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ABSTRACT: Comparison of the Midcontinent Rift, USA, and the East African Rift

Susan M. Landon

Hydrocarbon exploration has accelerated the acquisition of data in the 1 billion year old Midcontinent Rift, providing a more detailed geological interpretation. To further refine the interpretation, the Midcontinent Rift is compared to the well-studied, 40 million year old East African Rift. Structurally, both rifts are similar in scale. Geophysical data show that both have an asymmetric (half-graben) geometry which usually reserves at regularly spaced offsets along the trend. Volcanism, predominantly flood basalts, has been extensive in the rifts, and carbonatites occur at a rift offset in both. The stratigraphic packages are also similar, reflecting comparable depositional environments. Alluvial and fluvial conglomerates, sandstones and siltstones with lacustrine shale , stromatolites, and evaporites characterize both rifts.

The petroleum potential can also be compared. Lacustrine shales may be effective source rocks and seals. Organic contents appear to be dramatically different; the shales of the East African Rift may be ten times richer than the shales of the Midcontinent Rift with TOC values of 1-2%. Thermal histories and maturation are also different. The early history of the Midcontinent Rift would parallel the modern history of the East African Rift. As a result of relatively high heat flow, hydrocarbons are generated early in the history of rifts. Oil seeps in the East African Rift and hydrocarbons in fluid inclusions in sediments in the Midcontinent Rift document this early phase of generation. The subsequent history in the Midcontinent Rift is still debated. Potential reservoir rocks were deposi ed in deltas and fluvial channels. Diagenetic history in the Midcontinent Rift may be complex and the significant age of the sediments may be a problem in effectively sealing the potential reservoirs. Evaporites are present in both rifts and may provide effective seals for the billion year old reservoirs of the Midcontinent Rift.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990