Basin Research - Volume 30, Issue 6, 2018

Temperature impacts the quality of reservoir and source rocks, thereby representing an important aspect of petroleum prospect assessments of any basin. This case study of the Triassic basin of Edgeøya, eastern Svalbard, reevaluate earlier burial estimates that were solely based on organic matter maturation data from organic rich shales. Here, we estimate temperatures using a multi‐method approach: Rock‐Eval pyrolysis parameters, fluid inclusions in diagenetic quartz, and inorganic diagenesis signatures of sandstones. Data were collected from organic rich shales of the Botneheia and Tschermakfjellet formations and coal bearing sandstones of the De Geerdalen Formation. Rock‐Eval pyrolysis data indicate that Botneheia and Tschermakfjellet formations experienced burial temperatures of about 124–138°C while the De Geerdalen Formation experienced temperatures ≥92°C. Homogenization temperatures of the De Geerdalen Formation sandstones give similar diagenetic temperatures, from 70 to 124°C, while the kaolinite to dickite transformation implies the temperatures >90°C. Furthermore, the absence of illite formation associated with kaolinite suggest that temperature have never exceed 130°C. Integrating various methods validate spatial variations in temperature proxies and constrain the thermal history of this basin. Cretaceous intrusions have locally affected the temperatures and have obscured regional subsidence and uplift trends. The effect of igneous intrusions on inorganic matter is very limited, but intensive on organic matter. These differences between organic and inorganic thermal indices help in distinguishing of magmatic from burial heating. This study has therefore relevance in deciphering the thermal history of sandstones experiencing magmatic activity coupled with multiple burial and uplift events.

A new dimension has recently been added to provenance analysis by the rapid development of detrital‐geochronology techniques. The application of any dating method to sediments allows the definition of a unique age pattern of parent rocks, a “time structure” that represents an essential complement to the information on their lithological structure obtained by traditional petrographic and mineralogical methods. This detrital‐geochronology study illustrates the distribution of U‐Pb zircon ages in all parts of the Nile catchment from its equatorial headwaters to the Delta, and surveys how the provenance signal is formed, transmitted, and modified along this huge sediment‐routing system. Age‐spectra obtained by targeting specific parts of zircon grains after cathodo‐luminesce imaging and by our Automated Phase Mapping + LAICPMS “blind‐dating strategy” were compared. The former approach emphasises specific magmatic or metamorphic events in source areas, whereas the latter aims at minimising selection bias and focuses on consistency among samples. Grain‐size and hydraulic‐sorting controls were also checked, but found to have only a minor effect on zircon‐age distributions. The trimodal age spectrum of Kagera zircons sourced from the rift highlands of Burundi and Rwanda, characterised by prominent late Neoarchean (Aruan) and mid‐Mesoproterozoic (Kibaran) peaks with a wider mid‐Palaeoproterozoic (Ubendian) cluster, is lost in the Lake Victoria sediment sink. The sharp unimodal Aruan peak displayed by zircon grains in both Victoria and Albert Nile is supplemented and finally superseded by Neoproterozoic grains across the vast marshlands of South Sudan, where detritus produced in equatorial regions is eventually stored. All Nile tributaries in Sudan and Egypt carry zircon grains yielding predominantly Neoproterozoic ages, with a major peak around 0.6 Ga associated with clusters around 0.8 and 1.0 Ga. A few Oligocene zircons represent the key diagnostic feature of Ethiopian provenance, the Ariadne's golden thread that allowed retracing the palaeo‐Nile to its Ethiopian sources back in the Oligocene. The Nile presents a text‐book case of discontinuous transmission of provenance signals along a segmented ultra‐long drainage system. Zircon‐age fingerprints as well as all other detrital signatures are lost repeatedly in large Ugandan lakes, reconstituted or replaced, lost again in the vast marshlands of South Sudan, and finally homogenised downstream from Ethiopia and Sudan to the Mediterranean Sea.

