Mobilisation and withdrawal of the evaporites was therefore initiated and facilitated by the extension coeval with the opening of the basin, and continued during the subsequent Delamerian Orogeny.

Tectonosedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees

In this paper we describe the tectonosedimentary evolution and its subsequent inversion of a basin that underwent extreme crustal thinning in a transtensional setting ahead of a propagating ocean in the western Pyrenees. The Labourd‐Mauléon area situated in the western Pyrenees, at the termination of the V‐shaped Bay of Biscay, is an ideal natural laboratory to study how such complex basins evolve in time and space. Because of a mild inversion of the basin during Pyrenean compression, the rift structures and their relations to basement rocks and sediments are exposed and can be directly studied in the field. The basin shows a complex polyphase evolution that starts with left‐lateral dominated transtension in latest Jurassic–early Aptian time. This event is overprinted by a late Aptian–early Albian extension that is related to the counterclockwise rotation of Iberia away from Europe leading to the opening of the Bay of Biscay. During this stage, the Late Triassic to Jurassic carbonate platform was stretched, salt migrated, and detachment faults exhumed upper and lower crustal and mantle rocks to the seafloor. The final structure of the basin resembles a sag basin floored by exhumed rocks overlain by extensional allochthons and compartmentalized by N40° to N60° transfer faults. The sedimentary architecture is characterized by late Aptian synrift sediments (e.g., Urgonian limestones) that were deposited in fault‐bounded basins and are overlain by thick latest Aptian to Albo‐Cenomanian sediments (e.g., Flysch noir) that define a sag sequence. The complex tectonosedimentary evolution of the basin is associated with salt tectonics and overprinted by a major magmatic/thermal event that postdates mantle exhumation.

Description of Cretaceous Sedimentary Sequence of the Quantou Formation Recovered by CCSD-SK-Is Borehole in Songliao Basin: Lithostratigraphy, Sedimentary Facies and Cyclic Stratigraphy

The Quantou Formation recovered by CCSD-SK-Is borehole (China Cretaceous Continental Scientific Drilling-SongkeI-the south borehole) is 132.07 m long and the recovery of cores is 100%. The sequence and process of lithology-lithofacies and cyclostratigraphy are revealed by a detailed core description. Nine rock types and three kinds of sedimentary facies including meandering river, shallow lake, and delta front are recognized from the drilling core, there are ten sedimentary microfacies, which are point bar, natural levee, crevasse splay, crevasse channel, floodplain, flood lake, distributary mouth bar, interdistributary bay, mudstone of still water, and turbidite. The Quantou Formation represents seventy-six meter-scale cycles (sixth-order cycle), twenty-five fifth-order cycles, eight fourth-order cycles, and two third-order cycles. Meticulously depicted (centimeter level) continental red beds of the third and the fourth members of Quantou Formation will be a potential formulation profile for the studies of Cretaceous global oxygen-enriched events. The reservoir sandstones of the third and the fourth members were developed basin-wide as an inevitable product before the Songliao Basin was depressed or sagged on a large scale. The period of the third and the fourth members of Quantou Formation is a stage between the development of the fault basin and the sag basin on a large scale.

Mesozoic red bed sequences from SE Asia and the significance of the Khorat Group of NE Thailand

Abstract New geological data are presented and previously published information is reviewed to demonstrate that much of the Khorat Group (Phu Kradung to Khok Kruat Formations) of NE Thailand is Early Cretaceous in age. It is suggested that the Mesozoic red bed sequences of neighbouring Indochina are likely to be of similar age rather than spanning the entire Late Triassic to Early Cretaceous as previously assumed. Moreover, the Lower Nam Phong Formation dated as Late Triassic and previously included as the basal formation of the Khorat Group is now removed from this group, thus creating a hiatus within the Jurassic. There is therefore no clear relationship between the Indosinian Orogeny and the Triassic collision of the Sibumasu (also referred to as Shan-Thai) and Indochina Blocks and the subsequent deposition of the Khorat Group in a Late Triassic–Early Cretaceous thermal sag basin. It is now proposed that much of the sequence was deposited during the Late Jurassic–Early Cretaceous. Jurassic sediments may be absent across much of the Khorat Plateau whereas marine Jurassic sediments to the west and east show no sediments younger than Bajocian. Because sea levels were generally rising in the Middle and Late Jurassic it is likely that the Khorat region was uplifted at this time. It is suggested that the Khorat Group was originally deposited in a foreland basin setting rather than a thermal sag basin following Late Triassic rifting. Moreover, the original site of deposition was to the north in southern China, with the present-day location being the result of movement along the Red River Fault coupled with a clockwise rotation of the Indochina Block (on which the Khorat Group sits) with respect to the South China Block.

