Evidence For Uplift Research Articles

Windblown Hadean zircons derived by erosion of impact-generated 3.3 Ga uplifts, Barberton Greenstone Belt, South Africa

In the absence of a rock record older than about 4.03 Ga, Hadean (>4.0 billion-years-old) zircons provide a unique window into crustal development and surface conditions on the Earth within a few hundred million years of its formation. Recently discovered Hadean detrital zircons, 4.0–4.2 Ga, in the Barberton Greenstone Belt, South Africa, occur in a 3 to 5 m thick layer termed the Green Sandstone Bed (GSB) that is composed largely of mafic to komatiitic volcaniclastic debris and trace amounts of sand-sized quartz, chromite, and zircon. The youngest concordant zircons in the GSB and ages of zircons in immediately underlying strata suggest a depositional age of about 3.306 ± 6 Ga. Hadean zircons are concentrated in the lower 200 cm of the unit: higher parts of the GSB lack Hadean zircons and the uppermost 150 cm contain zircons 3.312 ± 5 Ga. The quartz and heavy minerals in the lowest 200 cm of the GSB appear to have been transported as windblown sediments in a subaerial environment. They were subsequently incorporated into mass flows represented locally by the lowest 200 cm of the GSB. The bulk of the overlying GSB sediments were worked and deposited in wave- and current-active, shallow-marine to intertidal settings. Cathodoluminescence studies suggest that a high proportion of the detrital GSB quartz is of plutonic origin although the GSB is underlain by 10–12 km of volcanic and volcaniclastic silicified sedimentary rocks with no evidence of uplift or terrigenous sediment sources. The GSB lies <2 m above impact layer S6 and we infer that much of the windblown sediment was derived by erosion of crustal rocks that were uplifted and emplaced at the surface during the S6 impact and of komatiites exposed subaerially between the impact and depositional sites.

First Results on the Mw=6.9 Samos Earthquake of 30 October 2020

On 30 October 2020 11:51 UTC, a Mw=6.9 earthquake struck the offshore region north of Samos Island, Greece, in the Gulf of Ephesos/Kuşadasi, causing two fatalities and 19 minor injuries at Samos Island, as well as 115 casualties and over 1,030 injuries in Western Turkey. Preliminary results indicate that the mainshock occurred on a north-dipping normal fault, with a focal mechanism of 270º/50º/-81º. The selection of the fault plane is supported by evidence of uplift at western Samos and over 10 cm of subsidence at the northernmost edge of the central part of the island. The distribution of relocated hypocenters shows clustering of events, east of the mainshock’s epicenter, where most major aftershocks have occurred. To the west, a smaller group of aftershocks is observed, separated by a spatial gap in seismicity. The latter is likely related to the region of the fault plane where most of the co-seismic slip occurred, with Coulomb stress-transfer towards the western and eastern margins of the rupture triggering aftershock activity. The apparent complexity of the mainshock’s source time function, supported by preliminary results, could indicate the rupture of more than one structures. This could explain the relatively weak magnitude of the largest aftershock (Mw=5.0). The mainshock caused damage mainly to non-engineered constructions, i.e. old residential buildings, churches and monuments in Samos Island, and minor damage to the majority of the building stock of the island built according to the National Seismic Code. On the other hand, it caused severe damage at Izmir, especially to high-rise buildings. The mainshock also triggered a small tsunami that reached heights of over 1 m, mainly affecting the Turkish coast.

Geochemistry and mineral chemistry of quartz mica schists within Iseyin-Oyan Schist Belt, Southwestern Nigeria

