Cretaceous Cover Research Articles

Interaction between basement detachment fault, rift onset unconformity, and overlying basin fills: An example from the Songliao basin of a Cretaceous active continental margin volcanic rift in northeast Asia

How does a giant rift basin begin? No precise examples describe this issue. Based on 7108 m of continuous coring data recovered from the whole stratigraphy of the Songliao Basin (SLB), an attempt has been made to answer this question. Through comprehensive geological, geochemical and seismic research it has been found that there is a triangular extensional domain (TED) beneath the sedimentary cover in the basement at the rift center, from where the initial rupture of the giant rift was created. The basement detachment fault (DF), rift onset unconformity (ROU), and overlying basin fills are interrelated elements of a sedimentary basin and are the key to interpreting the geological archives recorded in the basin. The interaction processes between them determine the location of the basin center and the style of basement subsidence, which reflect the tectonic properties of the basin. This interaction may have a coevolutionary relationship with the deep magmatic activities beneath the basin. The basement of a basin typically consists of a series of strata that the lower strata age older. The uppermost basement sequence of the Triassic in the SLB is particularly important for the overlying basin formation and evolution. The surface topography controlled the thickness of the overlying Cretaceous cover. An ROU and DF developed at the top and bottom of the Triassic, respectively.The long-term uplift, denudation, and subsequent volcanic deposition may be common features of an ROU in a volcanic rift basin. More so, DF is the controlling factor for basin subsidence in the syn-rift stage of basin evolution. The frequent intrusive association with graben faults suggests that sublithospheric mantle flow may affect the accommodation space of the overlying basin by cutting the basement blocks via feeder dykes and shaping the graben pattern on top of the basement.

A Regional Shallow Décollement at the Front of the Mexican Fold and Thrust Belt: Microstructure of the Santiago Shale

Abstract This contribution analyses the role played by the mechanical properties of a decollément shale layer in the evolution of the Mexican Fold and Thrust Belt (MFTB). The mobility of overpressured shales can accommodate large strains by grain-scale plastic mechanisms, and affect the folding and thrusting styles of the overburden. Research on shale deformation mechanisms is necessary to improve the knowledge of these processes and their influence on the structural style of fold and thrust belts. The ductile behavior of rocks involving grain-scale plasticity was documented in the Jurassic Santiago shale sequence using geological mapping, microstructural observations on thin-oriented sections, and scanning electron microscopy (SEM) imaging. Structural styles such as detachment folding, fault-bend folding, and shale-cored fold-thrusts were observed at the regional scale. At the outcrop scale, the shale developed strong foliation and pencil cleavage, with immersed packstone boudins. Observed structures include thrusting, soft and open folds, and buckle folding. In thin section, the ductile textures include a strong penetrative foliation with lenticular and wavy-parallel laminae composed of carbonates, ribbons of reoriented clays and organic matter (clay+OM), s-c structures, porphyroblasts microtextures, development of oblique cleavage concerning folded foliation (crenulation cleavage), and carbonates dissolution. The Santiago shale shows also evidence of brittle deformation including calcite-filled fractures and cataclastic gouges. X-ray diffraction (XRD) analysis of the clay size fraction suggests that the authigenic calcareous shale was deformed in conditions of the deep diagenetic zone (between 100 and 200°C) and fluid overpressure (>70 MPa). The results help to improve the understanding of ductile microstructure and its role in shale deformation cretaceous cover, promoting the formation of localized fault propagation folds in the overburden. This study aims to open new perspectives in the kinematics and rheology interpretations for this sector of the MFTB, highlighting the role of the décollement layers during the progression of the orogen.

Variscan Inheritance Induces Alpine Upper Crustal Delamination in East Spanish‐Portuguese Central System