Brazilian and African passive margins of the Central Segment of the South Atlantic Ocean: Kinematic constraints

The thinning of passive continental margins is usually explained by models using pure stretching or simple shear. These models imply hypothetical extensional structures and large horizontal movements between the two conjugate margins (more than 250 km for the Brazilian and Angolan Margins). Refraction/reflection data together with the most recent and tightest pre-opening fit using continental and oceanic, geological and geophysical constraints show that the substratum of the sag basin is divided into an autochthonous part (upper continental crust) and an allochthonous part (exhumed material). The thinning process, which evolves in an elevated position of the system until at least the break-up, seems to be depth dependent and to mainly concern the lower/middle crust, which we postulate is exhumed. This exhumation does not explain the entire thinning of the system: horizontal motions cannot alone explain the formation of the huge Angolan–Brazilian Basin. Similar observations are made on the whole Central Segment of the South Atlantic Ocean. Part of the lower continental crust is still missing and it seems most improbable that the continental crust maintains its integrity throughout the thinning process.

A new active tectonic model for the construction of the Northern Apennines mountain front near Bologna (Italy)

We integrate existing and new geologic data [REtreating TRench, Extension, and Accretion Tectonics (RETREAT project)], particularly on the origin, growth, and activity of the mountain front at Bologna, Italy, into a new model that explains Apennine orogenesis in the context of a slab rollback ‐ upper plate retreat process. The Bologna mountain front is an actively growing structure driving rock uplift ∼1 mm/year, cored by a midcrustal flat‐ramp structure that accommodates ongoing shortening driven by Adria subduction at a rate of ∼2.5 mm/year. The data we use are assembled from river terraces and associated Pleistocene growth strata, geodesy including releveling surveys, reinterpretation of published reflection lines, and a new high‐resolution reflection line. These data constrain a simple trishear model that inverts for blind thrust ramp depth, dip, and slip. Apennine extension is recognized both in the foreland, as high‐angle normal faults and modest stretching in the carapace of the growing mountain front, and in the hinterland, with larger normal faults that accomplish some crustal thinning as the upper plate retreats. This coevolution of extension and shortening shares some notable characteristics with other basement‐involved collisional orogens including the early Tertiary Laramide orogeny in the American West and the Oligocene to Miocene evolution of the Alps. We propose a possible relationship between underplating and the development of the Po as a sag basin as a Quaternary phenomenon that may also apply to past periods of Apennine deformation (Tortonian). Continued shortening on the structure beneath the Bologna mountain front represents by far the most important and underappreciated seismogenic source in the front of the northern Apennines.

Review: secular tectonic evolution of Archean continental crust: interplay between horizontal and vertical processes in the formation of the Pilbara Craton, Australia

AbstractThe Archean Pilbara Craton contains five geologically distinct terranes – the East Pilbara, Karratha, Sholl, Regal and Kurrana Terranes – all of which are unconformably overlain by the 3.02‐ to 2.93‐Ga De Grey Superbasin. The 3.53–3.17 Ga East Pilbara Terrane (EP) represents the ancient nucleus of the craton that formed through three distinct mantle plume events at 3.53–3.43, 3.35–3.29 and 3.27–3.24 Ga. Each plume event resulted in eruption of thick dominantly basaltic volcanic successions on older crust to 3.72 Ga, and melting of crust to generate first tonalite‐trondhjemite‐granodiorite (TTG), and then progressively more evolved granitic magmas. In each case, plume magmatism was accompanied by uplift and crustal extension. The combination of conductive heating from below, thermal blanketing from above, and internal heating of buried granitoids during these events led to episodes of partial convective overturn of upper and middle crust. These mantle melting events caused severe depletion of the subcontinental lithospheric mantle, making the EP a stable, buoyant, unsubductable continent by c. 3.2 Ga. Extension accompanying the latest event led to rifting of the protocontinent margins at between 3.2 and 3.17 Ga. After 3.2 Ga, horizontal tectonic forces dominated over vertical forces, as revealed by the geology of the three terranes (Karratha, Sholl and Regal) of the West Pilbara Superterrane. The c. 3.12‐Ga Whundo Group of the Sholl Terrane is a fault bounded, 10‐km‐thick volcanic succession with geochemical characteristics of modern oceanic arcs (including boninites and evidence for flux melting) that indicate steep Archean subduction. At 3.07 Ga, the 3.12‐Ga Sholl Terrane, 3.27‐Ga Karratha Terrane and c. 3.2‐Ga Regal Terrane accreted together and onto the EP during the Prinsep Orogeny. This was followed by development of the De Grey Superbasin – an intracontinental sag basin and widespread plutonism (2.99–2.93 Ga) as a result of orogenic relaxation and slab break off. Craton‐wide compressional deformation at 2.95–2.93 Ga culminated with 2.91‐Ga accretion of the 3.18 Ga Kurrana Terrane with the EP. This compression caused amplification of the dome‐and‐keel structure in the EP. Final cratonization was effected by emplacement of 2.89–2.83 Ga post‐tectonic granites.

