The distribution and composition of volcanic glass in late Quaternary loess deposits of southern South Island, New Zealand, and some possible correlations

Abstract

Late Quaternary loess deposits, subdividable into four layers (loesses 1–4), are widespread over lowland areas of southern South Island. Their accumulation is assumed to have been during cold climate episodes, but previously a chronology for the loess and direct correlation with the loess stratigraphy established elsewhere in New Zealand has not been possible. Tephra layers that are widespread in central New Zealand potentially provide a means of correlation. Aokautere Ash has previously been traced as far south as Timaru, and Mt Curl Tephra has been located in the southern Pacific Ocean to the southeast. Thus, volcanic glass was separated from three stratigraphic levels in loesses 1, 2 and 3 in the southern South Island loess. The chemical composition of the glass is more heterogeneous than found elsewhere and appears to be formed from discrete groups of shards mixed, probably mechanically, from different volcanic events. Homogeneous subgroups of analyses match the glass chemistries of four widespread North Island tephras—Aokautere Ash (c. 22 000 years), Griffins Rd upper and lower tephras (c. 150 000–200 000 years), and Mt Curl Tephra (c. 250 000–300 000 years). On the basis of stratigraphic position and these chemical similarities, correlations with the North Island loess stratigraphy are possible. Thus, loesses 1–3 recognised in Stewarts Claim Formation in Southland may have been deposited during δ¹⁸O stages 2, 6, and 7 or 8, respectively (i.e. between c. 12 000 years and c. 300 000 years ago). This finding suggests that southern South Island loess is older than previously supposed, and that there are fewer layers preserved than elsewhere in New Zealand. Stone lines and the absence of paleosols suggest that wind erosion may have removed whole layers of loess from some areas, and redeposition may have been responsible for the reworking of glass shards into younger loess.