AbstractThe Spanish‐Portuguese Central System (SPCS) is an Alpine Mountain range with crystalline basement characterized by a two‐layer rheological structure. This structure formed after primary (protolith) and secondary (tectonometamorphic) processes during the extensional collapse of the Variscan Orogen. The SPCS structure is usually controlled by foreland‐directed thrusts and strike‐slip faults. However, the eastern SPCS is dominated by NW‐directed, imbricate backthrusts and lacks the main thrust directed to the foreland basin located southeast of the mountain range (Madrid Cenozoic Basin). The SPCS exhibits a crustal root (>40 km depth) supporting SE‐directed crustal‐scale thrusting. Alpine backthrusts sole into an SE‐dipping décollement within the Variscan basement. Variscan extension‐related structures parallel the SE‐dipping geometry of Alpine backthrusts, so they provided favorably oriented rheological weaknesses to accommodate Alpine shortening. Backthrusts geometry, their hanging wall position within the fault that raised the SPCS and gravity modeling support an Alpine crustal delamination process. Tectonic wedging and delamination of the more competent basement occurred in the footwall of Variscan extensional faults (Daurius domain), which enforced the shearing off of a rheologically weaker upper layer of the crust, located in the hanging wall of the Variscan extensional faults (Arriaca domain) by inverting Variscan extensional faults. This led to NW‐directed incipient continental subduction of the weaker crust. Intraplate subduction and crustal delamination can be independent from lithosphere‐scale inheritance and be conditioned by structural inheritance in the overlying crust. Alpine shortening for the Cretaceous cover is around 17.7 km (10.5% shortening), and 11 km (7%) for the upper‐lower crust limit.

Structural overprinting and style of deformation at sierra Beauvoir and sierra de Apen: A geometric and kinematic model for the evolution of the internal thrust-fold belt, Fuegian Andes, Argentina

A detailed study of the structural fabrics and overprinting relationships exposed at sierra Beauvoir and sierra de Apen, which include folds, foliations and thrust faults, allowed us to settle a deformation sequence of the internal thrust-fold belt of the Fuegian Andes. Our results, together with previous geochronology results, indicate that ductile structures developed during a Late Cretaceous simple shear deformation that formed asymmetric folds and related foliations under very-low grade metamorphism. This deformation affected rocks not younger than the Campanian, and is here related to the front of the orogenic wedge formed during arc-continent collision associated with the closure of a prior back-arc basin. Thrust systems overprint this ductile deformation, and even though they were considered in previous theoretical models, this is the first time their detailed geometry and time constraints are reported from field data. We identified an older, brittle-ductile thrust system formed during the Danian, with the slight penetrative deformation and very gentle folding of the Maastrichtian-Danian Policarpo Formation representing the frontal part of this thrust wedge. This thrust system formed at intermediate structural levels and was linked with basement-involved thrusting in the hinterland, which caused the shortening accommodated in the internal thrust-fold belt. The Eocene Apen-Malvinera thrust system, formed by brittle faults, was responsible for later exhumation of the Cretaceous rocks deformed by prior brittle-ductile structures. This deformation sequence implies that a significant supply of detritus to the foreland basin, originated from the erosion of the Cretaceous cover of the brittle-ductile structures of sierra Beauvoir, should be not older than the Eocene.

Mineralogy as a proxy to characterise geochemical dispersion processes: A study from the Eromanga Basin over the Prominent Hill IOCG deposit, South Australia

The Early Cretaceous interbedded sandstone, siltstone and mudstone of the Cadna-owie Formation and the reduced mudstone of the Bulldog Shale overlying the Prominent Hill IOCG deposit in South Australia (total tonnage: 140 Mt. @ 1.2% Cu and 0.5 g/t Au; OZ Minerals Limited 2017) are shown to preserve Cu concentrations up to 5043 ppm Cu. These cover sequence materials are approximately 100 m thick over the basement rocks that host the deposit. This study investigates the source of elevated Cu in the cover material by assessing the Cu deportment in these formations and focuses on understanding element migration processes resulting in development of elevated Cu concentrations. Petrographic and QEMSCAN® analysis show that Cu resides within sulphides including high-temperature Cu-sulphides (e.g. bornite) in the Cadna-owie Formation. The Cu-sulphides are associated with Fe-oxides (magnetite) and minerals including pyroxene, amphibole, garnet, chlorite and plagioclase that are interpreted to be detrital and are likely to be of metamorphic origin. Based on these observations, lateral physical processes driven by sedimentary dynamics are interpreted to be the main processes responsible for elevated Cu concentrations in the Cadna-owie Formation in the Prominent Hill area. Chemical migration processes likely had only localised influence and resulted in minor Cu redistribution. QEMSCAN® data collected on samples from the weathering profile of the Bulldog Shale shows that weathering processes (e.g. pyrite oxidation, serpentinisation and feldspar hydrolysis) mostly affect the upper Bulldog Shale and that Eh-pH conditions gradually change with depth from the oxidised and acidic upper Bulldog Shale to the reduced and slightly acidic lower Bulldog Shale. Copper speciation and Eh-pH conditions are used to interpret downward chemical migration of Cu due to weathering as the main process for development of elevated Cu concentrations within the weathered Bulldog Shale in the Prominent Hill area. Upward chemical and lateral physical migration processes may have influenced Cu concentrations within the unweathered Bulldog Shale, however the degree of influence is yet to be determined. Overall this study shows that elevated Cu concentrations in the Early Cretaceous cover sequences overlying the Prominent Hill deposit can be linked back to mineralisation itself and that processes resulting in elevated Cu concentrations include chemical and physical migration.

