Maurice Streel

Spore stratigraphy and correlation with faunas and floras in the type marine devonian of the Ardenne-Rhenish regions

A spore zonation scheme comprising fifty-one zones is proposed for the marine Devonian strata of the Ardenne-Rhenish regions of Western Europe. The zonation comprises a series of Oppel and interval-type zones and these are closely intercalibrated with the associated marine faunal zonations to give a seventy-five level scale of correlation for the Devonian succession. The spore zonation provides stratigraphical dating of the Devonian megafloras of the region, particularly those from the Lower and Middle Devonian. The proposed spore zonation is closely compared with that erected for the Devonian of the Old Red Sandstone Continent.

Late Frasnian–Famennian climates based on palynomorph analyses and the question of the Late Devonian glaciations

Palynomorph distribution in Euramerica and western Gondwana, from the Latest Givetian to the Latest Famennian, may be explained, to some extent, by climatic changes. Detailed miospore stratigraphy dates accurately the successive steps of these changes. Interpretation is built on three postulates which are discussed: Euramerica at slightly lower latitudes than generally accepted by most paleomagnetic reconstructions; a conodont time-scale accepted as the most used available subdivision of time; and Late Devonian sea-level fluctuations mainly governed by glacio-eustasy. The Frasnian–Famennian timescale is also evaluated. The comparison, based on conodont correlations, between Givetian and most of the Frasnian miospore assemblages from, respectively, northern and southern Euramerica demonstrates a high taxonomic diversity in the equatorial belt and much difference between supposed equatorial and (sub) tropical vegetations. On the contrary, a similar vegetation pattern and therefore probably compatible climatic conditions were present from tropical to subpolar areas. A rather hot climate culminated during the Latest Frasnian when equatorial miospore assemblages reached their maximum width. The miospore diversity shows also a rather clear global Late Frasnian minimum which is also recorded during the Early and Middle Famennian but only in low latitude regions while, in high latitude, very cold climates without perennial snow may explain the scarcity of miospores and so, of vegetation. The Early and Middle Famennian conspicuous latitudinal gradient of the vegetation seems to attenuate towards the Late and Latest Famennian but this might be above all the result of the development, of cosmopolitan coastal lowland vegetations (downstream swamps) depending more on the moisture and equable local microclimates than on the probably adverse climates of distant hinterland areas. During that time, periods of cold climate without perennial snow cover and with rare vegetation may have alternated with less cold but wetter climates, thus giving rise to the development of mountain glaciers in high latitudes and explaining the jerking character of the global major marine regression. In high latitude regions, the development of an ice cap reaching sea level is only recorded by the end of the Latest Famennian, immediately below the DCB but, even if glacial evidences are not known at near the FFB, a short (0.1 Ma?) glaciation seems the best reasonable explanation of the major eustatic fall following the Kellwasser Event. The sudden growth and decay of a hypothetical Earliest Famennian ice sheet can be explained by the reduction and, later, increase in greenhouse capacity of the atmosphere. These changes in the atmospheric CO2 might have provoked changes in the mode of ocean-atmosphere operation. It may also be partly controlled by a volcanic paroxysm and/or bolide impacts. The Hony microtektites represent the best known evidence of the impact of extraterrestrial bodies on Earth corresponding to the UKW, and they are immediately followed by a regression, suggested by the quantitative analysis of acritarch assemblages. In the paleo-tropical Late Famennian, a peat-forming vascular plant community occurs for the first time and makes possible quantitative palynology of autochthonous sediments allowing the recognition of different swamp and near-swamp characteristic miospores. The early Latest Famennian starts with a widespread transgression which could correspond to the melting phases of the hypothetical Late Famennian mountain glaciers. During the end-Famennian, the coastal lowland vegetation has a worldwide distribution from sub-polar to equatorial regions. The climate has become less cold in high latitudes but wetter than before probably because the midlatitude cyclonic activity allows sufficient polar transportation of moisture to form large snow cover. Extensive coastal glaciers developed in different Bolivian and Brazilian basins, well dated by miospores. Rapid climatic changes characterize the onset of glaciation. The cyclic nature of climate allowed even intertropical marine faunas to reach occasionally the subpolar regions. For the plant kingdom, the crisis which follows the Hangenberg Event was more severe than the Late Frasnian Crisis.

