Jacques Thierry

Contrôle géodynamique de la sédimentation argileuse du Callovien-Oxfordien moyen dans l'Est du bassin de Paris: influence eustatique et volcanique

The Middle Callovian to Middle Oxfordian clay deposits drilled in the eastern Paris Basin (borehole HTM 102, ANDRA) show a strong mineralogical change occurring in the lowermost Oxfordian (Scarburgense subzone, Mariae zone). Such a change probably results from the initial development of connections between the young Atlantic Ocean and the Paris Basin. A bentonitic layer identified close to the boundary between Lower and Middle Oxfordian and also recognized in the subalpine Basin (South-East of France) at the same age, reflects a volcanic activity probably resulting from an extensional regime located in the North Sea.

Chronostratigraphy and geochronology: a proposed realignment

We propose a realignment of the terms geochronology and chronostratigraphy that brings them broadly into line with current use, while simultaneously resolving the debate over whether the Geological Time Scale should have a “single” or “dual” hierarchy of units: Both parallel sets of units are retained, although there remains the option to adopt either a single (i.e., geochronological) or a dual hierarchy in particular studies, as considered appropriate. Thus, geochronology expresses the timing or age of events (depositional, diagenetic, biotic, climatic, tectonic, magmatic) in Earth’s history (e.g., Hirnantian glaciation, Famennian-Frasnian mass extinction). Geochronology can also qualify rock bodies, stratified or unstratified, with respect to the time interval(s) in which they formed (e.g., Early Ordovician Ibex Group). In addition, geochronology refers to all methods of numerical dating. Chronostratigraphy would include all methods (e.g., biostratigraphy, magnetostratigraphy, chemostratigraphy, cyclostratigraphy, sequence stratigraphy) for (1) establishing the relative time relationships of stratigraphic successions regionally and worldwide; and (2) formally naming bodies of stratified rock that were deposited contemporaneously with units formally defined at their base, ideally by a GSSP (Global Boundary Stratotype Section and Point = “golden spike”) that represents a specific point in time. Geochronologic units may be defined and applied generally by either GSSPs or—as currently in most of the Precambrian—by Global Standard Stratigraphic Ages (GSSAs). Geochronologic units would continue as the time units eons/eras/ periods/epochs/ages, and chronostratigraphic units as the timerock units eonothems/erathems/systems/series/stages. Both hierarchies would remain available for use, as recommended by a formal vote of the International Commission on Stratigraphy in 2010. Geological context helps determine the appropriate usage of the component units.

Ammonite faunal exchanges betweenSouth Tethyan platforms and South Atlantic during the Uppermost Cenomanian-Lower most/Middle Turonian in the Benue trough (Nigeria)