The structure of the northern Austral Basin: Tectonic inversion of mesozoic normal faults

This paper analyzes the style, distribution, mechanics, and timing of deformation of the Andean retroarc zone between 47° and 50°S, in the southern Patagonian Andes during the Mesozoic times. This northern sector of the Austral Basin records almost the entire Meso-Cenozoic evolution of the Patagonia western margin. Two balanced cross sections were constructed in the north and south of the study area to show the contrasting structural styles of the fold and thrust belt and the foreland sector. The westernmost internal sector is characterized by a thick-skinned structure associated with a deep decollement, over which late Paleozoic rocks (basement) are uplifted. Towards the foreland, the external fold and thrust belt is dominated by partial positive tectonic inversion of normal faults that gently fold the Cretaceous cover. The restoration of the external fold and thrust belt structural sections has yielded a total minimum shortening of less than 3 km produced since the Cretaceous contractional stage continuing until the more recent Andean deformation. The age, distribution, and compositional features of the middle Early Cretaceous - Late Cretaceous magmatic rocks show a relative shift of the volcanic arc location, indicating interactions between the extensional and contractional regimes associated with different geodynamic settings.

Cretaceous and Eocene tectono-thermal events determined in the Inner Western Carpathians orogenic front Infratatricum

This paper re-assesses the tectono-thermal evolution of the frontal Infratatricum part of the Inner Western Carpathians orogen based on new geological-petrological and zircon fission track (ZFT) data and published 40Ar–39Ar and K–Ar data. The study area is in the Považský Inovec Mts. in Western Slovakia, where the Infratatric Inovec Nappe comprises a micaschist-gneiss basement and Upper Carboniferous–Lower Cretaceous cover with the Jurassic–Lower Cretaceous Humienec Succession; this latter is reconstructed from olistoliths in the Horne Belice Succession Upper Cretaceous flysch. The Infratatric Inovec Nappe posterior part is thrust over its frontal Humienec tectonic Slice with infolded Upper Cretaceous sediments. This nappe exhibits very low-temperature tectono-thermal overprinting, and this is documented by electron probe microanalysis of metamorphic phases and P–T estimates. The late Early Cretaceous age of this D1 stage event at approximately 115–95 Ma is recorded by 114 ±2 and 106 ±4 Ma phengitic white mica 40Ar–39Ar ages from a Lower Cretaceous cherty slate, and by 101 ±3 Ma 40Ar–39Ar age and 102±4 Ma ZFT age from Permian meta-sandstone. The Inovec Nappe was thus derived from an Albian–Early Cenomanian accretionary wedge. The hemipelagic Upper Turonian to Lower Santonian red marls and upward syn-orogenic Upper Santonian to Maastrichtian flysch were deposited on the frontal attenuated part of the Inovec Nappe-type basement in the inferred Pieninic–Vahic (~South-Penninic) zone. This flysch contains olistolithic to clastogeneous metamorphosed material, including the Lower Cretaceous slates, supplied from the posterior part of the Inovec Nappe. The 40Ar–39Ar age of 86 ±2 Ma from this nappes’ Permian meta-sandstone is consistent with D2 thrusting at approximately 95–85 Ma and Horne Belice foreland flysch basin supply in the D3 stage at ~85–65 Ma. The Inovec Nappe frontal Humienec tectonic Slice with infolded Upper Cretaceous sediments suggests formation of a Paleocene–Eocene accretionary wedge at approximately 65–40 Ma in the D4 stage. This event is constrained by 40Ar–39Ar age of 48 ±2 Ma from the Tatricum hanging wall blastomylonites and whole-rock K–Ar age of 46 ±3 Ma from a Permian basalt olistolith in the footwall Infratatricum Horne Belice flysch. The Infratatricum finally became part of an Eocene accretionary wedge north of the Hradok–Zlatniky thrust-fault, and reheating and exhumation cooling is documented by 57–37 Ma ZFT ages. Although the Infratatricum exhibits Late Cretaceous and Eocene tectono-thermal imprints, it is a remnant of the Early Cretaceous structure at the Inner Western Carpathian front. This imposes the Infratatricum as a distal continental margin of the Penninicum. Normal 0 21 false false false PL X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:Standardowy; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin-top:0cm; mso-para-margin-right:0cm; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:12.0pt; font-family:Arial,sans-serif; mso-bidi-font-family:CG Times 12pt; mso-fareast-language:EN-US;}