Background/Objectives: The Iseyin-Oyan schist belt is made up metasedimentary rocks, gneisses, granites and pegmatite intrusions. The study was aimed at identifying the schist within this belt and assess their metamorphism, geochemical characteristics and tectonic origin. Methods: Detailed geologic field mapping was undertaken where rocks were located, studied in-situ and identified. Samples of the schist were prepared for petrographic studies. Mineralogical contents were determined using X-Ray Diffraction technique. Polished sections were studied for mineral chemistry using Scanning Electron Microscope-Energy Dispersive Spectroscopy. Rock samples were analysed using X-Ray Fluorescence Spectroscopy and Inductively Coupled Plasma Emission Spectrometry. Geochemical data were elucidated using diverse geochemical discrimination diagrams. Findings: The schists are quartz mica schists and occur in close association with amphibolites, intrusive granitoids and pegmatites. The Mineral assemblage indicates upper (at the western part) to lower (at the central part) amphibolite facies grade metamorphism in the area. Pyrope-almandine garnets occur in quartz mica schist at the western parts reinforcing higher pressure-temperature metamorphic conditions. The concentration (in %) of SiO2 ranged from 56.4-71.6; Al2O3, 13.7-21.1; Fe2O3, 2-8; MgO, 0.7-2.4; and K2O, 2.1-5.5 supporting the evidence for differential degrees of metamorphism. Large iron lithophile and high field strength elements are similar to the average upper continental crust. Pronounced negative Europium anomaly pointed to the major roles played by feldspars during the geological processes. Plagioclase ranged from albite-oligoclase and oligoclase- andesine. The precursors of the quartz mica schist are possibly arkosic and greywacke sands deposited within the active continental margins. Evidence of uplift and overturning suggested for the differential metamorphism may be due to these events usually associated with active continental margins. Applications: This study has identified the once named undifferentiated schist in the study area to be quartz mica schist with details in their grades of metamorphism elucidated. Keywords: Quartz mica schist; geochemistry; mineral chemistry; Iseyin-Oyan schist belt; precambrian basement complex

Luminescence dating of Late Pleistocene faults as evidence of uplift and active tectonics in Sardinia, W Mediterranean

AbstractPrecise dating of the activity of Late Pleistocene to Holocene neo‐tectonic structures is crucial to quantify the rate of deformation in low‐seismicity regions. Sardinia is a relatively stable continental fragment set in the middle of the tectonically active Western Mediterranean belt. This paper provides evidences of significant uplift of northwest Sardinia that support an ongoing tectonic activity since the Marine Isotopic Stage 7 (MIS 7; ca. 220 ka). In particular, it documents for the first time Late Pleistocene to Holocene tectonics based on luminescence dating of travertine sealing a major NNE‐SSW fault.

Landscape evolution of the Blue Mountains revealed by longitudinal river profiles and Cenozoic basalts and gravels

For the past 200 years, there have been numerous investigations and much speculation concerning the formation of the dramatic valleys of the Blue Mountains of NSW. In this paper, further evidence for the uplift and erosional history of the Blue Mountains is obtained from longitudinal river profiles, detailed mapping of the Rickabys Creek Gravel on the Lapstone Structural Complex and consideration of the topographical position of Miocene basalts. Knickpoints on the main rivers flowing east from the Great Dividing Range are identified and interpreted to be due to uplift events linked to the northward movement of the Australian continent over mantle inhomogeneities. At the Lapstone Structural Complex on the eastern range front, the occurrence of the Rickabys Creek Gravel and the nature of the over-steepened reaches on the rivers and streams crossing the Complex, suggest a more recent ongoing phase of uplift and antecedent river erosion. The Miocene basalts provide evidence of this landscape 20–15 Ma. Their locations with respect to the current rivers and ridges are interpreted to show additional evidence for recent uplift that has resulted in the formation of the Lapstone Structural Complex. It is suggested that this uplift commenced 10–5 Ma when the contemporary compressive stress field was established.KEY POINTSLongitudinal profiles for major rivers in the Blue Mountains are consistent with a model of initial Cretaceous uplift followed by further Cenozoic uplift associated with dynamic topography.Mapping of Rickabys Creek Gravel within the Lapstone Structural Complex suggests the presence of antecedent rivers.Within the Lapstone Structural Complex, stream profiles, gravels and nearby outcrops of Miocene basalts are interpreted to indicate a third phase of uplift, possibly since 10 Ma.

