What lies beneath: geophysical mapping of a concealed Precambrian intrusive complex along the Iowa–Minnesota border

Abstract

Large-amplitude gravity and magnetic highs over northeast Iowa are interpreted to reflect a buried intrusive complex composed of mafic-ultramafic rocks, the northeast Iowa intrusive complex (NEIIC), intruding Yavapai province (1.8-1.72 Ga) rocks. The age of the complex is unproven, although it has been considered to be Keweenawan (ϳ1.1 Ga). Because only four boreholes reach the complex, which is covered by 200-700 m of Paleozoic sedimentary rocks, geophysical methods are critical to developing a better understanding of the nature and mineral resource potential of the NEIIC. Lithologic and cross-cutting relations interpreted from high-resolution aeromagnetic and airborne gravity gradient data are presented in the form of a preliminary geologic map of the basement Precambrian rocks. Numerous magnetic anomalies are coincident with airborne gravity gradient (AGG) highs, indicating widespread strongly magnetized and dense rocks of likely mafic-ultramafic composition. A Yavapai-age metagabbro unit is interpreted to be part of a layered intrusion with subvertical dip. Another presumed Yavapai unit has low density and weak magnetization, observations consistent with felsic plutons. Northeast-trending, linear magnetic lows are interpreted to reflect reversely magnetized diabase dikes and have properties consistent with Keweenawan rocks. The interpreted dikes are cut in places by normally magnetized mafic-ultramafic rocks, suggesting that the latter represent younger Keweenawan rocks. Distinctive horseshoe-shaped magnetic and AGG highs correspond with a known gabbro, and surround rocks with weaker magnetization and lower density. Here, informally called the Decorah complex, the source body has notable geophysical similarities to Keweenawan alkaline ring complexes, such as the Coldwell and Killala Lake complexes, and Mesoproterozoic anorogenic complexes, such as the Kiglapait, Hettasch, and Voisey's Bay intrusions in Labrador. Results presented here suggest that much of the NEIIC is composed of such complexes, and broadly speaking, may be a discontinuous group of several intrusive bodies. Most units are cut by suspected northwest-trending faults imaged as magnetic lineaments, and one produces apparent sinistral fault separation of a dike in the eastern part of the survey area. The location, trend, and apparent sinistral sense of motion are consistent with the suspected faults being part of the Belle Plaine fault zone, a complex transform fault zone within the Midcontinent rift system that is here proposed to correspond with a major structural discontinuity. Résumé :De grandes crêtes magnétiques et gravimétriques sur le nord-est de l'Iowa sont interprétées comme étant le reflet d'un complexe intrusif enfoui composé de roches mafiques et ultramafiques, soit le complexe intrusif du nord-est de l'Iowa (NEIIC), lequel pénètre les roches de la Province de Yavapai (1,8-1,72 Ga). L'âge du complexe n'a pas été prouvé, bien qu'il soit considéré comme datant du Keweenawien (~1.1 Ga). Puisque seulement quatre sondages ont atteint le co...

Geologic characterization of nonconformities using outcrop and core analogs: hydrologic implications for injection-induced seismicity

“…However, repeated brittle failure and mineralization, observed in Type I nonconformities, suggest that phyllosilicatedominated shear zones can act as a zone of mechanical weakness that can be reactivated, allowing for the development of fracture permeability. In this fractured nonconformity we observed alteration as deep as 5 m below the nonconformity in the crystalline rocks examined; however, previous work highlights the potential for fractures and connectivity to basement fault zones at much greater depths (Duffin et al, 1989). Preexisting basement shear zones that are reactivated may allow future fluid circulation during injection scenarios.…”

Geologic characterization of nonconformities using outcrop and whole-rock core analogues: hydrologic implications for injection-induced seismicity

“…CC BY 4.0 License. 1995).Northwest trending fault systems near the borehole were identified by magnetic lineaments and are likely part of the regional NW-SE Belle Plaine Fault Zone (Drenth et al, 2015).…”

The Decorah structure, northeastern Iowa: Geology and evidence for formation by meteorite impact

“…Compared to much of Iowa, bedrock exposures, especially along valley walls, are common, but Quaternary colluvium, loess, and patchy glacial till up to a few meters thick mantle large portions of the valley walls and uplands. The Paleozoic strata are underlain by a Precambrian (Mesoproterozoic) mafic to ultramafic complex thought to intrude Yavapai province (1.8-1.72 Ga) metagabbro and felsic plutons (Drenth et al, 2015). This basement complex is estimated to be present at depths of 490-620 m below the land surface.…”