Abstract

A 400 m thick sequence of Archean, apparently deep water subaqueous, aphyric, tholeiitic metabasalt flows is well exposed at Utik Lake. It comprises intimately interlayered pillowed and massive flows in about equal proportions, with each layer consisting of one or more flows of similar aspect. Flows were apparently linear in plan view with an average thickness of about 10 m and widths of about 1 km. Eruption of successive flows resulted in relief of up to 20 m on the surface of the volcano.Three flow types are present, and there is a lateral transition towards the flow margin from complex massive flows with autobrecciated flow tops, through simple massive flows, into pillowed flows. This lateral transition appears to represent a decrease in lava supply rate with most of the new lava being concentrated at the moving front of the flow, or an outward increase in viscosity due to cooling. The vertical interlayering of flow types may represent differences in flow rate or viscosity of flows erupted from the same vent, or pillowed flows may have been erupted from more distant vents. In the pillowed flows, most pillows are interconnected and apparently formed by budding and digital advance of lava tubes. Pillows also occur locally at the base of massive flows.

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