Timing and origin of the South Atlantic pre-salt sag basins and their capping evaporites

Abstract Continental extension between West Africa and Brazil was responsible, directly or indirectly, for the development of the pre-salt sag basins and the evaporites of the South Atlantic salt basin. Subsidence mechanisms to explain these basins and their capping evaporites include: (1) deposition on Barremian-aged ocean crust; (2) rift propagation from east to west across the West African margin such that post-rift subsidence commenced in the east while rifting was still occurring to the west; and (3) depth-dependent lithospheric extension. Predicted thermal subsidence of oceanic crust or rifted lithosphere is inadequate to generate sufficient accommodation for the evaporites. Within the Santos Basin, extensional faulting within the pre-salt sag basin occurs up to the base of the evaporites; extension clearly continued to the late Aptian. Time-equivalent onshore and offshore pre-salt sections across the West African margin, and the inability to generate sufficient subsidence if the sections are considered to be post-rift, disqualifies east to west rift propagation as a mechanism for the observed pre-salt basins and evaporites. Barremian–Aptian depth-dependent extension best explains the general rift and post-rift development of the West African and Brazilian margins and the paucity of syn-rift faulting, the strain balance being achieved by the lateral emplacement of lower crust and continental mantle out from under the adjacent continental lithosphere. Regional exposure and truncation of the top pre-salt sag section attests to a climate-induced lake level drawdown during the mid Aptian, and offers a simple mechanism to generate the shallow water environments for evaporite precipitation across the West African–Brazilian rift system. In the subsequent marine transgression the Gabon and Angolan salts and the evaporites within the conjugate Camamu-Almada, Jequitinhonha and Cumuruxatiba basins were deposited. Santos and Campos basin evaporites are younger. The barrier to southern Atlantic marine incursions and the possible delay in Santos and Campos evaporite deposition relates to the magmatic constructions of a proto-Walvis Ridge and the long-lived anomalous topography of the southeastern Brazilian highlands; Campos and Santos basin extension was necessarily superimposed on a broad, high-relief plateau.

Post-Variscan evolution of the lithosphere in the area of the European Cenozoic Rift System

Abstract The European Cenozoic Rift System (ECRIS) transects Variscan basement, Permo-Carboniferous troughs and Late Permian to Mesozoic series, deposited in thermal sag basins, which are exposed on rift-related arches. We have analysed processes governing the transformation of the orogenically destabilized Variscan lithosphere into end-Cretaceous stabilized cratonic lithosphere, prior to its renewed Cenozoic rift-related destabilization. In the ECRIS area, crustal and lithospheric thicknesses at present are in the range of 24-35 km and 60-120 km, respectively. The Variscan orogen was characterized at the time of its end-Westphalian consolidation by 45-60 km deep crustal roots, marking major sutures. During the Stephanian-Early Permian wrench-induced collapse of the Variscan orogen, subducted lithospheric slabs were detached, causing upwelling of the asthenosphere, thermal thinning and/or partial delamina-tion of the lithospheric mantle, and regional uplift. By mid-Permian times, the crust was thinned to 28-35 km owing to its regional erosional unroofing, localized mechanical stretching and the interaction of mantle-derived melts with its basal parts. By mid-Permian times, when the temperature of the asthenosphere returned to ambient levels, thermal subsidence of the lithosphere commenced, controlling development of a system of Late Permian and Mesozoic intracratonic basins. These experienced repeated minor subsidence accelerations, related to the build-up of far-field stresses, which did not involve renewed lithospheric destabilization. Modelling of observed subsidence curves indicates that during the mid-Permian lithospheric thicknesses ranged in the ECRIS area between 40 and 80 km, but had increased by the end of the Cretaceous to 100-120 km. Cenozoic rifting and mantle-plume activity caused renewed lithospheric thinning.

