The Jurassic-Cretaceous transition of the Fårarp-1 core, southern Sweden: Sedimentological and phytological indications of climate change

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The 116 m deep Fårarp-1 core drilled in the Vomb Trough in southernmost Sweden is dated by integrated terrestrial and marine palynostratigraphy. The lower part of the succession (ca 84 m) encompasses uppermost Jurassic to lowermost Cretaceous (uppermost Tithonian to Valanginian) strata. An unconformity separates the Valanginian strata from an overlying ca 1 m thick interval of upper Albian to Cenomanian Arnager Greensand Formation. The uppermost part of the core is a repetitive succession of lowermost Cretaceous sediments.

During the Jurassic–Cretaceous (J/K) transition NW Europe was located in mid latitudes, and comprised an archipelago of large and small islands separated by deeper grabens and epicontinental seaways that connected the Boreal Sea to the north with the warmer Tethys Ocean to the south. Boundary strata in England, France, the Netherlands and Germany are characterised by relatively prominent climatic change from arid/semi arid to subhumid/humid conditions. Southernmost Sweden was located on the margin of a large landmass comprising most of the Fennoscandian Shield bordering a large epicontinental sea to the west. By combining sedimentology, clay mineralogy and palynofacies the Tithonian to Valanginian cored succession of the Fårarp-1 core provides complementary information on how marginal deposits from the eastern part of the epicontinental sea reflect the climatological and environmental changes observed in other parts of NW Europe.

The Fårarp-1 core shows that during the Tithonian to earliest Berriasian deposition took place in a terrestrial but near-marine depositional setting, in coastal lakes or lagoons with little marine influence. A dry climatic regime favoured stagnant water conditions with common algal blooms of primarily Botryococcus and zygnemataceae. Palynofacies and sedimentology indicate limited transport of freshwater and material to the basin. The stagnant depositional environment was terminated by a marine flooding in the early Berriasian. During the remaining Berriasian and the early Valanginian conditions shifted between near marine and marine settings in a dynamic coastal environment, similar to contemporaneous assemblages reported from the Danish Island of Bornholm.

A shift in clay mineralogy, from a dominance of 10 Å minerals to increasing amounts of mixed layer and kaolinite indicates a change to more humid conditions in the latest Tithonian. Cheirolepidacean pollen (Classopollis) are present but never common in the cored succession, and a similar conspicuous decrease of these pollen, as previously reported from England, Germany and France, is not evident in the Fårarp-1 core. Instead a subsequent shift in both palynofacies and palynoflora, marked by an increase in abundance of heavy terrigenous material, i.e. wood and coal particles, upland pollen grains and reworked palynomorphs is also observed in the uppermost Tithonian–lowermost Berriasian interval. At the same level spores and pollen classified as warmer/drier elements decrease in abundance. This is interpreted as representing a shift to more humid climatic conditions with increased runoff from the hinterland. Thus, the combined sedimentological and palynological data from the Fårarp-1 core suggest that climatic conditions in the area changed from more seasonally dry (semi-arid) to more humid (semi-humid) across the J/K boundary (latest Tithonian to earliest Berriasian) and hence earlier than the mid-Berriasian climatic shift recorded from e.g. England and the Netherlands.
TidsskriftPalaeogeography, Palaeoclimatology, Palaeoecology
Udgave nummer3-4
Sider (fra-til)445-475
StatusUdgivet - 2011
Eksternt udgivetJa

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