Early Cretaceous Rifting Research Articles

Multiple controls on rift basin sedimentation in volcanic settings: Insights from the anatomy of a small Early Cretaceous basin in the Yanshan belt, northern North China

This paper deals with a small continental rift basin in a volcanic setting and tries to decipher potential controls on its synrift sedimentation. The Luanping basin, which is a well-exposed Early Cretaceous rift basin in the Yanshan belt on the northern periphery of the North China block, was chosen as a case study. We show that extensive and vigorous volcanism took place just prior to the onset of basin formation and continued into a synrift period. Basin fills consist predominantly of lacustrine, fan-delta, braid-delta, and volcaniclastic deposits. Lacustrine facies are composed mostly of deep-lake deposits that appear to have been established soon after basin initiation. Fan-delta systems developed along two high-angle border faults, the master northern Hongqi fault and the secondary western Xiaobaiqi fault. Fan-delta fronts were built up primarily by gravelly and sandy debrites and turbidites. Braid-delta deposits are restricted to the eastern corner of the basin and are characterized by an aggregation of braid-plain conglomeratic deposits. Underwater extrusions of magma and pyroclastic flows are inferred from the presence of volcanic breccias, ignimbrites, andesitic pillows, and peperites, which are enclosed by host lacustrine deposits. Deep-lake and fan-delta systems persisted throughout the synrift period, indicating a rapid subsidence rate, with both adequate sediment and water supply. The high-angle border faults are thought to have played a critical role in creating large accommodation space, and simultaneous displacements of the Hongqi and Xiaobaiqi faults led to the localization of a major depocenter in the western part of the basin. One-dimensional backstripping analysis of three stratigraphic sections shows that the Luanping basin experienced marked synrift tectonic subsidence up to 2 km at a subsidence rate close to 0.2 mm yr −1 . We propose that the high rate of basin subsidence might have been caused mainly by left-lateral transtensional faulting and was enhanced by the superposition of basement downsag induced by underlying magma withdrawal. A predominance of volcanic particles in both debrites and sandstones suggests that source areas were erodible volcanic edifices. A humid paleoclimate is inferred, which not only provided enough water supply, but also promoted weathering of volcanic rocks in source areas, thus maintaining both the deep-lake environment and sediment influx. We conclude that relatively small and isolated rift basins in volcanic settings can experience both rapid subsidence and receive thick lacustrine deposits if they are under the combined control of favorable structure, climate, and magmatism. The model proposed here is believed to be applicable to other rift basins in similar volcano-tectonic settings.

New information about the stratigraphic position and age of the sauropodAragosaurus ischiaticusfrom the Early Cretaceous of the Iberian Peninsula

AbstractThe sauropodAragosaurus ischiaticusSanz, Buscalioni, Casanovas & Santafé, 1987 was the first dinosaur to be described in Spain. The holotype was recovered from the site of Las Zabacheras (Galve, Teruel province). This site has traditionally been situated in the El Castellar Formation (in the lower part of the Wealden facies). Recently, it has been proposed that the remains ofAragosaurusstem from the Villar del Arzobispo Formation (late Tithonian–upper part of the early Berriasian), which would mean that the sauropod was almost 15 million years older than previously thought. Detailed field work has been carried out, making it possible to pinpoint the position of the low-angle unconformity between the Villar del Arzobispo Formation and the El Castellar Formation. This unconformity originated as a result of block tilting that occurred during the early stages of the Early Cretaceous rifting episode. The upper levels of the Jurassic sequence (i.e. the Villar del Arzobispo Formation) were exposed to erosion and karstification, leading to the formation of a discontinuous conglomeratic level. This level has been locally preserved at the bottom of the Wealden syn-rift sequence (i.e. the El Castellar Formation). The results of our detailed mapping demonstrate that theAragosaurusholotype was found in the lower part of the El Castellar Formation. Moreover, our revision of the existing datings suggests that the El Castellar Formation as a whole is Valanginian?–early Barremian in age. Given thatAragosauruswas located in its lower part, it is probably Valanginian?–Hauterivian in age.

