Understanding fluctuating magmatic and phreatomagmatic explosive activity in a large late Pleistocene basaltic maar: The Tower Hill Volcanic Complex, Newer Volcanics Province, southeastern Australia

The ~30 ka, basaltic Tower Hill Volcanic Complex, of the late Cainozoic Newer Volcanics Province, southeastern Australia, was produced by early phreatomagmatic maar forming activity followed by magmatic cone forming activity. Phreatomagmatic activity was caused by subsurface Tertiary sedimentary aquifers. However, fluctuating phreatomagmatic and magmatic explosive activity occurred during the early maar forming phase, represented in the maar rim succession by interbedded fine, accretionary lapilli bearing, base surge ash beds, poorly sorted base surge and fallout lapilli-ash beds with abundant sedimentary and basalt rock debris, and well-vesiculated, magmatic scoria fallout deposits. The interbedding could indicate simultaneously active “dry”magmatic and “wet” phreatomagmatic vents, or regular variation between “dry” and “wet”explosive activity from one vent system. The latter could be caused by variations in magma discharge rate, slow aquifer recharge rates, or episodic sealing of the conduit walls by impermeable linings of chilled magma. The maar rim succession contains repeated cycles beginning with ash facies, gradationally changing to coarsening up lapilli-ash, overlain by a scoria bed. Short-circuited cycles also occur. Each cycle represents short-term “drying” of the vent, suggesting aquifer recharge rate relative to magma discharge rate was not consistently high enough to sustain maximum efficiency phreatomagmatic explosive activity. Simple mass balance calculations will be used to estimate magma and aquifer flow rates. However, finally the aquifer must have become sealed by magma to explain late magmatic cone formation, including increased magma discharge rates leading to formation of spatter cones and agglutinates.