Upper Cretaceous shallow-marine deposits of the southwestern Münsterland (northwest Germany) influenced by synsedimentary tectonics

During the Late Cretaceous global sea level rise, marine nearshore sedimentary rocks were deposited in the Münsterland (northwest Germany). Thirteen borehole cores recently drilled in the southwestern part of the Münsterland have supplied new data on the lithostratigraphy and facies patterns of these deposits. By combining biostratigraphy (planktonic/benthic foraminifera, calcareous nannofossils), gamma ray data and lithological observations from the drill cores and form auxilliary chipped drillings a detailed correlation scheme of the Cenomanian–Campanian succession has been established. The successions studied show considerable variations in thickness and lithology over an area of 850 square kilometres. The western part of the study area is dominated by a homogeneous sequence of glauconitic sandstones and marlstones (greensands). Eastwards, the lithology becomes more variable and marls predominate, particularly in Coniacian and lower–middle Santonian strata. The new findings allow for a revision of the current lithological scheme used for Upper Cretaceous deposits in the western part of northern Germany. A major result is the evidence for synsedimentary tectonics as a driving force for the formation of an intrabasinal structural high which has been derived from spatial facies and thickness patterns of lower Turonian deposits. The Late Cretaceous synsedimentary inversion tectonics is examined in two examples, yielding a maximum movement rate of 0.1 mm/year for the early Santonian. Local geological mapping activities within the study area have provided a stripped map of the Cretaceous cover and a base Cretaceous subsurface contour map.

Modelling of the thermal history of the Carboniferous Lorraine Coal Basin: Consequences for coal bed methane

This paper proposes a new scenario for the thermal history of the Carboniferous Lorraine Coal Basin following the tectonic model developed by Averbuch et al. (2012) and establishes some consequences for the coal bed methane (CBM) exploitation. Organic matter maturation data (Vitrinite Reflectance) determined on eleven boreholes in the eastern Lorraine have been used to characterize the Lorraine Coal Basin evolution. Paleozoic and Mesozoic overburials have been calculated using a thermal modelling software (Petromod). Results show that (1) Paleozoic erosion may be estimated at a maximum of 1200m which represents a low amplitude event, (2) a little erosion occurred between Upper Paleozoic and Lower Mesozoic: paleotemperature offset is about 20°C and VR data range from 0.7 (Westphalian D) to 0.5 (Lower Triassic), (3) Cretaceous cover overburial reaches a maximum of 300m and decreases eastwards and (4) the variation of heat flows is in agreement with the compressive and extensive phases of Paleozoic and the extensive phase of Mesozoic. Consequences on the petroleum system are the following: organic matter is immature in the Jurassic and Triassic sedimentary rocks, mature (oil window) in the Permian, Stephanian and Westphalian C-D, highly mature (gas window) in Westphalian A-B-C and overmature in Namurian-Westphalian A. The high methane adsorption capacity of coal and the presence of natural fractures inside coal seams demonstrated by coal tomography allow a high CBM potential in this basin. The Lorraine Coal Basin is therefore a target for coal gas.

Mapping folds and fractures in basement and cover rocks using UAV photogrammetry, Cape Liptrap and Cape Paterson, Victoria, Australia