The linkage between fault throw and footwall scarp erosion patterns: an example from the Bremstein Fault Complex, offshore Mid‐Norway

AbstractStudies of normal fault systems in modern extensional regimes (e.g. Basin and Range), and in exhumed, ancient rift basins (e.g. Gulf of Suez Rift) have shown a link between the evolution of fault‐related footwall topography and associated erosional drainage systems. In this study, we use 3D seismic reflection data to image the footwall crest of a gravity‐driven fault system developed during late Middle Jurassic to Early Cretaceous rifting on the Halten Terrace, offshore Mid‐Norway. This 22‐km‐long fault system lacks significant footwall uplift, with hangingwall subsidence accommodating throw accumulation on the fault system. Significant erosion has occurred along the length of the footwall crest and is defined by 96 catchments characterized by erosional channels. These erosional channels consist of small, linear systems up to 750 m long located along the front of the fault footwall. Larger, dendritic channel systems extend further back (up to 3 km normal to fault strike) into the footwall. These channels are up to 7 km long, up to 50 m deep and up to 1 km wide. Fault throw varies along strike, with greatest throw in the centre of the fault decreasing towards the fault tips; localized throw minima are interpreted to represent segment linkage points, which were breached as the fault grew. Comparison of the catchment location to the throw distribution shows that the largest catchments are in the centre of the fault and decrease in size to the fault tips. There is no link between the location of the breached segment linkage points and the location and size of the footwall catchments, suggesting that the first‐order control on footwall erosion patterns is the overall fault‐throw distribution.

Extensional fault segmentation and linkages, Bonaparte Basin, outer North West Shelf, Australia

A detailed structural analysis of the Zone of Cooperation A-2 (ZOCA-2) three-dimensional (3-D) seismic survey on the outer northwestern edge of the Sahul Platform, northern Bonaparte Basin, outer North West Shelf, Australia, has identified three major populations of extensional faults. From oldest to youngest, these are (1) Jurassic north-south–trending extensional synrift faults, (2) Jurassic–Cretaceous east-west– to east-northeast–west-southwest–trending extensional faults, and (3) Neogene to present-day northeast-southwest–striking, right-stepping en-echelon faults. Seismic attribute analyses combined with fault-displacement analyses have illustrated the initial segmentation of all of the fault systems and characterized both the horizontal and vertical linkages formed by soft-linkage relay structures. The strongly segmented Neogene to present-day northeast-southwest–striking faults are interpreted to be a result of Neogene reactivation of underlying zones of weakness generated by preexisting east-northeast–trending Jurassic to Early Cretaceous faults. The Neogene to present-day northwest-southeast–directed extension was oblique to the underlying zones of weakness and thus formed characteristic strongly segmented en-echelon fault arrays. The Jurassic to Early Cretaceous extensional faults together with the overlying northeast-southwest–striking Neogene to present-day fault systems form a pseudoconjugate fault system separated by a Cretaceous sequence that acted as a decoupling horizon. Within this Cretaceous interval, which is characterized by polygonal fault systems, vertically segmented, isolated, and overlapping extensional fault arrays form a zone of soft linkage between the underlying Jurassic to Early Cretaceous rift faults and the overlying Neogene to present-day fault systems. This study shows that extension oblique to preexisting deeper fault systems produces en-echelon segmented extensional faults in the overlying sequences. Such en-echelon segmentation does not indicate major strike-slip deformation. The results of this research have implications for understanding the distribution, segmentation, linkages, and ages of extensional faults in many other rift basins as well as for the northern Bonaparte Basin. In addition, the complex 3-D linkage patterns shown in this study have significant implications for understanding trap geometries and fault-seal characteristics in other extensionally faulted basins.

Role of sandstone provenance in the diagenetic albitization of feldspars: A case study of the Jurassic Tera Group sandstones (Cameros Basin, NE Spain)

The Cameros Basin (Iberian Chain, NE Spain) formed during the latest Jurassic–Early Cretaceous rifting stage in an extensional regime characterized by high subsidence rates. Its sedimentary infill (thicker than 6000 m) has been subdivided into eight depositional sequences (DS) mainly composed of continental sediments. DS 1 and DS 2 represent the first rifting stage (Tera Group, Tithonian), mainly formed by fluvial and lacustrine sediments. Sandstone petrofacies evolved from quartz-sedimentolithic in DS 1 to quartz-feldspathic in DS 2 due to the rifting process. In DS 2, three different types of detrital feldspars (K-feldspars, albites and polysynthetic plagioclases) with similar sodium-rich compositions (mean: Ab 94.0 An 4.5 Or 1.5) can be recognized. Chemically pure nonluminescent albites (Ab > 99%) are common. In DS 2, diagenetic albitization of both plagioclases and K-feldspars is inferred from conventional microscopy observations, cathodoluminescence and electron microprobe analyses. DS 1 contains few plagioclase grains, which show no evidence of transformation into albite. Although the albitization is characterized as diagenetic it seems to be provenance-controlled since it affects the units showing higher original plagioclase/K-feldspar ratio (DS 2), due to the greater influence of plutonic and metamorphic source areas in DS 2. Possible Na sources are: (1) the percolation of moderate to high salinity residual brines from related alkaline lakes developed at top of DS 2 in the eastern sector of the basin, (2) clay mineral reactions (sodium smectite to illite and chlorite) indicated by mudstone composition in the interlayered mudstones, and (3) the replacement of detrital sodium plagioclases by carbonate. These three sources can be complementary.