Middle-upper devonian miospores from the Ghadamis Basin (Tunisia-Libya): Systematics and stratigraphy

Abstract Fifty-five miospores (including 7 new species and 3 new combinations) that range from Emsian to late Frasnian are identified, and 48 illustrated from 4 boreholes in the Ghadamis Basin. Their stratigraphic distribution is given and correlations made with established chronostratigraphic schemes. The Old Red Sandstone Continent miospore zonation can obviously be applied to this part of the Gondwanaland. The similarity of the miospore assemblages in both continents since at least the Emsian, suggests a close proximity of these continents during the middle and Upper Devonian.

Revision of the late Famennian Miospore Zonation scheme in eastern Belgium.

New palynological data have been obtained from the Late Famennian Evieux and Comblain au Pont Formations of the Chanxhe section located in the Ourthe Valley in the eastern part of the Dinant Basin. In the light of this new data the stratigraphic ranges of several zonally important miospore taxa are now significantly different to those previously recorded. This has necessitated a re-evaluation and revision of the late Famennian miospore zonation scheme for this region. Apiculiretusispora verrucosa and Vallatisporites hystricosus are now found to occur below the inception of Retispora lepidophyta. Consequently, a new biozone, the Apiculiretusispora verrucosa–Vallatisporites hystricosus VH Biozone is described for the late Fa2c interval. The problematical relationship between the Retispora lepidophyta– Apiculiretusispora verrucosa LV Biozone and the Retispora lepidophyta–Knoxisporites literatus LL Biozone is resolved, and part of the LL Biozone is now considered equivalent to the LV Biozone, which it consequently replaces. The Retispora lepidophyta–Indotriradites explanatus LE Biozone is recorded from the upper part of the Comblain au Pont Formation. A continuous succession of miospore zones is now established for the late Famennian Fa2c/Fa2d interval which permits more accurate correlations with other regions in Europe and North America. Correlation with the standard conodont biostratigraphy shows that the base of LL Miospore Biozone is correlated with the middle or late expansa Conodont Biozone and the base of LE Miospore Biozone with the early to middle praesulcata Conodont Biozone.

The biostratigraphical and palaeogeographical framework of the earliest diversification of tetrapods (Late Devonian)

Abstract The earliest diversification of tetrapods is dated as Late Devonian based on 10 localities worldwide that have yielded bone remains. At least 18 different species are known from these localities. Their ages span the ‘middle’–late Frasnian to latest Famennian time interval, with three localities in the Frasnian, one at the F/F transition (though this one is not securely dated) and six in the Famennian. These localities encompass a wide variety of environments, from true marine conditions of the nearshore neritic province, to fluvial or lacustrine conditions. However, it does not seem possible to characterize a freshwater assemblage in the Upper Old Red Sandstone based upon vertebrates. Most of the tetrapod-bearing localities (8 of 10) were situated in the eastern part of Laurussia (=Euramerica), one in North China and one in eastern Gondwana (Australia), on a pre-Pangean configuration of the Earth, when most oceanic domains, except Palaeotethys and Panthalassa, had closed.

EVIDENCE OF WEST EUROPEAN DEFINED MIOSPORE ZONES IN THE UPPERMOST DEVONIAN AND LOWER CARBONIFEROUS OF THE AMAZONAS BASIN (BRAZIL)

Abstract Uppermost Devonian and Lower Carboniferous miospore Zones previously defined in western Europe areidentified in a borehole in the Amazonas Basin in Brazil. They range with some discontinuity from the uppermost Famennian to the lowermost Visean. Except for a few species which are listed, most of the taxa are common to Euramerica and Gondwana.

