A Cenozoic-style scenario for the end-Ordovician glaciation

The end-Ordovician was an enigmatic interval in the Phanerozoic, known for massive glaciation potentially at elevated CO2 levels, biogeochemical cycle disruptions recorded as large isotope anomalies and a devastating extinction event. Ice-sheet volumes claimed to be twice those of the Last Glacial Maximum paradoxically coincided with oceans as warm as today. Here we argue that some of these remarkable claims arise from undersampling of incomplete geological sections that led to apparent temporal correlations within the relatively coarse resolution capability of Palaeozoic biochronostratigraphy. We examine exceptionally complete sedimentary records from two, low and high, palaeolatitude settings. Their correlation framework reveals a Cenozoic-style scenario including three main glacial cycles and higher-order phenomena. This necessitates revision of mechanisms for the end-Ordovician events, as the first extinction is tied to an early phase of melting, not to initial cooling, and the largest δ13C excursion occurs during final deglaciation, not at the glacial apex

Environnement de dépôt des formations à phtanites interstratifiés du protérozoïque supérieur armoricain (France): conséquences sur la genèse des phtanites

In the Cadomian belt (Upper Proterozoic) some sedimentary formations contain interbedded graphitic cherts. The sedimentological study carried out on the Lamballe Formation (Armorican Massif) shows that the associated terrigenous rocks were laid down between the shoreface zone and the inner shelf and were associated with storm wave action. These results question hypotheses about the origin of the black! cherts and the validity of using chert as a lithostratigraphic marker. (C) Academie des Sciences/Elsevier, Paris

Relationship between phosphogenesis and sequence architecture: Sequence stratigraphy and biostratigraphy in the Middle Ordovician of the Armorican Massif (NW France)

In the Armorican Massif (NW France), the Middle Ordovician formations contain abundant phosphatic (P) beds associated with terrigenous sediments laid down on a storm-dominated shelf Facies analysis and sequence stratigraphy carried out on sections located in the Crozon Peninsula (W Armorican Massif) allow us to distinguish four P-facies: (1) beds with P-nodules and crusts, contained within sediments laid down on the distal and median parts of the inner shelf, (2) amalgamated beds with P-nodules and crusts and (3) beds with lingulid brachiopods, heavy minerals and P-clasts associated with retrogradation phases; (4) P-clast-bearing conglomerates, associated with progradation phases. The genesis of these facies and their location in the depositional sequences are controlled by the fluctuation of terrigenous inputs during sea-level changes ("sediment volume partitioning"). We propose a genetic model integrating the superposition of very-high-frequency (VHF) and high-frequency (HF) eustatic signals. In this model, the amalgamated beds and beds with lingulid brachiopods, heavy minerals and P-clasts result from the superposition of several sea-level rise episodes of VHF cycles, occurring within the retrogradation phase of a HF cycle. The lateral extension of P-facies was investigated in the eastern part of the Armorican Massif (Benioc, Domfront/Les Tanneries and St Aubin-du-Cormier sections). Several early Darriwilian transgressive events associated with phosphogenesis episodes are dated through high-resolution chitinozoan studies. Some of these transgressive events are at least of regional scale (e.g., ornensis and protocalix transgressive events). The topmost calix transgressive event is clearly identified over the northern Gondwanan and Avalonian domains, and may reflect a global eustatic signature. (C) 2007 Elsevier B.V. All rights reserved

Relazione tra architettura deposizionale e composizione nei depositi di piattaforma terrigena: gli accumuli a minerali pesanti (placers) dell’ Ordoviciano della Sardegna e della Bretagna