Linking sedimentation in the northern Andes to basement configuration, Mesozoic extension, and Cenozoic shortening: Evidence from detrital zircon U-Pb ages, Eastern Cordillera, Colombia

Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analyses of 29 samples from the Eastern Cordillera of Colombia reveal the origin of northern Andean basement and patterns of sedimentation during Paleozoic subsidence, Jurassic–Early Cretaceous extension, Late Cretaceous postrift subsidence, and Cenozoic shortening and foreland-basin evolution. U-Pb geochronological results indicate that presumed Precambrian basement is mainly a product of early Paleozoic magmatism (520–420 Ma) potentially linked to subduction and possible collision. Inherited zircons provide evidence for Mesoproterozoic tectonomagmatic events at 1200–1000 Ma during Grenville-age orogenesis. Detrital zircon U-Pb ages for Paleozoic strata show derivation from Andean basement, syn depositional magmatic sources (420–380 Ma), and distal sources of chiefl y Mesoproterozoic basement (1650–900 Ma) in the Amazonian craton (Guyana shield) to the east or in possible continental terranes along the western margin of South America. Sedimentation during Jurassic–Early Cretaceous rifting is expressed in detrital zircon age spectra as Andean basement sources, recycled Paleozoic contributions, and igneous sources of Carboniferous–Permian (310–250 Ma) and Late Triassic–Early Jurassic (220–180 Ma) origin. Detrital zircon provenance during continued Cretaceous extension and postrift thermal subsidence recorded the elimination of Andean basement sources and increased infl uence of craton-derived drainage systems providing mainly Paleoproterozoic and Mesoproterozoic (2050–950 Ma) grains. By Eocene time, zircons from the Guyana shield (1850–1350 Ma) dominated the detrital signal in the easternmost Eastern Cordillera. In contrast, coeval Eocene deposits in the axial Eastern Cordillera contain Late Cretaceous–Paleocene (90–55 Ma), Jurassic (190–150 Ma), and limited Permian–Triassic (280–220 Ma) zircons recording initial uplift and exhumation of principally Mesozoic magmatic-arc rocks to the west in the Central Cordillera. Oligocene–Miocene sandstones of the proximal Llanos foreland basin document uplift-induced exhumation of the Eastern Cordillera fold-thrust belt and recycling of the Paleogene cover succession rich in both arc-derived detritus (dominantly 180– 40 Ma) and shield-derived sediments (mostly 1850–950 Ma). Late Miocene–Pliocene erosion into the underlying Cretaceous section is evidenced by elimination of Mesozoic– Cenozoic zircons and increased proportions of 1650–900 Ma zircons emblematic of Cretaceous strata.

3D architecture of a complex transcurrent rift system: The example of the Bay of Biscay–Western Pyrenees

The Parentis and Arzacq–Mauléon basins located in front of the V-shaped oceanic propagator in the Bay of Biscay present evidence for extreme crustal thinning. In this paper we investigate the 3D structure of these rift basins, based on field observations and the interpretation of seismic data. We compare these results with those obtained from two different and independent inversion methods: first a 3D gravity inversion and second the standard Euler deconvolution. For the Mauléon Basin our results show that the positive gravimetric anomaly identified above its southern margin is the consequence of two shallower high density bodies that are separated by the Pamplona fault and a deeper high density body. The high density bodies can be explained by the presence of mid-crustal and mantle rocks that were exhumed or uplifted at shallower depth during Early Cretaceous rifting before they were reworked and integrated to the Pyrenean chain during compression phase. Also, during this reactivation phase, some slices of the exhumed mid-crustal and mantle rocks were sheared off and were integrated in the present-day thrust belt in the Mauléon basin. For the Parentis Basin we can demonstrate, based on seismic data and gravimetric inversion methods, a decrease in extension from west to east, which is compatible with the V-shape geometry of the overall basin. Along strike, a change in the fault geometry from downward concave top-basement detachment faults to upward concave high-angle faults can be observed eastwards, i.e. towards the termination of the basin. A key structure, controlling the evolution of the Parentis Basin, is the east–west trending Ibis fault. We interpret this fault to have initially formed as a strike slip fault before it was reactivated during later crustal thinning. At present, it forms the limit between an upper plate sag basin to the north and a lower plate sag basin, floored at least locally by a top-basement detachment faults to the south. The strong asymmetry of the basin is supported by the shape of the basin and the results of standard Euler deconvolution.

