Oamaru Diatom Deposits



The objective of my study is to identify as many species as possible from 6,800 individually mounted diatoms in my collection from Oamaru. These were selected from 220 samples kindly given to me by colleagues in the Quekett Microscopical Society, among whom I am particularly indebted to the late Bernard Hartley and Klaus Kemp, also to A.R.Edwards in New Zealand. Most of the material was diatomite collected from 14 named exposures in a 16 x 4km area, among rolling hills, 5 – 10kms inland from the coastal town of Oamaru on New Zealand’s South Island.

I cleaned all my samples (see “Methods”) to separate individual frustules; material from each was examined in detail. The diatoms that I selected for identification were mounted individually by Klaus Kemp. I subsequently photographed and attempted to identify those in each mount. I made no attempt to make comparisons between the composition of samples from different outcrops within the Oamaru complex (see later section).


My study commenced in 1991 and I continued to acquire and search samples until 2000, with frequent breaks for other activities – I describe this as an “intermittent project”! Lately, I have been identifying the diatoms in my collection and labelling photographs. All photographic material is stored in my PC.

The existence of the Oamaru fossil marine diatom deposits was first publicised by Lautour (1888) and since then numerous Diatomists have sampled these exposures. They are now known to date from late Eocene to early Oligocene; 32 – 35 million years ago, and occur along with abundant foraminifera and calcarious nanoplankton (ref.).

The complex local stratigraphy has been described by Edwards (1991). He concluded that these extremely species-rich beds were deposited, over several million years, in a shallow, nutrient-rich tropical sea, with successive plankton blooms and subsequent speciation. The silica skeletons of the diatoms illustrated here do not decompose; they remain where they fell on the sea-bed and were covered and compressed by overlying layers of plankton to form a soft rock – “diatomite”; subsequently, they were further covered by volcanic deposits and earth movements. Remarkably, many of these diatoms remain intact and perfectly preserved over a period nearly one thousand times as long as from the birth of Christ to the present day.

This speciation resulted in, probably, the most diverse fossil marine diatom community ever recorded of over 700 species from about 120 genera. A few species have only been recorded from Oamaru (e.g. Kittonia sp.); others are known from widely dispersed Eocene – Oligocene fossil sites around the world – such as Barbados – supporting interconnected tropical seas during the late Eocene/early Oligocene (Redman and Carter). Some species are living today.

It has been reported by Edwards (1991) and Doig (1962) that the composition of the diatom flora varies markedly between samples taken from different points and depths within a site. This was confirmed by my own findings. Also, Edwards has shown that the abundance of some common genera differs between sites at Oamaru. He highlights Cocinodiscus, Stephanopyxus and Melosira varying as dominant species in this manner. I have not attempted to repeat these comparisons or to take them further with comparisons between the distribution of much rarer genera. During my search I have recorded less than ten individuals from a species and therefore it would require an impossibly long search to make any valid comparisons. Thus, I refer only to the presence of genera/species within the Oamaru complex as a whole.


Throughout my study I have followed the naming and order of diatom genera and species used in the monograph by Desikachary and Sreelatha (1989). They described and photographed 666 species (covering subspecies and varieties) which they had observed themselves in the collection at the British Museum. They also list a further 122 species that had been recorded by others but were not present in samples examined by themselves. Some of these may be accounted for as duplicates in which names have been changed and/or are misidentifications. I realise that there have been taxonomic revisions between 1989 and the present day. However, I have decided to use the naming by Desikachary and Sreelatha, without attempting to include recent revisions. This saves me as a relative newcomer and, hopefully, others, from too much confusion. Naming of species is difficult enough without having to follow changes in taxonomy.

Among the diatoms selected in my collection there are individuals that I have been unable to name so far. This is in part due to my inexperience (which, hopefully, is improving) but also because they don’t show the characters needed for identification. I will continue to examine these individuals; some will probably be named and later added to my list of species from this site. A very few of these species are new to Oamaru and they will be highlighted and discussed once verified. I should be very grateful for any criticisms and/or corrections you may have via my E-mail address (nigel.charles@pc2web.co.uk).

This website is presently under construction and there are still many photographs of different genera to be included. When this is complete it maybe thought worthwhile for others, willing to pass on their records, to loan slides or photographs of diatoms from Oamaru not recorded here. I think this is a decision for the future.