Proximal stratigraphy and event sequence of the c. 5600 cal. yr BP Whakatane rhyolite eruption episode from Haroharo volcano, Okataina Volcanic Centre, New Zealand

Abstract

The c. 5600 cal. yrBP Whakatane eruption episode consisted of a sequence of intracaldera rhyolite eruptions from at least five vents spread over 11 km of the Haroharo linear vent zone within Okataina Volcanic Centre. Initial ventopening eruptions from the Haroharo vent produced coarse lithic clast "blast beds" and pyroclastic density currents (surges). These were immediately followed by eruption of very mobile pumiceous pyroclastic surges from the Makatiti vent 6 km to the southwest. Major plinian eruptions from the Makatiti vent then dispersed Whakatane Tephra pumice fall deposits (bulk volume c. 6 km 3 ) across the northeastern North Island while smaller explosive eruptions produced pyroclastic flows and falls from the Haroharo-Rotokohu vents and at the Pararoa vent on the caldera rim 11 km northeast from Makatiti. The pyroclastic eruptions at all vents were followed by the extrusion of lava flows and domes; extruded lava volumes ranged from 0.03 km 3 for the Pararoa dome to 7.5 km 3 for the Makatiti-Tapahoro lava flows and domes. Minor variations in whole rock and glass chemistry show that the three main vent areas each tapped a slightly different high-silica rhyolite magma. About 10 km 3 of M-type magma was erupted from the Makatiti-Tapahoro vents; c. 1.3 km 3 of H-type magma from the Haroharo-Rotokohu vents, and 0.04 km 3 of P-type magma from the Pararoa vent.There are no significant weathering or erosional breaks within the Whakatane eruptive sequence, which suggests that all Whakatane eruptions occurred within a short time interval. However, extrusion of the Haroharo dome within the Makatiti pyroclastic eruption sequence suggests a duration of c. 2 yr for the main pyroclastic eruption phase. Emplacement of the following voluminous (7.5 km 3 ) lavas from the MakatitiTapahoro vents would have occurred over >10 yr at the c. 10-20 m 3 /s inferred extrusion rates.

Tephrochronology and its application: A review

“…Detailed studies by EPMA of thick sequences of proximal rhyolitic to rhyodacitic tephras erupted from Okataina volcano in northern New Zealand have revealed much more compositional diversity and heterogeneity within individual lapilli-sized clasts and at different azimuths around the volcanic centre than previously recognised (Smith et al, 2002Nairn et al, 2004;Kobayashi et al, 2005;Shane et al, 2005Shane et al, , 2008a. This heterogeneity, sometimes subtle, is a consequence of the mingling of separate batches of magma that were tapped simultaneously or sequentially, accompanied by changes in wind direction, as eruptions proceeded.…”

“…Intense, highly localized precipitation through an ash-bearing air mass would also deposit far more ash into a catchment by wash-out than would result from dry-air deposition alone, as shown by the Hekla 1947 eruption, when precipitation throughout southern Finland resulted in some of the heaviest and most localized deposition of ash along the eruption plume route (Pyne-O"Donnell, in press). Similarly, rain-flushing through co-ignimbrite ash clouds was invoked by Kobayashi et al (2005) to explain an occurrence of overthickened tephra beds in northern New Zealand. Pyne-O"Donnell (in press) noted that each factor additionally would be modulated by the general longer-term climatic regime (e.g., stadial versus interstadial).…”

“…An aeolian origin for such material both in New Zealand and elsewhere has been supported subsequently by grain-size, mineralogical, and quartz isotopic analyses (e.g., Benny et al, 1988;Mizota et al, 1990;Alloway et al, 1992;Yoshinga et al, 1992). Rain flushing through ash clouds may also lead to locally-overthickened tephra deposits (e.g., Kobayashi et al, 2005).…”

Geochemistry and magmatic properties of eruption episodes from Haroharo linear vent zone, Okataina Volcanic Centre, New Zealand during the last 10 kyr

“…Each eruption episode produced 1-3 km 3 (all volumes are in dense rock equivalent) of pyroclastics and >5 km 3 of lava. Explosive activity was predominantly magmatic, producing~30 km-high plinian columns that dispersed ash across much of the central North Island (e.g., Lowe 1988;Lowe et al 1999;Sandiford et al 2001;Shane and Hoverd 2002), and localised (<10 km from vent) pyroclastic density currents (e.g., Kobayashi et al 2005;Nairn et al 2001). Effusive activity generated a chain of overlapping lava domes and coulees, which form the 800 m-high Haroharo Volcanic Complex (Fig.…”

“…2 and 3c). The stratigraphy and sequence has been documented in detail by Kobayashi et al (2005). The eruption episode occurred from three vents spread over~11 km of the HLVZ.…”