Jurassic dinoflagellate cyst biostratigraphy of the Lusitanian Basin, West-central Portugal, and its relevance to the opening of the North Atlantic and petroleum geology

Correia, Vânia Dinora Pereira Fraguito

http://hdl.handle.net/10400.1/10828

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Authors

Correia, Vânia Dinora Pereira Fraguito

Advisor(s)

Fernandes, Paulo

Pereira, Zélia

Riding, James B.

Abstract(s)

The biostratigraphy of the Lusitanian Basin is based mainly on macrofossils and microfossils (foraminifera, nannofossils and ostracods). Compared to these works, palynological studies are scarce in the Lusitanian Basin. Dinoflagellate cysts are a powerful biostratigraphical and paleoenvironmental tool and, their study may be a major contribution for the Lusitanian Basin background. In this project, the Lower and Middle Jurassic of the Lusitanian Basin was investigated for palynological and palaeoenvironmental analyses. Eight sections were sampled: São Pedro de Moel, Brenha, Peniche, Fonte Coberta, Maria Pares, Vale das Fontes, São Gião e Cabo Mondego; 358 samples of marls and marly limestones were studied in detail. The palynological response to the Toarcian Oceanic Anoxic Event (T-OAE) was examined in the Lower Jurassic sections of Maria Pares, Vale das Fontes and Peniche. A low diversity dinoflagellate cyst flora, typical of the Sub-Boreal Realm, was recovered from the Emaciaticeras emaciatum and Dactylioceras polymorphum ammonite biozones (ABs). Prior to the T-OAE, dinoflagellates thrived in the Lusitanian Basin. The Hildaites levisoni AB represents the T-OAE and the overlying strata, and is characterised by a profound reduction in dinoflagellate cyst relative abundances, causing the extinction of Luehndea spinosa and disappearance of Nannoceratopsis spp. This dinoflagellate cyst “blackout” reflects significant environmental stress, such as marine anoxia, elevated temperatures and reduced salinity, with the former two probably being most important. A comprehensive investigation of the Early and Middle Jurassic stratigraphical palynology of the Lusitanian Basin was undertaken and a palynostratigraphical chart is proposed based, especially, in dinoflagellate cysts. The Sinemurian material examined in São Pedro de Moel was barren of dinoflagellate cysts, however the Pliensbachian and Toarcian successions are characterised by relatively low diversities, but biostratigraphically significant. Luehndea spinosa dominates the uppermost Pliensbachian–lowermost Toarcian, and is an index species. The Luehndea spinosa and Mendicodinium microscabratum dinoflagellate cyst biozones were defined, both of which are subdivided into two dinoflagellate cyst subbiozones. The Middle Jurassic samples of São Gião and Cabo Mondego successions are correlated to ammonite biozones spanning Pleydellia aalensis to Zigzagiceras zigzag. The Cabo Mondego section largely yielded relatively abundant palynomorph associations. By contrast, the São Gião outcrop, which only includes the Toarcian–Aalenian transition, produced sparse assemblages. The uppermost Toarcian to lowermost Bajocian is characterised by a low diversity dinoflagellate cyst association. Above this (Witchellia laeviuscula AB) is a markedly more diverse assemblage. The trend of increasing dinoflagellate cyst diversity continued at the Bajocian–Bathonian transition. The acritarchs, prasinophytes, pollen and spores observed in this study are typical of Jurassic assemblages worldwide. The palynostratigraphical scheme proposed in this project may be used in future hydrocarbon investigation of the Lusitanian Basin. The fluorescence colour of palynomorphs and spore-pollen colours (TAI) analyses, suggest that most of the Lower and Middle Jurassic sediments of the Lusitanian Basin are between the immature and mature phases for oil generation. This work is presented as a significant contribution to the Lusitanian Basin background, especially for biostratigraphy.