Fabrice Cordey

Radiolarian palaeoecology and radiolarites: is the present the key to the past?

Abstract Radiolarian productivity pulses and related radiolarite deposition are phenomena difficult to understand from an exclusively actualistic viewpoint. Evolutionary selection pressure among silica-secreting marine plankton, both radiolarians and diatoms, has led toward more economic usage of rapidly shrinking nutrient resources, including dissolved silica, of the photic zone in the late Cenozoic oceans, and, in particular, a substantial modification of oceanic cycle by the diatom explosive radiation. Even if there is a proved link between biomineralization and dissolved silica loading among the phytoplankton only, the relative independence of modern siliceous planktic biotas from the available silica pool reflects mainly their progressive physiological specialisation during evolutionary history. Oceanic chemistry and productivity, as well as patterns of circulation/upwelling have changed radically during the Phanerozoic. Radiolarites apparently represent an ‘anachronistic’ facies, as exemplified by their long-lived and ocean-wide distribution in palaeo-Pacific, and hitherto, highlighted actualistic models of localized intra-oceanic wind-driven upwelling loci are of largely questionable applicability. In addition to plate drift, hypersiliceous domains and intervals are explainable mostly by a large-scale volcano–hydrothermal activity during major plate-boundary reconfigurations, which, in many ways, favoured siliceous biotas acme, and their skeletal remains accumulation and preservation. Factors tied to rapid, voluminous submarine eruptions, such as thermal buoyant megaplumes and basin overturns, offer a viable alternative for traditional climatic/circulation scenarios in case of hypersiliceous high productivity events irrelevant to greenhouse-to-icehouse climatic change. The evolving carbon and silica cycles were coupled through the greenhouse effect and enhanced chemical weathering. Volcano-hydrothermal and tectonic uplift events, related mostly to extensive rifting and/or accelerated oceanic spreading, were the endogenous driving force that created this perturbation of the exogenous system. The present biogeochemical cycle is representative only for the overall silica-depleted post-Eocene oceanic ecosystems, which broadly correlates with a major expansion of diatoms groups extremely efficient in silica removal, and closely linking the silica budget with phosphorus and nitrogen cycles. Thus, an orthodox uniformitarian approach to biosiliceous sedimentation, based on a silica-starved vigorous ocean, is of limited significance when applied to the pre-Neogene settings, especially in the peculiar planktic habitats of epeiric seas, as well as during biotic crises marked by strong geotectonic overprint. The major turnovers in marine siliceous biota composition, in particular after the end-Permian radiolarite gap, may have been coupled with discernible changes in an increasing biological control on the long-term oceanic silica cycling (‘punctuated equilibrium’). The evolutionary turnovers have induced a stepdown decrease of dissolved silica levels through the Phanerozoic, contemporaneously with the general secular trend of upward scaling of nutrient-related ecological processes and increased effectiveness of resource utilization.

Significance of Jurassic radiolarians from the Cache Creek terrane, British Columbia

The discovery of new radiolarian localities in the western belt of the Cache Creek terrane in southern British Columbia possibly changes its upper age limit from Late Triassic to Early or Middle Jurassic. It favors a Middle Jurassic, rather than a Late Triassic, age of amalgamation for the Cache Creek terrane and Quesnellia (parts of superterrane I) in southern British Columbia. The new Jurassic ages also mean that the western Cache Creek terrane could be equivalent in age to the Bridge River–Hozameen terrane in British Columbia and to terranes containing the Tethyan fusulinid Yabeina in northwest Washington and the Klamath Mountains of California.

Coherent French Range blueschist: Subduction to exhumation in <2.5 m.y.?

Coherent oceanic strata in the French Range belong to the exotic Cache Creek terrane of the Canadian Cordillera. They were metamorphosed to blueschist grade, tectonically extruded, eroded, and intruded by plutons—perhaps in <2.5 m.y. Sodic amphibole overprint chert as young as late Pliensbachian to Toarcian age (ca. 191 to ca. 177 Ma). Blueschist mineral assemblages defi ne the early metamorphic fabric along with phengite dated by 40 Ar/ 39 Ar as 173.7

Late Jurassic magmatism, metamorphism, and deformation in the Blue Mountains Province, northeast Oregon