Examination of Tectonic Activity Based on Knickpoint Distribution and Movement Potential of Faults in Khuzestan Province, South West Zagros, Iran

Although the distribution of fluvial knickzones, as an important geomorphic feature in bedrock river morphology, has been studied by many investigators, the role of them in examination of tectonic activity has not been well investigated [1], specially based on its comparision with movement potential faults over a broad area. This study examines the tectonic activity of Khuzestan province, South West Zagros, by considering two different parameters: distribution of the fluvial knickzones along Mountain Rivers and evaluation of faults activity in the study area. A segment of a river long-profile that is steeper than adjacent segments is commonly referred to as a knickzone or a knickpoint if it is visibly steeper than the trend of the longitudinal profile. Knickzones are often observed along bedrock rivers and the most visible form is a waterfall. Knickzones are supposed to be a response to base-level changes or to alternations of local lithology [2]. Upstream migration of knickzones has been argued to cause rapid river incision and result in the formation of terraces and instability of valley-side slopes. Knickpoint evolution on a river can provide evidence for uplift of plate margins [3]. Knickpoints can be used as geomorphic markers in steep, rapidly eroding landscapes that commonly lack datable river terraces [4]. In this paper we use the method that was proposed by Hayakawa and Oguchi [5] to extract knickzones in broad areas using DEMs (Digital Elevation Models) and GIS (Geographical Information System). DEM analysis of the longitudinal profiles of rivers permits quantitative, reproductible, and efficient identification of knickzones. The obtained inventory of knickzones will provide a basis for objective analyses of the distribution of knickzones.

Reply to 'Unclear causes for subduction'

Keenan and Encarnacion suggest that the absence of pre-subduction inception basement in the drill core data taken from site U1438 raises ambiguity in our conclusion of spontaneous subduction initiation in the Izu–Bonin–Mariana system. However, there is no evidence for uplift in the earliest products of the Izu–Bonin–Mariana system preceding rifting preserved anywhere in the region. Three sub-parallel ridges — from north to south, the Amami plateau, Daito and Oki-Daito ridges (Fig. 1 from ref. 1) — comprise magmatic products of Mesozoic–Tertiary arcs formed prior to the Izu–Bonin–Mariana system. These ridges generally strike east–west orthogonally to the Kyushu–Palau ridge — the earliest stratovolcano chain of the Izu–Bonin–Mariana system. If compression preceded inception, as implied by the forced subduction model, we anticipate uplift of the old arc ridges, diminishing in effect westwards, and sediment shedding from uplifted regions into adjacent basins.

Mesozoic geology of southwestern China: Indosinian foreland overthrusting and subsequent deformation

The southwestern part of the South China Block (SCB) records Triassic and subsequent deformations and is a key region that provides evidence of the post-amalgamation tectonic evolution of Southeast Asia. Here, we outline the tectonothermal evolution of early Mesozoic orogenesis in this region using structural analysis and fission track thermochronology. This region is divided into three tectonic units (south to north): the Youjiang Fold-and-Thrust Belt (YFTB), the Qianzhong Massif (QZM), and the Thin-skinned Thrust Belt (TTB) of the southeastern Sichuan Basin, all three of which record three deformation events, here termed D1, D2, and D3. D1 deformation is represented by Triassic top-to-the-north thrusts (F1) and fault-related folds (f1) that are indicative of N–S shortening. The intensity of deformation decreases toward the north. The subsequent D2 and D3 deformations are marked by Cretaceous top-to-the-NW thrusting and fault-propagation folding, and Cenozoic NE–SW trending normal faults (F3), respectively. The temperature–time (t–T) path obtained by apatite fission-track modeling of samples from the YFTB, QZM, and TTB areas provides evidence of uplift and denudation. The D1 deformation at ∼230–210Ma is characterized by buried thrusts within the relatively stable YFTB and QZM, and an increase in depth of the TTB. This was coeval with the Late Triassic northward migration of uplift and denudation that is evidenced by a Late Triassic stratigraphic gap within the YFTB, a thin layer of coarse-grained continental clastics within the QZM, and a thick layer of clastic rocks within the TTB. The study area records a Triassic thrust system (D1) that progressively migrated northwards and represents a foreland fold-and-thrust belt that formed during the Indochina–SCB collision. This initial deformation was subsequently overprinted by Late Jurassic to Cretaceous NW–SE thrusting (D2) and the development of Cenozoic NW–SE extensional structures (D3).

