STRATIGRAPHY, SEDIMENTOLOGY, AND K-BENTONITE GEOCHEMISTRY OF THE GALENA GROUP, NORTHEASTERN IOWA, UPPER MISSISSIPPI VALLEY

The Ordovician (Mohawkian/Cincinnatian) Galena Group is a mixed carbonate-siliciclastic unit that was deposited in a near-equatorial epeiric sea that covered most of the North American craton. The Galena Group is widely distributed in the Upper Mississippi River Valley (UMV), and is well preserved in northeastern Iowa.

Various lithostratigraphic classification schemes have been applied to the Galena Group. All classifications generally recognize four formation-level subdivisions throughout the UMV. These are the basal Decorah Formation (mixed carbonate-siliciclastic, shale dominated), overlying Dunleith Formation (mixed carbonate-siliciclastic, carbonate dominated), overlying Wise Lake Formation (carbonate), and uppermost Dubuque Formation (equally mixed carbonate-siliciclastic). The classifications differ in their recognition of members throughout the Galena outcrop belt, an effect of local to regional facies variations. This study proposes a lithostratigraphic classification for the Dunleith, Wise Lake, and Dubuque Formations of the Galena Group in northeastern Iowa.

Generally, the Galena Group studied is composed of bioturbated wackestones and packstones that preserve abundant and diverse, predominantly benthic fauna. Specifically, four lithofacies groups and ten lithofacies are recognized. Depositional environments for the facies groups range from well-oxygenated open marine settings to dysoxic to anoxic marine settings. Vertical and lateral facies changes are a result of changes in eustatic sea level and sediment supply; this study recognizes four high-frequency sequences within one sequence for the Galena Group studied in northeastern Iowa.

Galena strata along a ~90 km northwest-to-southeast transect in NE Iowa contain thin, laterally continuous altered volcanic ash beds, or K-Bentonite beds. Recent studies proved that correlation of K-Bentonites via chemical fingerprinting of single apatite phenocrysts is regionally successful (Samson, 1986; Emerson, 2002). This study proposes a new correlation of Galena Group strata in NE Iowa based primarily on the chemical fingerprinting of single apatite phenocrysts obtained from a prominent and widespread K-Bentonite bed. The correlation is strengthened by recognition of key faunal zones and by gamma ray spectrometry.