INVESTIGATION OF AN AGGRADING PALEOSOL DEVELOPED INTO ANDESITIC RING-PLAIN DEPOSITS, RUAPEHU VOLCANO, NEW-ZEALAND

Within a sequence of andesitic volcaniclastic deposits on the northeas tern ring plain of Ruapehu volcano is a ca. 10 m-thick sequence of wea thered andesitic tephras. Weathering and paleosol development is most evident in 3.6 m of fine ash in this sequence. The ages of these tephr as are constrained between ca. 23-70 ka by dated rhyolite tephras erup ted from central North Island volcanoes. Mineralogy of the fine ash de posits reveals their origin, and the processes involved in their soil development. The fine ash deposits are almost totally locally derived either as primary volcanic ash or fines reworked from the ring plain i tself by aeolian processes. Aerosolic quartz input associated with reg ional loess deposition during the cool climatic episodes of mid-delta( 18)O stage 3 and delta(18)O stage 4 is very low, having been diluted b y rapid accumulation of andesitic tephras in these episodes. The obser ved weathering features and secondary minerals within the ash sequence were derived from a complex combination of factors including climate change, accretion rate, and post-depositional modification. Relatively strong weathering development in two parts of the ash sequence is cor related with two widespread soil development episodes during the Last Glacial observed throughout the southern North Island. The accretion r ate of the soil surface at these times also affected the expression of climate-related weathering. Formation of allophane (with an Al:Si rat io of 2.1) and ferrihydrite occurred near the soil surface as the ash was accreting. The amount of allophane and ferrihydrite through the se quence appears to be inversely related to the accretion rate of the so il surface. Upon burial of the ash materials by a thick (>20 m) sequen ce of lahar and tephra deposits, halloysite was later formed in the bu ried ash. The leaching of silica from the thick overburden of volcanic lastics into the ash material as well as perched water is thought to h ave decreased the Al:Si ratio in the soil solution and thus promoted t he formation of halloysite from weathering andesitic glass.