Abstract The Cenomanian and Turonian transgressions (Late Cretaceous) formed seaways through the Benue Trough (Nigeria) connecting the “Saharan Platforms” to the north (Niger) and the “Atlantic Margin” slope edge (“South Atlantic”) to the south. A biostratigraphical framework and precise correlations at the ammonite subzone and horizon level have been established providing a sound basis on which to compare palaeontological samples and to interpret the successive stages of colonization of the Trough by ammonites or exchanges among the various faunal groups. Quantitative study of palaeontological finds shows: several biogeographical entities are permanent features; the “Benue” assemblages consist principally of Vascoceratinae in the uppermost Cenomanian and are superseded bytthe Pseudotissotiinae in the Lower Turonian; the “Atlantic” assemblages are composed mainly of Acanthoceratinae and/or other Vascoceratine and/or Mammitinae, Desmocerataceae, Phylloceratina and Lytoceratina;and Lytoceratina; at the base of the Cenomanian series (Dumbli Horizon) and at the base of the Middle Turonian (Ingens Horizon) the assemblages are more uniform throughout the Trough; whichever biostratigraphical horizons are considered, faunal diversity and regularity increase from north to south. The “Benue” assemblages are characterized by taxa displaying little variation (overall shell shape), with very simple suture lines and short body chambers; they are generally well adapted to shallow environments. By contrast, the “Atlantic” assemblages are morphologically highly diverse, but are generally characterized by a long body chamber and a complicated suture line. These forms appear better suited to deeper and more open marine environments. Fossil finds reflect the biological associations; no evidence has ever been produced for substantial post mortem transportation of shells. Precise biostratigraphical correlations and knowledge of the geological phenomena occurring during the Cenomanian-Turonian transgressions indicate there were five major steps of colonization. These coincide with the N-S or S-N transgressive marine flows; they result from the interaction of three factors: ammonite mode of life, morphological adaptation to highly changeable environments and competition. 1) The Dumbli Horizon, a period of widespread faunal uniformity: the ubiquitous species Metengonoceras dumbli appears early in the north and south and is dominant in this horizon throughout the area. It emphasizes uniformity resulting from a double S-N and N-S faunal advance. 2) Latest Cenomanian colonizations (uppermost Juddii Zone) with a double trend which characterizes the end of the transgressive period and the late Cenomanian highstand: partial colonization of the Trough from the Niger (N-S) by groups related to the Saharan Vascoceratinae (Nigericeras gadeni and N. cauvini); initially restricted and then large-scale colonization by globose Vascoceras evolved from Atlantic Acanthoceratinae (S-N). 3) Faunal regression at the onset of the Turonian (Coloradoense Zone) which concides with a clear regressive trend at the onset of the stage: the globose Vascoceratinae are confined to the Atlantic margin with no palaeontological evidence of a seaway between the Saharan platforms and the Atlantic margin. 4) Recolonization in the early Turonian (Nodosoides Zone): the “Pseudotissotiinae”, derived from Vascoceras, reach the Upper Benue (Thomasites gongilensis), then the Niger (Pseudotissotia nigeriensis), indicating a new S-N flow. During this period Saharan influences in the Trough are thought to be indicated by the permanent if numerically small presence of Choffaticeras. This step reflects a new transgressive period and subsequent relative highstand in the early Turonian; 5) A new period of substantial uniformity in the mid-Turonian (Hoplitoides ingens Horizon) which marks the onset of a regressive period: the index species in found north of the Saharan platforms (Maghreb) and throughout the Trough but is unknown in Niger (continental deposits?); no N-S biogeographical connection is found for the earliest mid-Turonian. It should be emphasized that: episodes 1 and 5 correspond to times of substantial morphological uniformization with oxycone shells predominant in both instances (Metengonoceras and Hoplitoides); strictly Atlantic ammonites (Mammitinae, Acanthoceratinae, Fagesia, Neoptychites, etc.) are rare in the Trough; they are found particularly at the end of the transgressive periods and at periods of relative highstand when a few dispersed units are mixed with Upper Benue faunas.

Settlement down again patterns of a carbonated platform following up a sedimentary crisis: example of the middle-late Jurassic boundary in the southeastern part of the Paris Basin

