NEW INSIGHTS INTO THE DEVELOPMENT OF THE MANSON IMPACT STRUCTURE CENTRAL PEAK

The Manson Impact Structure (MIS) is a complex crater consisting of a central peak, crater moat, and disturbed terrace terrain. The structure was discovered beneath the town of Manson (Iowa) after the drilling of two water wells (1905 and 1928) into the southern flanks of the central peak. With declining water levels in the old wells, Manson searched for new water, drilling seven test wells. Each well penetrated about 250 m of crater moat materials before encountering rocks shed off the central peak and crater walls. Samples, logs, and down-hole videos are providing insight into the evolution of the crater’s central peak. When a large asteroid impacted the shallow eastern margin of the Cretaceous Seaway about 74 million years ago, it blasted a 7 km-deep, 24 km diameter transient crater, and lifted its rim about 2 km above the pre-impact landscape. As impact energy diminished, the uplifted rim collapsed, transferring energy to the crater center and driving the molten crater floor rapidly upward, a central peak of brecciated granitic basement rock. The collapsing rim mobilized saturated clastic rocks overlying the crystalline basement, pushed them into the crater moat region, then up the sides of the rising central peak. As the impact energy diminished and the central peak reached its maximum height, impact melt rock on the top began to flow off its edges and it began to mushroom. The main body of the peak also began to collapse, with downward pressure blowing its sides out producing huge debris flows of granitic blocks which moved out into the crater and onto the mobilized clastic rocks. This boulder flow was soon overtaken by the collapsing molten crater floor which had entrained other central peak debris and displayed many of the characteristics of a volcanic pyroclastic surge. Meanwhile a tsunami of sea water displaced by the impact blast was returning to the crater. As this resurge flowed into the crater, it engulfed the granitic block debris flow and pyroclastic-like melt rock surge, quickly overtopping the collapsing central peak and completely filling the crater with resurge materials. In the following 74 million years, erosion removed all intervening sediments and some resurge rocks, exposing a small area of crystalline rocks at the top of the central peak. Today the MIS remains buried beneath only 30-100 m of glacial sediments.