Registration for the AIPG MN Section's May 5th, 2026, technical talk is now open online!

The cost is $10 for non-members and $5 for members. Students may attend free of charge. 

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Unearthing Minnesota’s First Impact Crater: The Latest from Pine Bend

Presented by Julia Steenberg, Geologist with the Minnesota Geological Survey

Presentation Overview

During geologic mapping of Dakota County for the County Geologic Atlas mapping program, geologists at the Minnesota Geological Survey identified evidence for a large, deeply buried impact structure in Inver Grove Heights, MN, now named the Pine Bend Impact Structure. Quartz sand grains from archived drill cuttings contain planar fractures and planar deformation features diagnostic of a meteorite impact. Subsurface mapping methods including analysis of drill cuttings, geophysical logs, water-well driller’s records, and a newly obtained drill core, delineate an approximate 4 km wide, ~190 m thick zone of structural disruption buried beneath up to 120 m of Quaternary glacial sediment. Within this zone, Cambrian and Mesoproterozoic strata occur out of stratigraphic order and in places, are completely overturned. The new drill core from near the structure’s center reveals steeply dipping to fully inverted, intensely fractured and fluidized intervals of the Middle Cambrian Mt. Simon Sandstone. Ongoing stratigraphic analysis continues to support a complex crater morphology, including extensive subsurface deformation, a central uplift, and terraced rims. The impact is tentatively dated to approximate the Cambrian-Ordovician boundary (~490 Ma), based on the youngest overturned unit (Jordan Sandstone, Late Cambrian), the apparent absence of Lower Ordovician Prairie du Chien Group carbonates within the disrupted section, and possible shock-related features observed at this contact 10 km from the structure.  This presentation will summarize the evidence for the Pine Bend Impact Structure, highlight results from the new drill core, and share emerging findings from recent student research.

Biography

Julia Steenberg, Julia is a geologist with the Minnesota Geological Survey (MGS), a research and service arm of School of Earth and Environmental Sciences at the University of Minnesota, where she leads County Geologic Atlas mapping programs. She maps the distribution and structure of the Paleozoic and Cretaceous aged sedimentary bedrock across the state and is involved in research to characterize bedrock properties that influence groundwater flow. She is also the technical map editor, reviewing all geologic maps and products produced by MGS. She has been with the MGS for 18 years. Julia is active with the Minnesota Ground Water Association, served as president in 2020 and is an active board member on the Foundation. Julia holds a master’s degree in Geosciences from Idaho State University and a bachelor’s degree in Geology from Gustavus Adolphus College.