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Kaolinite peaks in early Toarcian profiles from the Polish Basin-an inferred record of global warming

Brański P.

Geological Quarterly

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2010

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Vol. 54, No 1

15-24

EN

In lower Toarcian clay deposits (Ciechocinek Fm., VIII depositional sequence of the Lower Jurassic) from three boreholes from the Polish Basin, illite-dominated sedimentation representing the lower part of studied interval was interrupted by enhanced kaolinite input. Levels of high kaolinite/illite ratio at the VIIIb/VIIIc parasequence boundary suggest strong continental weathering in a humid-subtropical to tropical climate related to the phase of the early Toarcian global warming recorded at the top of the tenuicostatum Zone and correlated with isotope curves from a number of European sections. Kaolinite enrichment may be locally enhanced by reworking ofpre-Jurassic kaolinitic rocks and differential settling. Diagenetic processes were not sufficient enough to transform the initial kaolinite,but may have altered smectite and mixed-layers into illite and/or chlorite.

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Variability changes in the ammonite family Dactylioceratidae during the Lower Toarcian

Morard A., Guex J.

Volumina Jurassica

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2006

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Vol. 4, no. 1

194-195

EN

The comparison of successive dactylioceratid samples collected in the Moroccan Middle-Atlas, the Spanish Betic Cordillera, the Lusitanian Basin and the Causses Basin, allowed us to better characterize the range of variability of these forms from the very base of the Toarcian to the Lower-Middle Toarcian transition. From a biostratigraphic point of view, the Tethyan subgenus Eodactylites (Schmidt-Effing 1972) clearly precedes the NW European subgenus Orthodactylites – as recorded in the Lusitanian Basin succession for example, the latter then giving rise to the Dactylioceras s.s. Both early subgenera can be distinguished morphologically, and separated in a number of “species”, although isolated specimens may be difficult to identify. The main point is that Eodactylites, as well as early Orthodactylites form complete series of continuous covariation, the latter progressively branching into two main lineages in the Semicelatum Subzone. Following the occurence of some rare forerunners, known from the Middle-Upper Domerian, the evolution of the macroconch representatives of this ammonite family can be summarised in three main steps: 1. A sudden “mass apparition” of Eodactylites defines the base of the Toarcian. Their variability spectrum is immediately quite large, with a covariation series between a densely ornamented, somewhat involute and compressed pole (E. mirabile) and a more robust very evolute form with distant primary ribs and possible tuberculation (E. pseudocommune). 2. Ornamental variability tends to disappear in the outer whorls of Orthodactylites, whereas their inner whorls still display a wide covariation series (from slightly compressed forms with dense simple ribs to stout pantuberculate ones), as already noted by Howarth (1962). Intermediate forms seem to disappear in the Semicelatum Subzone. 3. Two lineages are then clearly distinct, one leading from isocostate to variocostate non tuberculate serpenticones (Dactylioceras s.s.), the second developing depressed whorls prone to tuberculation, particularly in inner whorls (Nodicoeloceras). A further diversification occurs at the Lower-Middle Toarcian transition (Guex 1971). In this new evolutionary frame, divergent views on the taxonomy of the Toarcian Dactylioceratidae can eventually be reconciled, as different authors in the past based their classification schemes only on a limited portion of the succession presented here.

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The Lower Toarcian of the High-Atlas, Morocco (Todrha-Dades area):stratigraphy and sedimentation

Ettaki M., Chellai E.

