MIDCONTINENT RIFT AS A FRONTIER HYDROCARBON TARGET.

Graben development by crustal extension approximately 1. 1 Ga, followed by infilling by numerous basaltic flows, and isostatic sinking of volcanic rock created erosion and deposition of older (Oronto Group) sedimentary rock with reactivation and uplift of the central (St. Croix) horst. The Oronto Group was eroded and the sediment deposited as Bayfield Group sedimentary rock in the flanking basins; erosion of St. Croix Horst continued into Paleozoic time until terminated by transgression of Lower Paleozoic units. The basic structure of the Midcontinent Rift System southwest of Lake Superior is a central horst block flanked by basins. The trend of the central St. Croix Horst is not continuous. In Wisconsin the Oronto Group is associated with the Ashland-Lake Superior Syncline. To the southwest a 580 m column of Oronto-like sedimentary rock was drilled south of Minneapolis-St. Paul in search of suitable natural gas storage reservoir.