Dolomitization of the Lower Ordovician Prairie du Chien Group in southern Wisconsin and southeastern Minnesota: A case for confined and unconfined aquifer systems

Conference · Tue May 01 00:00:00 EDT 1990 · AAPG Bulletin (American Association of Petroleum Geologists); (USA)

OSTI ID:6642672

Smith, G L ^[1]

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1. Univ. of Wisconsin, Madison (USA)

The Lower Ordovician Prairie du Chien Group overlies the Cambrian-Ordovician Jordan Formation and underlies the Middle Ordovician St Peter Formation. The Prairie du Chien Group contains the Oneota Formation and the New Richmond and Willow River Members of the Shakopee Formation. The Prairie du Chien Group and associated formations form a repetitive sequence of alternating dolomites and sandstones: Jordan (sand), Oneota (dolomite), New Richmond (sand/dolomite), Willow River (dolomite), St. Peter (sand), and Platteville/Galena (dolomite/limestone). Prairie du Chien and Platteville/Galena carbonates thin over the Wisconsin arch and thicken eastward and westward. Petrography, cathodoluminescence, and electron microprobe analysis were used to identify and differentiate dolomite zones. The Oneota contains dolomite zones 1 to 3; the Shakopee contains zones 2 and 3; the Platteville/Galena only contains zone 3. Electron microprobe analysis of zone 3 reveals systematic decreases in dolomite stoichiometry and increases in iron and manganese trace-element composition along a transect from the Wisconsin arch to southeastern Minnesota. Zone 3 probably precipitated within a confined aquifer with recharge on the Wisconsin arch and flow toward southeastern Minnesota. In analogous modern systems, pore waters become progressively more reducing downflow, favoring trace-element enrichment. Zone 2 dolomites have low, uniform iron and manganese contents and uniform stoichiometries. Zone 2 compositions are consistent with precipitation in an unconfined and/or well-mixed aquifer associated with a continent-wide pre-St. Peter sea level drawdown and paleokarsting. Trace-element distributions within zone 1 dolomites are intermediate, suggesting precipitation within a semiconfined aquifer during pre-New Richmond exposure.

• OSTI ID: 6642672
• Report Number(s): CONF-900605-; CODEN: AABUD
• Journal Information: AAPG Bulletin (American Association of Petroleum Geologists); (USA), Vol. 74:5; Conference: Annual convention and exposition of the American Association of Petroleum Geologists, San Francisco, CA (USA), 3-6 Jun 1990; ISSN 0149-1423
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
CARBONATE ROCKS
DIAGENESIS
MINNESOTA
WISCONSIN
AQUIFERS
ELECTRON MICROPROBE ANALYSIS
GEOLOGIC FORMATIONS
IRON
MANGANESE
ORDOVICIAN PERIOD
SANDSTONES
TRACE AMOUNTS
CHEMICAL ANALYSIS
ELEMENTS
FEDERAL REGION V
GEOLOGIC AGES
METALS
MICROANALYSIS
NORTH AMERICA
PALEOZOIC ERA
ROCKS
SEDIMENTARY ROCKS
TRANSITION ELEMENTS
USA
580000* - Geosciences