The late Rhaetian transgression in southern Sweden: Regional (and global) recognition and relation to the Triassic–Jurassic boundary
Palynological investigations of Rhaetian outcrops and drillcores in combination with subsurface data from wells in Scania, southern Sweden, have revealed the presence of marine dinoflagellate cysts in sedimentary strata that were previously considered to have been deposited in a mainly terrestrial environment. Two distinct dinocyst events are identified. One older event where persistent, rare to common occurrences of Rhaetogonyaulax rhaetica and Lunnomidinium scaniense indicates deposition in shallow marine and marginal marine environments. This Lunnomidinium interval is preceded and succeeded by assemblages with rare to common R. rhaetica, and can be correlated with the mid to late Rhaetian R. rhaetica Zone. It is associated with spore/pollen assemblages of the mid Rhaetian Rhaetipollis-Limbosporites Zone. The younger event, the R. rhaetica maximum interval, is characterised by mass-occurrence of R. rhaetica, with less common or absent Dapcodinium priscum, in association with a dark grey to black mudstone/shale. The introduction of D. priscum in association with the over-whelming abundance of R. rhaetica allows correlation with the transition between the R. rhaetica Zone and the succeeding D. priscum Zone, and signals fully marine conditions. The R. rhaetica maximum interval is associated with spore/pollen assemblages of the late Rhaetian Ricciisporites-Polypodiisporites Zone, and is in Sweden succeeded both lithostratigraphically and palynostratigraphically by the T-J transition. The R. rhaetica maximum interval corresponds to a late Rhaetian maximum flooding event that took place in a shallow, gently sloping embayment that covered the Danish Basin area during the Late Triassic-Early Jurassic. This maximum flooding event can be recognised in late Rhaetian strata from different parts of the Northern Hemisphere. It is an important temporal constraint on the T-J boundary as it is associated with the disappearance of Triassic marine faunas, appears to coincide with the onset of the main pulse of CAMP volcanism, immediately precedes the initial carbon isotope excursion and the global sea-level drop that characterises the T-J transition.