The Mesozoic–Cenozoic interior sag basins of Central Africa: The Late-Cretaceous–Cenozoic Kalahari and Okavango basins

The Kalahari basin formed as a response to down-warp of the interior of the southern Africa, probably in the Late Cretaceous. The down-warp, along with possible uplift along epeirogenic axes, back-tilted rivers into the newly formed Kalahari basin and deposition of the Kalahari Group sediments began. Initial deposition of basal gravels occurred in the channels of the Cretaceous rivers, with other unsorted gravel beds deposited at the base of scree slopes along the edges of valleys and fault-bounded structures. The accumulation of gravels continued as the down-warp of the basin progressed with interbedding of the gravel layers with sand and finer sediment carried by the rivers. Thick clay beds accumulated in the lakes that formed as a result of the back-tilting of rivers, with sandstone being deposited in braided streams interfingering with the clays and covering them in some areas as the shallow lakes filled up with sediment. A period of relative tectonic stability during the mid-Miocene saw the silcretisation and calcretisation of older Kalahari Group lithologies, and this was followed in the Late Miocene by relatively minor uplift of the eastern side of southern Africa and along certain epeirogenic axes in the interior. More significant uplift that followed in the Pliocene along epeirogenic axes may have elevated Karoo Supergroup and basal Kalahari Group sedimentary rocks above the basin floor where they were exposed to erosion. The eroded sand was washed into the basin where it was reworked and redeposited by aeolian processes during drier periods, resulting in the extensive dune fields that are preserved today. Propagation of the East African Rift System into southern Africa has led to the formation of local depressions and the disruption of drainages, notably of the Kwando and Okavango Rivers. Lakes periodically occupied these depressions in the past. Large alluvial fans have developed in the depressions as a result of loss of flow confinement, and they currently support extensive wetlands (Okavango and Linyanti swamps).

Evolution of the lithosphere in the area of the Rhine Rift System

The Rhine Rift System (RRS) forms part of the European Cenozoic Rift System (ECRIS) and transects the Variscan Orogen, Permo-Carboniferous troughs and Late Permian to Mesozoic thermal sag basins. Crustal and lithospheric thicknesses range in the RRS area between 24–36 km and 50–120 km, respectively. We discuss processes controlling the transformation of the orogenically destabilised Variscan lithosphere into an end-Mesozoic stabilised cratonic lithosphere, as well as its renewed destabilisation during the Cenozoic development of ECRIS. By end-Westphalian times, the major sutures of the Variscan Orogen were associated with 45–60 km deep crustal roots. During the Stephanian-Early Permian, regional exhumation of the Variscides was controlled by their wrench deformation, detachment of subducted lithospheric slabs, asthenospheric upwelling and thermal thinning of the mantle-lithosphere. By late Early Permian times, when asthenospheric temperatures returned to ambient levels, lithospheric thicknesses ranged between 40 km and 80 km, whilst the thickness of the crust was reduced to 28–35 km in response to its regional erosional and local tectonic unroofing and the interaction of mantle-derived melts with its basal parts. Re-equilibration of the lithosphere-asthenosphere system governed the subsidence of Late Permian-Mesozoic thermal sag basins that covered much of the RRS area. By end-Cretaceous times, lithospheric thicknesses had increased to 100–120 km. Paleocene mantle plumes caused renewed thermal weakening of the lithosphere. Starting in the late Eocene, ECRIS evolved in the Pyrenean and Alpine foreland by passive rifting under a collision-related north-directed compressional stress field. Following end-Oligocene consolidation of the Pyrenees, west- and northwest-directed stresses originating in the Alps controlled further development of ECRIS. The RRS remained active until the Present, whilst the southern branch of ECRIS aborted in the early Miocene. Extensional strain across ECRIS amounts to some 7 km. Plume-related thermal thinning of the lithosphere underlies uplift of the Rhenish Massif and Massif Central. Lithospheric folding controlled uplift of the Vosges-Black Forest Arch.

Sedimentary aspects and paleoenvironmental evolution of a rift basin: Salta Group (Cretaceous–Paleogene), northwestern Argentina

The rift history of the Salta basin is related to the evolution of the Central Andes and to the activity of the Pacific margin, owing to its geographic location. Sedimentation occurred from the Neocomian to the Paleogene, with deposits reaching up to 5,000 m in thickness. Paleoenvironmental analysis reveals an evolutionary history controlled by tectonic and climatic changes. Isolated grabens characterized the early synrift stage; differential subsidence provoked distinct environments in the southern and northern subbasins. In the southern subbasins, alluvial-fan, fluvial-fan and lacustrine deposits prevail, whilst in the northern subbasins eolian and fluvial environments dominate. During the Maastrichtian, two major factors controlled the basin fill: the decrease in tectonic subsidence and a relative sea-level rise as recorded in South America. An extensive and shallow Atlantic marine ingression installed a carbonate system coincident with mainly humid conditions until the Danian. Until the Middle Eocene, the fluvial and lacustrine environmental evolution of the sag basin was controlled especially by the alternation of temperate with dry and humid periods. Paleontological records reflect these climatic changes and show their relationship to the sedimentation regime.

