Foraminiferal Biostratigraphy of the Aalenian–Bajocian in the Iberian Plate

Abstract

In the last decades, several attempts have been made to establish biostratigraphic scales based on Jurassic benthic foraminifera, as this group is an essential tool in log dating and correlation, namely in the context of hydrocarbon exploration. But besides biostratigraphy these marine microfossils are also important to support paleoenvironmental reconstructions and evolutionary studies, and to understand extinction events. The Aalenian–Bajocian transition (Middle Jurassic) presents a significant fossil record in some basins located around the Iberian Plate, which enabled the establishment of a detailed ammonite-based biostratigraphical framework displaying suprabacinal value. In this work, a zonal scale for that stratigraphic interval based on benthic foraminifera is proposed, well calibrated with the ammonite-based scale, as a result of detailed sampling in sections located in the Western Iberian Margin (the Lusitanian Basin, Portugal) and in the South Iberian Margin (External Zone of the Subbetic Domain of the Betic Cordillera, Spain). For the Aalenian–Bajocian transition two biostratigraphic units have been recognized in both areas: the Lenticulina quenstedti Zone, based on the first occurrence of the species Lenticulina quenstedti (Gümbel) in the Bradfordensis Zone (middle Aalenian) in the Lusitanian Basin, and in the Concavum Zone (upper Aalenian) in the Betic Cordillera; the Ramulina spandeli Zone, based on the first occurrence of Ramulina spandeli Paalzow, at the base of the Bajocian (Discites Zone) in the Lusitanian Basin. This index fossil, which has also been recognized at the base of the Bajocian in the Iberian Cordillera, displays a different stratigraphic range in the Betic Cordillera, where its first occurrence has been recorded earlier, in the middle Aalenian (Gigantea Subzone). However, another bioevent with biostratigraphic relevance can be used in both areas to recognize the Aalenian–Bajocian boundary: the decrease in abundance and diversity of the foraminiferal assemblages’ composition, which occurs in the uppermost part of the Limitatum Subzone (upper Aalenian) – Discites Zone (lower Bajocian), which represents a useful proxy in log interpretation. This bioevent has been previously recognized not only in other Iberian Basins, as the Basque-Cantabrian Basin, but in other European areas as Great Britain and Poland, showing as a consequence a wider geographical usefulness. Ongoing detailed studies in other basins around the Iberian Plate will allow the establishment of other useful biostratigraphic tools based on benthic foraminifera and other microfossil groups, such as calcareous nannoplankton and dinoflagellates.

AAPG Datapages/Search and Discovery Article #90325 © 2018 AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin, Lisbon, Portugal, May 2-3, 2018