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.