Plain Language Summary
Highly diverse and exceptionally well-preserved Jurassic fossil plants are found in sedimentary rocks of Mount Flora, a classic geological localities of the Antarctic Peninsula, firstly discovered by a Swedish expedition in 1903. The geology indicates a dynamic landscape of lakes, emerging mountain chains and active volcanoes that harbored a diverse plant community of horsetails, ferns, seed ferns, conifers and cycads. Remarkably similar Jurassic flora once covered the northern Patagonia at a time when both regions were part of the Gondwana supercontinent and were dominated by a warm, humid climate. Our high-precision radioisotopic age determination by the U-Pb technique, along with a collection of more than 1400 new plant specimens indicate that this plant association dominated the Antarctic Peninsula in late Middle Jurassic, nearly 17 million years after it disappeared in Patagonia. Therefore, the composition of the Jurassic flora was strongly dependant on climatic and environmental conditions as the main driver of floral migrations, as opposed to the result of simple, simultaneous evolution of the plants throughout the southern hemisphere. Our results further portrait a period of tectonic upheaval that led to the formation of new sedimentary basins and culminated in the break-up of Gondwana in the Late Jurassic and Cretaceous.
1 Introduction
The Mount Flora of the Hope Bay area (Fig. 1) near the northern tip of the Antarctic Peninsula is one of the most famous geological and geographical localities of Antarctica, known for its diverse and remarkably well preserved Jurassic fossil flora, which is found in the strata of the Mount Flora Formation. Because of the historical, biostratigraphic and paleoecological significance of the flora, the plant-bearing outcrops are included in the Antarctic Specially Protected Area (ASPA) 148 of the Antarctic Treaty. The age of the paleoflora has been broadly assigned, ranging from the Early Jurassic (e.g., Rees, 1993a; Rees and Cleal, 2004) to the Early Cretaceous (e.g., Gee, 1989), on the basis of transcontinental correlations. Besides its biostratigraphic significance, the precise age of the paleoflora is critical to the depositional history of the Larsen Basin in the Antarctic Peninsula (Fig. 1). A latest Jurassic to earliest Cretaceous age interpretation (e.g., Farquharson 1984; Macdonald et al., 1988; Medina et al., 1989) would suggest that the Larsen Basin developed as a back arc to the emerging Early Cretaceous arc along the Proto-Pacific margin of Gondwana. On the other hand, an Early Jurassic age assumed by others (e.g., Hathway, 2000) would invoke tectonic linkage to the continental rift basins of similar age throughout southern South America. Recent U-Pb zircon geochronology by in situ techniques (Pankhurst et al., 2000; Hunter et al., 2005) proposed a Middle Jurassic age for the Botany Bay Group, which incorporates the Mount Flora Formation, on the basis of ages from Hope Bay (Mapple Formation), Botany Bay (Camp Hill Formation) and Tower Peak (Tower Peak Formation) localities (Fig. 1). Although highly informative, these age data lacked the necessary resolution to distinguish among individual formations. No direct radioisotopic age has been reported from the Mount Flora Formation at Hope Bay in spite of the conspicuous tuffaceous intercalations in the lower part of the succession recognized since the pioneering work of Croft (1947) in Bibby (1966) and Elliot and Gracanin (1983).
Here we report two high-precision U-Pb zircon dates by the chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) from tuff deposits intebedded within the Mount Flora Formation, with clear stratigraphic relationship to the highly fossiliferous strata. The tuffs were sampled during the 2017 austral summer field season in the Hope Bay area together with a large collection of fossil plants (c. 1400 hand samples). The results yield a reliable temporal framework, though preliminary, for the Mount Flora Formation and its paleoflora, and allow a biostratigraphic and paleoecologic analysis of the flora in comparison to analogous Jurassic assemblages from Patagonia. Finally, the depositional history of the Botany Bay Group succession in the context of Jurassic tectonic evolution of southern Gondwana is discussed.