Post-rift landscape development of north-east Brazil

The evolution of the landscape of north-east Brazil in relation to the burial and exhumation history of both onshore and offshore areas is the focus of a research project carried out for StatoilHydrodo Brasil and Petrobras from 2007 to 2009 by the Geological Survey of Denmark and Greenland in collaboration with Geotrack International. In hydrocarbon exploration it is important to understand the regional tectonic framework and thus also to consider the volumes of rocks that may have been present and then removed during the geological past. For example, the timing of hydrocarbon generation and changes in migration routes can be assessed when the timing and magnitude of uplift and erosion is known. Studies in West Greenland have demonstrated the usefulness of large-scale, low-relief, high-level landscapes as markers of uplift events, and in particular the strength of combining the denudation history from landscape analysis with the cooling history from apatite fission-track analysis (AFTA) data and the stratigraphic record (Bonow et al. 2006, 2007; Japsen et al. 2006, 2009). In the study area, there are two plateaux with elevations up to c. 1300 m above sea level (a.s.l.). The plateaux are currently being dissected by deeply incised fluvial valleys, and escarpments separate the two plateaux. The lowlands cut across Early Cretaceous rift systems along the Atlantic margin, including the intracontinental Recôncavo–Tucano–Jatobá (RTJ) Rift and also the Camamu Basin, of which the western margin is exposed onshore (Fig. 1). The RTJ Rift is a mature hydrocarbon province, whereas the deep-water parts of the Camamu Basin are the target of frontier exploration (e.g. Magnavita et al. 1994; Davison 1999; Cobbold et al. 2008). The post-rift sequence in the RTJ Rift and the inshore Camamu Basin is thin or absent. However, it has been estimated that up to 2000 m of sedimentary cover once was present, but has now been removed (Magnavita et al. 1994).

Geoid and gravity anomaly data of conjugate regions of Bay of Bengal and Enderby Basin: New constraints on breakup and early spreading history between India and Antarctica

Timing of breakup of the Indian continent from eastern Gondwanaland and evolution of the lithosphere in the Bay of Bengal still remain as ambiguous issues. Geoid and free‐air gravity data of Bay of Bengal and Enderby Basin are integrated with shipborne geophysical data to investigate the early evolution of the eastern Indian Ocean. Geoid and gravity data of the Bay of Bengal reveal five N36°W fracture zones (FZs) and five isolated NE‐SW structural rises between the Eastern Continental Margin of India (ECMI) and the 85°E Ridge/86°E FZ. The FZs meet the 86°E FZ at an angle of ∼39°. The rises are associated with low‐gravity and geoid anomalies and are oriented nearly orthogonal to the FZs trend. The geoid and gravity data of the western Enderby Basin reveal a major Kerguelen FZ and five N4°E FZs. The FZs discretely converge to the Kerguelen FZ at an angle of ∼37°. We interpret the FZs identified in Bay of Bengal and western Enderby Basin as conjugate FZs that trace the early Cretaceous rifting of south ECMI from Enderby Land. Structural rises between the FZs of Bay of Bengal may either represent fossil ridge segments, possibly have extinct during the early evolution of the Bay of Bengal lithosphere or may have formed later by the volcanic activity accreted the 85°E Ridge. Two different gravity signatures (short‐wavelength high‐amplitude negative gravity anomaly and relatively broader low‐amplitude negative gravity anomaly) are observed on south and north segments of the ECMI, respectively. The location of continent‐ocean boundary (COB) is at relatively far distance (100–200 km) from the coastline on north ECMI than that (50–100 km) on the south segment. On the basis of geoid, gravity, and seismic character and orientation of conjugate FZs in Bay of Bengal and western Enderby Basin, we believe that transform motion occurred between south ECMI and Enderby Land at the time of breakup, which might have facilitated the rifting process in the north between combined north ECMI‐Elan Bank and MacRobertson Land and in the south between southwest Sri Lanka and Gunnerus Ridge region of East Antarctica. Approximately during the period between the anomalies M1 and M0 and soon after detachment of the Elan Bank from north ECMI, the rifting process possibly had reorganized in order to establish the process along the entire eastern margins of India and Sri Lanka.

What we know (and what we don't) about the petroleum prospectivity of the Northland Basin, North Island, New Zealand