The Stellwagen Bank region off eastern Massachusetts: a Wisconsin glaciated Cenozoic sand banks/delta?

Stellwagen and Tillies banks, Jeffreys Ledge, the erosional remnants shoreward of the banks in Massachusetts Bay, Cape Cod Bay and beneath Cape Cod may define an Eocene sand banks/delta. The present configuration of the region is due to a combination of pre-Wisconsin fluvial erosion, Wisconsin proglacial, subglacial and marine processes and Holocene marine processes. Wisconsin sediments south of Jeffreys Ledge experienced deformation and the formation of sag basins by the sinking of the overlying sediments into the underlying fine grained mass with the sediments between the basins being squeezed up to producing diapirs. The western part of Jeffreys Ledge rising tens of meters above its surroundings is a moraine created during a local re-advance when the overall Laurentide ice was retreating prior to 14 000 years ago. This re-advance resulted in the deformation of Coastal Plain and Wisconsin strata.

Post-Variscan evolution of the lithosphere in the Rhine Graben area: constraints from subsidence modelling

Abstract In the area of the Cenozoic Rhine rift system, crustal and lithospheric thicknesses range between 24 and 35 km, and 60 and 120 km, respectively. This rift system transects the deeply truncated Variscan Orogen and superimposed Permo-Carboniferous wrench-induced troughs, and Late Permian and Mesozoic thermal sag basins. At the time of its Westphalian consolidation, the Variscan Orogen was probably characterized by 45–60 km deep-crustal roots that were associated with its Rheno-Hercynian-Saxo-Thuringian, Saxo-Thuringian-Bohemian and Bohemian-Moldanubian sutures, all of which are transected by the Cenozoic Rhine rift system. During the Stephanian-Early Permian wrench-induced disruption of the Variscan Orogen, subducted lithospheric slabs were detached causing upwelling of hot mantle material. During the resulting thermal surge, partial delamination and/or thermal thinning of the continental mantle-lithosphere induced regional uplift. At the same time the Variscan orogenic roots were destroyed and crustal thicknesses reduced to 28–35 km in response to the combined effects of mantle-derived melts interacting with the lower crust, regional erosional unroofing of the crust and, on a more local scale, by its mechanical stretching. Towards the end of the Early Permian, the potential temperature of the asthenosphere returned to ambient levels. With this, regional, long-term thermal subsidence of the lithosphere commenced, controlling the development of a new system of Late Permian and Mesozoic thermal sag basins. However, the evolution of these basins was repeatedly overprinted by minor short-term subsidence accelerations that reflect the build-up of far-field stresses related to rifting in the Tethyan and Atlantic domains. Comparison of observed and modelled subsidence curves suggests that in the area of the Rhine rift system the lithosphere had equilibrated with the asthenosphere at the end of the Cretaceous at depths of 100–120 km, before it became thermally destabilized again by Cenozoic rifting and plume-related magmatism. Modelled subsidence curves indicate that by the end of Early Permian times the thermal thickness of the remnant mantle-lithosphere ranged between 10 and 50 km in areas that were later incorporated into Mesozoic thermal sag basins; this corresponds to mid-Permian thermal lithosphere thicknesses of 40–80 km.

Geology and tectonic evolution of Palaeoproterozoic basins of the eastern Capricorn Orogen, Western Australia

