THE MIDCONTINENT RIFT SYSTEM IN IOWA

The geology and history of the Keweenawan Midcontinent Rift System (MRS) in Iowa has been investigated using gravity modeling constrained by interpretations of aeromagnetic surveys and petroleum industry seismic profiles, study of limited drill data, and comparisons with related rocks in the Lake Superior outcrop region. This modeling identified a failed rift system characterized by the axial Iowa Horst, dominated by mafic volcanic and plutonic rocks, capped in some areas by shallow sedimentary basins, and flanked by deep clastics-filled basins. MRS stratigraphic units in Iowa are apparently correlative with Keweenawan rocks of the Lake Superior area. The mafic-dominated volcanic rocks were erupted into an actively subsiding graben, as two discrete packages, an early sequence erupted in time of reversed magnetic polarity, unconformably overlain by a late normal polarity sequence. A short transform fault (the Sheeder Prairie Structural Zone) divided the graben into two segments during the early phase of volcanism, but it was apparently overwhelmed by the late phase extrusives. As volcanism waned, graben subsidence continued and an early sequence of post volcanic clastic sediments (the Lower Red Clastic Series) buried the graben. Compressional reactivation of the graben-bounding fault zones thrust the graben-fill volcanic rocks upward through the Lower Red Clastic rocks which eroded from the uplifting horst and shed (as the Upper Red Clastic Series) over the Lower Red Clastic rocks flanking the horst. During this uplift, a block of the eastern edge of the horst (the Ames Block) split from the main body of the horst along the Perry-Hampton Fault Zone. Post uplift erosion removed most of the clastic rocks from the main body of the Iowa Horst, but the central area of the Ames Block preserves both Lower and Upper Red Clastics strata. Interpretations suggest that the horst-flanking clastic rocks are contained in three interconnected basins on the east and two connected basins on the west. Clastic sediments are present in model thicknesses of up to 10 km and volumes in excess of 150,000 km3. Keweenawan flood basalts and related crystalline rocks and their underlying feeder systems extend nearly to the mantle (43 to50 km deep) along most of the trend of the Iowa Horst.