The offshore Northland Basin is a major sedimentary accumulation lying to the west of the Northland Peninsula of New Zealand. It merges with the Taranaki Basin in the south and its deeper units are separated from Deepwater Taranaki by a buried extension of the West Norfolk Ridge. Sedimentary thicknesses increase to the northwest and the Northland Basin may extend into Reinga. Its total area is at least 65,000 km2 and if the Reinga Basin is included, it may be up to 100,000 km2. As in Taranaki, petroleum systems of the Northland Basin were thought to include Cretaceous to Recent sedimentary rocks. Waka Nui–1 was drilled in 1999 and penetrated no Cretaceous sediments, but instead drilled unmetamorphosed Middle Jurassic coal measures. Economic basement may be older meta-sediments of the Murihiku Supergroup. Thick successions onlap the dipping Jurassic unit and a representative Cretaceous succession is likely to be present in the basin. Potential source rocks known to be present include the Middle Jurassic coal measures of Waka Nui–1 and the Waipawa Formation black shale. Inferred source rocks include Late Jurassic coaly rocks of the Huriwai Beds, the Early Cretaceous Taniwha Formation coaly sediments, possible Late Cretaceous coaly units and lean but thick Late Cretaceous and Paleogene marine shales. Below the voluminous Miocene volcanoes of the Northland arc, the eastern margin of the basin is dominated by a sedimentary wedge that thickens to more than two seconds two-way travel time (TWT), or at least 3,000 m, at its eastern margin and appears to have been thrust to the southwest. This is interpreted to be a Mesozoic equivalent of the Taranaki Fault, a back-thrust to subduction along the Gondwana Margin. The ages of sedimentary units in the wedge are unknown but are thought to include a basal Jurassic succession, which dips generally to the east and is truncated by an erosional unconformity. A southwestwards-prograding succession overlies the unconformity and its top surface forms a paleoslope onlapped by sediments of Late Cretaceous to Neogene ages. The upper succession in the wedge may be of Early Cretaceous age—perhaps the equivalent of the Taniwha Formation or the basal succession in Waimamaku–2. The main part of the basin was rifted to form a series of horst and graben features. The age of initial rifting is poorly constrained, but the structural trend is northwest–southeast or parallel to the Early Cretaceous rifting of Deepwater Taranaki and with the Mesozoic Gondwana margin. Thick successions overlie source units which are likely to be buried deeply enough to expel oil and gas, and more than 70 slicks have been identified on satellite SAR data suggesting an active petroleum system. Numerous structural and stratigraphic traps are present and the potential of the Northland Basin is thought to be high.

Analysis of large thermal maturity datasets: Examples from the Canadian Arctic Islands

Abstract This paper describes an approach for verifying thermal maturity data in a large historical dataset from the Canadian Arctic Islands. A compilation of more than 6000 maturity measurements (vitrinite reflectance and Rock-Eval Tmax) collected over the span of three decades involved a rigorous assessment of data quality. Some common anomalies in interpreting thermal maturity dataset include: (i) elevated thermal maturity due to Cretaceous igneous intrusion in the region, (ii) reworking of refractory material from older rocks into younger strata during the Triassic period, (iii) suppression of vitrinite reflectance and Tmax in hydrogen-rich samples, (iv) low maturity values due to cross-contamination by the younger sediments during drilling process (caving), and (v) offset maturity values obtained from different maturity measurements. The study discusses various independent checks to verify the obtained maturity parameters. The comparison between thermal maturity data with the sonic velocity of shale resulted in a satisfactory correlation. While such a correlation may vary in different sedimentary basins, it produces a useful independent assessment of thermal maturity. The results indicate that increased heat flow during the Jurassic–Early Cretaceous rifting of the Canada Basin may have caused the elevated maturity beyond the expected burial level as suggested by the discrepancy between thermal maturity and sonic velocity data. Given the fact that vitrinite reflectance records only the maximum temperature to which the enclosing rocks were exposed, deviation of the collected reflectance values from the current depth of burial serves as an indicator for the amount of geological uplift.

Sedimentary dynamics and extensional structuring related to early Cretaceous rifting of Neocomian and Barremian deposits of the interior basin of Gabon

Recent field and subsurface data about the early Neocomian N’dombo series and the Neocomian to mid-Barremian Schistes series of the interior basin of Gabon further our understanding of the initial stages of early Cretaceous N40–60°E extensional rifting. The syn-rift series comprise fluvial–lacustrine claystones–sandstones, rare conglomerates, and carbonates. The syn-rift fill begins with braided-stream feldspathic sandstones. These are overlain first by fluvial–lacustrine deposits and then by predominantly lacustrine–palustrine claystones, which are potential petroleum source rocks. The claystones are eroded in part and are capped by the pre-Aptian angular unconformity marking the end of Cretaceous rifting in the interior basin. This change in syn-rift facies and depositional environments reflects a rise in base level in response to accelerated subsidence after the initial stage of rifting. The syn-rift deposits form two fining-upward sequences several 100–1000 m thick.

