The Foulden Maar Diatomite, a terrestrial Konservat-Lagerstätte in southern New Zealand

by Uwe Kaulfuss and Daphne Lee |

In green fields grazed by sheep and cattle at Foulden Hills near the township of Middlemarch in Otago, southern New Zealand lies a small, inconspicuous diatomite pit which holds the most significant record of terrestrial life in Zealandia at the beginning of the Miocene period (~23 mya). Dark brown when fresh and white/beige when weathered, the finely laminated diatomite holds a plethora of exquisitely preserved fossil leaves, fruits, flowers, spiders, insects and freshwater fish of a long-vanished lake and evergreen forest ecosystem (Lee et al. 2022). In drilling cores, the diatomite deposit is 120 m thick but only the uppermost 10 m are exposed at the surface.

The localized, near circular occurrence of diatomite near Middlemarch is the sedimentary infill of a small (1500–2300 m diameter) but deep (200–300 m) ancient maar lake that occupied a crater formed by violent phreatomagmatic eruptions when rising basanitic magma interacted explosively with groundwater close to the pre-eruptive landscape surface. The partly eroded maar-diatreme volcano lies within the Dunedin Volcanic Group, an intraplate volcanic field made up of hundreds of small volume eruption centres and one, much larger, composite volcano, the Dunedin Volcano on which is situated the city of Dunedin, home to New Zealand’s oldest University, the University of Otago (Scott et al. 2020).

The existence of a small deposit of ‘diatomaceous earth’ in the area was first mentioned in 1875 when mineralogist George Ulrich and paleontologist Frederick Hutton published a Report on the Geology and Gold Fields of Otago. It is not clear if either of them actually visited the locality, but they (probably Hutton) wrote that ‘the sample is chiefly made up of three or four minute species of Cymbella or Cocconema’ and ‘although I have not been able to determine any of them, I have no doubt but that they are freshwater species’ (Hutton & Ulrich 1875).

The dominating diatom at Foulden Maar is Encyonema jordaniforme Krammer 1997, siliceous frustules of which are the principal component of the diatomite (Harper et al. 2019). This pennate and possibly mucilaginous freshwater diatom must have entered the Foulden Maar lake system at an early stage (possibly brought in by waterfowl) and from then on it completely dominated the algal ecosystem over the lake’s lifespan of ~130.000 years. Each spring/summer, Encyonema and other algae bloomed in vast numbers on or near the lake surface and, after death sank slowly through the water column to form a 0.5–1mm thick, evenly spread blanket across the anoxic lakebed. In wetter seasons (autumn/winter), thin, black laminae of clay, organic matter and freshwater sponge spicules were deposited on the lake floor. These dark winter laminae are in stark contrast to the white summer laminae, giving the diatomite a ‘barcode-like’ appearance.

The exceptional preservation of fossils at Foulden Maar may be due in part to the mucilaginous nature of the diatoms, which, coupled with anoxia and the absence of bioturbation and currents in the bottom waters, effectively “pickled” organic material such as leaves, flowers and insects. Several other diatom taxa, both pennate and centric, occurring in minor amounts in the Foulden Maar diatomite are yet to be identified.

References

Harper, M.A., van de Vijver, B., Kaulfuss, U. & Lee, D.E. 2019. Resolving the confusion between two fossil freshwater diatoms from Otago, New Zealand: Encyonema jordanii and Encyonema jordaniforme (Cymbellaceae, Bacillariophyta). Phytotaxa, 394 (4): 231–243.

Hutton, F.W. & Ulrich, G.H.F. 1875. Report on the Geology and Gold Fields of Otago. Mills, Dick & Co., Dunedin, 244 pp.

Krammer, K. 1997. Die cymbelloiden Diatomeen. Eine Monographie der weltweit bekannten Taxa. Teil 2. Encyonema part, Encyonopsis and Cymbellopsis. Bibliotheca Diatomologica, 37: 1–469.

Lee, D.E., Kaulfuss, U. & Conran, J.G. 2022. Fossil Treasures of Foulden Maar: A window into Miocene Zealandia. Otago University Press, Dunedin, 216 pp.

Scott, J.M., Pontesilli, A., Brenna, M., White, J.D.L., Giacalone, E., Palin, J.M. & le Roux, P.J. 2020. The Dunedin Volcanic Group and a revised model for Zealandia’s alkaline intraplate volcanism. New Zealand Journal of Geology and Geophysics, 63 (4): 1–20.

Fig 1 – Near-circular Foulden Maar surrounded by schist tors and diatomite pit near the centre.

Fig 2 – A team of paleontologists in the diatomite pit at Foulden Maar.

Fig 3 – Cross-section of Foulden Maar diatomite with fine ‘barcoding’ lamination. Height of section 6 cm.

Fig 4 – SEM image of Foulden Maar diatomite showing densely packed frustules of pennate diatoms.

Fig 5 – Siliceous frustule of the pennate diatom Encyonema jordaniforme Krammer 1997.