The aim of the present study is to investigate the stratigraphic significance of some heavy minerals concentrations. It is based on a sequence analysis and a petrophysic characterisation (natural radioactivity) of placers located in ordovician formations of SE Sardinia (S Italy) and W Armorican Massif (W France). In these formations, the heavy minerals are mainly represented by titaniferous minerals (rutile and anatase), zircon and monazite; the tourmaline is less common. The modal analyses show that their concentration can reach 40 %. Several gamma ray facies have been identified. A high radioactivity facies is represented by sandy beds enriched in heavy minerals; the high radioactivity (until 88000 cpm) is linked to abundance of U and Th in zircon and monazite. Two litho facies with medium radioactivity (4000 to 6000 cpm) have been recognized; their radioactivity is linked to the K in the silty clayey intercalations. The gamma ray facies have been located in the depositional sequences and a stratigraphic model is proposed. This model is based on the sediment volume partitioning and on the superposition of eustatic signals of different frequencies. On the shallow environments of shoreface and proximal inner shelf, the storm wave action favours the reworking of high-density minerals into storm beds. During major-order cycles sea-level rises, the decreasing of terrigenous inputs in the marine environments and the amalgamation of several very high frequency sequences lead to concentrations of the heavy minerals. On the contrary, during sea-level falls, the high-density minerals are diluted by abundant terrigenous flux

Sea-level curve for the Middle to early Late Ordovician in the Armorican Massif (western France): Icehouse third-order glacio-eustatic cycles

This contribution deals with the Middle to early Late Ordovician of the Armorican Massif (western France). The aim is to reconstruct a multi-order eustatic curve from the stratigraphic record. The studied section (Crozon Peninsula area) shows sediments deposited in tidal to storm-dominated shelf environments. A sequence analysis (facies and stacking pattern analyses), combined with a gamma-ray record and a backstripping procedure, was carried out. Chitinozoan assemblages provide the biostratigraphic framework including time calibration. The stratigraphic succession is dominated by the stacking of sequences representing different orders of sea-level change i.e. very high- (b400 kyr), high- (400 kyr) and low- (3rd order) frequencies. Atime-calibratedsea-level high frequency curve, corrected for compaction, sediment load and tectonic subsidence, has been calculated based on a backstripping procedure. The smoothing of this curve permits identification of eleven thirdordersequences with seven episodes of significant (50 to 80 m) and rapid (less than 100 kyr) sea-level falls during the Darriwilian and the Sandbian, and two major transgressive events located in the formosa and pissotensis biozones of the Darriwilian. The reconstructed sea-level curve reveals thatmost of the third-order cycles comprise three fourth-order cycles of assumed 400 kyr duration. This 1.2 myr duration of the Ordovician thirdordersequences suggests that they may be related to long-period obliquity cycles, an orbital forcing characteristic of icehouse sequences in the late Cenozoic and the Quaternary. These results imply that themajor part of theMiddle and early Late Ordovician was characterized by icehouse conditions and third-orderglacio-eustatic cycles. A time calibration of the chitinozoan biozones was developed from the sequential analysis. It suggests that the early Darriwilian biozones (from henryi to calix biozones) have short durations of several 10 to several 100 kyr whereas the pissotensis Biozone has a temporal extent of several myr, extending into the early Sandbian

Biostratigraphical calibration of third order Ordovician sequences on the northern Gondwana platform

This study illustrates bathymetric trends across the northern Gondwana platform during the Ordovician. The sedimentary records of the Algerian Sahara, central Morocco. and western France are investigated to document related environmental changes through the period Outcrop and subcrop sections have been selected with regard to their completeness and to the precision of the available biostrarigraphical data, based on chitinozoans. This group provides the most continuous biostratigraphical record in the terrigenous Ordovician successions of northern Gondwana. For each area, the chitinozoan data have been assembled, updated and referred to one of the 26 Ordovician chitinozoan biozones defined in northern Gondwana regions A sequential analysis based on the identification of genetic units is presented The stacking pattern of these units is established for each locality, and the resulting curves express variations in sedimentary environments partly driven by relative sea-level changes. From successive smoothings of these curves, we define 16 main cycles at lower frequencies Uncertainties concerning the precise chronostratigraphic position of some of these cycles are discussed The proposed northern Gondwana relative sea-level variation curves are compared to the recently-published global Ordovician sea-level curve and to other regional curves available for the major Ordovician palaeoplates (Laurentia, Baltica, South China, north-eastern Gondwana) (C) 2010 Elsevier B V All rights reserve

