Abstract:
This thesis examines the chronology of the Kokoamu Greensand and associated formations at Landon Creek, southeastern South Island. Landon Creek is the type locality for the Duntroonian Stage (late Oligocene). Biostratigraphic and recent chemostratigraphic evidence is reviewed and new data is provided from paleomagnetic measurements and studies of glaucony. The latter have provided a sound basis for K-Ar age determinations. A correlative greensand in the upper Aotea Sandstone, Waitomo, centra North Island is also studied and dated.
A 24m-thick section at Landon Creek comprising the Totara Limestone (bottom - 1.5m thick), Kokoamu Greensand (7.5m), Otekaike Limestone (7m) and Gee Greensand (top - 8m) was selected for detailed study on the basis of good exposure of all formations. Three samples from 109 horizons were taken at 20cm intervals up the section for paleomagnetic analysis, of which 88% were found to contribute usefully to a magnetostratigraphy. However, the number of reversals in the corresponding interval of the Geomagnetic Polarity Time Scale, and the error associated with the available datums, prevented a unique correlation.
Thin section and electron microprobe analysis of greensand samples (3 from the Kokoamu Greensand, 2 from the Gee Greensand and 3 from the upper Aotea Sandstone) showed that the glaucony was predominantly detrital and evolved (terminology from Odin, 1982). Density and magnetic separation techniques were used to obtain pure detrital glaucony separates of 3 samples (and a duplicate) for K-Ar dating. Stratigraphically consistent 'moment of closure' ages were obtained for all 4 samples, being for the basal Kokoamu Greensand 24.8 and 24.7 +/- 0.4 Ma (duplicated), for the upper Kokoamu 24.2 +/- 0.3 Ma, and for the upper Aotea Sandstone (biostratigraphically correlated with the Kokoamu) 24.4 +/- 0.3 Ma. These ages represent a time after the glaucony had evolved and become an isotopically closed system through burial, but pre-date its redeposition, and thus must be regarded as a maximum age for the greensand. The basal Kokoamu age (base of the Duntroonian) is 2.5 m.y. younger than recent age estimates from 87Sr/86Sr analyses from the basal Kokoamu Greensand at Squire's Farm, 50km to the north of Landon Creek, suggesting that the shell material on which the analyses were made was reworked.
The depositional age of the basal Kokoamu Greensand at Landon Creek is constrained by the 'moment of closure' age of detrital glaucony within the formation to lie between 24.8 and 24.2 Ma, while that of the upper Aotea Sandstone, Waitomo, must be less than 24.4 Ma. The lower age of 24.8 Ma gives an upper age limit for the Marshall Unconformity at Landon Creek. Formation of the glaucony is interpreted as relating to the latest Oligocene transgression following the series of mid-Oligocene sea level lowstands (Haq et al, 1988).