Brittle and ductile deformation of alternating layers of Devonian sandstone and mudstone at Cape Liptrap, Victoria, Australia, resulted in upright folds with associated fold accommodation faults and multiple fracture sets. Structures were mapped at the Fold Stack locality at Cape Liptrap using high-resolution aerial photographs acquired by a digital camera mounted on an unmanned aerial vehicle (UAV). Subsequent photogrammetric modelling resulted in georeferenced spatial datasets (point cloud, digital elevation model and orthophotograph) with sub-cm resolution and cm accuracy, which were used to extract brittle and ductile structure orientation data. An extensive dataset of bedding measurements derived from the dense point cloud was used to compute a 3D implicit structural trend model to visualise along-strike changes of Devonian (Tabberabberan) folds at the Fold Stack locality and to estimate bulk shortening strain. This model and newly collected data indicate that first generation shallowly south-southwest plunging upright folds were gently refolded about a steeply plunging/subvertical fold axis during a Devonian low-strain north–south shortening event. This also led to the local tightening of first generation folds and possibly strike-slip movement along regional scale faults. In order to distinguish fractures associated with Devonian compression from those that formed during Cretaceous extension and later inversion, we compared the five fracture sets defined at Cape Liptrap to previously mapped joints and faults within the overlying sedimentary cover rocks of the Cretaceous Strzelecki Group (Gippsland Basin), which crop out nearby. An east-southeast trending fracture set that is not evident in the Strzelecki Group can be linked to the formation of Devonian folds. Additionally, hinge line traces extracted from the Fold Stack dataset are aligned parallel to a dominant fracture set within the overlying cover sediments. This suggests that basement structures (folds and coeval parallel faults) have an important influence on fault and joint orientations within Cretaceous cover rocks.

Lower Paleogene denudation of Upper Cretaceous cover of the Morvan Massif and southeastern Paris Basin (France) revealed by AFT thermochronology and constrained by stratigraphy and paleosurfaces

Abstract Low-temperature thermochronology data have been acquired from outcrop samples from the southeastern edge of the Paris Basin (Morvan). They provide indications about the former sedimentary cover, its denudation and about inversion tectonics in the Paris Basin. Apatite fission-track data acquired on Variscan granites and Triassic sandstones reveal that the basement was subjected to temperatures of about 70 ± 10 °C during the Late Cretaceous. We suggest that the peak temperature was generated by an Upper Cretaceous sedimentary cover of low thermal-conductivity chalk. Additional stratigraphic, sedimentological, paleogeographic, paleoweathering, and continental paleosurface data support the reconstruction of this chalk paleocover and of its erosion. This former blanketing sequence was removed during the Early Paleogene regional inversion episode which affected the southern edge of the Basin intensively and was related to far-field compression between Europe and Africa. This study shows that (i) for the SE border of the intracontinental Paris Basin, kilometer-scale vertical deformation occurred, (ii) the sedimentary sequences of the Paris Basin covered considerably larger areas than the present contours of the sediment fill, (iii) paleoweathering and erosion processes later removed the sedimentary cover rather quickly (

Detrital zircon data reveal the origin of Australia’s largest delta system

The Late Cretaceous Ceduna Delta is the largest deltaic system on the Australian continent, yet its source is unknown. Apatite fission-track data reveal widespread Late Cretaceous exhumation across the southern Australian margin. New detrital zircon analysis of 786 grains from the Gnarlyknots-1 well, which penetrated the offshore delta top, show that the upper part of the delta (Santonian–Maastrichtian) was sourced largely from recycled Permian to Early Cretaceous cover and underlying basement eroded from the margin, proximal to the basin. This challenges the widely accepted model involving distal provenance of >2000 km from the eastern margin of Australia. Supplementary material The 2D seismic reflection data, results for detrital zircon LA-ICP-MS and zircon fission-track analyses, including the LA-ICP-MS method, and a list of sample intervals and ages are available at www.geolsoc.org.uk/SUP18582 .

The mid-Cretaceous transition from basement to cover within sedimentary rocks in eastern New Zealand: evidence from detrital zircon age patterns

AbstractDetrital zircon U-Pb ages for 30 Late Jurassic and Cretaceous sandstones from the Eastern Province of eastern New Zealand, combined with previously-published geochronological and palaeontological data, constrain the time of deposition in the Pahau and Waioeka terranes of the Cretaceous accretionary margin of Zealandia, and their adjacent cover strata. The zircon age patterns also constrain possible sediment source areas and mid-Cretaceous geodynamic models of the transition from basement accretionary wedge to passive-margin cover successions. Pahau Terrane deposition was mainly Barremian to Aptian but continued locally through to late Albian time, with major source areas in the adjacent Kaweka and Waipapa terranes and minor inputs from the inboard Median Batholith. Waioeka Terrane deposition was mainly Albian, with distinctive and exclusive sediment sources, principally from the Median Batholith but with minor inputs from the Western Province. Alternative tectonic models to deliver such exclusive Median Batholith and Western Province-derived sediment to the mid-Cretaceous Zealandia continental margin are: (1) the creation of a rift depression across Zealandia or (2) sinistral displacement of South Zealandia with respect to North Zealandia, to expose Western Province rocks directly at the Zealandia margin. Detrital zircon age patterns of Cretaceous cover successions of the Eastern Province of eastern New Zealand demonstrate purely local sources in the adjacent Kaweka and Waipapa terranes. Cretaceous zircon components show a decline in successions of late Early Cretaceous age and disappear by late Late Cretaceous time, suggesting the abandonment or loss of access to both the Median Batholith and Western Province as sediment sources.

