Three-dimensional Structure of the Decorah Impact Structure: Constraints from AEM and potential-field modeling

Abstract:

Terrestrial impacts are poorly understood, despite considerable interest in their resource potential and the prominent role they play in extinction events. We will present three-dimensional models of the mid-Ordovician Decorah Impact Structure (DIS) in Northeastern Iowa, USA, based upon airborne and ground geophysical measurements that cover the impact area and the surrounding undisturbed stratigraphy. The DIS is characterized by a circular conductive feature in electrical resistivity models and a negative gravity anomaly. It has no magnetic signature, suggesting that igneous Precambrian basement was not significantly affected by the impact. Resistivity models of the DIS reveal a clear disruption of regional stratigraphy to >300 m depth across a 5.6 km diameter region. An electrically conductive post-impact shale, unique to the impact area is of similar diameter, suggesting both a moderate erosional level and an imaged impact diameter close to that of the original transient crater. A variable thickness of impact breccia is imaged above a prominent resistive unit, which together with borehole data indicate intact stratigraphy, atop a central uplift. The entire region inside the eroded DIS was uplifted by 200-300 m, bracketed by offset stratigraphy and the lack of evidence for uplifted igneous basement. Topography on the crater floor is suggestive of block faulting with offsets in excess of 100 m. Modeling of the airborne gravity data, constrained by the geometry revealed by the resistivity model and measured density on samples of pre- and post-impact rocks, indicates a considerable mass deficit. Only a fraction of the mass deficit is derived from the low-density breccia and post-impact shale, with the largest contribution resulting from density reduction within fractured rock beneath the crater floor.