The continental margins of East Africa and West Madagascar are a frontier for hydrocarbon exploration. However, the links between the regional tectonic history of sedimentary basins and margin evolution are relatively poorly understood. We use a plate kinematic model built by joint inversion of seafloor spreading data as a starting point to analyse the evolution of conjugate margin segments and corresponding sedimentary basins. By correlating megasequences in the basins to the plate model we produce a margin‐scale tectono‐stratigraphic framework comprising four phases of tectonic development. During Phase 1 (183–133 Ma) Madagascar/India/Antarctica separated from Africa, first by rifting and later, after breakup (at ca. 170–165 Ma), by seafloor spreading in the West Somali and Mozambique basins and dextral strike‐slip movement on the Davie Fracture Zone. Mixed continental/marine syn‐rift megasequences were deposited in rift basins followed by shallow‐marine early postrift sequences. In Phase 2 (133–89 Ma) spreading ceased in the West Somali basin and Madagascar became fixed to the African plate. However, spreading continued between the African and Antarctic plates and deposition of the early postrift megasequence continued. The onset of spreading on the Mascarene Ridge separated India from Madagascar in Phase 3 (89–60 Ma). Phase 3 was characterized by the onset of deposition of the late postrift megasequence with continued deep marine sedimentation. At the onset of Phase 4 (60 Ma onward) spreading on the Mascarene ridge ceased and the Carlsberg Ridge propagated south to form the Central Indian Ridge, separating India from the Seychelles and the Mascarene Plateau. Late postrift deposition continued until a major unconformity linked to the development of the East African Rift System marked the change to deposition of the modern margin megasequence.

Intermontane basins are often the result of regionally variable uplift in tectonic settings. Wedge‐top basins, a type of intermontane basin, form along thrust faults within a fold and thrust belt, and provide an ideal environment to study the regional fluvial and surface response to local variations in rock uplift. This study simulates the formation and evolution of an intermontane basin using a landscape evolution model. The modelling results demonstrate that large trunk streams maintain connectivity during basin formation for two reasons: (1) their stream power is enhanced by the capture of smaller streams, enabling them to incise through the uplifting downstream region, and (2) they acquire increased sediment yield to completely infill the upstream accommodation space rather than forming an endorhic basin. During active deformation of the fold‐and‐thrust belt, both channel slope and erosion rates are reduced upstream of the intermontane basin and these changes propagate as a wave of low erosion into the uplands. For a uniform background uplift rate in a landscape previously at steady state, this reduced rate of erosion results in a net surface uplift upstream of the basin. Following the eventual breach of the basin's bounding structural barrier, a wave of high erosion propagates through the basin and increases the channel slope. This onset of increased erosion can be delayed by up to several million years relative to the onset of downstream uplift. Observed paleoerosion rates in paired wedge‐top and foreland basin sequences, and present‐day stream profiles in the Argentine Precordillera support our modelling results. Our results may be extrapolated to other foreland systems, and are potentially identifed using low‐temperature thermochronometers in addition to paleoerosion rates.

The evolution of Yanshanian structures, especially the timing of onset, is a key problem in reconstructing the Late Mesozoic structural framework of the North China Craton. Growth strata and progressive unconformities, coupled with related structures, have been recognized for the first time in the Jurassic Shiguai Basin, thus allowing us to constrain the timing of Yanshanian structures with greater precision. Growth geometries and detailed structural analysis indicate that the late Middle Jurassic Changhangou Formation can be considered as the earliest sedimentary response to the onset of Yanshanian deformation during 170–160 Ma. At this time interval, shortening deformation was dominantly controlled by N–S compression driven by the closure of the Mongol‐Okhotsk Ocean, and growth structures constituted a roughly forward thrusting sequence with a slow uplift rate in the early stages and a suddenly accelerated uplift rate in the final stage. Subsequently, due to the superposition of NW–SE compression during the Late Jurassic (160–145 Ma), the initial E–W‐trending structures were converted into a stepped geometry marked by several structural reentrants developing along strike. In addition, out of sequence thrusts represented by the Beilinshan thrust and its splays began to be active in the middle portion of the previous forward thrusting sequence. Northwestward subduction of the Izanagi plate along the eastern margin of Asia is generally considered to be responsible for this phase of deformation.

Recent subsalt petroleum discoveries associated with rifted‐margin salt basins have piqued interest in the presalt geology of the Gulf of Mexico margin. Available subsurface data does not uniquely constrain the subsalt geometry, so creating an interpretation of the crustal architecture requires the application of geological models for crustal extension and breakup. However, published interpretations of the nature of the transition from continental rifting to seafloor spreading range from magma‐rich to magma‐poor. To address this uncertainty, we present 2D forward kinematic models for crustal configurations generated by diverse models (symmetric extension, depth‐dependent extension, and volcanic extension). Through a series of conceptual balanced cross‐sections grounded in a ~600 km 2D seismic line from the NE Gulf of Mexico, we demonstrate the implications of each model for the limit of oceanic crust, basement morphology, crustal architecture, and hydrocarbon prospectivity. We discuss evidence for the dominant crustal processes, including geodynamic factors and structural and stratigraphic observations. Based on our observations and the geologic history, we favour an asymmetric, magma‐poor to ‐intermediate margin interpretation for the NE Gulf of Mexico, but suggest that the degree of volcanic input and width of the ocean‐continent transition zone may vary along strike. The along‐strike variability highlights the importance of understanding all potential presalt crustal configurations, their key features, and their implications. With increased data availability on the presalt geology in the Gulf of Mexico the relevance of these scenarios can be assessed, allowing development of comprehensive geodynamic and tectonic models of the margin and consideration of petroleum system elements in the presalt sequence.

