Pillow formation and layered flows in the Circum-Superior Belt of eastern Hudson Bay

Abstract

Detailed sections mapped across well exposed Aphebian tholeiitic and komatiitic flow assemblages on the islands of eastern Hudson Bay afford an insight into mechanisms of pillow development. Most of the flows with distinguishable boundaries are pillowed in their lower parts and massive in their upper parts. Pillow tubes are envisaged as developing, therefore, at the thin leading edge of a flow; as the volume.of flow is thus choked the lava behind overrides the impediment and repeats the process farther forward. Continued repetition of tube formation at flow fronts and the subsequent overriding or diversion of advancing lava result in formation of a carpet of pillow tubes over which the massive, flowing lava meanders in the manner of a prograding river. When lava production declines or ceases, lava streams and trapped ponds distributed over the surface of the pillow mat freeze into the massive tops observed in section.Layered flows are a development of the same process, illustrated in this paper by the Hudson's Bay layered flow. In this flow, a massive top thickens from 20 to 75 m over a lateral distance of 200 m as a result of damming behind a pillow barrier of its own making. Subsequent spills of lava over the pillow dam increased its height until the drainage, adapting to the new topography, aligned itself parallel to the barrier and flowage resumed along the thickened lens thus created. Accompanying flow differentiation formed an olivine-rich zone suspended above and conforming to the shape of the base of the massive lens.