Palynology and terrestrial ecosystem change of the Middle Triassic to lowermost Jurassic succession of the eastern Danish Basin

Abstract

The pre-Rhaetian Triassic succession in the Danish Basin is generally dominated by red bed deposits unsuitable for preservation of organic material. However, during the late Middle Triassic a temporary change to wetter conditions resulted in preservation of diverse palynofloral assemblages, dominated by corystospermous seed fern pollen of Alisporites/Falcisporites and characterized by high abundances of the conifer pollen Protodiploxypinus gracilis, as well as common to abundant lycophyte spores assigned to Aratrisporites spp. A mid-Ladinian to early Carnian (earliest Julian) age is inferred for this interval based on the presence of the probable gymnosperm pollen Staurosaccites quadrifidus, rare Retisulcites perforatus (of unknown affinity) and Enzonalasporites vigens of probably voltzialean affinity. No marine palynomorphs were recorded, only rare specimens of Ovoidites sp. and Plaesiodictyon mosellanum. Thus, together with previously recorded other fossils (a fish microfauna, an ostracod, plant remains and characeans) the recorded palynoflora indicates semi-arid conditions and deposition in a terrestrial to marginal fresh- or brackish water environment. After this, red bed deposition resumed in the area, and it is not until the mid-Rhaetian that well preserved and diverse palynofloras are again encountered in the succession. The Rhaetian palynofloras are significantly different to the Ladinian–early Carnian assemblages, being dominated by tree fern spores (Deltoidospora spp.), taxodiacean/cupressacean pollen (Perinopollenites elatoides), and the enigmatic gymnosperm pollen assigned to Ricciisporites tuberculatus. As much as 62% of the mid-Ladinian to Early Carnian spore-pollen taxa were not recorded in the Rhaetian, suggesting that major restructuring of the terrestrial ecosystem took place in this area during the Carnian and Norian. In comparison, 34% of the Rhaetian spore-pollen taxa recorded in this study, e.g. R. tuberculatus, Polypodiisporites polymicroforatus, Limbosporites lundbladiae and Cingulizonates rhaeticus, disappeared during the well-recognized end-Triassic event. The extinction interval is also characterized by a dramatic increase in the number of reworked palynomorphs, indicating erosion of the hinterland at this time. There is a marked drop in diversity from the Late Triassic to the earliest Jurassic. The palynology indicates that the established post-extinction vegetation was dominated by tree ferns (Deltoidospora) and pinacean conifers (Pinuspollenites minimus), along with common taxodiacean/cupressacean conifers, ginkgos/cycads and corystospermous seed ferns, and that it remained relatively stable during the Hettangian to early Sinemurian.