Following a sedimentary crisis which begins in the late Lower Callovian and spans all the early Oxfordian, the settlement down patterns of a platform with carbonated sedimentation are analysed in a southeastern area of the Paris Basin (fig. 1). Ten lithostratigraphic units (reefal formations, associated bioclastic facies and marly distal lateral facies ; fig. 2) are defined (fig. 3). New ammonite and brachiopod faunas, collected in situ, allow to date accurately the sedimentary units with a precision matching an ammonite subzone of the standard bio-chronostratigraphic scale (fig. 4) of the middle-late Oxfordian (from the Parandieri Subzone, at the base of the Transversarium Zone, to the Planula Subzone, at the top of the Planula Zone). The sedimentologic analysis coupled with the study of the benthic and pelagic faunal communities allow to define twelve type-facies (tabl. I and II). Regrouped into three associations, these characterise depositional environments which occur in succession, following three platform models (fig. 5). As witnesses of the evolution of the accomodation/sedimentation ratio, the resulting time succession of sedimentary bodies shows a depositional dynamics organised into three sequence tracks (fig. 5 and 6) : - a retrogradation phase is characterised by a moderately deep and open platform, dominated by low energy and marly sedimentation, which ranges from the Middle Oxfordian (Plicatilis Zone and Transversarium Zone) to the lowermost late Oxfordian (Bifurcatus Zone) ; - during the late Oxfordian (Bimammatum Zone, from the Semimammatum Subzone to the Bimammatum Subzone) an aggradation phase corresponds to the installation of three successive shallow platforms with contrasted morphology. Indicating the re-initiation of carbonated production, these platforms are well limited and represent high energy shallows with reef buildings, which lateraly grade into dismantling bioclastic facies, then secondly and more laterally again into low energy and medium deep marly facies ; - the upper part of the late Oxfordian (Hauffianum Subzone, in the uppermost Bimammatum Zone, and Planula Zone) shows the wide extent of a low energy and morphologically very little contrasted distal platform. This one has a high potential of carbonated production characterised by bioclastic, oolitic and micritic facies which illustrate a progradation phase ; such a phase proceeds in the early Kimmeridgian. The collected data and the corresponding phenomenons pointed out on this area of the southeastern border of the Paris Basin are discussed and replaced in the general framework of the sedimentary, biologic, palaeogeographic and palaeoclimatic events henceforth recognised at the middle-late Jurassic boundary on the peri-Tethyan intracratonic domains of western Europe. The demise of the carbonate production seems to be correlated with a global cooling of both marine waters and atmosphere, which is considered as a limiting factor. During the Middle Oxfordian, the re-initiation of carbonate production with the developement of reef buildings should correspond to a large scale warming of the marine waters still observed elswhere on the Russian Platform, in the North Sea and in the Paris Basin. However, the geographic distribution and the chronologic succession of the facies and deduced palaeoenvironments is probably equally related to a synsedimentary tectonic activity which operates as a favorable factor at both a local and regional scale.

Modalites sequentielles de la transgression aaleno-bajocienne sur le sud-est du Bassin parisien

Sedimentologic, biostratigraphic and diagenetic studies made on Middle Aalenian to Lowermost Bathonian deposits in 25 outcrops of the Burgundy High (southeastern Paris basin) lead to a sequence stratigraphy subdivision of the early Dogger record into 7 third order depositional sequences. These sequences make up a second-order transgressive phase whose lower limit is a tectonically enhanced unconformity (the Mid-Cimmerian unconformity), associated with a large scale uplift during the Upper Toarcian and the Lower Aalenian. In the largest outcrops, where the geometry of sedimentary bodies is observable at the hectometre to kilometre scale, the combination of diagenetic, biostratigraphical, spatial and sedimentologic studies is useful to understand the evolution of sediment distribution in the sequences. This approach also allows the measurement of sea level falls at the origin of the sequence boundaries. For example, we show that Lower Bajocian sequences end with decreases of accommodation space

Stage boundaries, global stratigraphy, and the time scale: towards a simplification

This paper examines four facets of stratigraphic terminology and usage considered faulty and proposes corrective measures. The four perfectible areas are: (1) The system of dual nomenclature requiring discrete terminologies for the superpositional and temporal aspects of rock units. (2) The premise that a GSSP establishes the base of a stage as being coincident with the top of the preceding stage rather than simply defining it as the boundary between stages. (3) The rejection of supplementary (auxiliary) sections that would broaden the knowledge of a GSSP and enlarge the area in which it is easily usable. (4) The current dual system of nomenclature for Precambrian and Phanerozoic strata is accepted, but a third system is proposed for strata formed in the last 3 to 5 Ma. In addition, the paper advocates a broader use of a limited number of conventions but warns against their proliferation; units shortened thereby would be more difficult to recognize. It points out the clear distinction between these conventions - a matter of administration and general compliance - and knowledge - a domain where free expression of opinion is indispensable -.

Séries condensées et indice de préservation d'unité biostratigraphique: exemple de l'ennoiement de la plate-forme nord-bourguignonne (France) au Callovo-Oxfordien

Abstract Several depositional environments associated with Callovian-Oxfordian condensed series of the southeast of the Paris Basin are defined. As sedimentation rates were very low, variations in palaeodepth, which were synchronous at regional scale, were taken to directly reflect fluctuations in accommodation potential. Within a major trend toward increased accommodation potential, five sharp upturns and one sharp downturn are detected. A biostratigraphic unit preservation index is denned to quantify and discuss the low preservation of sediments in these condensed series. This index is seen to vary with accommodation potential.