Volumina Jurassica

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2006

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Vol. 4, no. 1

160-160

EN

The Toarcian represents an important stage in the evolution of the Atlasic domain develped during the rifting on the West Tethyan margin during the Early Jurassic. In the Todrha-Dades area, the ammonites are apparently absent in deposits of the Tagoudite Formation, but a rich fauna of foraminifera and ostracods is meet in the marls and marly sandstones particularly in the lower member of the formation. The micropaleontological study indicates the Early Toarcian age (Polymorphum-Levisoni zones) for the Tagoudite Formation. In the Todrha-Dades area, the Lower Toarcian deposits increase in thickness from SW to NE. Towards SW, the sedimentation is characterized by large terrigenous sediment supply. In the NE direction the marly-sandstone succession of the Tagoudite Formation was deposited in a deep basin (umbilicus) created by a block tilting unfavourable for development of the microfauna. The impact of the tectonics during the Early Toarcian (Polymorphum Zone) is recorded by normal faults and plastic distortions.Tectonic structure analysis shows that the sedimentation took place in a transtensional tectonic regime. In general, the Todrha-Dades area is characterized during the Early Toarcian by the lateral passage between the different domains of sedimentation. The change in the sedimentation conditions took place after the Synpolymorphum Crisis. This change, accentuated by distensive tectonic, begins in the Early Toarcian and it is shown by sedimentary hiatus occurring in the platform and the seamounts ridges (individualized already during Pliensbachian). The neptunian dykes in Jbel Akenzoud and Tarhia n’Dades are associated with this level. Afterwards the detritical sedimentation occurred before the Middle Toarcian. Sedimentologic and paleontologic analysis prove that the Tagoudite Formation has been deposited in a relatively deep marine area. It marks a radical sedimentary change in the whole of the central High-Atlas domain after the Polymorphum Crisis during the Early Toarcian. The distensive tectonic was responsible for isolation of the basin and led to the development of anoxic conditions that controled the replacements of the fauna.

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The Early Toarcian of the Moroccan Atlasic Domain (Middle Atlas and High Atlas central in particular): important period of geodynamic and sedimentary changes

Sadki D., Akasbi A.

Volumina Jurassica

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2006

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Vol. 4, no. 1

64-64

EN

In the western Tethys, the Early Toarcian constitues one of the Jurassic stages marked by major palaeo-environmental and palaeo-oceanographic events. The changes could be linked to one of the most important and active phases of rift of Atlantic and Mesogee. In the Moroccan Atlasic chains (Middle Atlas and High Atlas in particular), these changes are responsible for important geodynamic, sedimentary and palaeogeographic events. From a geodynamic point of view: The Early Toarcian represents in Moroccan Atlasic chains, an intense distensive episode which corresponds to the peak of the break-up of the Liassic platform. This tectonic phase, which can be qualified as a "sudden rifting" is located in the Levisoni Zone that marked the formation of individual Atlasic basins, framed by residual platforms. The indices of this synsedimentary tectonic events are: lateral variations of thickness and facies, stratigraphic wedging, progressive unconformities, slumps, slides and resedimented blocks, synsedimentary normal faults, neptunians dykes. From a sedimentary point of view: This period corresponds to a change in sedimentary conditions: Sedimentation - becomes essentially marly, ending the carbonate episode of the Early and Middle Lias. These changes of depositional conditions are accompanied by sedimentary perturbations which are recorded as phases of erosion and non deposition in high zones (internal ridges and basin borders) or by allodapic deposits in depocenters as sandy turbidites ("Niveau brun" of Middle Atlas and turbiditic member of the Tagoudite Formation of the High Atlas). This turbiditic event of the Levisoni Zone is well known at a regional scale (Portugal) expressing the peak of the formation of individual basins within the western Tethys.

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Lower Toarcian organic-rich sediments from the Tuscan Succession (Northern Apennines, Italy)

Perilli N., Veiga de Oliveira L., Rodrigues R., Catanzariti R.