Crust beneath the northwestern Deccan Volcanic Province, India: Evidence for uplift and magmatic underplating

AbstractThe northwestern Deccan Volcanic Province of India and its pericratonic rift basins were reactivated during different stages after the breakup of India from the Gondwanaland and collision with the Asian plate. In this study, we present results of crustal thickness and average crustal Vp/Vs ratios beneath this plume‐affected region using common conversion point imaging and H‐k stacking analysis of 6893 receiver functions using data from a network comprising 58 broadband seismic stations sited on diverse tectonic terrains. We find large variations in crustal thickness, with the Moho depths varying from 28 to 43 km in the Kachchh rift, 28 to 38 km in the Cambay rift, 39.5–41.5 km in the north and eastern parts of the Cambay rift, and 29 to 39 km in the Saurashtra region and South Gujarat. A Moho upwarp of 6 to 7 km in the Saurashtra region can be attributed to positive buoyancy and uplift due to thermal influx affected by the Reunion plume. High crustal Vp/Vs ratios beneath the Kachchh rift (1.8 to 2.05), coastal areas of Saurashtra (1.75 to 2.06), and North Gujarat (1.81 to 1.85) indicate dominance of a mafic/ultramafic crust. High regional heat flow, high electrical conductivity, large intracrustal S wave velocity reduction, and high average crustal Poisson's ratios are consistent with partial melt related to the process of magmatic underplating in the lower crust. At other stations, the crust appears to be felsic with Vp/Vs ratios in the range of 1.57 to 1.76.

Numerical modelling of Quaternary terrace staircase formation in the Ebro foreland basin, southern Pyrenees, NE Iberia

AbstractThe southern foreland basin of the Pyrenees (Ebro basin) is an exorheic drainage basin since Late Miocene times. Remnants of an early exorheic Ebro drainage system are not preserved, but morphology provides evidence for the Pliocene–Quaternary drainage development. The incision history of the Ebro system is denoted by (i) extensive, low gradient pedimentation surfaces which are associated with the denudation of the southern Pyrenean piedmont around the Pliocene–Quaternary transition and (ii) deeply entrenched Quaternary river valleys. Presumably since the Middle Pleistocene fluvial incision intensified involving the formation of extensive terrace staircase in the Ebro basin. Terrace exposure dating in major Ebro tributary rivers indicates climate‐triggered terrace formation in response to glacial–interglacial climate and glacier fluctuations in the Pyrenean headwaters. The overall (semi)parallel longitudinal terrace profiles argue for progressive base level lowering for the whole Ebro drainage network. The landscape evolution model, TISC, is used to evaluate climatic, tectonic and base level scenarios for terrace staircase formation in the Ebro drainage system. Model simulations are compared with morpho‐climatic, tectonic and chronologic data. Results show that climatic fluctuations cause terrace formation, but the incision magnitudes and convergent terrace profiles predicted by this climate model scenario are not consistent with the (semi)parallel terraces in the Ebro basin. A model including previous (late Pliocene) uplift of the lower Ebro basin results in rapid base‐level lowering and erosion along the drainage network, small late stage incision magnitudes and terrace convergence, which are not in agreement with observations. Instead, continuous Quaternary uplift of both the Pyrenees and the Ebro foreland basin triggers (semi)parallel terrace staircase formation in southern Pyrenean tributary rivers in consistency with the observed longitudinal terrace profiles and Middle–Late Pleistocene incision magnitudes. Forward model simulations indicate that the present Ebro drainage system is actively incising, providing further evidence for uplift.

