THE COPPER HARBOR CONGLOMERATE (MIDDLE KEWEENAWAN) ON ISLE ROYALE, MICHIGAN, AND ITS REGIONAL IMPLICATIONS
BY ROGER G. WOLFF1 and N. KING HUBER
The Copper Harbor Conglomerate on Isle Royale is the north limb counterpart of the type Copper Harbor Conglomerate exposed on the south limb of the Lake Superior syncline on the Keweenaw Peninsula. In both areas the formation, of middle Keweenawan age, overlies the middle Keweenawan Portage Lake Volcanics and is largely derived from older Keweenawan volcanic source terranes that shed debris into the subsiding Lake Superior basin from opposite sides; for Isle Royale, the source terrane would have been the North Shore Volcanic Group in Minnesota. The deposits appear to he piedmont fan deposits and flood-plain deposits formed under a combination of fluvial and lacustrine conditions.
Various sedimentary features on Isle Royale indicate that the sediments were transported generally eastward, with a range in direction from northeast to southeast. The formation also increases in thickness and in textural and compositional maturity in this direction; it varies from a boulder conglomerate, through a mixture of cobble and pebble conglomerates and sandstone, to sandstone alone. This elastic wedge thickens within a distance of 20 miles from a minimum of 1,500 feet to more than 6,000 feet between stratigraphic marker horizons; the top of the formation is nowhere exposed, however, and the total thickness probably is considerably greater.
The clastic materials in the formation are predominantly derived from older Keweenawan volcanic rocks. Felsic varieties are slightly dominant over mafic varieties. Metamorphic rocks are minor components, and clasts of intrusive igneous rocks appear to be absent. The overall local composition of the formation, chiefly reflecting degree of sediment maturity, is closely related to grain size; rocks of similar grain size have the same composition throughout the formation. Calcite cement is ubiquitous; in the conglomerate and coarse-grained sandstone, it commonly amounts to about 15 percent. As the textures become finer, rock fragments and calcite cement decrease in abundance, whereas quartz, feldspar, and opaque mineral percentages increase.
The felsic and many of the mafic Keweenawan rock types that occur as clasts in the Copper Harbor Conglomerate are not known to occur as flows within the Portage Lake Volcanics on the island. Most of the rock types, however, are known to occur as flows in the North Shore Volcanic Group, which is stratigraphically lower than the Portage Lake Volcanics and is present in the direction from which the clastic debris was transported. Several lines of evidence suggest the presence of an unconformity between the North Shore Volcanic Group and the Portage Lake Volcanics. Such an unconformity would facilitate the erosion of the North Shore Volcanic Group and the deposition of the erosional debris basinward on top of the Portage Lake Volcanics.
The situation would be analogous to that on the south side of the Lake Superior syncline. There many clasts in the Copper Harbor Conglomerate appear to have been derived from the lowermost Keweenawan volcanic terrane, volcanic rocks of the South Trap Range which underlie the Portage Lake Volcanics.
Last Updated: 22-Jan-2009