Detrital zircon geochronology and sandstone provenance of basement Waipapa Terrane (Triassic–Cretaceous) and Cretaceous cover rocks (Northland Allochthon and Houhora Complex) in northern North Island, New Zealand

AbstractDetrital zircon U–Pb ages are reported for 14 sandstones of mainly Cretaceous age from the Northland Allochthon, Houhora Complex and Waipapa Terrane of northern North Island, New Zealand. Results from the Waipapa Terrane samples, selected from sequences in the Bay of Plenty, Coromandel Peninsula and Great Barrier Island, show that deposition continued into late Early Cretaceous time and, as in the Torlesse Composite Terrane, finally waned at c. 110–114 Ma. Upper Lower Cretaceous and Upper Cretaceous sedimentary successions in the Houhora Complex and Northland Allochthon have dominant sediment sources derived from local, contemporary volcanism, with a minor older contribution from the Murihiku Terrane to the west. As in eastern North Island, upper Upper Cretaceous sandstones lack major Albian magmatic components and their sources are solely in the Murihiku Terrane, and possibly the Western Province. We propose a Cretaceous palaeogeographic model that invokes a recently extinct orogen and a partially submerged continental borderland, dissected by rivers supplying submarine fans.

First sedimentary record of the pre-obduction convergence in New Caledonia: formation of an Early Eocene accretionary complex in the north of Grande Terre and emplacement of the ‘Montagnes Blanches’ nappe

Abstract New Caledonia lies at the northern tip of the Norfolk ridge, a continental fragment separated from the east Gondwana margin during the Late Cretaceous. Stratigraphic data for constraining the convergence that led to ophiolitic nappes being obducted over Grande Terre during the Eocene are both few and inaccurate. To try and fill this gap and determine the onset of the convergence, we investigated the lithology, sedimentology, biostratigraphy and geodynamic context of the Late Cretaceous – Palaeogene sedimentary cover-rock succession of northern New Caledonia. We were able to establish new stratigraphic correlations between the sedimentary units, which display large southwest-verging overfolds detached along a basal argillite series, and reinterpret their interrelationships. The sediments from the Cretaceous-Paleocene interval were deposited in a post-rift pelagic environment and are mainly biogenic with minimal terrigenous input. From the base up, they comprise black organic-rich sulphide-bearing argillite, black chert (silicified equivalent of the argillite), micritic with chert, and micrite rich in planktonic foraminifera. These passive-margin deposits are found regionally on the Norfolk Ridge down to New Zealand, and on the Lord Howe Rise, and were controlled primarily by regional or global environmental factors. The overlying Eocene deposits mark a change to an active-margin regime with distal calciturbidite and proximal breccia representing the earliest Paleogene flysch-type deposits in New Caledonia. The change from an extensional to a compressive regime marks the beginning of the pre-obduction convergence and can be assigned fairly accurately in the Koumac–Gomen area to the end of the Early Eocene (Late Ypresian, Biozone E7) at c 50 Ma. From this period on, the post-Late Cretaceous cover in northern New Caledonia was caught up and recycled in a southwest-verging accretionary complex ahead of which flysch was deposited in a flexural foreland basin. The system prograded southwards until the Late Eocene collisional stage, when the continental Norfolk ridge entered the convergence zone and blocked it. At this point the autochthonous and parautochthonous sedimentary cover and overlying flysch of northern New Caledonia was thrust over the younger flysch to the south to form a newly defined allochthonous unit, the ‘Montagnes Blanches’ nappe, that is systematically intercalated between the flysch and the obducted ophiolite units throughout Grande Terre.