An early to mid-Mesozoic record of sedimentation, magmatism, and metamorphism is well developed in the Blue Mountains Province of northeast Oregon. Detailed studies-both north and south of the Blue Mountains Province (e. g., terranes of the Intermontane belt, Klamath Mountains, and western Sierra Nevada) have documented a complex Middle to Late Jurassic orogenic evolution. However, the timing of magmatic, metamorphic, and deformational events in the Blue Mountains, and the significance of these events in relationship to other terranes in the western North American Cordillera remain-poorly understood. In this study, we investigate the structural, magmatic, and metamorphic histories of brittle to semibrittle deformation zones that indicate widespread Late Jurassic orogenesis in the Blue Mountains Province. Folding and faulting associated with contractional deformation are primarily localized along terrane boundaries (e. g., Baker-Wallowa and Baker-Izee-Olds Ferry boundaries) and within the composite Baker oceanic melange terrane (e. g., Bourne-Greenhorn subterrane boundary). These brittle to semibrittle deformation zones are broadly characterized by the development of E-W-oriented slaty to spaced cleavage in fine-grained metasedimentary rocks of the Baker terrane (e. g., Elkhorn Ridge Argillite), approximately N-S-bivergent folding, and N- and S-dipping reverse and thrust faulting on opposite flanks of the Baker terrane. Similarly oriented contractional features are also present in late Middle Triassic to early Late Jurassic (i.e., Oxfordian Stage, ca. 159 Ma) sedimentary rocks of the John Day and Huntington areas of northeast Oregon. Radiometric age constraints from youngest detrital zircons in deformed sedimentary rocks and crystallization ages of postkinematic plutons, which intrude the deformation zones, limit deformation to between ca. 159 and ca. 154 Ma. We suggest that the widespread, approximately N-S-directed contractional features in the Blue Mountains Province record a short-lived, intense early Late Jurassic deformational event and preserve an example of upper-crustal strain localization associated with terminal arc-arc collision between the Olds Ferry and Wallowa island-arc terranes. The age interval of deformation in the Blue Mountains Province is younger than Middle Jurassic deformation in the Canadian Cordillera and Klamath Mountains (Siskiyou orogeny) and predates classic Nevadan orogenesis

Long-lived Panthalassic remnant:the Bridge River accretionary complex, Canadian Cordillera

Newly identified radiolarians from ribbon chert of the Bridge River complex in the southeastern Canadian Coast Mountains range in age from Mississippian to late Middle Jurassic. The Bridge River complex and the associated Cadwallader arc and Tyaughton and Methow basins lie between the Intermontane superterrane to the east and the Insular superterrane to the west. Triassic-Middle Jurassic development of the Bridge River subduction-accretion complex records an important component of convergence between these superterranes. The time span represented in the Bridge River complex (∼170 m.y.), one of the longest known age ranges for chert sedimentation, suggests that the Bridge River complex contains remnants of a long-lived, potentially far-traveled Panthalassic oceanic domain.

The Middle Jurassic-Lower Cretaceous Rosso Ammonitico succession of Monte Inici (Trapanese Domain, western Sicily); sedimentology, biostratigraphy and isotope stratigraphy

The pelagic Jurassic successions of the Trapani region of western Sicily are dominated by the characteristic Tethyan Rosso Ammonitico lithofacies. In order to revise the stratigraphy (sedimentology, microbiofacies, ammonite and calpionellid biostratigraphy, isotope stratigraphy) and to define a reference section, a well exposed succession of Rosso Ammonitico, spanning the Callovian-Upper Berriasian interval, has been selected in the eastern sector of Monte Inici, Trapani province. A sedimentological study has enabled us to distinguish three subfacies within the Rosso Ammonitico: nodular, pseudo-nodular, and intraclastic nodular subfacies. The two latter subfacies are characterized by the presence of intraclasts produced largely by mechanical abrasion, that suggest a formation by hydrodynamic processes. Increasing hydrodynamic energy is inferred from Lower Kimmeridgian to Upper Berriasian deposits. The vertical evolution of the microfacies shows the greatest skeletal diversity in the Middle Oxfordian, coinciding with a peak of delta 13 C. Ammonites are abundant in the Middle Oxfordian-Lower Kimmeridgian, where several ammonite zones have been recognized. With the aid of calpionellids we have been able to identify the Jurassic-Cretaceous boundary and to recognize high frequency biostratigraphic intervals in the Berriasian, that can be correlated throughout palaoetethys. The delta 13 C curve shows a marked positive peak at the base of the section (Lower to Middle Callovian) and a second, composite, positive excursion in the Middle-Upper Oxfordian. From the base of the Kimmeridgian, delta 13 C values slowly decrease, remaining low in the Berriasian. Oxygen isotope values exclude late burial diagenetic overprinting and the carbon peaks are interpreted as reflecting carbon cycle perturbations due to high biological productivity that may mirror global palaeoenvironmental variations. The Middle Oxfordian peak coincides with the highest skeletal diversity, with the first occurrence of the nodular facies, and with the onset of cherty limestones (lateral equivalents of radiolarites) in the southern sector of Monte Inici, and in localities to the west and north. The heteropy between Rosso Ammonitico and radiolarites s.l. suggests that sedimentation was controlled by local palaeotopography.

