Discovering a negative anomaly: Gravity survey of the Pine Bend impact
structure
Peter Berganross, Iowa State University
Abstract
The Pine Bend impact structure, discovered in 2023, is an ∼2 by 4 km crater (elongated NS) formed ∼490 m.y.a. in flat-lying sedimentary rocks in Dakota County, Minnesota. The crater was identified with drill cuttings, well logs, and a targeted core, which revealed beds uplifted as much as 50 m in the center of the structure, overturned bedding around the rim, and shocked quartz. However, it is difficult to constrain the 3D geometry using these data sources. I collected 137 new gravity stations in the northern half of the crater, with regional data from 80 stations, to map the location of the crater and model subsurface structures. The residual Bouguer anomaly gravity field shows a -2 mGal anomaly. The second derivatives and Gauss curvature of the vertical component of the Bouguer anomaly highlight the impact structure as a nearsurface anomaly and were used to cross-check the crater boundaries. Gravity data and the second vertical derivatives suggest a larger crater than was previously inferred, with dimensions of ∼6 by 8 km. I used forward models to infer a density contrast of -0.1 g/cm3 with crater walls dipping ∼30°. These findings suggest that the Pine Bend impact structure is a complex crater exhibiting the negative gravity anomaly expected for small impact structures formed in sedimentary target rocks.
Keywords: Local gravity survey, impact structure, complex crater.