Provenance of fluvial sandstones at the start of late Jurassic–Early Cretaceous rifting in the Cameros Basin (N. Spain)

The Cameros Basin (Iberian Chain, Central Spain) developed during the latest Jurassic–Early Cretaceous in an extensional regime characterized by high rates of subsidence. Its sedimentary fill has been subdivided into eight depositional sequences (DS) mainly composed of continental sediments. DS 1 and DS 2 represent the first rifting stage (Tera Group, Tithonian). The purpose of this study is to characterize the Tera Group in the eastern part of the basin based on provenance criteria derived from fluvial sandstones. In this area of the basin, the Tera Group can be subdivided into three formations: the Ágreda Formation, the Magaña Formation and the Sierra de Matute Formation. These formations are composed of alluvial-fan deposits, meandering fluvial sediments and lacustrine–palustrine mudstones. A quantitative petrographic study indicated the presence of three main petrofacies in the Tera Group. The close correlation between petrofacies and lithostratigraphic units indicates that sandstone composition is a powerful tool for deciphering the tectonic processes active during the initial rift stages of the Cameros Basin. Petrofacies 1 is sedimentolithic (mean: Qm 54F 3Lt 43) and represents erosion of the Jurassic marine pre-rift substratum (mainly Kimmeridgian limestones) during deposition of the DS 1 alluvial fan deposits (Ágreda Fm.). Petrofacies 2 is quartzofeldspathic, and can be subdivided into Petrofacies 2A, with an average composition of Qm 84F 15Lt 1 and Petrofacies 2B, whose average composition is Q 71F 23Lt 6. Petrofacies 2 was generated by the erosion of low to medium-grade metamorphic terranes and plutonic source rocks. It characterizes the Magaña Fm. (DS 2). Petrofacies 3 is quartzolithic (mean: Qm 67F 16Lt 17), and is attributed to tectonic reactivation of the basin. This petrofacies characterizes the Sierra de Matute Fm. (DS 2). Thus, the provenance evolution of this basin is characterized by erosion of the pre-rift sedimentary substratum, followed by unroofing of the basement, as recorded in other ancient and modern rifted basins.

Cretaceous tectonostratigraphy and the development of the Cauvery Basin, southeast India

The late Jurassic to early Cretaceous rifting between India/Australia and India/Antarctica resulted in the formation of a number of NE–SW-trending basins in the Indian Precambrian crystalline basement. The Cauvery Basin is the southernmost basin along the eastern margin of the Indian Sub-Continent, covering much of this part of India and extending a considerable distance offshore. The basin comprises several ‘depressions’, or sub-basins, with the Ariyalur–Pondicherry Depression in the north. The exposed successions are in the southern part of this sub-basin. The result of fieldwork (1994–8) has been a reassessment of the lithostratigraphy and the tectonostratigraphic history of the Ariyalur outcrop. Three major sedimentary groups were identified: the syn-rift Gondwana Group (of early Cretaceous age), the syn-rift Uttatur Group (of Albian to Coniacian age) and the post-rift Ariyalur Group (of Santonian to Maastrichtian age). Both microfaunal and macrofaunal information were used to develop a biostratigraphic framework for the basin and a new tectonostratigraphic model. This new model for the development of the basin is significantly different to that used by the Oil and Natural Gas Commission of India. Structures exposed onshore, which have been interpreted as Albian reefs, are interpreted here as irregularly shaped limestone olistoliths/olistostromes produced by intra-Cretaceous rifting and slumping within the basin. The paper discusses this model for the basin history which is calibrated by updated foraminiferal (and macrofossil) biostratigraphy.

Illitizing fluids as precursors of hydrocarbon migration along transfer and boundary faults of the Jeanne d’Arc Basin offshore Newfoundland, Canada

Illite content in mixed-layer illite/smectite (I/S) and the degree of ordering in the fine fraction (<0.1 μm) of argillaceous rocks invariably increase with burial depth and temperature in 22 deep exploration wells in the Jeanne d’Arc Basin, offshore Newfoundland, Canada. The illitization of I/S mixed-layer clays, however, displays significant anomalies with respect to organic-matter maturation in the Trans-Basinal Fault (TBF) area, which is one of the major transfer zones of the Late Triassic–Early Cretaceous rift basin. Illitization is particularly rapid in the 30–70% illite range causing step-shaped %I/depth profiles. In comparison, illitization trends in the Southern and Northern Jeanne d’Arc Basin show a more or less linear increase in illite content with depth lacking the stepwise illite profiles. In the anomalous TBF zone and in the boundary fault zone on the western flank of the Outer Ridge Complex (ORC), the transition from random to (R1)-ordered I/S is advanced compared to the main parts of the basin as it occurs at lower vitrinite reflectance values. This indicates that factors other than temperature-controlled I/S evolution, possibly the chemistry of the pore fluids. Advection of deep K +-bearing fluids along faults could have accelerated I/S diagenesis. The concentration of all major oil discoveries in the TBF zone and ORC suggests that the upward migration of hydrocarbons to the reservoirs located up to 2.5 km above the source rocks may have followed the same pathways as the illitizing fluids. The regional pattern of I/S diagenesis thus supports the concept of fault-assisted fluid flow for hydrocarbon migration.

