David I. M. Macdonald

Tectonic and paleoenvironmental evolution of Mesozoic sedimentary basins along the Andean foothills of Argentina (32°–54°S)

Abstract Chronoenvironmental and tectonic charts are presented for Mesozoic basins located along the Andean foothills of the South American plate. On the basis of the main tectonic events, pre-Andean basins, break-up-related basins, extensional back-arc basins, and Andean foreland basins are recognized. The pre-Andean basins were formed by continental extension and strike-slip movement before the development of the Mesozoic–Cenozoic Andean magmatic arc. Upper Permian to Middle Triassic extension along Palaeozoic terrane sutures resulted in rifting, bimodal magmatism (Choiyoi group), and continental deposition (Cuyo basin). From the Late Triassic to the Early Jurassic, continental extension related to the collapse of the Gondwana orogen initiated a series of long, narrow half-grabens that filled with continental volcaniclastic deposits. These depocenters were later integrated into the Neuquen basin. Coeval development of the shallow marine Pampa de Agnia basin (42–44°S) is related to short-lived extension, probably driven by dextral displacement along major strike-slip faults (e.g. the Gastre fault system). Widespread extension related to the Gondwana breakup (180–165 Ma) and the opening of the Weddell Sea reached the western margin of the South American plate. As a result, wide areas of Patagonia were affected by intraplate volcanism (Chon Aike province), and early rifting occurred in the Magallanes basin. The Andean magmatic arc was almost fully developed by Late Jurassic times. A transgressive stage with starvation and anoxia characterized the Neuquen basin. In western Patagonia, back-arc and intra-arc extension produced the opening of several grabens associated with explosive volcanism and lava flows (e.g. Rio Mayo, El Quemado). To the south, a deep marginal basin floored by oceanic crust (Rocas Verdes) developed along the back-arc axis. In mid-to late Cretaceous times, Andean compressional tectonics related to South Atlantic spreading caused the inversion of previous extensional structures and the beginning of a retro-arc foreland phase in the Neuquen and Austral basins.

Transtensional deformation in the evolution of the Bohai Basin, northern China

Abstract Extensional basins with an element of strike-slip deformation can form because of a perturbation in a strike-slip fault zone (pull-apart and fault wedge basins), or where extension is oblique to the margins of the deforming zone (transtensional basins). Transtensional basins are characterized by en echelon arrays of normal faults which are individually oblique to the basin margins. The Bohai Basin, northern China, has previously been modelled as either (1) a giant pull-apart between NNE-SSW trending dextral strike-slip faults, or (2) a rift basin caused by WNW-ESE extension, without significant strike-slip deformation. We present a model for the Bohai Basin’s rift history in which the basin formed as a result of dextral transtension. The Bohai Basin is one of a family of early Tertiary extensional basins present within eastern Asia from northeastern Russia to southeast China. The structural grain in this basin was inherited from a phase of late Mesozoic sinistral transpression. Tertiary extension began in the Paleocene. Most half-grabens in the eastern and western regions of the Bohai Basin have master faults with a NE-SW or NNE-SSW orientation. Secondary normal faults strike oblique to the main structures, in en echelon arrays which indicate a component of dextral transtension. The central part of the basin, the Bozhong Depression, became a significant depocentre for the first time in the middle Eocene. It formed when activity on transtensional zones to its east and west created an extensional overlap between them. Thus the basin as a whole resembles a giant pull-apart basin, with the Bozhong Depression as its central depocentre, but dextral transtension rather than simple strike-slip controlled the deformation. The component of dextral deformation in the Bohai Basin is shared by other early Tertiary east Asian extensional basins, and is consistent with the sense of shear implied by the oblique convergence of the Pacific and Asian plates: an east-west convergence vector applied to a NE-SW trending plate boundary. The consistency of this dextral shear along the Asian margin, and the fact that several of these basins pre-date the India-Asia collision, supports an origin by subduction roll-back of the oceanic Pacific plate from Asia. The extrusion model for east Asian basin formation, whereby extension was caused by lateral transport of lithospheric blocks out of India’s northward path following the India-Asia collision, is not applicable to major basins east and northeast of the Red River Fault.