Abstract The Palaeoproterozoic Yerrida, Bryah, Padbury, and Earaheedy Basins extend for about 700 km along the northern margin of the Yilgarn Craton and are part of the Capricorn Orogen. The basins developed between ca. 2.2 and 1.8 Ga, recording periods of sedimentation, volcanism, rifting, accretion and passive-margin tectonism. The oldest is the 2.17 Ga Yerrida Basin, which began as an intracontinental sag within which siliciclastics and evaporites accumulated. At about 2.0 Ga convergence of the Yilgarn Craton and the Glenburgh terrane closed an oceanic arm, resulting in the accretion of oceanic crust and plateaux onto the Yilgarn margin and the western side of the Yerrida sag basin. This accretion produced the Narracoota Formation, which together with the clastic and chemical (banded iron-formation) sedimentation that followed, forms the Bryah Basin. Continuing collision resulted in the establishment, around 1.96 Ga, of the Padbury foreland basin, developed on top of the Bryah Basin rocks. The ca. 1.83 Ga convergence and collision of the Pilbara and Yilgarn Cratons (Capricorn Orogeny), resulted in deformation of the Bryah–Padbury Basins and orogenic uplift with coarse clastic sedimentation accompanied by the eruption of flood basalts into the Yerrida Basin. To the east of the collision zone, and possibly postdating collision, a passive margin was present. Sedimentation along it consisted of clastic and chemical sediments (granular iron-formation), which form the Earaheedy Basin. During the ca. 1.79 Ga Yapungku Orogeny, due to the convergence of the North Australian and Western Australian cratons, the northern margin of the Earaheedy Basin was deformed (Stanley Fold Belt). This last deformation event concludes the Palaeoproterozoic history of the eastern Capricorn Orogen basins.

Sequence stratigraphy and evolution of the Paleoproterozoic intracontinental Baker Lake and Thelon basins, western Churchill Province, Nunavut, Canada

Abstract The Baker Lake and Thelon basins lie along the border between the Rae and Hearne domains of the western Churchill Province. Basin fill comprises the Dubawnt Supergroup, a ca. 1.85–1.70 Ga succession of predominantly continental clastic and intercalated volcanic rocks that nonconformably and unconformably overlies granitic and supracrustal rocks, most of late Archean age. The Dubawnt Supergroup comprises three second-order sedimentary sequences that record deposition within a rift basin, a modified rift basin and a thermal sag basin, respectively. The Baker sequence is inferred to represent the principal phase in the development of Baker Lake Basin, a series of generally elongate, northeast-striking, half-graben and fault-bounded troughs filled with continental redbeds and coeval voluminous ultrapotassic volcanic rocks. Faults parallel to the basin margin exhibit evidence for strike-slip ca. 1.83–1.81 Ga. The nature and timing of basin development and associated magmatism allows that it may be a transtensional basin that formed by lateral escape in response to crustal thickening during the Trans-Hudson orogeny, similar to the formation of Mesozoic and Cenozoic strike-slip and associated rift basins on the margins of the Tibetan Plateau. The overlying Whart sequence was deposited in small basins formed by block-faulting and tilting of the Baker sequence. Basin fill comprises eolian and alluvial redbeds with intercalated rhyolite flows and epiclastic rocks. Distribution of facies and stratigraphy suggest that faulting occurred throughout deposition of the Whart sequence, but ceased before deposition of the overlying Barrens sequence (ca. 1.72 Ga). Deformation and basin formation can be linked to a regional episode of granite emplacement and associated thermal metamorphism (ca 1.76–1.75 Ga). The Barrens sequence represents deposition over a broader area, primarily in Thelon Basin. Strata generally are thinner and flatter than underlying sequences, and display lateral continuity, indicating minimal influence from syndepositional faulting. The depositional record reflects progressive upward fining and eventually the first record of marine transgression within the Dubawnt Supergroup. These features suggest that the Barrens sequence was deposited over a broad region of thermal subsidence, likely related to cooling of previously attenuated continental lithosphere. The Barrens sequence may be a remnant of a huge cratonic sand sheet that included the Thelon, Athabasca, Amundsen and Elu basins. A regional subsidence mechanism to account for these basins may be mantle downwelling linked to the late-stage amalgamation of Laurentia. The tripartite sequence evolution of the Baker Lake and Thelon basins represents a framework for future sequence correlation among late Paleoproterozoic basins in Laurentia, and shares characteristics with the evolution of contemporaneous intracontinental basins from north-central Australia and southeastern Brazil.

Jaibaras trough: an important element in the early tectonic evolution of the Parnaı́ba interior sag basin, Northern Brazil

