Outcrops of Miocene coral-rich limestones mapped as Trealla Limestone are widespread along the coast of north-west Western Australia. Despite their central importance in Australian Miocene stratigraphy, the age of these outcrops is poorly constrained as they were only dated in their type area, and as the age assignation was based only on off-centred sections of sparse planktonic foraminifera and a broad correlation based on larger benthic foraminiferal species.
This study aims to refine the dating of the Trealla Limestone in its type area, and to provide the first dating of outcrops located elsewhere in the Cape Range Anticline. Results are based on microscopic observations made on large acetate peels, which allowed the observation of numerous planktonic foraminifera, and also of serial sections of species.
The coral-rich deposits mapped as “Trealla Limestone” can be divided into three age-based units in the Cape Range Anticline: (1) upper Burdigalian, (2) Langhian to lower Serravallian, and (3) Serravallian. This revised stratigraphic framework provides a standard for future correlations with sections located elsewhere in Australia. The Trealla Limestone at its type section on Mt Lefroy contains mainly upper Burdigalian strata of the mid-neritic zone, with the top of the formation marked by an erosion surface. It may in part be equivalent to the Tulki Limestone mapped elsewhere in the anticline. Further north in the anticline, the Trealla Limestone ranges stratigraphically higher into the Langhian and Serravallian in shallower water facies. An influx of quartz sand in the Cape Range Anticline, probably indicative of a wetter environment in the hinterland to the east, started during or after the Serravallian.
This study also brings new insight into the environmental and stratigraphic ranges of larger benthic foraminifera from north-west Western Australia. Austrotrillina asmariensis was recorded above levels with Orbulina universa. This suggests that the species has a more extended stratigraphic range in this area than in Asia. A re-examination of the soritids reveals that Marginopora, previously recorded here, is not present in the deposits. Borelis is replaced by Flosculinella after the end of the Te Letter Stage, a zone based on larger benthic foraminifera recognized throughout the Indo-West Pacific region. The morphology of Flosculinella may be environmentally controlled, with the most globular specimens more abundant in protected and shallow environments. Finally, Cycloclypeus from the Cape Range Anticline had a similar bathymetric range to its modern analogue found in the lower part of the photic zone, as indicated by its co-occurrence with minute planktonic foraminifera.