Stratigraphy and sedimentology of the Permian of the Falkland Islands: lithostratigraphic and palaeoenvironmental links with South Africa

Comparison of the facies, ichnology and palaeocurrent patterns of the Permian of the Falkland Islands and the Dwyka, Ecca and Beaufort (part) groups in South Africa supports the hypothesis that the Falkland Islands lay east of South Africa in a rotated position prior to continental break-up. Key sections in the Falklands (NW Lafonia) and the Eastern Cape (Ecca Pass) would have lain about 200 km apart in Permian time. In the absence of accurate chonostratigraphic and biostratigraphic information from the Permian of the Falkland Islands, emphasis is placed on lithostratigraphic comparison. The Permian successions in both areas pass from diamictite through basin floor sediments, including organic-rich shale, rhythmites and turbidites. Mud-dominated delta front deposits culminating in channel deposits of a generally subaqueous delta top overlie the basin floor sediments. Points of close lithostratigraphic comparison are: (1) the diamictites have identical appearance, facies, and compatible palaeocurrents; (2) glacial sections are overlain by organic-rich black shales; (3) K-bentonites occur in basin-floor successions; (4) presence of basin-floor turbidite sandstone units with interbedded rhythmite units, and a distinctive Umfolozia/Undichna ichnofauna; (5) coarsening-up delta-front sequences are overlain by fining-up channel sandstone facies; (6) there is a close petrographic similarity indicating derivation of sandstones from a contemporaneous volcanic arc. The Permian successions of the two areas form parts of the fill of the eastern end of the main Karoo Basin, and were subject to the same influences of tectonic development, sediment derivation, eustatic variation and climatic change.

A Depositional and Sequence Stratigraphic Model for Cold-Water, Spiculitic Strata Based on the Kapp Starostin Formation (Permian) of Spitsbergen and Equivalent Deposits from the Barents Sea

This article summarizes the lithostratigraphy of two sections through the Kapp Starostin Formation on Spitsbergen having contrasting distal and proximal depositional settings. These outcrop sections are compared with an approximately age-equivalent spiculitic unit penetrated in two exploration wells 800 km to the southeast on the Finnmark Platform. Eight main facies associations comprise these strata, including limestones (matrix-rich, matrix-poor, and mudstone), siliciclastics (shale and sandstone), spiculite (dark/tight and light-colored/porous), and phosphatized surfaces. A sequence stratigraphic model is proposed wherein siliciclastic-limestone intervals represent lowstands of relative sea level, and the spiculites are mainly highstand deposits. Based on this model, seven depositional cycles (sequences) are recognized in the distal, high-accommodation locality on Spitsbergen (Akseloya), four in the proximal locality (Dickson Land), and two on the Finnmark Platform. A speculative model for correlation between these localities is presented, wherein lateral amalgamation of sequences, due to landward pinch-out of some lowstand intervals, results in recognition of fewer sequences in areas of lower accommodation. On both Dickson Land and the Finnmark Platform, an upward change in spiculite lithology from dark/tight to light/porous is shown, and Dickson Land also shows an upward change in siliciclastic lithology from shale to glauconitic sandstone. These changes record a long-term shoaling trend approaching the Permian-Triassic boundary. This model provides a basis for interpreting the basin-scale (Begin page 2062) distribution of lithofacies and predicting the occurrence of porous zones representing potential hydrocarbon reservoirs.