A Cretaceous-Eocene depositional age for the Fenghuoshan Group, Hoh Xil Basin: Implications for the tectonic evolution of the northern Tibet Plateau

The Fenghuoshan Group marks the initiation of terrestrial deposition in the Hoh Xil Basin and preserves the first evidence of uplift above sea level of northern Tibet. The depositional age of the Fenghuoshan Group is debated as are the stratigraphic relationships between the Fenghuoshan Group and other terrestrial sedimentary units in the Hoh Xil Basin. We present new radiometric dates and a compilation of published biostratigraphic data which are used to reinterpret existing magnetostratigraphic data from the Fenghuoshan Group. From these data, we infer an 85–51 Ma depositional age range for the Fenghuoshan Group. U-Pb detrital zircon age spectra from this unit are compared to age spectra from Tibetan terranes and Mesozoic sedimentary sequences to determine a possible source terrane for Fenghuoshan Group strata. We propose that these strata were sourced from the Qiangtang Terrane and may share a common sediment source with Cretaceous sedimentary rocks in Nima Basin. Field relationships and compiled biostratigraphic data indicate that the Fenghuoshan and Tuotuohe Groups are temporally distinct units. We report late Oligocene ages for undeformed basalt flows that cap tilted Fenghuoshan Group strata. Together, our age constraints and field relationships imply exhumation of the central Qiangtang Terrane from the Late Cretaceous to earliest Eocene, followed by Eocene-Oligocene deformation, and shortening of the northern Qiangtang and southern Songpan-Ganzi terranes. Crustal shortening within the Hoh Xil Basin ceased by late Oligocene time as is evident from flat-lying basaltic rocks, which cap older, deformed strata.

Tectonic geomorphology of the southern Sierra Nevada Mountains (California): Evidence for uplift and basin formation

Hypotheses regarding uplift of the Sierra Nevada Mountains (Sierra), California, USA, vary from a single slowly tilting block to rapid Pliocene and/or Quaternary uplift in the south exclusively. To test these hypotheses, we examined geomorphic indices of the western Sierra. We interpret longitudinal profiles of the larger westerly flowing rivers, mountain front sinuosity, valley floor-to-width to height ratio, and relief ratio to show that relative tectonic activity is greater in the southern Sierra near the Kern River Gorge fault. The Sierra range crest profile indicates that the increased tectonism is related to more recent uplift in the southern Sierra. Only westward migration of Basin and range extension is consistent with the locus of uplift in the southern Sierra. We hypothesize that the Sierra topography is the result of Pliocene delamination-related uplift in the central Sierra and post-Pliocene interaction of the San Andreas, Garlock, and Sierra Nevada Frontal fault zones.

Beta Regio, Venus: Evidence for uplift, rifting, and volcanism due to a mantle plume

Geophysical data have led to the interpretation that Beta Regio, a 2000 × 25000 km wide topographic rise with associated rifting and volcanism, formed due to the rise of a hot mantle diapir interpreted to be caused by a mantle plume. We have tested this hypothesis through detailed geologic mapping of the V-17 quadrangle, which includes a significant part of the Beta Regio rise, and reconnaissance mapping of the remaining parts of this region. Our analysis documents signatures of an early stage of uplift in the formation of the Agrona Linea fracture belts before the emplacement of regional plains and their deformation by wrinkle ridging. We see evidence that the Theia rift-associated volcanism occurred during the first part of post-regional-plains time and cannot exclude that it continued into later time. We also see evidence that Devana Chasma rifting was active during the first and the second parts of post-regional-plains time. These data are consistent with uplift, rifting and volcanism associated with a mantle diapir. Geophysical modeling shows that diapiric upwelling may continue at the present time. Together these data suggest that the duration of mantle diapir activity was as long as several hundred million years. The regional plains north of Beta rise and the area east and west of it were little affected by the Beta-forming plume, but the broader area (at least 4000 km across), whose center-northern part includes Beta Regio, could have experienced earlier uplift as morphologically recorded in formation of tessera transitional terrain.

