|mhorn||Дата: Sunday, 12.10.2014, 20:36 | Сообщение # 1|
|Jain S. (2014) The Arkelli Chronozone: A new early Middle Bathonian standard ammonite zone for Kachchh, western India (southern Jurassic Tethys) // Zitteliana A, Bd.54. P.91-136. |
Based on the widespread distribution of the nominal and Madagascan ammonite species Procerites (Gracilisphinctes) progracilis arkelli Collignon [M] in Kachchh (western India), a new early Middle Bathonian Arkelli Chronozone is proposed and correlated with the Western European early Middle Bathonian Progracilis Zone. The nominal species is now firmly established as a geographic subspecies of the Western European P. (G.) progracilis Cox and Arkell [M]. The recorded assemblage within the Kachchh Mainland from this chronozone now contain Micromphalites (Clydomphalites) clydocromphalus Arkell [M], Procerites hians Waagen [M], Procerites (Siemiradzkia) cf. verciacensis [m], Sivajiceras congener (Waagen) [M and m], Procerites (Gracilisphinctes) progracilis arkelli Collignon [M and m], P. (G.) progracilis aff. arkelli Collignon [M], P. (G.) intermedius n. sp. [m], Macrocephalites cf. etheridgei Spath [m], M. bifurcatus transient aff. bifurcatus Boehm [m], M. bifurcatus transient intermedius (Spath) [M and m], M. bifurcatus transient cf. intermedius Boehm [M], Macrocephalites triangularis Spath [M and m], Parapatoceras distans (Baugier and Sauzé) [M], Reineckeia sp. A and B [M] and Eutrephoceras sp. [M]. Coeval sediments at the Island Belt had previously yielded the presence of Procerites cf. schloenbachi, Micromphalites (Clydomphalites) cf. clydocromphalus, M. (Clydomphalites) sp. indet., M. aff. hourcqui, Clydoniceras triangulare, C. pachchhamensis, Clydoniceras sp. and Bullatimorphites (s.s.). These finds, thus, now extend the Kachchh non-macrocephalitid records to Saudi Arabia and on to the NW European localities, and for the macrocephalitids, into the High Himalayas and further outside the Indo-Madagascan Province, to the East Indies and into New Zealand. Recent nannofossil data from the same ammonite-yielding bed (the Yellow Bed at Jumara, Kachchh Mainland) is reinterpreted with updated chronology. New data suggests that this condensed Yellow Bed encompasses a 2.6 Ma interval spanning from 168.67 Ma (Late Bajocian; Garantiana Zone) to 166.07 Ma (Middle Bathonian; Morrisi Zone). However, for both Garantiana and Morrisi zones, the ammonite records are lacking in Kachchh, so far. Previous benthic foraminiferal data from the same bed had also yielded a broad Middle Bathonian age. Although, an integrated approach is employed here (using ammonite, foraminifers, nannofossils and lithostratigraphy), but for the Bathonian, among the fauna, the ammonites provide a far better age resolution. Globally, Macrocephalites bifurcatus transient cf. intermedius Boehm [M] is recorded here as the earliest macrocephalitid. Its evolutionary path follows from the Late Bajocian to Early Bathonian Satoceras to Praetulites and on to M. bifurcatus in the early Middle Bathonian. The Macrocephalites entry into Kachchh was possibly facilitated by a transgressive event (the T10 event of Hallam and Haq et al.’s ebbing LZA-2.2). This contribution also discusses the implications of these new records to regional and global biostratigraphy.
Middle Jurassic - Lower Cretaceous ammonites & aptychi