Subaqueous eruption and emplacement of OT2 in the Middle Miocene Iizuka Formation, Noto Peninsula, Japan

OT2 (Okada Tuff 2) is one of six rhyolitic tuff beds that occur in a Middle Miocene marine diatomite (lower lizuka Formation) and are known as the Okada Tuff. OT2 is well-sorted, and thins and thickens infilling channels 1-3 m across and 030 cm deep with a thickness up to 30 cm. Small channels 10-30 cm across and 1-10 cm deep are commonly superimposed within wider channels. A thin diatomite layer is ripped from the substrate up to a level immediately above the small channels. The basal part beneath the ripped-up diatomite layer is dominated by ash components with vague wavy stratifications. Above the ripped-up diatomite layer, diatomite clasts are dispersed, and pumice lapilli increase upward in amount and size with local interruption at a middle lower level by a poorly defined, pumice lapilli poor thin layer. Many glass and pumice shards are blocky with curviplanar surfaces and jigsaw cracks, presumably produced by phreatomagmatic explosions. The poorly defined two layers of OT2 are interpreted to be ash turbidites individually produced by discrete phreatomagmatic explosions. Lenticular bedforms and channels indicate that the currents were fully turbulent and unsteady, fed by the submarine eruptions. Upward coarsening of pumice probably represents settling lag of hot pumice. OT2 perhaps provides potential diagnostic features to identify eruption-fed turbidites. (c) 2005 Elsevier B.V All rights reserved.