Vertical and lateral changes in organic matter from the Mesozoic, eastern Paris Basin (France): Variability of sources and burial history

Abstract The Callovian–Oxfordian claystones located at 500 m depth in the eastern Paris Basin are currently being investigated by ANDRA (the French National Radioactive Waste Management Agency) for long-term geological nuclear waste disposal. For this purpose, a recent 2000 m deep well was drilled close to the ANDRA underground research laboratory. This borehole penetrated the Mesozoic sedimentary series for a detailed study of the sediments located beneath the target formation. It also provides the opportunity to study the vertical evolution of the sedimentary organic matter and its thermal maturity in this part of the Basin, where the thickness of sedimentary eroded cover is poorly constrained. To extend this study at the scale of the eastern Paris Basin, 5 more wells and numerous outcrops were sampled. Sedimentary organic matter characterization was performed on kerogen (Rock-Eval pyrolysis, organic petrography and mean huminite and vitrinite reflectance measurements) and on the extractible organic fraction (Gas Chromatography–Mass Spectrometry). Results show that inputs of organic matter during the Mesozoic were essentially of terrigenous origin, except during the Toarcian where marine contributions are predominant. The major macerals present are huminite (ulminite), vitrinite (collotelinite) and inertinite (inertodetrinite and fusinite). Vitrinite shows a regular increase of reflectance with depth in the 2000 m depth well. Correlations between T max , vitrinite reflectance, and biomarker data define the thermal maturity of the Mesozoic sediments: maturity is reached at 1300–1500 m present day depth in the 2000 m well, in the Upper Triassic. At the regional scale, the organic matter maturity of each sedimentary formation decreases towards the eastern part of the basin, probably in relation to a decrease of the Jurassic and/or Cretaceous cover thickness, which is now eroded. All results indicate the lack of hydrocarbon generation in the Mesozoic series in this part of the basin.

秩父南帯のジュラ紀-白亜紀前期付加コンプレックスと被覆層：四国東部の津乃峰亜帯を例として

秩父南帯のジュラ紀-白亜紀前期付加コンプレックスと被覆層：四国東部の津乃峰亜帯を例として

Upper Cretaceous feldspars in the Cenozoic Limagne Basin: A key argument in reconstructing the palaeocover of the Massif Central (France)

The northern part of the N–S-trending basin of the Limagne graben, located in the northern part of the Massif Central (France), contains Oligocene and Miocene calcareous lacustrine deposits. The area is known for its stromatolite reefs surrounded by marl, clay and uncemented carbonate sand with oncolites. Because the origin of the carbonate accumulated by cyanobacteria has long been debated, the clayey deposits associated with the stromatolites were analysed using XRD, SEM and K–Ar dating methods. The results show a monotonous assemblage of carbonates, dioctahedral smectite, illite–smectite (Reichweite) R = 0, glauconitic illite and abundant orthoclase. Detrital minerals such as quartz and detrital illite are found near the basin's borders but are very scarce in its central part, indicating a lack of detrital input from the regional basement. Hence, the abundant orthoclase found in all the clayey deposits of the Limagne Basin cannot be linked to erosion of the surrounding Hercynian basement. The SEM analysis showed the orthoclase crystals to have a tabular morphology with sharp boundaries and to be embedded within a clay matrix, whilst the K–Ar dating of the orthoclase and glauconitic illite yielded ages ranging from 90 to 66 ± 2 Ma, i.e. Turonian to Maastrichtian. These two results clearly indicate that the fill of the Limagne Basin, composed mainly of carbonate, clay and orthoclase, must have been partly due to the erosion of an Upper Cretaceous sedimentary blanket once covering the Massif Central basement. This cover was probably constituted of chalk, dioctahedral smectite, I/S and glauconitic illite, associated with authigenic orthoclase and flint. Flints are found reworked in the alluvial formations of the Late Pliocene Lower Bourbonnais sands and clays (‘Sables et argiles du Bourbonnais’) and in the Pleistocene terraces. Moreover, relicts of the Upper Cretaceous cover were preserved during the sedimentary filling of the Cenozoic Limagne Basin. Authigenic orthoclase of similar shape has been described in the Chalk Group of the Paris Basin and from localities in northern France and Belgium. The mineral is rare in the Turonian chalk but increases rapidly at the base of the Coniacian to Campanian chalk; it has also been noted that the number of orthoclase crystals increases as the quantity of clastic components decreases. Similar orthoclase crystals were found associated with dioctahedral smectite in the decarbonated residues of the Coniacian to Santonian chalk from Fécamp and Etretat (northern France). Moreover, our results are consistent with other data from within and around the Paris Basin, such as the palaeogeography and facies distribution of the Chalk formations, the residual flints in the clay-with-flints of the southern Paris Basin and around the Morvan, and apatite fission-track thermochronology data from the Hercynian basement of the Massif Central and the Morvan. All these data indicate a major connection between the Paris Basin and the Tethys, and an extensive palaeocover on the Massif Central and Morvan basement.