The Late Ordovician glacio-eustatic record from a high-latitude storm-dominated shelf succession: The Bou Ingarf section (Anti-Atlas, Southern Morocco)

Evidences of glaciation at the end of the Ordovician are widespread in western Gondwana Some authors consider the glaciation was restricted to the Hirnantian time, but occurrences of glacial deposits in the Lower Silurian strata of South America indicate that the Gondwana glaciers did not completely disappear after the Hirnantian glaciation. In addition, numerous studies based on palaeoecology, sedimentology, sequence stratigraphy and stable isotopes have suggested that ice sheets formed as soon as the Early/Middle Katian The objective of this work is to investigate the high-frequency eustatic signals in a stratigraphic succession corresponding to the Katian and the Hirnantian (similar to 10 My comprising the uppermost part of the Ordovician) The studied section (Bou Ingarf section >600 m) is located in the Central Anti-Atlas, southern Morocco It shows an almost continuous succession of siliciclastic platform deposits. The lower package, Katian to early Hirnantian in age, corresponds to a shelf succession, within which storm dynamics prevailed The upper package, middle to upper Hirnantian in age, comprises a glaciation-related succession made up of a suite of coastal to flood-dominated fluvio-glacial deposits The latter have essentially filled in large palaeochannels interpreted as subglacial tunnel valleys From high-resolution facies and sequential analysis, a curve of variation of depositional environments was established at very high, high and low frequencies Time calibration was performed based on a high-quality biostratigraphic control mainly derived from chitinozoan biozones through the whole succession. Assuming a constant tectonic subsidence and a bathymetric model (shoreface/upper offshore boundary 30 m, upper offshore/lower offshore boundary: 120 m), changes in facies-based water depths are converted into an eustatic sea-level curve using a one-dimensional backstripping procedure The eustatic sea-level curve shows that the stratigraphic succession is dominated by the stacking of sequences at very high- (40 m) and coeval forced regressions were identified during the Katian They represent glacial episodes of significant extent prior to the Hirnantian, but essentially lacking a glacial record Strata reflecting the Hirnantian glaciation include two differentiated events and an intermediate but important transgression. The second Hirnantian glacial event, the only one associated with subglacial erosion in the Bou Ingarf area, corresponds to the Late Ordovician glacial climax characterised by a continental-scale ice sheet Ice sheets permanently occupying the centre of the Gondwana landmass throughout the Late Ordovician may reconcile moderate Hirnantian eustatic sea-level fall amplitude (4080 m) and palaeoglacial reconstructions that show a Hirnantian ice sheet covering the main part of western Gondwana. (C) 2010 Elsevier B V All rights reserved

Provenance and tectonic setting of Miocene siliciclastic sediments, Sibuti formation, northwestern Borneo

Provenance and tectonic setting of sandstone and mudstone units of the Miocene Sibuti Formation from northwest Borneo have been studied based on the mineralogy, major and trace element geochemistry data. The X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) data revealed that the sandstones and mudstones were abundant in quartz, pyrite, clay, and heavy minerals such as zircon, rutile, and some detrital cassiterite. Geochemically, the sandstones and mudstones are classified into quartz arenite, litharenite, sublitharenite, arkose, and wacke. Quartz arenites are enriched with SiO2, Zr, and Th and depleted in Al2O3, CaO, and other elements compared to other sandstone types, indicating high maturity and intensive weathering. Chemical index of alteration (CIA: 77–90), plagioclase index of alteration (PIA: 86–100), and A-CN-K diagram suggest intense weathering in the source area. Elemental ratios such as La/Sc, Th/Sc, Cr/Th, La/Co, and Th/Co are similar to sediments derived from the felsic rocks. Also, the provenance discrimination diagrams suggest recycled continental nature of these clastic sediments which are mostly derived from metasedimentary source (Rajang Formation). Discriminant-function diagram for the tectonic discrimination of siliciclastic sediments revealed that the sediments of Sibuti Formation were derived from a collision zone, which is consistent with the geology of the study area