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Oronto Group: A Petroliferous Rift-Fill Sequence of the Mid-Continent Rift System

Paul A. Daniels, Jr.

The Oronto Group of northern Michigan and Wisconsin includes the Copper Harbor Conglomerate, Nonesuch Formation, and Freda Sandstone. These formations are part of a volcaniclastic sequence created in response to the formation of the Mid-Continent rift system. Although intercalated volcanics are found in the lower one-third of the Oronto Group, a sedimentary depositional regime was dominant.

Lithologically, the Copper Harbor Conglomerate is a red-brown, basinward-thickening wedge of volcanogenic clastics and subordinate volcanics that fine distally and upsection. Depositionally, the Copper Harbor represents a prograding alluvial fan complex.

Overlying and interfingering with the Copper Harbor Conglomerate is the petroliferous, metalliferous Nonesuch Formation, an unoxidized sequence of gray-black siltstone, shale, and sandstone. In the model proposed, the Nonesuch is perceived as having been deposited in a rift-flanking, anoxic, lacustrine environment. As with the underlying Copper Harbor Conglomerate, the contact between the Nonesuch Formation and the overlying Freda Sandstone is gradational in character.

The Freda Sandstone is a ferruginous, lithic, cyclic sequence of sandstones and mudstones. Similar in appearance to some sandstones of the Copper Harbor Conglomerate, the Freda is of greater compositional maturity overall, with conglomeratic facies uncommon. The Freda is comprised dominantly of fluvial facies and is interpreted to have over-ridden the Nonesuch environments.

Although complex in detail, an overall depositional model for the Oronto Group is one of relatively simple transgressive-regressive relationships between alluvial, lacustrine, and fluvial environments. Important aspects of such a model include (1) all Oronto Group formations are genetically related; (2) the intervening Nonesuch Formation is, at least in part, equivalent in age to the upper Copper Harbor and the lower Freda; and (3) such rift settings can have significant hydrocarbon potential.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.