The Jaibaras Trough (JT) is a late Precambrian to Early Paleozoic basin that is related to the subsequent formation of the Paleozoic–Mesozoic Parnaı́ba intracratonic basin in northern Brazil. Based on available geological and geophysical data, we propose that the JT is a prototypical rift basin formed by brittle reactivation processes along ancient shear zones associated with the NE–SW Transbrasiliano Lineament of the Brazilian–Panafrican Orogeny. The JT evolution is partially constrained by the stratigraphic sequences outcropping at the border of the basin or drilled by exploratory boreholes. The preserved sedimentary column records, in a broad sense, the sedimentary and tectonic controls on the early subsidence in the Parnaiba Basin. These sequences are constituted by basal fault-scarp-related paraconglomerates followed laterally and vertically by fluvial–lacustrine sandstones interbedded with reddish shale. The JT geological evolution is also constrained by petrology of four magmatic events (Coreaú Dike Swarm, Mucambo and Meruoca plutons and mafic Parapuı́ Suite) related to continental rifting. The Late Precambrian–Early Paleozoic age of JT is perfectly compatible with continental fragmentation during the processes of lithospheric stretching that affected western Gondwana at that time. The contemporaneous development of a magmatic-sedimentary system, dated as Cambrian to Ordovician and confined to the rift trend, raised the thermal gradient and produced a mantle softening discontinuity, giving rise to a broad uplifted region. This might explain the close temporal and spatial association between the Transbrasiliano Lineament and the Early Paleozoic depositional axis of the Parnaı́ba Basin. An integrated interpretation of the JT and the early tectonostratigraphic behaviour of the Parnaı́ba Basin indicates that the JT might constitute a precursory stage that was slightly inverted before the initiation of sag deposition in the intracratonic basin.

Syn-rift regional subsidence across the West African continental margin: the role of lower plate ductile extension

Abstract New ostracode data from the West African margin indicate that the Outer Basin Sediment Wedge (also termed the ‘pre-salt wedge’ and the ‘pre-salt sag basin’) is Neocomian to Aptian in age and is contemporaneous with syn-rift deposits developed inboard of the Atlantic hinge zone. Despite the fact that the Outer Basin Sediment Wedge is clearly a syn-rift deposit, it does not exhibit any of the diagnostic characteristics of brittle deformation, such as the existence of normal faults and the faulting and rotation of crustal blocks. Such features are common between the Atlantic and Eastern hinges for the early stages of rifting between West Africa and Brazil, which occurred as a series of extensional phases commencing in the Berriasian and culminating in the Late Aptian. To reconcile the concomitant development of fault-controlled subsidence between the hinges and across the Atlantic hinge zone and sag-basin development seaward of the Atlantic hinge zone requires that: (1) extension seaward of the Atlantic hinge is the result of strain-partitioning between a relatively non-deforming upper crust (i.e. the upper plate) and a ductile-deforming lower crust and lithospheric mantle (i.e. the lower plate) during the second and third rift phases, while (2) between the hinges, early brittle deformation (normal faulting) progresses to ductile deformation in the third rift phase. During the third rift phase, lower plate ductile deformation across the entire region generated regional subsidence both seaward of the Atlantic hinge and between the hinges with little attendant brittle deformation. This extension style produced, directly or indirectly, a sequence of crucial events across the West African margin: (1) the development of the pre-Chela unconformity as lake level dropped in the Early Aptian, exposing the prograding deltas of the Argilles Vertes Formation; (2) the regional development of the Chela unconformity and transgressive lag deposits of the Chela Formation in the Mid-Aptian; (3) the development of regionally extensive, shallow-water, restricted marine conditions across the entire margin (between West Africa and Brazil) immediately prior to evaporite precipitation; and (4) the development of significant post-rift accommodation (deposition of the Late Cretaceous, Paleogene and Neogene formations) in the same region previously characterized by minor syn-rift faulting, repeated dessication cycles (allowing the precipitation of thick evaporites) and negligible erosional truncation of earlier syn-rift units. Previous workers have suggested that the Loeme evaporites were formed as part of the rapid, early post-rift phase of basin subsidence as the region became inundated by sea water across the Walvis Ridge. In this model, it is difficult to develop the restrictive environments required to deposit the thick (>1 km) evaporites of the Loeme Formation (and the equivalent Ezanga and Ibura evaporites of Gabon and Brazil, respectively) across the entire West African-Brazilian rift system. The existence of shallow-water environments across the entire region is not consistent with water depths determined from the relief of clinoform foresets existing immediately prior to evaporite deposition thus requiring tectonic uplift of the deep-water regions. These evaporites, therefore, appear to be part of the late-stage syn-rift sediment package and the break-up unconformity, if it exists, separates the Loeme evaporites below from the overlying Albian carbonates. A direct consequence of ductile extension is one of increased heat input accompanying the rift stage in those areas dominated by syn-rift sag-basin development. The distribution and amplitude of the heat pulse is governed by the geometry of the mid-crustal weak zone and the distribution and amplitude of the lower plate extension. Seaward of the Atlantic hinge zone, the maximum heat flow is predicted to be in excess of 200 mW m −2 , whereas between the hinge zones, the heat flow is significantly less and ranges between 20mW/m 2 and 100 mW/m 2 . Because sediment temperature is a function of thermal conductivity and thickness of sediment overburden, the viability of syn-rift sources and prospectivity of the deep-water West African margin will, to a large degree, depend on the delicate interplay between the cooling of the extended lithosphere and subsequent burial of source rocks as a function of time.