Differentiating compressional growth strata from halokinetic sequences is not straightforward in foreland fold‐and‐thrust belts where compressional and diapiric processes were coeval. Although there are numerous studies on the role of salt layers in fold‐and‐thrust belts, very few focus on syntectonic evaporites where they are thick enough to develop diapirism. The tectonic structures of the northern Dezful Embayment, along the footwall of the Zagros Mountain Front Fault, show a complex evolution interpreted in this study by the concurrence of folding, thrusting and pre‐ and syn‐shortening diapirism of the lower Miocene Gachsaran evaporites. We explore this tectonic–diapiric–sedimentary interplay by combining field studies and remote‐sensing mapping, detailed stratigraphy and sedimentology, high‐quality seismic data, and step‐by‐step balanced tectono‐sedimentary restorations. Initial diapirism occurred soon after the deposition of at least 1.5 km thick Miocene Gachsaran evaporites during the sedimentation of up to 5 km thick Aghajari, Lahbari and Bakhtyari formations, which display syntectonic mixed growth strata and halokinetic sequences. These sedimentary sequences filled salt‐related minibasins, from which the Azanak Minibasin is the deepest, limited by growing salt walls and salt anticlines (Azanak thrust‐weld and the Saland–Lali anticline). Palaeocurrents and clasts compositions document fluvial diversions and exposure and erosion of Gachsaran evaporites along the active salt walls. Although, by further shortening, the precursor salt walls squeezed and reactivated as thrust‐welds structuring a duplex of overriding minibasins. During the deposition of the middle‐late Miocene Aghajari Formation, Kazhdumi and Pabdeh source rocks were buried to oil‐expulsion depths, and Sarvak and Asmari reservoirs were mildly folded in large and open anticlines. The Gachsaran system in the northern Dezful Embayment is collated with the structures of preshortening Hormuz salt and syn‐shortening Fars salt to highlight the important role of these ductile units in the tectonic evolution of the Zagros fold belt that is compared to syn‐compressional salt‐related Pyrenees, Carpathians, Sivas and Kuga fold belts.

The Late Cenozoic is typically considered a time of widespread episodic tectonic uplift along the West Greenland continental margin (36–2 Ma), similar to other margins across the North Atlantic, such as Norway, East Greenland and the UK. The present study re‐examines and remodels onshore thermochronological data from central West Greenland and the Cretaceous Nuussuaq Basin, utilising a Bayesian modelling approach and new concepts related to radiation damage within apatite. These new thermal histories indicate that slow‐protracted cooling has occurred across the southern extent of the margin during the Mesozoic and Cenozoic, whereas those from within the Nuussuaq Basin display reheating through the Late Cretaceous/Palaeogene and cooling to present. Results suggest that no significant Late Cenozoic uplift has occurred along the southern margin, while cooling in the Nuussuaq Basin is consistent with events outlined in the basin's stratigraphy and implies uplift during volcanism and an isostatic response to the unroofing of the lithosphere has elevated the modern topography. These results imply significant tectonism in the region ceased by ~45 Ma, yet have wider implications regarding how low temperature thermochronology data are treated and our understanding of the postrift evolution of passive margins.

The Messinian Salinity Crisis (MSC) involved the progressive isolation of the Mediterranean Sea from the Atlantic between 5.97 and 5.33 Ma, and a sea‐level fall whose timing, modalities, and magnitude remain actively debated. At that time, the central Mediterranean was undergoing strong tectonic activity due to the rollback of the Adria slab and eastward migration of the Apenninic belt. The combined effects of the post‐evaporitic MSC sea‐level drop and morphostructural changes (due to the Intra‐Messinian phase) resulted in a regional unconformity, which shows erosive markers and conformable relationships with the Messinian and Mio–Pliocene boundary in the Po Plain and Northern Adriatic Foreland. Here, we produce a palaeotopographic reconstruction of the Po Plain‐Northern Adriatic region (PPNA) during the Messinian peak desiccation event based on such regional unconformity. We mapped this surface through wells and 2D seismic data form Eni's private dataset. The unconformity shows V‐shaped incisions matching the present‐day southern Alpine valleys and filled with Messinian post‐evaporitic and Pliocene deposits, suggesting that the modern drainage network is at least of late Messinian age. The Messinian unconformity has been restored to its original state through flexural‐backstripping numerical modelling. The resulting landscape suggests a maximum sea‐level drop of 800–900 m during the MSC peak, and is consistent with stratigraphic and sedimentologic data provided by previous works. The modelled shoreline separates the subaerially eroded land from an elongated basin composed by two ca. 400 and 1,000 m deep depocentres during the maximum sea‐level drop. These results suggest that the Mediterranean was split in at least three sub‐basins subject to independent base levels, fresh‐water budgets, and flexural responses during the maximum lowstand.