A dynamic model of rifting between Galicia Bank and Flemish Cap during the opening of the North Atlantic Ocean

Abstract A finite-element model is used to simulate Late Jurassic–Early Cretaceous rifting between the Flemish Cap and Galicia Bank continental margins. The model results show that variations in the thickness of the continental crust on these margins at wavelengths greater than about 75 km can be explained as a consequence of the interaction of two pre-existing weaknesses in the lithosphere. A weakness in the crust, attributed to structural fabrics in the Variscan front, controls the location of crustal extension during the early stages of rifting in the model. This results in formation of a broad rift basin similar to the Galicia Interior Basin. A deep-seated weakness located 110 km further west, attributed to the thick crust beneath the central Variscan Orogen, controls the location of mantle necking. Extension in this region is initially diffuse, but accelerates and becomes more focused with time. Approximately 13 million years after rifting begins, the locus of crustal extension shifts from the region of pre-weakened crust into the region of pre-weakened mantle. This marks the end of subsidence in the Galicia Interior Basin and the onset of subsidence in the Flemish Cap and Galicia Bank marginal basins. Extension in these areas continues for another 12 million years before continental breakup. The asthenosphere does not ascend to depths shallow enough for decompression melting to begin until less than 5 million years before the onset of sea-floor spreading. The model predicts that all late-stage synrift magmatism during this period is limited to within 45 km of the rift axis, and production of melt thicknesses greater than 2 km is restricted to within 35 km of the rift axis. Mantle potential temperatures of 1250–1275 °C, ∼5–30 °C cooler than normal, result in 3.1–4.5 km-thick oceanic crust at the time of breakup, in general agreement with the 2–4 km thick crust observed adjacent to these margins.

Upper crustal response to Mesozoic tectonism in western Liaoning, North China, and implications for lithospheric delamination

Abstract A widespread and well-documented episode of Late Jurassic–Early Cretaceous rifting followed multiple events of mid- to late Mesozoic crustal contraction in NE China. This extensional deformation was closely associated with widespread Mesozoic magmatism, thought to be related to lithospheric delamination and destabilization of the previously stable North China craton. Early Cretaceous rift-related sedimentary basins in the western Liaoning region of NE China comprise numerous discrete, largely lacustrine half-graben basins bounded by NW-rooting low-angle normal faults that sole into older thrusts or mid-crustal shear zones. These basins characteristically lack post-rift thermal subsidence and significantly postdate most of the Mesozoic volcanism in the region. Instead, magmatism that has been attributed to lower crustal foundering, and hence lithospheric delamination (perhaps as old as 160 Ma) accompanied continuing crustal thickening in eastern North China. Thus, although widespread magmatism plausibly played a role in thermally weakening the crust prior to extension, there is little upper crustal evidence that wholesale removal of the lithosphere and lower crust occurred during Mesozoic time. The expansive Cenozoic rift basins of Eastern China, which do contain thick post-rift sequences, constitute a more viable response to lithospheric delamination.

The Aquitaine Basin

The Aquitaine Basin is located in the southwest of France, between the Gironde Arch in the north and the Pyrenean Mountain Chain in the south. It is a triangular-shaped domain, extending over 35 000 km 2 . From north to south, six main geological provinces can be identified: (1) the Medoc Platform located south of the Gironde Arch; (2) the Parentis sub-basin; (3) the Landes Saddle; (4) the North Aquitaine Platform; (5) the foreland of the Pyrenees (also known as the Adour, Arzacq and Comminges sub-basins); and (6) the Pyrenean fold-and-thrust belt. Only the Parentis sub-basin, the foreland of the Pyrenean Chain and a minor part of the fold-and-thrust belt itself are proven hydrocarbon provinces. The Aquitaine Basin, in turn, is subdivided into four sub-basins – the Parentis, Adour–Arzacq, Tarbes and Comminges areas. The lozenge shape of these depocentres is related to the Hercynian tectonic framework of the Palaeozoic basement, reactivated during Early Cretaceous rifting. This rift phase aborted at the end of the Albian (prior to the development of an oceanic crust) in response to the beginning of the subduction of the Iberian plate under the European plate. During the Upper Cretaceous, continued subduction led to the creation of northwards-migrating flexural basins. In the Eocene, a paroxysmal phase of compression was responsible for the uplift of the Pyrenean Mountain Chain and for the thin-skinned deformation of the foreland basin. The resulting structuration is limited to the south by the internal core of the chain and to the north by the leading edge of the fold-and-thrust belt, where the Lacq and Meillon gas fields are located. Four main petroleum provinces have been exploited since the Second World War: (1) the oil-prone Parentis sub-basin and (2) salt ridges surrounding the Arzacq and Tarbes sub-basins; and (3) the gas-prone southern Arzacq sub-basin (including the external Pyrenean fold-and-thrust belt and the proximal foreland sub-basin) and (4) Comminges sub-basin. Two major distinct vertically drained petroleum systems (PS) can be identified: the Upper Kimmeridgian–Barremian is the main PS, based on Type II shaly-carbonate source rocks; and the Lias PS based on Type II–III source rocks. The latter is restricted to the Comminges and Tarbes sub-basins. Reservoirs consist of fractured and diagenetically modified carbonates, and clastics. Shaly-marly deposits of regional extent associated with transgressive systems provide the main seals. Formation of petroleum traps results from a complex polyphase geodynamic evolution. They are developed mainly along Palaeozoic inherited palaeostructures. Oil and gas migration took place from Albian times up to the present, depending on the respective structural position and histories of the traps. These petroleum provinces of the Aquitaine Basin have been exploited since 1939 and a total of 11.5×10 12 SCF of gas and 470×10 6 BBL oil recoverable reserves have been proven to date (the total amount is in the order of 2.5×10 9 BOE).

