Mid–Cretaceous South Polar Palaeoenvironments and Eastern Gondwanan Rifting (~80°S): New Data and Insights from the Tupuangi Formation, Chatham Islands, Southwest Pacific

The mid–Cretaceous was an interval of climactic tectonic, biotic and climatic change across Gondwana. Supercontinental break-up was nearing completion, the global greenhouse climate was reaching an unparalleled zenith, and the rise of angiosperms caused a global floral turnover which far surpassed the Cretaceous–Paleogene mass extinction.

Albian–Turonian strata from the Chatham Islands represent the southernmost–latitude palaeoenvironments ever studied to date; recent palaeogeographic reconstructions estimate a latitude of ~80°S at the time of deposition. The Chatham Islands is an island complex ~860km east of Christchurch, New Zealand, near the eastern end of the submerged subcontinent “Zealandia”. The rifting of Zealandia began during the Albian which formed a sedimentary graben-basin between Zealandia and Marie Byrd Land, West Antarctica. New data support the notion that this rifting occurred in a pulsed, two–stage process: firstly, a failed–rift system resulting in the Bounty Trough (~105–99 Ma); and the second stage culminated in the ongoing rifting of Zealandia from West Antarctica during the Late Cretaceous (since 99 Ma). Biostratigraphic analysis of the Tupuangi Formation indicates the onset of sedimentation during the late Albian (105 Ma). This is incongruent with current palaeomagnetic seafloor data between Antarctica and Zealandia, suggesting that the Chatham Islands' mid–Cretaceous strata accumulated during the earlier failed–rifting. Sedimentological analyses reveal a spectrum of terrestrial settings (deltaic, lacustrine and fluvial with minor, intermittent coal beds). There is no reliable marine sedimentological or palaeontological evidence, suggesting that the failed rifting produced a system of elongated intra–continental rift-lakes analogous to that of the modern East Africa Rift complex.

The fossil content comprises a purely terrestrial biotic assemblage, with diverse, well–preserved gymnosperm, fern and angiosperm leaf compressions, spore and pollen as well as probable coleopteran insect remains. Future work on these fossils will help elucidate the climate and floral adaptations to the intrinsic and seasonal polar extremes near the mid–Cretaceous South Pole. The Chatham Islands provide a palaeontological window into terrestrial polar environments and biotas during this critical period of floral evolution, the most southern mid–Cretaceous biotic outpost yet discovered.