Summanen, a new meteorite impact structure in Central Finland

Abstract

The Summanen structure (62°39.0′N, 25°22.5′E) is located within the Paleoproterozoic Central Finland Granite Belt, Fennoscandian Shield. The structure is hidden under Lake Summanen and not directly observable. It owes its discovery to low‐altitude airborne geophysical data, which revealed a circular, ~2.6 km wide electromagnetic in‐phase, and resistivity, anomalies. Two field campaigns were conducted in 2017 to search for impact signatures. The fieldwork concentrated on the southeastern side of the lake following the ice flow direction of the latest (Weichselian) glaciation. In addition, the islands and the SE peninsulas of the mainland were investigated for outcrops and glacial erratics. A few tens of erratic boulders with shatter cones and striated features, and a few brecciated rocks were discovered. Lamposaari Island in the eastern part of the lake revealed one fractured outcrop containing in situ porphyritic granite with converging striated features. Microscopic shock metamorphic features in two shatter‐cone‐bearing samples of porphyritic granite were found. These are planar deformation features (PDFs; up to two sets) in quartz and kink bands in biotite. Based on these geological, geophysical, and petrographic results, we suggest that Lake Summanen hides a relatively small, probably simple, meteorite impact structure, the twelfth confirmed one in Finland, of so far unknown age.

Earth's Impact Events Through Geologic Time: A List of Recommended Ages for Terrestrial Impact Structures and Deposits

“…After several hundred Myr, this crater (dashed circle) is vaguely recognizable by its morphology, and most of the impact crater geology is known from drillings (Grieve, 1978(Grieve, , 2006. (H) Satellite image of the recently discovered *2.6 km-diameter Summanen impact structure in Finland of uncertain age (Plado et al, 2018). After perhaps hundreds of millions of years of exposure, the crater (dashed circle) has been significantly overprinted by erosion, sedimentary infill, and glaciation and is today concealed by a lake.…”

The terrestrial impact crater record: A statistical analysis of morphologies, structures, ages, lithologies, and more

“…Such low‐relief magnetic zones have resulted in the identification of several smaller ancient impact structures, such as Summanen in Fennoscandia (Plado et al. 2018). The weak relief is related to a variety of possible causes including shock‐induced demagnetization of rocks in the crater, the random distribution of magnetic vectors due to chaotic displacements related to the cratering process, oxidization of magnetite, and the deposition of postimpact sediments with a weak magnetization.…”

Summanen structure: Further geological and geophysical evidence of a meteorite impact event in Central Finland

“…The structure was initially discovered using low‐altitude airborne geophysical data, which revealed circular electromagnetic in‐phase and resistivity anomalies 2.6 km in diameter (Lerssi et al., 2007). Further field campaigns and laboratory studies conducted in 2017 led to the discovery of erratic boulders containing shatter cones and planar deformation features (PDFs), indicating that the Summanen structure represents an old, eroded impact structure (Plado et al., 2018).…”

“…(b) Bedrock map (© Geological Survey of Finland) of Summanen after Nironen (2003). The dashed ring outlines the aeroelectromagnetic anomaly (AEM), 2.6 km in diameter (Plado et al., 2018). Shatter cone findings, multifrequency deep sounding electromagnetic (FrEM), and gravity profiles are shown.…”

“…Lake Summanen is located within an area characterized by strong regional magnetic anomalies, with a general SW to NE trend, reaching up to 1000 nT. However, the lake area itself is characterized by weak magnetic relief, lacking prominent anomalies (Plado et al., 2018). This pattern is not a diagnostic feature for an impact origin, but it appears to be typical for many impact structures in Fennoscandia, particularly those that lack highly magnetic impact melt rocks, such as Jänisjärvi (Elo et al., 2000), Karikkoselkä (Pesonen et al., 1999), Keurusselkä (Raiskila et al., 2013), Suvasvesi South (Donadini et al., 2006), and Tvären (Ormö & Blomqvist, 1996).…”