THE SEARCH FOR WATER IN THE MANSON IMPACT STRUCTURE

The Manson Impact Structure (MIS) was initially discovered when the small northwest Iowa town of Manson drilled two water wells in 1905 and 1928, encountering bedrock strata never before seen anywhere in the region. Core drilling in the 1990s, along with seismic data collected by Amoco in the 1970s, revealed that the MIS is a complex crater that formed ~74 Ma in the shallow eastern flanks of the Cretaceous Western Interior Seaway. The MIS features an uplifted central peak consisting of three distinct units (in ascending order): the crystalline rock megabreccia (CRM), suevite breccia (SB), and impact melt breccia (IMB). The central peak is surrounded by a crater moat filled with the Phanerozoic Clast Breccia (PCB), a mixture of rocks from the crater footprint as well as ejecta, which were swept into the crater by the resurgent tidal wave. The PCB not only filled the crater moat but also covered the central peak, but after 74 million years of erosion more than 500m of material have been removed, exposing portions of the central peak to overlying glacial sediments.

Over a century of steadily declining water levels in both wells necessitated the need for a third water source to sustain the community into the future. A hydrogeologic assessment in 2011 concluded that old, soft water from the “central peak aquifer” was being mined out and replenished by younger hard water from the overlying glacial sediment package. The study determined that the “central peak aquifer” is likely the CRM. Although the first two wells were successful, a total of seven test wells have been drilled since 2011 and none are viable. Logging of the cuttings produced by the recent test wells has illuminated a possible twist in the story. Beneath the PCB in two of the three deepest test wells lies a unit that appears to be a welded breccia. The depth of this unit corresponds with the lowermost unit found in both Manson wells, the presumed “central peak aquifer”. However the test wells extend several hundred feet deeper than Manson’s wells where, below this unit lies sandstones and siltstones of the Precambrian Red Clastics. The Red Clastics fill deep basins that flank the Keweenawan Midcontinent Rift (MCR) throughout central Iowa. The presence of Red Clastics beneath the presumed “central peak aquifer” further convolutes the current model of what the Manson aquifer truly is.