The Keweenawan-age Oronto Group of northern Michigan and Wisconsin includes the Copper Harbor Conglomerate, Nonesuch (Shale) Formation, and Freda Sandstone. These formations are part of a volcanic-clastic sequence created in response to the formation of the Midcontinent Rift System. Although intercalated volcanics are found in the lower one-third of the Oronto Group, a sedimentary depositional regime was dominant. Along the Keweenaw Peninsula, paleocurrent indicators for all three formations show that the predominant depositional directions were northerly. On Isle Royale, the opposite side of the rift, sedimentary structures in the Copper Harbor Conglomerate indicate that flow was to the south and east. Lithologically, the Copper Harbor Conglomerate is a red-brown, basinward-thickening wedge of volcanogenic clastics and subordinate volcanics that fines distally and upsection. Maximum thickness for this formation is about 1830 m. The dominant sandstone type is lithic graywacke. Conglomerate facies are primarily clast-supported and comprised of volcanic clasts with a ratio of mafic to silicic + intermediate clasts of about 2:1. The heavy-mineral suite for the Copper Harbor Conglomerate (as well as the other Oronto Group formations) mainly consists of ilmenite and similar opaque minerals and epidote. Depositionally, the Copper Harbor Conglomerate represents a prograding alluvial fan complex. Interfingering with the Copper Harbor Conglomerate is the Nonesuch (Shale) Formation, an unoxidized sequence of gray-black siltstone, shale, and sandstone with a maximum thickness of 215 m. Besides having been deposited in a reducing environment, the Nonesuch differs from the enclosing redbed sequences by its increased textural maturity and its sulfide and hydrocarbon content. The heavy-mineral suite of the Nonesuch also differs from that of the redbeds only by relative enrichment of chlorite. The Nonesuch is perceived as a rift-flanking lacustrine environment, probably initiated through disruption of existing drainages by alluvial, volcanic, or tectonic processes. As with the underlying Copper Harbor Conglomerate, the contact with the overlying Freda Sandstone is gradational in character. The Freda Sandstone is a ferruginous, lithic sequence of cyclic sandstone and mudstone exceeding 3660 m in maximum thickness. Although similar in appearance to some sandstones of the Copper Harbor Conglomerate, the Freda, overall, is of greater compositional maturity, and conglomerate facies are uncommon. The Freda is dominantly fluvial in origin and appears to have “overridden” the Nonesuch environments. Although complex in detail, the overall depositional model for the Oronto Group is one of simple transgressive-regressive relationships between alluvial fan/lacustrine/fluvial environments. Important aspects of such a model are that (1) all the Oronto Group formations are genetically related with no major unconformities between them; and (2) the intervening Nonesuch Formation is, at least in part, equivalent in age to the upper Copper Harbor and the lower Freda.