Facies architecture and paleohydrology of a synrift succession in the Early Cretaceous Choyr Basin, southeastern Mongolia

The Choyr Basin is one of several Early Cretaceous rift basins in southwestern Mongolia that developed in specific regions between north–south trending fold-and-thrust belts. The eastern margin of the basin is defined by high-angle normal and/or strike-slip faults that trend north-to-south and northeast-to-southwest and by the overall geometry of the basin, which is interpreted to be a half graben. The sedimentary succession of the Choyr Basin documents one of the various types of tectono-sedimentary processes that were active in the rift basins of East Asia during Early Cretaceous time. The sedimentary infill of the Choyr Basin is newly defined as the Khalzan Uul and Khuren Dukh formations based on detailed mapping of lateral and vertical variations in component lithofacies assemblages. These two formations are heterotopic deposits and constitute a third-order fluvio-lacustrine sequence that can be divided into transgressive and highstand systems tracts. The lower part of the transgressive systems tract (TST) is characterized by sandy alluvial-fan and braided-river systems on the hanging wall along the western basin margin, and by a gravelly alluvial-fan system on the footwall along the eastern basin margin. The alluvial-fan and braided-river deposits along the western basin margin are fossiliferous and are interpreted to have developed in association with a perennial fluvial system. In contrast, alluvial-fan deposits along the eastern basin margin do not contain any distinct faunas or floras and are interpreted to have been influenced by a high-discharge ephemeral fluvial system associated with fluctuations in wetting and drying paleohydrologic processes. The lower part of the TST deposit fines upward to siltstone-dominated flood-plain and ephemeral-lake deposits that constitute the upper part of the TST and the lower part of the highstand systems tract (HST). These mudstone deposits eventually reduced the topographic irregularities typical of the early stage of synrift basin development, with an associated decrease in topographic-slope gradients. Finally, a high-sinuosity meandering river system drained to the south during the late highstand stage in response to the northward migration of the depocenter. The upper HST deposits are also fossiliferous and are interpreted to have been influenced by a perennial fluvial system, although the average annual discharge of this system was probably less than 5 percent of that involved in the formation of the lower TST deposits along the western basin margin.

Oil reservoirs in foreland basins charged by thrustbelt source rocks: insights from numerical stress modelling and geometric balancing in the West Carpathians

Abstract The West Carpathian thrustbelt advanced northeastwards over the European Platform. Its thrust sheets comprise sediments of the Early Cretaceous rifts that evolved on a passive margin of the European Platform, the Late Cretaceous-Paleocene basins formed by rift inversion, and the Eocene-Oligocene flexural basin. Geochemical analyses established a clear link between pooled oils in the foreland and the Oligocene Menilite Formation inside the thrustbelt. In order to understand the driving forces for this oil migration scenario, finite-element models of fault-propagation and fold-bend folds are used to study the mean stress distribution in the thrust sheets and the foreland. Mean stress has a profound control on the pore fluid pressure through the relationship affected by sediment porosity, and sediment skeleton and fluid compressibilities. Modelling results suggest that only fault-propagation folds are capable of generating foreland-directed mean stress gradients as they are characterized by a large foreland area of decreased mean stress, by coupled increased/decreased mean stress areas on advancing/receding sides of the ramp tip, and an overall mean stress decrease inside the thrust sheet in the direction towards the foreland. This interpretation is in accordance with the dominant fold-and-thrust style in the Western Carpathians inferred from balanced cross-section restoration. It shows that frontal fault-propagation folding was active during the late Oligocene-Early Miocene, providing an effective tectonic driving force for hydrocarbon migration from source rocks inside the thrustbelt towards reservoirs in the foreland.