Age and provenance of basement rocks of the Chatham Islands: An outpost of Zealandia

Rb‐Sr metamorphic and U‐Pb detrital zircon ages are reported for basement metasediments and cover sandstones of the Chatham Islands. Rb‐Sr metamorphic ages of the Chatham Schist are 180–198 Ma (Early Jurassic), with initial 87Sr/86Sr ratios 0.7075–0.7091, grouping this distinctively with Torlesse Supergroup of the Eastern Province of New Zealand. These ages impose an Early Jurassic minimum age for the sedimentary protoliths. Detrital zircons from two Chatham Schist metagrey wackes have principal age peaks in the interval Late Triassic to Late Permian, and the youngest, 230–250 Ma, suggest maximum early Late Triassic to early Early Triassic ages for sedimentation. The patterns of minor, but significant, Paleozoic components and scattered Precambrian ages in these greywackes resemble those of Permian Torlesse Supergroup greywackes, but that in a greywacke from the Forty Fours islets is closer to possible latest Permian Caples Group greywackes in Otago. A Cretaceous cover sandstone from Pitt Island has a detrital zircon age pattern dominated by several Albian, 101–119 Ma, age components, reflecting contemporary volcanism, with a minor older component resembling that from a Chatham Schist source.

The deep seismic reflection MARCONI-3 profile: Role of extensional Mesozoic structure during the Pyrenean contractional deformation at the eastern part of the Bay of Biscay

The new MARCONI-3 deep seismic profile allows recognition of the upper crustal structure of the eastern part of the Bay of Biscay and the main features of its Alpine geodynamic evolution. It denotes that the easternmost part of the Bay of Biscay consists of a thick wedge of uppermost Cretaceous to Cenozoic synorogenic sediments lying unconformably on the top of a thinned continental crust with the Mesozoic Parentis Basin to the north and the coeval Landes High to the south. The Parentis Basin appears as a major half-graben bounded southwards by a north-dipping planar fault. It is filled by a thick sequence of Jurassic–Upper Cretaceous carbonates affected by salt domes and squeezed diapirs made up of Triassic evaporites and mudstones. These salt tectonic structures also affect the overlying uppermost Cretaceous to Lower Miocene synorogenic deposits which are folded upon these structures. The Landes High includes a thin pre-Upper Cretaceous cover tilted to the south. In the Basque shelf, it is deformed by a basement-involving thrust wedge emplaced during the Late Eocene–Miocene that constitutes the North-Pyrenean contractional front. Geometric relationships and thickness variations depict that this overall structure results from the following. • Mesozoic extensional stage which includes a Late Jurassic (?)–Late Aptian syn-rift stage in which the Parentis Basin formed; and an Albian–early Late Cretaceous post-rift stage in which diapirs of Triassic evaporites grew close to this major fault. • Compressive deformational stage coeval to the Pyrenean orogeny which led to (1) the development of a latest Cretaceous up to Middle Miocene foreland basin; and (2) from Late Eocene, the formation of a basement-involving thrust wedge in the innermost foreland basin, the squeezing of the diapiric structures formed previously in the Parentis Basin and, later (Middle Miocene), the partial inversion of some pre-existent faults located in the southern Parentis Basin margin. This geodynamic evolution together with the structure of the area evidences that the extensional structure resulting from the opening of the Bay of Biscay played an important role both in the location of the North-Pyrenean front and in the North-Pyrenean foreland contractional deformation features. Specially, the lack of significant inversion structures in the Parentis Basin, despite that it belongs to a severely thinned crustal area before the Alpine compression, denotes that the Mesozoic Landes High acted as an important buffer for the propagation of the Pyrenean contractional deformation to the north. This deformational buffer was active until Early Miocene and probably vanished afterwards during the last stages of Pyrenean orogen development when some basement faults reactivated in the Parentis Basin.