Preliminary analysis of the paleofloral assemblages (Charophytes and Palynomorphs) from the uppermost Jurassic of Valmitão (Lusitanian Basin, Portugal)

Abstract

The Upper Jurassic biostratigraphy of the Lusitanian Basin is well defined in charophyte assemblages from the Oxfordian (Pereira et al., 2003) and Late Tithonian (Pereira et al., 2010) stages. The limited charophyte records in the Early-Middle Tithonian interval between these two reference levels create a biostratigraphic resolution gap at the basin scale. This study examines whether the Early-Middle Tithonian charophyte gap in the Lusitanian Basin, reflects paleoenvironmental dynamics, rather than a depositional hiatus, based on preliminary data from the Valmitão coastal section near Ribamar, Lourinhã (western Portugal). Within the scope of the study, charophyte and palynological analyses were performed on 10 samples systematically collected along the Valmitão section. The investigations revealed that gyrogonites were restricted to the lowermost productive levels, VLM1 and VLM2, among the 10 sampled intervals. High-resolution sampling within the VLM2 interval revealed a systematic upward decrease in gyrogonite abundance, accompanied by increasing pyritization and the eventual disappearance of charophyte remains. Palynofacies analyses showed that the levels studied clustered in the “Field 1” (high phytoclast, low AOM, and palynomorph) on the Tyson Diagram. These data indicate a phytoclast-dominated proximal depositional setting with high terrestrial organic matter input. These findings suggest that local paleoenvironmental and taphonomic conditions, particularly facies variability and redox fluctuations, may have influenced charophyte occurrence and preservation. In conclusion, these preliminary results from the Valmitão section suggest that local paleoenvironmental and taphonomic factors may have contributed to the apparent scarcity of Early-Middle Tithonian charophyte records in the Lusitanian Basin, rather than solely reflecting a true stratigraphic hiatus.