Geological Survey Professional Paper 754—C The Portage Lake Volcanics (Middle Keweenawan) on Isle Royale, Michigan

THE PORTAGE LAKE VOLCANICS (MIDDLE KEWEENAWAN) ON ISLE ROYALE, MICHIGAN

By N. KING HUBER

The Portage Lake Volcanics is the upper part of the total volcanic sequence of Keweenawan (Precambrian) age in the Lake Superior region. On Isle Royale the formation consists of more than 10,000 feet of basaltic and andesitic lava flows with interbedded pyroclastic rocks and fluvially deposited sedimentary rocks. Certain stratigraphic units representing flows or groups of flows can be identified and traced in the field on the basis of their texture or other distinctive characteristics. Twelve such flow units have been distinguished and named on the island, and these key units provide stratigraphic and structural control within the volcanic sequence.

Correlation between the Portage Lake Volcanics on Isle Royale and the type area some 50 miles southeast on the Keweenaw Peninsula on the opposite side of the Lake Superior syncline is excellent, and at least one individual lava flow is interpreted as being continuous across the basin.

The Portage Lake Volcanics on Isle Royale does not appear to stratigraphically overlap the lower part of the Keweenawan volcanic sequence on the north side of the syncline, represented by the North Shore Volcanic Group in Minnesota, and there may be a stratigraphic break between them. The North Shore Volcanic Group is the source terrane for most of the clastic debris in the interbedded sedimentary rocks in the Portage Lake Volcanics on Isle Royale as well as in the overlying Copper Harbor Conglomerate.

The flood basalts of the Portage Lake Volcanics were erupted from a major rift zone of continental proportions that trends in an arc through Lake Superior. Similar flood basalts underlie most or all of a belt nearly 100 miles wide and more than 1,000 miles long. The depositional basin was subsiding as it was being filled, and during intermittent periods of volcanic quiescence, clastic debris was swept into the basin from its margins to form the sedimentary deposits interbedded with the lava flows. When volcanism ceased, subsidence continued, permitting the accumulation of a thick sedimentary sequence, the Copper Harbor Conglomerate and younger Keweenawan sedimentary rocks.