Reply to comment by R. Bilham on “Interpreting the style of faulting and paleoseismicity associated with the 1897 Shillong, northeast India, earthquake: Implications for regional tectonism”

Reply to comment by R. Bilham on “Interpreting the style of faulting and paleoseismicity associated with the 1897 Shillong, northeast India, earthquake: Implications for regional tectonism”

Does Kamchatka belong to North America? An extruding Okhotsk block suggested by coastal neotectonics of the Ozernoi Peninsula, Kamchatka, Russia

This paper addresses one part of an outstanding tectonic problem regarding the nature of the plate boundary between Eurasia and North America in northeastern Russia. In this region, the northwestern corner of the Pacific plate interacts either simply with the North American plate, or more complexly with one or more blocks independent of North America. North of this corner, evidence of uplift, tilting, and convergence contradicts the prevailing, simpler model. On the Ozernoi Peninsula, similar to 150 km north of the subducting Pacific plate, marine terraces indicate uplift rates of 0.1 to 0.3 mm/yr, with faster rates to the east. Historic and paleoseismic records provide evidence for recurring tsunamigenic, thrust earthquakes offshore of the Ozernoi Peninsula, the most recent a Mw 7.7 earthquake in 1969. A multiplate model where an eastward-moving Okhotsk block, including most of Kamchatka, is converging with a clockwise- rotating Bering block better explains these observations than does the unbroken North American plate model.

Isotopic evidence for uplift through extension? A comparison of the cooling histories in two adjacent orthogneiss suites in Fiordland, New Zealand

Isotopic evidence for uplift through extension? A comparison of the cooling histories in two adjacent orthogneiss suites in Fiordland, New Zealand

Eustatic and palaeoenvironmental assessment of the mid-Cretaceous Bathurst Island Group of the Money Shoals Platform, northern Australia

Abstract The Cretaceous System of the Money Shoals Platform consists of the Bathurst Island Group which is extensively developed on the continental borderland of northern Australia. The platform experienced a simple post-Cretaceous tectonic history and in its southern part the Bathurst Island Group is nonconformable on Precambrian basement, an ideal circumstance for evaluating eustacy and palaeoenvironmental succession. The group records a long-term sea-level cycle of late Aptian-mid-Turonian age. Its lowest part represents a transgressive systems tract developed on a pediment surface of very low local and regional relief. Compositionally mature glauconitic sandstone and radiolarian mudstone are characteristic facies, indicative of terrigenous sediment starvation for the shallow transgressive sea which received wind-driven, plankton-bearing currents from the shelf edge 400 km to the north. A district-scale nodular phosphorite horizon and associated highly condensed section capping the transgressive suite represents the culmination of deepening. It is succeeded by a shelfal mudstone succession comprising the bulk of the Bathurst Island Group and interpreted as a highstand systems tract. Clay mineralogy of this and the underlying radiolarian mudstone is uniform and dominated by smectite/mixed layer species of poor crystallinity. The clay assemblage is attributed to a pedigenic origin associated with weathering of moderate intensity on a poorly drained, low relief landscape. The upper part of the group comprises bioturbated glauconitic and quartzose sandstone displaying sedimentary evidence of high-energy depositional conditions consistent with regression. It is viewed as a prograding/aggrading, shelf margin systems tract which accumulated under circumstances of declining accommodation. A disconformity at the top of the group, cut on strata of mid-Turonian age, completed the sedimentary cycle. Calculated sedimentation rates indicate a major change in detrital sediment supply broadly coincident with the passage from transgressive to highstand systems tract and unrelated to eustacy. The change is attributed to mid-Cretaceous landscape rejuvenation. It is also reflected in the late Albian-Cenomanian sedimentary record of the epicontinental Great Artesian Basin where enhanced sediment supply likewise overprinted and obscured the eustatic signature. Mid-Cretaceous landscape rejuvenation reflected in the sedimentary record broadly correlates with evidence of uplift in eastern and southern Australia now recognised from fission track studies. It also correlates with a widely recognised palaeomagnetic overprint attributed to a mid-Cretaceous thermal event affecting the crust of southern and eastern Australia. Although short-term (third order) sea level cycles are not apparent for the Bathurst Island Group, three eustatic episodes related to global patterns can be recognised: late Aptian transgressive onlap, early Albian maximum flooding and late Turonian regression.