Givetian-Frasnian phytogeography of Euramerica and western Gondwana based on miospore distribution

Abstract The Givetian and Frasnian miospore distributions in western Gondwana and southern Euramerica show a rather uniform vegetation prevailing from palaeo-polar to palaeo-tropical regions. Similar climatic conditions are certainly required to explain this but it is concluded from a discussion on the dispersal of homosporous vegetation that no wide oceans separated these regions at the time. Frasnian northern Euramerica vegetation seems different and might correspond to an equatorial belt. Heckel & Witzkes palaeogeographical reconstruction fits much better with the miospore distribution than other maps.

Lower and middle devonian miospore-based stratigraphy in Libya and its relation to the megafloras and faunas

Abstract A review of miospores, faunas and megafloras of the Lower and Middle Devonian in Libya allows new conclusions to be drawn. The application of the Ardenne-Rhenish Devonian miospore zonation in Libya gives the following results. In the North Hammadah Basin, the base of the Tadrart Formation might well be strongly diachronous depsite the lithostratigraphical correlations shown on the base of “Gamma-Ray/Neutron” logs. At least the lower part of the Tadrart Formation in borehole MG-1 is within the lowermost part of the Lochkovian not at its base. In the southern margin of the Hammadah Basin, the Tadrart and Ouan-Kasa formations are probably not older than late Emsian, maybe early Eifelian. The Ouenine Formation I is often absent there or strongly reduced. The isochroneity of the (discordant) lower limit of the Tadrart Formation across the Hammadah Basin is far to be demonstrated. The Caledonian age of the discordance on the southern margin of the Hammadah Basin can obviously be challenged. In the eastern Murzuk area, the plant-bearing Tadrart-Emi Magri formations have a Middle Devonian age according to the plant fossils themselves. The discordance which separates these formations from the Acacus Sandstone at Dor el Goussa has to be late Early Devonian or Middle Devonian because the Acacus Sandstone “Psilophytes” cannot be older than Pragian and can even be younger (Emsian). There is obviously no available argument which could counteract the fossil floral data and prove that the discordance in the eastern Murzuk has the same age as the Silurian/Devonian transitional beds in the northern Hammadah Basin.

Palynological evaluation of the Famennian Protosalvinia (Foerstia) Zone in the Amazon basin, Northern Brazil : a preliminary study.

Miospore assemblages recovered from a shallow borehole (Caima PH-2) and outcrops near the Tapajos River (Itaituba area), on the southern margin of the Amazon Basin, permit the correlation of the Protosalvinia Zone in this region with the equivalent of the uppermost part of the VCo miospore Zone in the eastern USA. Therefore it is dated as late (not latest) Famennian (postera to early expansa conodont Zones). The Protosalvinia Zone also exists in the eastern USA but cannot be considered there as one single floral zone, its age varying, from place to place, from the middle to the late (not the latest) Famennian (one of the marginifera Zones to one of the expansa conodont Zones). As demonstrated in well 1RX1AM, miospore evidence points to an important intraformational gap separating the lower and upper parts of the Curiri Formation. The lower section (which contains the Protosalvinia Zone) lacks diamictites, and rests conformably on the upper, non-radioactive shales of the uppermost Barreirinha Formation, of similar late Famennian (VCo) age. The ‘upper Curiri unit’ contains diamictites, and invariably displays a latest Famennian (late ‘Strunian’, LE-LN) age; it may constitute the whole formation in areas where ‘lower Curiri’ strata are missing, due to either erosion or non-deposition.

A comprehensive pollen- and tephra-based chronostratigraphic model for the Late Glacial and Holocene period in the French Massif Central

This paper discusses about 250 conventional C-14 ages obtained on peat and gyttja samples that apply to pollen features or tephra beds in the Massif Central region of France. A method to estimate the migration periods of eight Late Glacial to Holocene pollen features typical of the Massif Central is described. A review of all C-14 ages pertaining to tephras of this region enabled the most appropriate age, or age range, of the tephras to be estimated. The chronostratigraphic relationships between pollen features and tephras are discussed and a new comprehensive stratigraphic model is presented. The model shows that pollen features are diachronous and should not be used therefore as proxy stratigraphic markers. Copyright (C) 1996 INQUA/Elsevier Science Ltd