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

Source areas for the debris in the Copper Harbor Conglomerate were on opposite sides of the Lake Superior syncline, and the major sediment transport directions were generally toward the axis of the syncline. The relative abundance of various sedimentary features in the formation on Isle Royale (table 5) best fits that summarized for a fluvial environment by Picard (1967), and the coarser facies are characteristic of alluvial fan deposits (Bull, 1972). These features, together with lateral changes in texture and compositional maturity, suggest a piedmont fan deposit grading outward into a flood-plain deposit. Although some sedimentary features in the finer grained rocks, such as crossbeds and ripple marks, indicate that they were deposited in bodies of standing water, the abundance of desiccation cracks (fig. 15) and occasional raindrop impressions indicate that such bodies of water were ephemeral. Other more permanent bodies of water may have existed in the central part of the Lake Superior syncline.

FIGURE 15.—Desiccation cracks in fine-grained sedimentary rock on Menagerie Island.

A depositional environment such as this has been proposed by White and Wright (1960) for the Copper Harbor Conglomerate on the Keweenaw Peninsula. The Isle Royale study not only supports their interpretation, but provides an exceptional illustration of the transition from fan deposits to flood-plain deposits over a distance of more than 20 miles along the general direction of sedimentary transport.

White (1960) has clearly demonstrated that the Lake Superior syncline was subsiding during alternating eruption of the lava flows and deposition of the interflow sediments of the middle Keweenawan Portage Lake Volcanics. Presumably, the subsidence continued into at least part of the subsequent Keweenawan, for the deposition of the Copper Harbor Conglomerate indicates the gradual demise of volcanic activity without any radical changes in the depositional environment. In fact, on the Keweenaw Peninsula a few last surges of volcanic activity are represented by flows within the Copper Harbor Conglomerate (refer to Cornwall, 1955; White and Wright, 1960); one group of flows is only 300 feet below the top of the formation (White and others, 1953). The wedge-shaped depositional geometry of the Copper Harbor Conglomerate on Isle Royale is related to continuing subsidence of the Lake Superior basin.

White's analysis (1960), which indicates periodic reversals of slope between flows and interflow sediments, further indicates rather low gradients for the streams that deposited the Copper Harbor Conglomerate. Hamblin and Horner (1961, p. 210) suggested that the small standard deviation in orientation of current structures and the coarse texture of the sediments on Isle Royale indicate streams with straight courses and steep gradients. These sedimentary features, however, may well be compatible with quite low gradients under the prevegetation hydrologic conditions that existed during the Precambrian. As Schumm (1968, p. 1585) recently pointed out, "before the appearance of significant plant cover, denudation and runoff rates were high and floods were large, and coarse sediments were spread over piedmont areas even in humid climates." These processes would also help explain the extensive sheetlike nature of the interflow conglomerates.