The Middle Jurassic palynostratigraphy of the northern Lusitanian Basin, Portugal

Abstract

A composite largely Middle Jurassic succession spanning the Toarcian-Aalenian transition to the lowermost Bathonian exposed at Cabo Mondego and Sao Giao in the northern Lusitanian Basin, western Portugal, was examined palynologically. The 129 samples are correlated to ammonite biozones spanning Pleydellia aalensis to Zigzagiceras zigzag. The Cabo Mondego succession comprises the type section of the Cabo Mondego Formation and spans virtually the entire interval studied. This is a significant interval because it includes the Global Stratotype Section and Point (GSSP) and the Auxiliary Stratigraphical Section and Point (ASSP) for the Bajocian and Bathonian stages respectively. The Cabo Mondego Formation largely yielded relatively abundant palynomorph associations in the 68 productive samples recovered. By contrast, the Povoa da Lomba Formation at Sao Giao only includes the Toarcian-Aalenian transition; the 21 productive horizons produced sparse assemblages. The uppermost Toarcian to lowermost Bajocian is characterised by a low diversity dinoflagellate cyst association, typified by Nannoceratopsis. Above this is a markedly more diverse assemblage. This influx, in the Witchellia laeviuscula ammonite biozone AB, represents a global evolutionary radiation which may be linked to sea level rise. The trend of increasing dinoflagellate cyst diversity continued at the Bajocian-Bathonian transition. The Middle Jurassic dinoflagellate cyst assemblages of the Lusitanian Basin are significantly less diverse than coeval palynobiotas from eastern and northern Europe, and the Arctic. The Toarcian Oceanic Anoxic Event (T-OAE) profoundly inhibited cyst-forming dinoflagellates in this depocentre, and the recovery was protracted. Hence the T-OAE may have suppressed dinoflagellate cyst diversity well into the Middle Jurassic. This phenomenon may have been exacerbated by the absence of typically Arctic taxa through latitudinal controls and/or global cooling during the early Aalenian. These low levels of dinoflagellate cyst species richness may also be related to the palaeogeography of the Lusitanian Basin. This relatively isolated deepwater depocentre close to the Proto Atlantic, may have precluded extensive biotal exchange with the widespread shelfal areas of the western Tethys. The absence of Dissiliodinium giganteum in the Lusitanian Basin is consistent with this scenario. The pollen and spores observed in this study are typical of Middle Jurassic assemblages worldwide. Araucarian pollen, largely Callialasporites, diversified and became prominent during the Aalenian.