Abstract: Facies relations in the San Onofre Breccia: Evidence for Uplift and Volcanism in the Early Miocene, Santa Cruz Island

Abstract: Facies relations in the San Onofre Breccia: Evidence for Uplift and Volcanism in the Early Miocene, Santa Cruz Island

Low‐Mg calcite marine cement in Cretaceous turbidites: origin, spatial distribution and relationship to seawater chemistry

ABSTRACTLower Cretaceous (Hauterivian) bioclastic sandstone turbidites in the Scapa Member (North Sea Basin) were extensively cemented by low‐Mg calcite spars, initially as rim cements and subsequently as concretions. Five petrographically distinct cement stages form a consistent paragenetic sequence across the Scapa Field. The dominant and pervasive second cement stage accounts for the majority of concretions, and is the focus of this study. Stable‐isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions. Nevertheless, trends from whole‐rock data, augmented by cement separates from synlithification fractures, indicate an early calcite δ18O value of+0·5 to ‐1·5‰ PDB. As such, the calcite probably precipitated from marine pore fluids shortly after turbidite deposition. Carbon isotopes (δ13C=0 to ‐2‰ PDB) and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution. Textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation, as expected for a mineral‐controlled transformation.Electron probe microanalyses demonstrate that early calcite cement contains &lt;2 mol% MgCO3, despite its marine parentage. Production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine Mg2+/Ca2+ ratio in early Cretaceous ‘calcite seas’, relative to modern oceans. Scapa cement compositions concur with published models in suggesting that Hauterivian ocean water had a Mg2+/Ca2+ ratio of ≤1. This is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics. In contrast to the dominant early cement, late‐stage ferroan, 18O‐depleted calcites were sourced outwith the Scapa Member and precipitated after 1–2 km of burial.Our results emphasize that bioclast dissolution and low‐Mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence for uplift and meteoric diagenesis.

Stratigraphic record of the structural evolution of the western extensional margin of the Subalpine Basin during the Triassic and Jurassic, Ardeche, France

The nature and the geometry of Mesozoic deposits in the Ardeche region reflect the different stages of structural evolution of the western extensional margin of the Subalpine Basin. Through stratigraphic analysis of a broad transect passing from the Uzer block, through the GPF (“Géologie Profonde de la France”) borehole Balazuc 1 and two petroleum exploration wells on the different blocks of the margin, it has been possible to integrate data from borehole BAI in a sequential stratigraphic and regional dynamic framework, and to attempt to specify the modes of tectonic control on the organization of the deposits along a profile of this extensional palaeomargin. The Triassic and Jurassic deposits are composed of six major transgressive-regressive cycles for which the geometric and stratigraphic characteristics indicate that they were controlled by the tectonic evolution of the basin, and more specifically by variations in the type and rate of subsidence. The first two cycles, Anisian-early Sinemurian in age, correspond to the initial structural creation of the margin with a west-east sedimentary polarity. Extensional tectonism, which affected the Palaeozoic basement, in part controlled the duration and intensity of the transgressive phases, with the regressive phases reflecting a decrease in regional subsidence. The third and fourth cycles, early Sinemurian-middle Bathonian in age, were contemporaneous with the Tethyan rifting. In particular, they record evidence of decreasing regional subsidence, and even uplift, associated with block faulting. This caused strong tectonic decoupling and highly variable subsidence along the margin profile; episodes that are represented by that part of each sedimentary cycles deposited around the maximum regression, i.e. end of the regressive phase and beginning of the transgressive phase (Sinemurian and Aalenian). A regional phase of subsidence followed which allowed maximum expansion of the transgression (Toarcian and Bajocian transgressive peaks). The fifth and sixth cycles, middle Bath on ian-Kimmeridgian in age, were deposited during a period of widespread subsidence of the margin that was synchronous with the onset of Tethyain oceanic spreading. They preferentially record episodes of accelerated subsidence giving rise to transgressive phases generally associated with a synsedimentary tectonic activity that incurred relatively heterogeneous subsidence across the entire palaeomargin (late Bathonian and early Oxfordian). The regressive phases are subsequently marked by a decreased, but uniform subsidence with no evidence of uplift. The sediments of these last two cycles were deposited in a post-rift context and are liable to record effects of geodynamic events outside the basin. Characterization of the different stages in the evolution of the Ardeche palaeomargin by their stratigraphic expression provides geological constraints for modelling fluid migration throughout this extensional margin, the objective of the 'GPF Ardeche' project.