Studies of the earthquake flat breccia formation and other unwelded pyroclastic flow deposits of the central volcanic region, New Zealand

Abstract

The Earthquake Flat Breccia is a rhyolitic ash and pumice deposit of unwelded pyroclastic flow deposition, covering a 110 km2 area surrounding Earthquake Flat situated 16 km south-east of Rotorua. The Formation consists of at least l4 pyroclastic flow units, plus intercalated airfall units, erupted from a fissure marked by a 5 km line of explosion craters of which Earthquake Flat is the largest. The breccia has physical characteristics typical of unwelded pyroclastic deposition - very poor sorting, non-stratification of flow units, and intercalation of airfall units. Grain size distribution studies show the Earthquake Flat Breccia to fall in the ash-flow size range. Tephra stratigraphy demonstrates that the Earthquake Flat Breccia Formation was erupted immediately following the much larger Rotoiti Breccia pyroclastic flow eruptions from the Haroharo Centre some 26 km to the north-east. The Rotoiti Breccia has been radiocarbon dated at c.42,000 years B.P. Widespread tephra showers accompanied the Earthquake Flat eruptions but have not been previously recognised as a separate deposit. No erosion or weathering breaks are present within the Earthquake Flat Breccia Formation, which was erupted as a single cooling unit. The Earthquake Flat Breccia deposits lie across the Taupo Fault Belt of closely spaced traces, and together with mantling tephra deposits provide surfaces for the dating of late Quaternary fault movements. Much of the area underlain by the Earthquake Flat Breccia has recently been developed for pastoral farming, and spectacular erosion has since occurred at many sites. The erosion is closely related to physical properties of the breccia - properties dependent on modes of eruption and deposition, and thus the paleovolcanology of the deposits. The grain size distribution, considerable depth, and weak consolidation of the deposits together with the geohydrology are the main erosion inducing factors; clay mineralogy and chemical content are shown to have little significance. Some of the worst erosion is related to the many faults crossing the breccia deposits. The history of dissection of the breccia deposits can be followed by study of the mantling tephra beds which indicate most dissection of the Earthquake Flat Breccia prior to 20,000 years B.P., and little erosion between c.13,000 years B.P. and the present day. A new erosion cycle may have been recently initiated.