Geological Survey Professional Paper 754—B The Copper Harbor Conglomerate (Middle Keweenawan) on Isle Royale, Michigan, and its Regional Implications

The Copper Harbor Conglomerate, in its type area on the Keweenaw Peninsula of Michigan, was named and defined so as to include a thick sequence of sedimentary rocks, previously separated (in ascending order) into the Great, Middle, and Outer Conglomerates, with intervening lava flows, the Lake Shore Traps (Lane and Seaman, 1907, p. 690-691; Lane, 1911, p. 37-40). On the Keweenaw Peninsula, the Copper Harbor Conglomerate conformably overlies the Portage Lake Volcanics (middle Keweenawan), and locally the two formations interfinger (fig. 2; White and Wright, 1960). The Portage Lake Volcanics consists primarily of lava flows; minor sedimentary rocks, similar to those within the Copper Harbor Conglomerate, are intercalated between flows (hereafter referred to as interflow sedimentary rocks). The transition between the two formations reflects a gradual cessation of volcanic activity and the growing dominance of a sedimentary regime. The Copper Harbor Conglomerate is overlain by the Nonesuch Shale and Freda Sandstone (upper Keweenawan).

FIGURE 2.—Nomenclature for middle and upper Keweenawan rocks of northern Michigan and adjacent parts of Wisconsin (after White, 1972). The unnamed volcanic formation occurs only in the Porcupine Mountains area (fig. 1).

Irving (1883) placed the base of what he called the "Upper Division" of the Keweenawan Series at the base of his Outer Conglomerate; this boundary falls within the Copper Harbor Conglomerate as originally defined by Lane and Seaman (1907) and as currently used on the Keweenaw Peninsula. White, Cornwall, and Swanson (1953), in later work, arbitrarily chose to place the base of the upper Keweenawan at the base of the Copper Harbor Conglomerate but failed to discuss this apparent departure from prior usage. Because of confusion arising from this dual usage, White (1972) recently reviewed the difficulties of applying either of these alternative boundaries for the base of the upper Keweenawan and concluded that a third alternative, the top of the Copper Harbor Conglomerate, was, for both theoretical and practical reasons, a more meaningful stratigraphic boundary. He therefore proposed that the top of the Copper Harbor Conglomerate be considered the top of the middle Keweenawan; this usage places the entire Copper Harbor Conglomerate within the middle Keweenawan (fig. 2) and is followed in this report.

Details regarding the geology of the Copper Harbor Conglomerate on the Keweenaw Peninsula and farther west in Wisconsin are provided by White and Wright (1960), Hite (1968), and White (1971; 1972); Halls (1966) summarizes the regional stratigraphy. The following brief description is based primarily on these sources.

The Copper Harbor Conglomerate crops out in a discontinuous belt from the east end of the Keweenaw Peninsula westward into Wisconsin (fig. 1). The observed range in thickness in Michigan is about 200 feet to more than 6,000 feet. As the formation is wedge shaped, thickening rapidly to the northwest toward the axis of the Lake Superior syncline, the thickness at any one place depends on the proximity of the place measured to the margin of the depositional basin. And as neither the basin margin nor the outcrop belt is linear, much apparent thickening and thinning is found parallel to the regional strike. The stratigraphy is further complicated in the vicinity of the Porcupine Mountains, where a thick lenticular unit of volcanic rock lies between the Portage Lake Volcanics and the Copper Harbor Conglomerate. There the conglomerate is locally less than 200 feet thick (White and others, 1971; White, 1971, 1972).

The formation consists chiefly of red to brown arkosic conglomerate and sandstone; most of the pebbles are of volcanic origin and predominantly felsic. Metamorphic and granitic rocks are generally very minor constituents (<1 percent). Hite (1968) notes that near the Michigan-Wisconsin border, the relative percentage of metamorphic and granitic clasts as compared with volcanic increases upward in the section. He ascribes this change to a change in provenance, the metamorphic and granitic clasts coming from the Animikean terrane after streams had cut through the lowermost Keweenawan lava flows, the source for the lower part of the Copper Harbor Conglomerate. Boulders and smaller clasts of pre-Keweenawan rocks are also locally abundant in the Copper Harbor Conglomerate on Manitou Island (Cornwall and White, 1955).

The texture of the conglomerate is indicative of high energy conditions. Most of the conglomerate clasts are less than 10 inches in diameter, but boulders more than 2 feet in diameter are present. Modal size is generally in the 2- to 8-inch range. Most clasts are subrounded to well-rounded and bladed or prolate in shape. These pebbles, cobbles, and boulders lie closely packed in a sandy matrix cemented with calcite; near the top of the formation, packing is more open (Hite, 1968). The sandy units are poorly sorted, and the grains are angular to subangular.

White and Wright (1960, p. B6) have recognized a separate "red facies" within the Copper Harbor Conglomerate near the Porcupine Mountains (fig. 1). This unit is a platy fine- to medium-grained sandstone and is redder than the rocks of the formation as a whole.

It has been demonstrated that the general direction of sediment transport for both the Copper Harbor Conglomerate and the interflow conglomerates in the Portage Lake Volcanics was toward the Lake Superior basin (fig. 3); the source area is thus located to the southeast of the present outcrop belt (White, 1952, 1960; White and Wright, 1960; Hamblin and Horner, 1961). The Copper Harbor Conglomerate, on the Northern Peninsula of Michigan, has been interpreted as a piedmont fan stretching northward away from foothills near the margin of the Lake Superior basin toward an alluvial plain or body of water occupying the center of the basin (White and Wright, 1960; White, 1960, 1971). Hamblin (1961, 1965) has further discussed the regional sedimentary environment and paleogeography of the Lake Superior basin during Keweenawan and subsequent periods; he concludes that a highland extended through northern Michigan and Wisconsin, acting as a source of sediment for the Lake Superior basin from at least late Keweenawan through Late Cambrian (Dresbachian) time.

FIGURE 3.—Distribution of the Copper Harbor Conglomerate and paleocurrent directions on the Northern Peninsula of Michigan: 1, average direction of current flow from crossbeds in conglomeratic sandstone. 2, average direction of current flow from cross-beds in "red facies" 3, direction of current flow indicated by imbrication of pebbles. Number of individual observations included in average is shown by each arrow, Modified from White and Wright (1960). (click on image for a PDF version)