--> Abstract: Precambrian Midcontinent Rift, by S. M. Landon; #91012 (1992).

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ABSTRACT: Precambrian Midcontinent Rift

LANDON, SUSAN M., Consultant, Denver, CO

Few significant exploration wells (Texaco #1 Poersch, Kansas, and Amoco #1 Eischeid, Iowa) have been drilled in the billion-year-old Midcontinent Rift supplementing the geologic framework established by outcrop studies.

This trend, as defined by the gravity anomaly map, extends approximately 1200 mi in a horseshoe shape from central Kansas, through Iowa and southeastern Minnesota, to the region beneath Lake Superior and the Michigan Basin. Geophysical data show that the Midcontinent Rift developed as a series of half-grabens, with asymmetry reversing at regularly spaced offsets. After extension and associated faulting had ceased, but prior to the Cambrian transgression, the rift was compressed. The normal faults were reactivated as high-angle reverse faults, creating an axial horst.

Volcanism was predominant during the early phase of extension. The overlying stratigraphic package includes alluvial and fluvial conglomerates, sandstones, and siltstones, with lacustrine shales mirroring depositional environments in modern rifts. Indications of evaporites are also present.

The Nonesuch Formation, in the Lake Superior area, is lacustrine shale containing adequate organic matter to be an effective source rock. Similar shales have been identified in the subsurface in Minnesota and Iowa. These shales have documented TOC values as high as 3%. Analyses indicate thermal maturity values from peak oil to advanced. Surface seeps, fluid inclusions, and modeling indicate the potential for more than a single phase of generation. Potential reservoir rocks were deposited in deltas and fluvial systems. Effective seals for structural or stratigraphic traps may include lacustrine shales and evaporites.

The Midcontinent Rift is a relatively unexplored frontier hydrocarbon province in the heart of North America.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)