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On the Pliensbachian/Toarcian boundary in the Lower Jurassic Toyora Group in southwest Japan

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Nakada K. , Matsuoka A.

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tom Vol. 7, no. 1

47-54

EN

The Toyora Group, consisting of continental shelf sediments, is exposed in southwest Japan. It is divided into three formations, the Higashinagano, the Nishinakayama, and the Utano formations in ascending order. The Nm Member, the lower part of the Nishinakayama Formation, has yielded abundant ammonites, indicating successive ages of Early Toarcian. The ammonite zonation was established on these materials, but the Pliensbachian/Toarcian (P/T) boundary was not defined clearly in previous studies. A total of 99 ammonite specimens including 11 genera and 16 species were collected along the Sakuraguchi-dani Valley. The first occurrence of Dactylioceras helianthoides is the most suitable candidate for drawing the P/T boundary in the Toyora Group, the boundary being drawn at least 35.5 m lower than the previously indicated.

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The Pliensbachian/Toarcian boundary in the Lower Jurassic Toyora Group in southwest Japan

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Nakada K. , Matsuoka A.

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tom Vol. 4, no. 1

197-197

EN

The Toyora Group is the continental shelf sediment exposed in the western part of Yamaguchi Prefecture, southwest Japan. It is mainly composed of sandstone and mudstone, and divided into three formations, that is, the Higashinagano, the Nishinakayama, and the Utano formations in ascending order. The Sakuraguchidani Mudstone Member, the lower part of the Nishinakayama Formation, is rich in ammonoids. Almost all ammonite specimens examined are classified into Hildocerataceae, and indicate successive ages from Upper Pliensbachian to Lower Toarcian. Hirano (1973) established three ammonite zones, in ascending order, the Fontanellense Zone, the Nipponicum Zone, and the Helianthoides Zone, and defined the Pliensbachian/Toarcian boundary within the P. nipponicum Zone. Furthermore, the black shale, a main component of the member, is known as a layer representing global Early Toarcian Ocean Anoxic Event (Tanabe 1991). The purpose of this study is to revise the Pliensbachian/Toarcian boundary in the Lower Jurassic Toyora Group by adopting the recently established global standard. Along the Sakuraguchidani route, the type locality of the Sakuraguchidani Mudstone Member, 99 ammonite specimens including 11 genera and 16 species were collected. The ammonite fauna is characterized by the abundance of Dactylioceras helianthoides (Yokoyama), Protogrammoceras nipponicum (Matsumoto), and Cleviceras chrysanthemum (Yokoyama). Paltarpites paltus (Buckman) and Fontanelliceras fontanellense (Gemmellaro) were obtained from this member. The Pliensbachian/Toarcian boundary is determined by adopting the definition which is used in Europe (e.g. Macchioni 2002). Among the various indexes, the first appearance of the genus Dactylioceras and P. paltus, mainly used in the Mediterranean area, is the most suitable for the Toyora Group. The boundary is drawn within the Fontanellense Zone, 23 m lower than that in the previous studies (Hirano 1973; Tanbe 1991). The Toyora Group has high potential for elucidating the timing of the Toarcian Ocean Anoxic Event off eastern Asia.

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Importance of the Mellala section (Traras Mountains, northwestern Algeria) for the correlation of the Pliensbachian-Toarcian boundary