Reverse reactivation of normal faults, also termed “inversion”, has been extensively studied, whereas little is known about the strike‐slip reactivation of normal faults. At the same time, recognizing strike‐slip reactivation of normal faults in sedimentary basins is critical, as it may alter and impact basin physiography, accommodation and sediment supply and dispersal. Motivated by this, we present a study of a reactivated normal fault zone in the Liassic limestones and shales of Somerset, UK, to elucidate the effects of strike‐slip reactivation of normal faults, and the inherent deformation of relay zones that separate the original normal fault segments. The fault zone, initially extensional, exhibits a series of relay zones between right‐stepping segments, with the steps between the segments having subsequently become contractional due to sinistral strike‐slip movement. The relay zones have therefore been steepened and are cut by a series of connecting faults with reverse and strike‐slip components. The studied fault zone, and comparison with larger‐scale natural examples, leads us to conclude that the relays turned contractional steps are associated with (a) complex fault and fracture networks that accommodate shortening, (b) anomalously high numbers of fractures and faults, (c) layer‐parallel slip and (d) folding and uplift. Comparison with published statistics from global relay zones shows that whereas the reactivated relay zones feature aspect ratios similar to those of unreactivated relay zones, bed dips within reactivated relay zones are significantly steeper than unreactivated relay zones. Given the potential of reactivated relay zones to form areas of local uplift, they may affect basin structure and may also form potential traps for hydrocarbon or other fluids. The elevated faulting and fracturing, on the other hand, means reactivated relays are also likely loci for enhanced up‐fault flow.

Previous studies on the Lower‐Middle Jurassic Cuyo Group in the southern Neuquén Basin, Argentina, have discussed evidence for pre‐ and syn‐depositional structural inversion during pre‐Andean shortening. While the Cuyo Group sequence stratigraphic and facies framework are well understood, the effects of structural inversion and progressive postrift thermal subsidence on sediment provenance and dispersal, as well as the timing and magnitude of deformation during and after Cuyo Group deposition, remain poorly constrained. The Cuyo Group comprises both reservoir‐quality fluvial to deep‐marine siliciclastic deposits and a petroleum source‐rock. Thus, the temporal relationship between the onset of deformation and sediment dispersal are crucial aspects for an improved understanding of both the basin evolution and petroleum system. This study presents new detrital zircon (U‐Th)/(He‐Pb) double dating from the Los Molles and Lajas Formations of the Cuyo Group in the southern area of SW Zapala to evaluate the influence of early rift inversion on sediment routing, provenance, and palaeogeography, and to provide crucial chronostratigraphic constraints. The youngest concordant Jurassic detrital zircon (DZ) U‐Pb and He ages from the Lajas and Los Molles Formations in Lohan Mahuida and La Jardinera areas suggest a late Middle Jurassic depositional age. The DZ U‐Pb provenance analysis confirms that both formations are part of the same Middle Jurassic shelf margin and were both sourced from the Choiyoi basement, Late Triassic to Middle Jurassic Andean magmatic arc and pre‐Cuyo Group strata. The detrital He (DZHe) ages provide additional provenance constraints by recording three discrete cooling events, (1) early‐Late Triassic, (2) early‐Early Jurassic, and (3) Middle‐Late Jurassic, documenting tectonically driven exhumation during rifting and contractional stages prior to and during early Neuquén basin evolution. Triassic‐Jurassic He cooling ages of zircons derived from the Choiyoi Group document the existence of pre‐Cuyo Pangean extensional structures and basins in the source area. Furthermore, the abundance of rapidly cool DZHe ages requires rapid exhumation of both Choiyoi and Carboniferous basement. All of this evidence suggests that structural inversion in the Huincul ridge region started in the Middle Jurassic, earlier than previously proposed. Hence, this challenges conventional Cuyo Group tectono‐depositional models that advocated postrift thermal sagging as the primary control on subsidence and deposition. The occurrence of first‐cycle volcanic (U‐Th)/(He‐Pb) ages implies that the Cuyo Group burial never reached depths >4–5 kilometres. Exhumation to shallower crustal depths was spatially partitioned and driven by Cenozoic Andean shortening as constrained by AHe ages.