Geology and Magnetic Signatures of the Neugrund Impact Structure, Estonia

The Early Cambrian Neugrund crater, Estonia, has a rim-to-rim diameter of 7 km. In addition to the structural elements common in impact structures (crater cavity, rim wall, crater filling, ring fault), there occurs a specific, up to 70 m deep and 200 – 500 m wide circular canyon between the rim wall and the crater. Theoretically, Neugrund should also have a central uplift, but there is neither geological nor geophysical evidence as yet. The seismic reflection data show that the disturbed strata around the structure are spatially delimited by the ring fault, which has a diameter of about 21 km. The ring fault is interpreted as the outer limit of the impact structure. The true depth of the crater, as well as the distribution of post-impact sediments and impactites within the crater, is still unknown. Magnetic modelling and comparison with other impact structures of the Fennoscandian shield (e.g., Kärdla, Lappajärvi, Lockne) suggest that the maximum depth of the interface between autochthonous and allochthonous breccias is approximately 700 m. A distinct, slightly elongated negative magnetic anomaly about 5 km in diameter marks the crater cavity. This magnetic low is caused mainly by weakly magnetic post-impact sedimentary infill and its screening effect, but possibly also by allochthonous breccias and post-impact oxidation processes. The absence of high-amplitude local anomalies within the minima suggests a lack of highly magnetic melt bodies. The magnetic data show no evidence of a central uplift.