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

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tom Vol. 7, no. 1

37-45

EN

In the Mellala profile, located in the Traras Mountains (Tlemcen Domain, NW Algeria), the hemipelagic Bayada Fm. includes the transition from the Pliensbachian to the Toarcian within a continuous succession of alternating marls and marly limestones, whose thickness reaches approximatively 50 m.The Bayada Fm. begins in the Upper Domerian. The Solare Subzone of the Emaciatum Zone is proved by the occurence of several Pleuroceras solare (Phillips) in the lower part, associated with Emaciaticeras upwards. The Elisa Subzone is characterized by Tauromeniceras elisa (Fucini), Canavaria finitima (Fucini), Paltarpites bettonii (Fucini) associated with numerous Phymatothyris kerkyreae (Renz) and rare Lobothyris punctata (Sowerby). Leioceratoides gr. serotinus (Bettoni) has been found in the upper part of the subzone. The base of the lowermost Toarcian (Mirabile Horizon, Paltus Subzone) is marked by a decimetric bed (no. 38) with Paltarpites paltus (Buckman) but which has not yielded Eodactylites in the present state of the research. The following calcareous bed (no. 40) has yielded several Dactylioceras (Eodactylites) polymorphum (Fucini), D. (E.) mirabile (Fucini) and D. (E.) pseudocommune (Fucini). Upwards, Eodactylites is abundant over 10 to12 m, especially in Bed no. 44. At 3.50 m below the top, a marker-bed corresponds to a lenticular level of bioclastic quartz-rich limestones that marks the limit between the two members of the formation. The overlying 15 m are attributed to the Semicelatum Subzone (Tethyan nomenclature). At the base a level with D. (Orthodactylites) crosbeyi (Simpson) allows a good correlation with the Clevelandicum Subzone (or Horizon) of Northwestern Europe. The topmost 5 m of the outcrop are dated to the Levisoni Zone with Eleganticeras sp. The anoxic event of the beginning of this Zone is indicated only by an abnormal variability of the foraminifera. The presence of Lenticulina obonensis Ruget indicates stressing conditions. The occurrence, from the Upper Domerian to the Polymorphum Zone, of a fauna comprising the brachiopod Koninckella is important because it confirms that the environment was restricted within a deep, strongly subsiding basin (the "umbilicus").

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The Pliensbachian/Toarcian boundary in the Almonacid de la Cuba section (Iberian Range, Spain)

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Goy A. , Comas-Rengifo J. , Gomez J. , Gonzales J. , Herrero C. , Palencia A. , Perilli N. , Rodrigo A.

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tom Vol. 4, no. 1

164-166

EN

The Almonacid de la Cuba section, representative of the Pliensbachian-Toarcian transition in the Iberian Range (Fig. 1), is reviewed. It is an expanded section where no important discontinuities have been detected. Four successive assemblages of ammonites, which are characterized by the presence of Pleuroceras (BH14-CU14), Canavaria (CU16-CU32), Dactylioceras (E.) (CU35.2-CU44) and Dactylioceras (O.) (CU44-CU87), are distinguished. The Pliensbachian/Toarcian boundary is located at the base of level CU35.2 with the first record of Dactylioceras (Fig. 2). These assemblages are mainly constituted by taxa typical of the NW European Province, such as Pleuroceras, Dactylioceras (O.) and P. paltum. However, frequent Mediterranean Province taxa such as Emaciaticeras, Canavaria, Lioceratoides, Neolioceratoides, Dactylioceras (E.) and P. madagascariense, are also recorded. In the Tenuicostatum Zone, dactylioceratidae are dominating with respect to harpoceratinae. In the Mirabile Subzone, species of Dactylioceras (E.) are coexisting with P. paltum. Brachiopods show two successive assemblages. The lower one is composed generally of the Pliensbachian taxa and the upper assemblage includes more endemic taxa. Coinciding with the Early Toarcian OAE, almost all these species disappeared at the end of the Tenuicostatum Chron. Foraminiferal assemblages are rich and diversified. Calcareous hyaline taxa are dominated by suborder Lagenina, agglutinated foraminifera are scarce, the suborders Spirillinina and Miliolina are represented by few specimens and taxa, and specimens of Robertinina have been recovered throughout the whole stratigraphic interval. The main biostratigraphical foraminiferal events can be recognized and compared with other sections of the Iberian Range and with another ones of selected NW European Basins. Ostracod assemblages of the Spinatum Zone are dominated by healdiids and cytheraceans, which decrease at the base of the Tenuicostatum Zone, where the cypridaceans are better represented. In the Semicelatum Subzone, coinciding with the disappearance of the healdiids, the cytheraceans become dominants.Calcareous nannofossils assemblages are rich and well preserved. This allowed locating precisely the biochronostratigraphical position of the main markers and events and comparing them with these recorded in other basins of Western Tethys. A magnetic polarity column for the Pliensbachian/Toarcian boundary has been constructed on the basis of the polarities of the 2C Component (Fig. 2). The lower boundary of the Toarcian is located within the R2 magnetozone. A relatively large magnetozone N3 of normal polarity is located within the Tenuicostatum Zone.