Vegetation communities of a high palaeolatitude Middle Jurassic forest in New Zealand

Abstract

Fossil evidence of a high southern palaeolatitude (approximately 75–78° South) Middle Jurassic (Temaikan) forest from Gondwana is investigated from the Murihiku Supergroup exposed at Kawhia Harbour, North Island, New Zealand. Horizons of fossilised tree stumps, preserved in growth position and rooted within coal-rich layers at bedding contacts, are exposed within cross-bedded medium-grained sandstone on the present-day shore platform. Interbedded siltstone and fine-grained sandstone contain terrestrial palynomorphs, fossilised leaves and other macroflora material (including cones and seeds). The sediments are interpreted as three fluvial facies ( vegetated bar top, major sheet flood and channel fill and migration) within a sandy braided river palaeoenvironment. The fossil flora suggest the original vegetation was multi-layered and dominated by gymnosperms and ferns. Lycopods, bryophytes, fungi and algae also grew on the forest floor and in bar top pools. A model of plant communities is presented reflecting changing habitats up the side of interdistributary bars. The model is based on stratigraphically successive taphocenoses within vegetated bar top facies and the preferred growing conditions and life forms of comparable modern relatives. Lycopods and bryophytes on the saturated channel banks became less frequent higher up the bar on the damp, partially drained soil preferred by ferns and cycad trees. Favouring the drier bar top, arborescent gymnosperms thrived around probably ephemeral pools containing lycopods, bryophytes and algae. The characteristics of this fossil forest at Kawhia Harbour compare well to a similar Middle Jurassic site at Curio Bay in South Island, New Zealand, and other high southern palaeolatitude fossil floras from the late Mesozoic. A humid, warm climate with high average annual rainfall and large storms is interpreted from the Kawhia Harbour fossil forest for the New Zealand Middle Jurassic. These palaeoclimatic conditions are comparable to the modern mesothermal zone.