Abstract
Foulden Maar is a latest Oligocene–earliest Miocene maar–diatreme volcano located in a vent cluster of the monogenetic Waipiata Volcanic Field in Otago, New Zealand. The present–day erosion level exposes a 1000 x 650 m wide maar crater that is filled by fossiliferous diatomite. At nearby Gladsmuir, stratified phreatomagmatic pyroclastic tuff, lapilli tuff and breccia of the upper diatreme zone of a second maar–diatreme volcano are capped and transected by coherent basanite that indicates late–eruptive magmatic explosive and effusive activity. Other volcanic remnants in the vicinity represent eroded lava plugs and lava flow units of similar age but without genetic relation to Foulden Maar. The pre–eruptive stratigraphy above the schist basement is largely eroded in the area but a sedimentary sequence consisting of unconsolidated late Eocene–Oligocene glauconitic sands and fluvial–palustrine sediments of presumably late Oligocene–early Miocene age (Dunstan Formation) can be reconstructed from local erosion remnants and accidental lithic clasts in maar tephra at Gladsmuir and reworked maar tephra at Foulden Maar.
At Foulden Maar, a ~188 m thick crater–infilling sequence was investigated in surface pits and two drilling cores. Four distinct lithozones (LZ 1–4) were recognized, each including a characteristic assemblage of lithofacies types that reflect the post–eruptive sedimentation history of the maar crater and the environmental factors that operated during the crater–infilling period. A massive breccia rich in country rock fragments in an ash and mud matrix in the basal LZ 1 was deposited by a voluminous, cohesive mass–flow in the central part of the immediate post–eruptive crater, when the crater walls and the inner slopes of the tephra rim were steep and unstable. A 62 m thick sequence of massive or graded, coarse to fine–clastic lithofacies types in lithozone 2 represents subaqueously emplaced debris flow and turbidity current deposits. The existence of a maar lake at this depositional stage is evidenced by textural features of lithofacies types in LZ 2 and by limnic microfossils such as sponge spicules and algal remains. Mainly dense and blocky, vitric ash and lapilli–sized clasts that are present in LZ 2 are reworked juvenile clasts of the tephra rim and exhibit textural features that are in agreement with a phreatomagmatic explosive eruption style at Foulden Maar. Quartz sand grains and glauconite, mud– and sandstone clasts and lignite fragments found in mass–flow horizons in LZ 2 represent accidental lithic clasts of the tephra rim and support inferences about the pre–eruptive stratigraphy from field mapping. Facies types in lithozone 3, at 115.18–101.55 m in core FM 2, include massive and stratified clastic mass–flow deposits with smaller grain–size and bed thickness than in LZ 2. They reflect sedimentation mainly from turbidity currents and minor cohesive debris flows from proximal slopes of the maar lake basin. Deep, meromictic conditions and seasonally increased bioproductivity within the maar lake are evidenced by thin intervals of laminated diatomite in LZ 3. Lithozone 4 consists of 106 m of laminated and non–laminated diatomite that indicates a biogenic fall–out sedimentation mainly controlled by bioproductivity of diatoms and sponges, and sporadic mass–flow events within the maar lake basin. The preserved lamination, the presence of organic matter, articulated fish and insects, and the absence of bioturbation confirm a calm and anoxic sedimentary environment at the lake bottom. The constant composition and thickness of lamination throughout LZ 4 reflects stable environmental conditions over a period of ~127,000 years, during which time mass–flow events decreased in frequency due to infilling of the lake basin. At this depositional period, the lake was surrounded by a subtropical evergreen forest. The lack of fluvial and eolian sediments in the studied sequence indicates a small catchment area for the crater sedimentation and a hydrologically closed system for the maar lake. Four tephra beds present in LZ 3 and 4 indicate nearby strombolian eruptions contemporaneous with maar lake sedimentation.
In addition to previous paleontological studies that were mainly focussed on plant fossils, numerous new arthropods, fish, and trace fossils discovered and described in this study provide valuable new insights into the paleoecosystem at Foulden Maar and a novel perspective on the terrestrial biodiversity of Zealandia about 23 million years ago. Arthropods were a key component of the forest ecosystem at the maar lake, including representatives of the Coleoptera, Hymenoptera, Isoptera, Hemiptera, Diptera, possibly Blattodea, Araneae and Acari. Among 13 confirmed insect families, the curculionid beetles (weevils) and Formicidae (ants) are most diverse. The majority of insects are ground–dwelling, forest litter and wood–living taxa with low dispersal ability. Flying and aquatic insects are rare. Arthropod–plant interactions on plant fossils include external foliage feeding, galling, leaf mining, piercing–and–sucking and seed predation as well as foliar domatia and flowers with insect pollination syndrome, which indicates a well established terrestrial ecosystem in Zealandia at the Oligocene–Miocene boundary. The taxonomic composition of the arthropod taphocoenosis is in agreement with a warm temperate to subtropical paleoclimate. Collectively, these fossils represent the first pre–Quaternary terrestrial arthropod fauna from New Zealand.
The first evidence of larval stages of Galaxias effusus Lee, McDowall & Lindqvist 2007 supports previous suggestions that this fish species was non–migratory and locked to the maar lake. The predominance of juvenile stages of this species is likely related to seasonal diatom blooms. Anguilla sp. described in this study represents the first non–European fossil of freshwater eels. It contradicts previous suggestions of an absence of Anguilla from Miocene New Zealand and challenges the timing of radiation of freshwater eels determined from molecular phylogeny. Three morphotypes of coprolites can be linked to ontogenetic stages of Galaxias effusus and Anguilla sp. but most of the morphologically and compositionally diverse coprolite types found in this study represent vertebrate taxa that are currently not known from body fossils. Together, the fossil plants and animals from Foulden Maar provide a unique window into a mid-latitude, low elevation southern New Zealand terrestrial ecosystem. Some of the plants and animals have close living representatives in the modern New Zealand biota, but many taxa are now extinct in New Zealand.
This is the first detailed account of a pre-Quaternary maar deposit in the Southern Hemisphere.