Inertinite-rich tertiary coals from the Zeya-Bureya Basin, Far Eastern Russia

Selected Tertiary coals from the Zeya–Buryea Basin, Far Eastern Russia, were investigated for aspects of their coal type, rank, depositional environment and post-depositional history. The coals have been examined in outcrop (lithotype logging), microscopically (maceral, reflectance and fluorescence), and geochemically (proximate analysis). Two laterally extensive coal-bearing horizons occur: one of Palaeocene age and the other of early Miocene age. The Palaeocene coals were investigated in active open-cut mines at Raichikhinsk and Yerkovtsi and the early Miocene deposit in an abandoned open-cut mine at Cergeyevka. Palaeocene coals at Raichikhinsk and Yerkovtsi were indistinguishable from each other macroscopically, microscopically, and geochemically. The deposits were sufficiently coalified that brightness logging could be undertaken. Dull coals, with numerous fusainous wisps, were dominant. Four dulling-up sequences, which represent stacked peat deposits, were observed at Raichikhinsk. At Yerkovtsi, only a small section of the middle of the seam, which was mostly dull and muddy coal, was investigated. Petrographically, these coals were dominated by inertinite group macerals, which is unusual in non-Gondwanan coals and rare in the Tertiary. Rank classification was problematic with volatile matter (VM) content of vitrain (daf), macroscopic appearance, and microscopic textures suggesting subbituminous B rank, but carbon content, moisture content and specific energy indicating a lignite rank. Notwithstanding complications of rank, estimates of the maximum-range burial depths were calculated. Taking the VM (daf) content of vitrain as 48%, burial depth estimates range from 900 m for a high geothermal gradient and long heating time to a maximum of 3300 m for a low geothermal gradient and short heating time. These estimates are maxima as the coal rank may be lower than implied by the VM. The Cergeyevka deposit is a soft brown coal. Limited sampling of the upper-most portion indicated a high moisture content (75% daf) and an unusual, hydrogen-rich geochemistry. Lack of identifiable liptinites using either reflected light or fluorescence microscopy suggested a significant bituminite component. Otherwise, the coals appear to be typical for the Tertiary. An estimate of 125 m maximum burial depth was obtained using the bed-moisture content of the coal, which is around the present burial depth. Comparison of present-day thicknesses with inferred burial depths suggests that at least 500 m of section is missing between the Palaeocene coals and the early Miocene coals. Palaeoenvironmental considerations suggest that fire played a significant role in the accumulation of the peats at Raichikhinsk and Yerkovtsi. At Cergeyevka, peat accumulation ended by drowning of the mire. Two tuff beds were recognised within the seam at Raichikhinsk and one in the seam at Yerkovtsi. Correlation of the tuff beds is uncertain but they should prove useful in regional coal seam correlation and interpreting coal depositional environments. Geochemical analysis by XRF was complicated by high loss-on-ignition (LOI) values. Despite extensive alteration, an acid igneous source is implied from the presence of free quartz and TiO2/Al2O3 ratios of 0.02 to 0.05.

Gondwana break-up related magmatism in the Falkland Islands

Jurassic dykes (c. 182 Ma) are widespread across the Falkland Islands and exhibit considerable geochemical variability. Orthopyroxene-bearing NW–SE-oriented quartz-tholeiite dykes underwent fractional crystallization at >1 GPa, and major element constraints suggest that they were derived by melting of a pyroxenite-rich source. They have εNd182 in the range −6 to −11 and 87Sr/86Sr182 >0.710 and therefore require an old lithospheric component in their source. A suite of basaltic-andesites and andesites exhibit geochemical compositions transitional between Ferrar and Karoo magma types, and are similar to those seen in the KwaZulu-Natal region of southern Africa and the Theron Mountains of Antarctica. Olivine-phyric intrusions equilibrated at <0.5 GPa, and have isotopic compositions (εNd182 1.6–3.6 and 87Sr/86Sr182 0.7036–0.7058) that require limited interaction with old continental lithosphere. A suite of plagioclase-phyric intrusions with 87Sr/86Sr182 c. 0.7035 and εNd182 c. +4, and low Th/Ta and La/Ta ratios (c. 1 and c. 15, respectively) also largely escaped interaction with the lithosphere. These isotopically depleted intrusions were probably emplaced synchronously with Gondwana fragmentation and the formation of new oceanic lithosphere. Estimates of mantle potential temperature from olivine equilibration temperatures do not provide unequivocal evidence for the presence of a plume thermal anomaly beneath the Falkland Islands at 182 Ma. Supplementary materials: Mineralogical data, XRF major and trace element data and Ar-Ar results for Falkland Islands intrusions are available at http://www.geolsoc.org.uk/sup18873.

Evolution of the west Greenland margin: offshore thermostratigraphic data and modelling

The origin and age of topography along the west Greenland margin is a matter of continued debate. Evidence for tectonically driven Neogene uplift has been argued from interpretations of offshore seismic surveys, onshore fission-track data and inferred episodes of cooling. Here, analysis of seismic reflection profiles and 1D modelling of exploration wells along the Greenland margin of Davis Strait demonstrate that the data are consistent with a model of ancient continental topography affected by late Cretaceous–early Palaeocene rifting followed by thermal subsidence where offshore Neogene tectonic uplift is not required. This interpretation for the offshore evolution of the west Greenland margin has implications for the adjacent onshore evolution and for other continental margins developed throughout the Atlantic–Arctic rift system.