Alpine ocean seafloor spreading and onset of pelagic sedimentation: new radiolarian data from the Chenaillet-Montgenèvre ophiolite (French-Italian Alps)

Radiolarians of Middle Jurassic age (tentatively middle Bathonian) provide the first direct age determination from oceanic sediments associated with the Chenaillet-Montgenèvre ophiolite (Piemonte zone, French-Italian Alps). This datum obtained from radiolarites of the Lago Nero- Replatte thrust sheet is older than those previously established on ophiolite sedimentary covers from this segment of the western Alps. It also shows that Lago Nero-Replatte basal radiolarites are anterior to the youngest intrusives from the overlying Chenaillet s.s. thrust sheet. This chronological relationship implies either a late seafloor spreading-related magmatic activity in places younger than adjacent initial pelagic sedimentation, or more likely that the Lago Nero-Replatte and the Chenaillet s.s. thrust sheets are distinct and distant pieces of lithosphere that were eventually stacked together: the Lago Nero-Replatte unit was trapped within the accretionary wedge while the Chenaillet s.s., of a younger age and in a more distal position with regards to the European margin, was obducted. Regionally, the Lago Nero-Replatte sediments appear to be coeval to other Bathonian supraophiolitic radiolarites exposed in the western Alps. These results strengthen the Bathonian correlation of widespread seafloor spreading in both western Tethys and the central Atlantic ocean.

Middle to Late Jurassic Radiolarians from Sicily (Italy)

Abstract Middle and Upper Jurassic radiolarian faunas from two sections of Sicily are compared with the previously proposed zonations. The ages of the faunas are estimated by correlation with these zonations and by concurrent range zone. The faunas could be assigned to the Bathonian or earlier late Tithonian or early Berriasian. Two new spumellarian species (Bernoullius furcospinus andBernoullius rectispinus) from Middle Jurassic are described.

Dating the Tethyan Ocean in the Western Alps with radiolarite pebbles from synorogenic Oligocene molasse basins (southeast France)

For the first time in the Western Alps, radiolarite pebbles collected from Tertiary foreland molasse conglomerates are treated for microfossil extraction and dated. Among forty pebbles collected in the field, seven of them released diagnostic radiolarian assemblages ranging in age from Late Bajocian–Early Callovian to Middle Oxfordian–Early Tithonian. These ages overlap previous biochronological data obtained from in situ localities of the Schistes Lustrés Piemont zone of the French-Italian Alps and triple the number of diagnostic radiolarite samples known so far in this segment of the chain. The diagnostic pebbles are characterized by low grade metamorphism, showing that some eroded thrust-sheets from the oldest parts of the ocean escaped any tectonic burial during the Alpine convergence. Mixing of low and high-grade radiolarites, mafics and ultramafics pebbles implies that various ocean-derived units were exposed. This tectonic scenario involves tight refolding and severe uplift of the Eocene subduction wedge in the Early Oligocene.

Integrated biostratigraphy and regional correlations of Upper Jurassic–lowermost Cretaceous series in northern Tunisia

Integrated biostratigraphy and palaeogeographical interpretations of Upper Jurassic–Lower Cretaceous (Callovian–Berriasian) successions are synthesised on the basis of published data and our recent investigations of key sections from the “Tunisian Dorasale” (TD) and “Tunisian Trough” (TT) domains of northern Tunisia. After a revised biostratigraphy had been proposed for the “rosso ammonitico” series, bed-by-bed sampling led to a radiolarian-based first direct dating of the partly coeval biosiliceous series in the TT. Upward within the Jurassic column, an integrated biostratigraphy (ammonites, calpionellids and associated biomicrofacies) allowed precise biozonation for the Kimmeridgian–middle Berriasian interval. The heterogeneity of the Late Jurassic facies in northern Tunisia is the result of two main geodynamical and palaeogeographical events. The first corresponds to the fragmentation of the initial Early Jurassic platform linked to Tethyan rifting. The second event, coeval to the radiolarian-bearing series of the TT, is expressed by a significant deepening in north-west Tunisian palaeoenvironments. During Kimmeridgian–Tithonian times, previous troughs evolved back into carbonate platforms. Within the Maghrebian Belt, Upper Jurassic–lowermost Cretaceous series comprising three major facies groups are easily correlated; minor differences in age are interpreted as due to local tectonic control. Regional correlations confirm, on the one hand, a common geological history for the external segments of the African margin and, on the other hand, a distinct palaeogeographical evolution of the inner domain including the “Dorsales calcaires” and ancient massifs, both being separated by intermediate flysch zone segments.