Volumina Jurassica

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2006

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Vol. 4, no. 1

203-204

EN

This work is aimed to improve lithostratigraphy and biostratigraphy, based on calcareous nannofossils, of the deep-marine Lower Toarcian sediments belonging to Tuscan Nappe (Northern Apennines). The sampled lower part of the Calcari e marne a Posidonia (Posidonia Marls) correlates with the Marne del Monte Serrone of the Umbria-Marche Basin, which is characterized by the presence of the Lower Toarcian black shales. The Calcari e marne a Posidonia consists of grey to greenish hemipelagic to pelagic bivalves-bearing marlstones and limestones with interbedded grey to grey dark, sometimes reddish, clayey marlstones and marly claystones. In some localities, in the lower portion of this formation is recognized a thin to medium thick organic-rich interval of black marlstones and marly claystones. The recovered calcareous nannofossil assemblages allow to assigne the basal portion of the Marne a Posidonia to the Lower Toarcian with the thin organic-rich interval comprised between the appearances of the genera Carinolithus and Discorhabdus.

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Importance of the Mellala section (Traras Mountains, NW Algeria) for the correlations of the Pliensbachian/Toarcian boundary

Elmi S., Marok A., Sebane A., Almeras Y.

Volumina Jurassica

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2006

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Vol. 4, no. 1

158-160

EN

The Mellala profile has a great importance as international reference section because it exposes a continuous and very fossiliferous record through the Pliensbachian/Toarcian boundary (Fig. 1). This boundary is situated within the hemipelagic Bayada Formation (alternating marls and marly limestones) yielding Zoophycos and Steinmannia bronni (shells and “filaments”). It has been deposited in a small subbasin (umbilicus) that was strongly subsiding and deepening during the Late Pliensbachian and the Early Toarcian. The sequence evolution is globally stratodecreasing and deepening upward until the middle part of the Early Toarcian (early Levisoni Zone). The Bayada Fm. begins in the Upper Domerian. The Emaciatum Zone is divided in two parts. The lower (Solare Subzone, thickness 8 m) is documented by several Pleuroceras solare (Phillips) occurring alone in the lower part of the subzone, but associated with Emaciaticeras upwards. Brachiopods include Phymatothyris kerkyrae (Renz) and Quadratirhynchia quadrata Buckman. The upper part (Elisa Subzone) is characterized by Tauromeniceras elisa (Fucini), Canavaria finitima (Fucini), Paltarpites bettonii (Fucini) associated with numerous P. kerkyreae (Renz) and rare Lobothyris punctata (Sowerby). The main part of the brachiopod assemblage consists of small sized species: Nannirhynchia pygmoea (Davidson), Koninckella liasina (Davidson) and Cadomella cf. moorei (Davidson). This is the Koninckella fauna (previously named Leptaena fauna). Leioceratoides gr. serotinus (Bettoni) has been found in the upper part of the Elisa Subzone. The base of the lowermost Toarcian (Mirabile Subzone) is marked by a decimetric bed (n° 38) with Paltarpites paltus (Buckman) but without Eodactylites at the present state of the research. The following calcareous bed (n° 40) has yielded several Dactylioceras (Eodactylites) polymorphum (Fucini) and D. (E.) pseudocommune (Fucini). The brachiopods are represented by Liospiriferina subquadrata (Seguenza), Lobothyris sp. and the Koninckella fauna: C. moorei, N. pygmoea and K. liasina. Upwards, the Eodactylites becomes abundant at 10 to 12 m, especially in the bed n° 44. At 3.50 m under the top of the subzone, a marker-bed (48) corresponding to a lenticular level of bioclastic quartziferous limestones occurs. The overlying 15 m are attributed to the Semicelatum Subzone (Tethyan nomenclature). At the base, there occurs a level with D. (Orthodactylites) crosbeyi (Simpson) associated with Lobothyris arcta (Dubar) which allows a good correlation with the Clevelandicum Subzone (or Horizon) of northwestern Europe. The topmost 5 m of the deposits are dated to the Levisoni Zone with Eleganticeras sp. The anoxic event at the beginning of this Zone is only indicated by an abnormal variability of the foraminifera. The presence of Lenticulina obonensis (Cubaynes) mg Planularia indicates stressing conditions.