Introduction

The Oamaru District has long been regarded as one of the classic areas of New Zealand Tertiary geology due to the variety of lithologic types it contains and the ease with which well preserved fossils belonging to many different groups can be collected from certain horizons. This paper is an attempt to synthesize the evidence provided by these diverse fossil groups for marine climate in this particular area during the late Kaiatan (upper Eocene), Runangan (upper Eocene) and the early Whaingaroan (lower Oligocene).

The distribution and climatic implications of the various groups were provided by Messrs A. J. Doig (diatom occurrences only), N. de B. Hornibrook (Foraminifera), I. W. Keyes (scleractinian corals), P. A. Maxwell (Mollusca) and their generous co-operation has made this paper possible. The writer is responsible for the climatic implications suggested for the diatoms, the occurrence and implications given for the calcareous nannoplankton and for the arrangement of the data in stratigraphic order.

Ages of Stratigraphic Units

Occurrence and Significance of Fossil Groups

The stratigraphic distribution and climatic implications of species and genera considered useful in estimating the local climate during the late Kaiatan, Runangan and early Whaingaroan are page 76

Table 1: Occurrence of temperature indicative fossils in the late Kaiatan to early Whaingaroan strata of the Oamaru district.

page 77 given in Table 1. Brief comments on the occurrence and significance of each fossil group are given below.

Although scleractinian corals probably occur in every formation dealt with here the coral evidence is entirely based on selective species present in small assemblages in the Waiareka and Deborah Volcanic Formations owing to the paucity of the fauna in the other formations. The corals seem to have a preference for tuffs but this may be due to collector bias resulting from their conspicuousness or better preservation in tuffs. The absence of hermatypic (reef building) corals almost certainly indicates the absence of truly tropical conditions since shoal areas almost certainly existed during this period in what is now the coastal area.

The molluscan evidence for marine climate is, of necessity, based on genera selected from special assemblages associated with particular facies. For example the only molluscan evidence available for late Kaiatan climate comes from the rather unique assemblage occurring in the Waiareka Volcanic Formation at Williams’ Bluff (Lorne). Mollusca are rare in the Oamaru Diatomite and, apart from the well known early Runangan Trig M. locality, are also rare in the Totara and McDonald Limestones. The Deborah Volcanic Formation contains common Mollusca only at Everett's Quarry and Gee's Point. At first the molluscan evidence appears to indicate fairly uniform near-tropical conditions throughout the entire period. However the upward decrease in the number of genera prefering tropical or subtropical conditions plus the distribution of Hyalocylis, a pelagic which should be independent of depth changes, suggests that the temperature gradually declined as time passed.

The Foraminifera is the only group that occurs abundantly in every stratigraphic unit discussed in this paper. However even these ubiquitous microfossils are sporadic in their distribution within the Waiareka and Deborah Volcanic Formations and are represented in the former by special assemblages at Williams’ Bluff (Lorne). Forrester's Hill and Fortification Hill. Further details of the climatic implications and occurrence of Amphistegina are given by Hornibrook (this issue). Asterocyclina and Hantkenina became extinct within the Runangan according to Hornibrook, 1958. The absence of Lepidocyclinidae in the Whaingaroan (Hornibrook, (this issue) suggests that the marginally tropical conditions formerly prevailing did not extend into the Whaingaroan.

The calcareous nannoplankton are extremely abundant in the Oamaru Diatomite and are often common in certain, usually tuffaceous, horizons within the Totara and McDonald Limestones. They do not occur in the Waiareka or Deborah Volcanic Formations. The prominent upward increase in the abundance of Ericsonia ovalis sensu lato and the upward decrease in Braarudosphaera bigelowi clearly suggests gradually declining temperatures (see Edwards, this issue).

The diatoms are totally restricted to the Oamaru Diatomite where they are often abundant (especially in the lower part). The upward decrease in the abundance of Stephanopyxis grunowii, a close relative of S. turris which, according to Crosby and Wood, 1958 is a East Australian Current indicator, forms the main evidence provided by these diatoms for changing climate.

Conclusions

The evidence for marine climate provided by the planktonics suggests that subtropical conditions prevailed during the late Kaiatan and early Runangan whereas a warm temperate climate probably occurred in late Runangan to early Whaingaroan time. On the other hand the benthonic organisms strongly suggest that marginally tropical conditions occurred during late Kaiatan to early Runangan time and that subtropical conditions probably prevailed during the late Runangan and early Whaingaroan. The discrepancies between the planktonic and benthonic evidence are probably due to the different habitats they occupy.

The oxygen isotope estimates obtained by Devereux (this issue) for “surface’ temperatures using planktonic Foraminifera appear, as would be expected, to follow fairly closely the estimates obtained using the temperature implications of the planktonic groups. However there are serious inconsistencies between Devereux's estimates for bottom temperatures and those indicated by the benthonic groups. Although this disagreement may be due to sampling of different habitats (only one locality in common) this explanation seems unlikely since Devereux's estimate for a sample in which abundant Asterocyclina having tropical implications occur is 12.8° Centigrade.

Discussion

Mr. I. Devereux. The isotope results I gave earlier which have been cited by Mr. Edwards should possibly be clarified. The Runangan results are from planktonic foraminifera whereas the Whaingaroan results are from benthonic material because the formation contained practically no planktonic material. The difference in temperature between planktonic and benthonic material in all the formations sampled in the Oamaru area was only two to three degrees so the Whaingaroan results should be near the true surface temperature and hence suggest a marked cooling compared with the Runangan results.

Mr. A. R. Edwards. Looking at the various groups studied it seems that the planktonic organisms give a better picture than the benthonics.

Mr. I. Devereux. I think this could be extended to the other papers given so far. The planktonic organisms appear more sensitive to temperature change than the benthonic.

Dr. N. deB. Hornibrook. I think that Mr. Edwards has shown us the value of working in detail on one area. Especially if various people can work on different animals in the same beds and then see where the results agree or don't agree.