A Unified Lower-Middle Cambrian Chronostratigraphy for West Gondwana
|Title||A Unified Lower-Middle Cambrian Chronostratigraphy for West Gondwana|
|Publication Type||Journal Article|
|Year of Publication||2004|
|Authors||Geyer, G, Landing, E|
|Journal||Acta Geologica Polonica|
|Keywords||Agdzian Stage, Atlasian Series, Australia, Cambrian, Celtiberian Series, chronostratigraphy, Cordubian Series, Israel, Jordan, Morocco, Poland, Siberia, South China, Spain, Trilobita|
Similarities in biotic successions support a unified, composite chronostratigraphy for the Lower-Middle Cambrian of the Iberian and Moroccan margins of West Gondwana. The Cordubian Series (emended from an Iberian stage-level unit) comprises the sub-trilobitic Lower Cambrian of West Gondwana. This series represents ca. half of the Cambrian (ca. 25 m.y.), has a base defined at the lowest occurrence of diagnostic Cambrian ichnogenera in central Spain, and is comparable to the Placentian Series of Avalon. The overlying trilobite-bearing Lower Cambrian (ca. 9 m.y. duration), or Atlasian Series (new), of West Gondwana consists of two stages originally defined in highly fossiliferous (trilobites and archaeocyaths) Moroccan sections where they have great potential for precise U-Pb geochronology. The penultimate Lower Cambrian stage, the Issendalenian, with the near-joint lowest occurrence of trilobites and archaeocyaths, is approximately comparable with the sparsely fossiliferous, Iberian "Ovetian Stage" (designation abandoned). The "Ovetian" as currently revised is invalid because it is an objective homonym of SDZUY’s original “Ovetian.” The "Ovetian" is not a practically applicable unit as its basal stratotype horizon lacks any identified fossils, and its trilobites are so rare that it cannot serve as a standard for interregional correlation. By contrast, the Banian Stage as defined in Morocco is suitable as the terminal Lower Cambrian stage of West Gondwana. It is based on very fossiliferous successions that allow recognition of three successive trilobite zones and is similar in concept to the Iberian “Marianian Stage” (designation abandoned). The “Marianian” lacks a designated lower boundary stratotype locality, lacks a biostratigraphically defined base at a stratotype, and is so sparsely fossiliferous that no biostratigraphic zonation exists.
Long-term problems involving regional definition of the West Gondwanan Lower–Middle Cambrian boundary are resolved. The lowest occurrence of paradoxidid trilobites, the classic index for the base of the Middle Cambrian, is diachronous in West Gondwana and other regions, with the group showing a delayed appearance in Iberia, Sweden, and Bohemia, by comparison with Morocco. Taxonomic revision of Iberian trilobites and new documentation of their ranges in Morocco show that the Iberian terminal “Lower Cambrian” “Bilbilian Stage” (designation abandoned) correlates with the Moroccan upper Banian and lower and middle “Tissafinian” Stages and with the Siberian upper Toyonian (traditionally assigned to the Lower Cambrian in Siberia) and lower Amgan (assigned to the Middle Cambrian in Siberia) Stages. The “Bilbilian” thus includes Middle Cambrian strata in a traditional international concept. Faunas from the base of the Iberian “lowest” Middle Cambrian “Leonian Stage” (designation abandoned) correlate largely into the second trilobite zone of the “Tissafinian” and are now understood to appear above an intra-Middle Cambrian faunal break. This faunal break is still poorly understood and may be an artifact of collection failure immediately above the sparsely fossiliferous, problematical “Valdemiedes event” interval. The base of this “Middle Cambrian”, now termed Celtiberian Series (new designation), in West Gondwana is best defined by the base of the lower Middle Cambrian Agdzian Stage (new designation). The Agdzian includes the “Tissafinian” and overlying lower “Toushamian Stage” designations abandoned) of Morocco, and has its top defined by the base of the middle Middle Cambrian Caesaraugustian Stage, as defined in Spain at the lowest occurrence of Badulesia tenera. The Languedocian, originally defined at the lowest occurrence of Solenopleuropsis (Manublesia) thorali in the southern Montagne Noire of France comprises the third, and terminal stage of the Celtiberian Series. We suggest that the designations “Lower Cambrian” and “Middle Cambrian” be regarded as descriptive and nonchronostratigraphic terms in discussions of the Cambrian.