Vacuum desorption of light hydrocarbons adsorbed on soil particles: A new method in geochemical exploration for petroleum

Light hydrocarbons in soils are the most important indicators in geochemical hydrocarbon exploration. However, the methods actually used to detect light hydrocarbons in soil sometimes provide data with a low signal-to-noise ratio and/or high level of interference. This research explores vacuum desorption, a new method for light hydrocarbon analysis. Vacuum desorption is a partial extraction technique designed to identify light hydrocarbons that are tightly adsorbed on soil particles. This method was used in the Huimin sag (basin) of east China. Microseepage anomalies are developed over oil pool S105 of the Shanghe field in this basin and larger seepage anomalies are developed over faults. In contrast to the method of acid-extraction of soil hydrocarbons, the vacuum desorption method provides a low-noise background that enables a more effective identification of anomalies. The new method is also cost and time effective and is capable of covering large areas.

Proterozoic (1.85–1.75 Ga) igneous suites of the Western Churchill Province: granitoid and ultrapotassic magmatism in a reworked Archean hinterland

Paleoproterozoic igneous rocks in the Archean hinterland of the Paleoproterozoic Trans-Hudson orogen (THO) consist of voluminous late syn-orogenic to post-orogenic monzonite to granite (Hudson granitoids; ≈1850–1810 Ma), and contemporaneous ultrapotassic lamprophyre dykes and volcanic rocks (Dubawnt minettes) that are interbedded with alluvial fan and fluvial deposits (Baker Lake Group, lower Dubawnt Supergroup). They were followed at approximately 1750 Ma by rapakivi granite (Nueltin granite) and porphyritic rhyolite associated with aeolian sandstone (Pitz Formation, middle Dubawnt Supergroup). The tectonic cycle ended with the deposition of conglomerates and sandstones in a large sag basin (Thelon Formation, upper Dubawnt Supergroup, ≈1.72 Ga). The Hudson granitoids, which are strongly concentrated northwest of the THO, were broadly synchronous with terminal collision between the Archean Churchill and Superior cratons and the development of NE-trending ductile structures in the Western Churchill Province (WCP) that may be related to tectonic escape to the northeast. They were emplaced at mid-crustal level and no volcanic equivalents are preserved. Fault-bounded basins containing the minette volcanic rocks are located farther west in a domain dominated more by brittle faulting. The Nueltin granites, emplaced during a period of active extensional faulting, are present in a band extending southwest from the minette basins toward a preserved remnant of the sag basin (the Athabasca basin). Hudson granitoids are largely absent from this band but reappear west of it, indicating a higher crustal level of exposure in a downdropped Nueltin ‘corridor’. The Nd isotope composition of the three suites is similar (minettes: ε Nd,1830 Ma=−5 to −11; Hudson granitoids: ε Nd,1830 Ma=−7 to −13.5; Nueltin suite: ε Nd,1750 Ma=−7 to −10.5), and they have late Archean model ages that match those of average Archean WCP rocks. The Hudson granitoids are rich in inherited Archean zircon, and both granitoid suites are interpreted as crustal melts. Some Nueltin granites and Pitz rhyolites are mingled with basalt, and the Nueltin suite fits a commonly cited model for rapakivi granite production, which postulates injection of basalt into extending, brittly faulted crust. The Hudson granitoids are similar to late syn- to post-orogenic plutons in numerous other collisional hinterlands, which are typically associated with ultrapotassic lamprophyres. The minettes, which have high mg# and bear mantle xenocrysts, must have a mantle source component, and their source region could have been subduction-enriched lithospheric mantle. However, their source had only slightly lower time-integrated LREE enrichment than did that of the granitoids, and the incompatible element signatures of the two suites are strikingly similar. The minette source region may have been in a zone of mixed crust and upper mantle, formed during a shortening event which resulted in crustal thickening and subsequent melting at mid-crustal layers to form the Hudson granitoid plutons. The generation and emplacement of minette melts may have been promoted by extension related to a combination of slab breakoff, gravitational collapse of thickened crust, and strike-slip faulting in the deforming hinterland. Subsequent anorogenic rapakivi granite-basalt activity may have been triggered by lithospheric mantle delamination. The hinterland tectonic cycle of the WCP was repeated in other large Archean terranes that were deformed during the early Proterozoic, but the igneous and sedimentary record is unusually complete in the WCP.