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Ammonoidea Toarcian biohorizons in the Basque-Cantabrian Basin (N Spain): a correlation between the Iberian Range and the Mediterranean Province standard scale

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Goy A. , Martinez G. , Ureta S.

tom Vol. 4, no. 1

166-167

In the Basque-Cantabrian Basin several detailed biostratigraphical studies in Toarcian sediments have been carried out, and all the zones and the most subzones from the NW European Province standard scale have been characterized. Recent investigations allow us to complete the reference scale and define 35 successive ammonoidea biohorizons that can be identified in the whole basin with rare exceptions. Biohorizons have been established with similar criteria to those suggested by Page (1995), and when possible, the evolution of a particular taxonomic group has been taken into account. The use of taxa with an important record overlap in the basin has been avoided. • Tenuicostatum Zone (1 – simplex, 2 – mirabile, 3 – crosbeyi, 4 – tenuicostatum, 5 – semicelatum). Successive species of Dactylioceras with reference sections in Camino (1, 2, 3, 5) and San Miguel de Aguayo (4). • Serpentinus Zone (6 – elegantulum, 7 – exaratum, 8 – elegans, 9 – pseudoserpentinus, 10 – douvillei). Species of Harpoceratinae (Eleganticeras, Cleviceras, Harpoceras) and Hildoceratinae (Orthildaites) with reference sections in Tudanca (6, 7, 8, 9) and San Andrés (10). • Bifrons Zone (11 – sublevisoni, 12 – tethysi, 13 – lusitanicum, 14 – apertum, 15 – bifrons, 16 – semipolitum). Successive species of Hildoceras with reference section in San Andrés. • Variabilis Zone (17 – variabilis, 18 – illustris, 19 – phillipsi, 20 – vitiosa). Species of Haugia with reference section in San Andrés. • Thouarsense Zone (21 – “Grammoceras”, 22 – “Essericeras”, 23 – fallaciosum). The presence of discontinuities in numerous sections and the poor and casual record of Grammoceras and Essericeras, challenging the establishment of the lowers biohorizons in this Zone. Their reference sections are Cillamayor (21, 22) and Castillo Pedroso (23). • Dispansum Zone (24 – cappuccinum, 25 – pachu, 26 – gruneri). The firsts two are successive species of Hammatoceras and the third one is a Gruneria species with a extensive record in the basin. All of them have the reference section in Cillamayor. • Pseudoradiosa Zone (27 – levesquei, 28 – munieri, 29 – pseudoradiosa, 30 – tectiforme). Successive species of Dumortieria and Paradumortieria with reference section in Cillamayor. • Aalensis Zone (31 – mactra, 32 – subcompta, 33 – aalensis/fluitans, 34 – falcifer, 35 – buckmani). Successive species of Pleydellia with reference section in San Andrés. This succession of biohorizons has numerous similarities with the one for the Iberian Range and considerable differences with sucessions from regions further south in the Iberian Peninsula. Moreover, the presence of the same taxa as other W. Tethys regions, where detailed biozonations have been made, enable the establishment of a precise correlation between the Basque-Cantabrian Basin and the Mediterranean Province, for some particular intervals and specifically for the base of Tenuicostatum/ Polymorphum Serpentinus/Levisoni, Bifrons, Dispansum/Speciosum and Aalensis zones. The discontinuous records of Collina gemma and Dumortieria meneghinii provide difficulties in correlation between Variabilis/Gradata, and Pseudoradiosa/Meneghinii zones, respectively. The present knowledge on the Basque-Cantabrian Basin doesn’t allow the precise correlation of the base of Thouarense/Bonarelli Zones.