Tying catchment to basin in a giant sediment routing system: a source-to-sink study of the Neogene–Recent Amur River and its delta in the North Sakhalin Basin

Abstract This paper uses an extensive dataset from more than 200 samples to provide a comprehensive source-to-sink analysis of the Amur River and its delta in the Russian Far East. The majority of sand-sized sediment in the Amur River and its former delta comes from upstream of the Lesser Khingan Ridge, shown by uniformity of sediment composition in the lower 1700 km of the river. Stable mineral ratios, U–Pb age spectra and garnet geochemistry show little stratigraphic provenance-specific variation in the Neogene delta. This renders Miocene–Pliocene drainage capture models unlikely. The onset of uplift in the delta is marked by a decrease in the apatite–tourmaline index (ATi) in Upper Pliocene offshore well samples, caused by dissolution of apatite as sediments were uplifted and eroded onshore Sakhalin. These wells also show variable ATi and garnet–zircon index (GZi) values in Lower Miocene samples, which could potentially be used for stratigraphic correlation. A positive correlation between GZi values and distance from the river mouth is attributed to hydrodynamic sorting across the delta system. This has negative implications for the use of this stable mineral index and others of a similar hydraulic equivalence as regional correlation tools on a basin scale (>100 km). Supplementary material: Heavy mineral data, petrographic data, geochronometric data, sample locations available at www.geolsoc.org.uk/SUP18643.

Long-term exhumation of a Palaeoproterozoic orogen and the role of pre-existing heterogeneous thermal crustal properties: a fission-track study of SE Baffin Island

New apatite fission-track data from SE Baffin Island exhibit central ages that range from just under 200 Ma to 440 Ma, and mean track lengths that vary between c. 12 and 13.3 µm. First-order analysis of the data (a plot of central age v. mean track length) reveals an approximate ‘boomerang’ trend, typical of samples that have experienced contemporaneous cooling from an array of initial temperatures. One-dimensional inverse thermal modelling of single samples suggests that cooling through the partial annealing zone (c. 120–60 °C) occurred over discrete periods ranging from 100 to 300 Ma. Modelling the 3D exhumation of a heterogeneous crust with flat topography demonstrates that some of the variability in observed fission-track ages could be attributed to heterogeneity in crustal heat production and thermal conductivity. The remaining variability in the observed dataset is attributed here to differential erosion from a variable initial topography. However, age discontinuities over short distances require other explanations such as faulting and/or unidentified compositional effects. Collectively, these results suggest that the observed data are consistent with a simple exhumation scenario where the present-day high topography is a remnant of that created during Palaeoproterozoic orogenies. The new data do not require any recent (Cenozoic) periods of exhumation. Supplementary material: Data locations, and isotopic and petrographic results are available at www.geolsoc.org.uk/SUP18657.

Quaternary landscape evolution over a strike-slip plate boundary: Drainage network response to incipient orogenesis in Sakhalin, Russian far east

This study focuses on the fluvial and tectonic landscape of the North Sakhalin Basin (Russia), where 5 km of Neogene deltaic sediments were deposited across the Okhotsk-Amur plate boundary. The homogeneous, poorly lithified sedimentary sequence created a flat landscape without structural inheritance. These sediments are now being actively deformed by oblique compression. This allows us to investigate the early stages of orogenesis in a strike-slip plate boundary and the response of drainage networks in such a setting, and to construct a model for the topographic evolution along 220 km of the plate boundary. We use fluvial geomorphological indicators (planform morphology, concavity, steepness indices, and knickpoint distribution) as evidence for active landscape deformation. Tectonics and topography are strongly coupled, and neotectonic activity can be observed directly from the landscape. Knickpoints are mostly located on fault planes, suggesting geologically recent activity, or in areas of drainage capture, where they are associated with low concavity indices. Geomorphic indications from longitudinal river profiles and planform morphology suggest that creation of anticlines and disruption of drainage patterns appears to be diachronous in the North Sakhalin Basin, with deformation propagating north and east through time. Minimum uplift and strike-slip displacement rates in the northeast of Sakhalin are 0.63 mm a -1 and 1.95 mm a -1 , based on exhumed stratigraphy and offset drainage networks, respectively.