Sous-espèces géographiques et/ou contrôle environnemental de la variabilité morphologique chez “Thomasites” gongilensis (Woods, 1911), (Ammonitina, Acanthocerataceae, Vascoceratinae) du Turonien inférieur de la Haute Bénoué (Nigéria)

The ammonite “Thomasites” gongilensis (Woods, 1911) is a classical species from the Lower Turonian limestones of the Upper Benue Basin (Nigeria). In all the known sections, the species characterises a bed considered as isochronous in all this area; within the ammonite fauna, “Thomasites” always represents the main part with a ratio of 80 to 98%. The huge range of morphological variation of the shell of this ammonite is well known in the type population of the Ashaka quarry and described as a species represented by several “morphs”, which extreme morphologies are gongilense and compressum, and with instermediates like tectiforme, crassicostatum, etc… But, this variability exists not only within the population but in several samples, coming from other localities, situated in a marginal position according to the population of the type section. So, this interpopulation variation seems to be related to several factors: on the one hand, as it is commonly admited, to internal ones (genetical) showing a certain phenotypic potentiality linked to ontogenetical heterochrony; on the other hand, to external constraints, in connection with environmental conditions (energy and deepness, concurence with other organisms). Owing to all these facts, three taxonomical solutions are proposed. The first one, not retained here, considers the possibility of two sympatric species. The second one, refering to a polytypism and to ecological variations is proposed to explain the large variability; “Thomasistes” gongilensis can be split into two geographical groups which can be considered as subspecies. On the one hand, “Thomasites” gongilensis gongilensis which is present in the center, the Norhtern and the North-Western part of the basin; this subspecies can be secondary split into three subgroups which morphology is aconditioned by external constraints and considered as ecological variants (ecophenotypes). On the other hand, “Thomasistes” gongilensis inflatum which is restricted to the South-Eastern quadrant of the basin. The third solution, proposed as an alternative to the previous one, admits a unical species highly variable below internal (ontogenetical heterochrony) and external (environments) constraints. These investigations open the way to new researches on the taxonomical status, and especialy the generic status of the Woods species.

Biogéographie des ammonites jurassiques et reconstitution palinspastique de la Téthys

Resume— La distribution geographique des ammonites jurassiques est regie par deux causes fondamentales : l’ecologie et l’histoire evolutive (evenements vicariants). L’analyse conjointe de ces deux facteurs permet de reconnaitre : d’une part des taxons dont la distribution est essentiellement controlee par des contraintes ecologiques (Phylloce-ratinae et Lytoceratinae, lies a des environnements oceaniques profonds; Clydoniceratidae et Proplanulitinae lies a des plates-formes assez superficielles); d’autre part des taxons dont l’histoire evolutive est associee a des evenements vicariants (Ammonitina liasiques euro-boreales et Cardioceratidae-Kosmoceratidae boreaux du Jurassique moyen).Les exemples developpes ici permettent d’analyser en terme de paleobiogeographie la position de l’Europe centro-meridionale (Bulgarie, Hongrie, Roumanie) incluse au Jurassique dans la bordure nord-tethy-sienne. Il est montre que Phylloceratinae et Lytoceratinae sont tres abondants dans les regions supposees profondes a cette e...

Integrated Stratigraphy of the Potential Candidate Oxfordian GSSP at Thuoux and Saint-Pierre d’Argençon (France)

The Thuoux and Saint-Pierre d’Argencon sections (Subalpine Basin, southeastern France) are proposed as a potential GSSP candidate for the Callovian–Oxfordian boundary. Several aspects of stratigraphy that have recently been applied in the two selected sections are discussed, including ammonite and nannofossil biostratigraphy, palynology, physical stratigraphy, and cyclostratigraphy.