The Neugrund Marine Impact Structure (Gulf of Finland, Estonia)

Abstract

The Early Cambrian (approximately 535 Ma) Neugrund marine impact structure is located on the southern side of the entrance to the Gulf of Finland, immediately eastward of Osmussaar Island, Estonia. The origin of the structure was noted already in 1995–1998, but data obtained during the expeditions of 2000 and 2001 have shed new light on its morphology. The impact structure is about 20 km in diameter and spatially delimited by a ring fault between dislocated rocks and mostly intact target rocks. The structure has a central depression (crater deep or crater proper) 5.5 km in diameter, surrounded by a 50–100 m high and anomalously wide (2.5–3 km) 3-ridge shaped rim wall. The crater deep is filled with post-impact Early Cambrian and Early Ordovician siliciclastic rocks and covered with Middle and Late Ordovician calcareous rocks. The slight (some metres) uplift of limestone beds in the centre of the crater suggests that a central uplift also exists. The Ordovician erosion-resistant limestone forms a circular Central Plateau (Neugrund Bank) above the crater proper about 4.5 km in diameter. The plateau is surrounded by a 200–500 m wide and 20–70 m deep canyon (Ring Canyon). A 3–5 km wide circular depression where the crystalline target rocks are dislocated lies outside the rim wall. Sedimentary target rocks are eroded in the northern part of the structure. Outside the ring fault (outer boundary of the structure), sedimentary target rocks are dislocated within about 10 km, obviously due to the Neugrund impact. The 1–2 m thick ejecta layer consists of sandstones with abundant shock-metamorphosed quartz grains with well-developed planar deformation features (PDFs). Erratics consisting of Neugrund Breccia, derived by glacial action from the exposed parts of the impact structure, spread in an area of more than 10 000 km2.