R.R. Anderson, K.E. Windom, and K.E. Seifert

Geological Society of America
1995 Abstracts with Program, v. 27, no. 3, p. 34


The Precambrian basement of Iowa comprises numerous terranes and terrane boundaries, plutons, and supracrustal rocks ranging in age from approximately 2.9 Ga in extreme northwestern Iowa to the <1.1 Ga clastic rocks of the Midcontinent Rift System (MRS). Terranes formed between these age extremes include the southern edge of the Archean Superior Craton (Lyon County gneiss approx. 2.5 Ga), the Early Proterozoic Penokean Province (approx. 1.82-1.89 Ga), and the Central Plains Province (approx. 1.7-1.8 Ga) and the Eastern Granite Rhyolite Province (approx. 1.4 Ga). Despite the exceptional concentration of terrane boundaries within this geographically restricted region, only limited geochemical and geochronological data are available because of the cover of Phanerozoic sedimentary rocks. Drilling is the only viable method to sample these basement rocks.

We present arguments for drilling selected targets within Iowa to obtain information regarding the nature of the various Precambrian terranes and associated rocks. Two series of relatively shallow holes (<3 km) are identified along with other single-hole targets. The first series would consist of 3 to 4 holes along the MRS. This major feature crosses both the Penokean and Central Plains provinces. Holes would be sited to sample clastic rocks atop and flanking the MRS and volcanic rocks with the MRS that have traversed mantle and crust associated with these two major provinces. Data from recovered samples would provide information about the nature of the mantle and lower crust beneath these terranes. A second series of holes is proposed across the Archean-Proterozoic boundary in northwestern Iowa. A 2.5 Ga gneiss and a 2.9 Ga layered gabbro complex occur at depth but the geological association is unknown. The nature of the boundary between the Archean and Proterozoic terranes is also unknown. In addition to these traverses, we propose several single-hole targets be drilled in southeastern, northeastern, and western Iowa; virtually no basement samples have been recovered from these regions of the state, thus we have no information regarding the boundaries of several major Precambrian provinces. Drilling is the only method capable of providing these important samples.