Structural and petrographic characterization of the Mien impact structure, Sweden

Abstract

The Mien structure, a partly lake-filled depression in southern Sweden, was confirmed to be an impact structure in the 1960s. The structure formed about 120 Myr ago. Poor exposure of outcropping bedrock in and around the lake, due to a thick cover of glacial sediments, has made morphological mapping and impactite classification challenging. Constrains on crater diameter and morphological features such as the central uplift of the structure are solely based on geophysical measurements from the 1980s and 1990s. Impactites have been found as glacial boulders along the southern shore of Lake Mien and in drill cores retrieved from the island Ramsö in the central part of the lake. The latter are the only findings of in situ impactites. This study combines field mapping with a review of geophysical measurements, as well as petrographic and petrological analysis of sampled impactites and pre-existing thin sections from earlier studies on Mien. This has led to an allocation of the impact structure’s centre and central uplift, constrains on the distribution of impact melt rock and the identification of Ramsö as a likely crater fill remnant. In this thesis two types of lithic breccia, two types of suevitic breccia and five types of impact melt rock have been identified. Findings of larger structural features, such as rim faults and radial fractures, that could assist the assignment of crater diameter and overall morphology, proved unsuccessful due to the advanced erosion of the impact structure and the poor access and quality of outcropping bedrock.

Abstract (Swedish)

Mienstrukturen, en delvis vattenfylld sänka i södra Sverige, bekräftades vara ett astroblem på 1960-talet. Strukturen bildades runt 120 miljoner år sedan. Dålig exponering av berggrunden i och runt sjön, på grund av tjocka lager glaciala sediment, har gjort geomorfologisk kartering och klassificering av impaktbergarter till en utmaning. Data angående kraterdiameter och morfologiska särdrag som exempelvis central uplift är enbart baserade på geofysiska mätningar från 1980– och 90-talet. Impaktiter har hittats i form av flyttblock längs sjön Miens södra strand, samt i borrkärnor från ön Ramsö som ligger i den centrala delen av sjön. De senare är de enda fynden av in situ impaktiter. Denna studie kombinerar kartering i fält med en granskning av geofysiska mätningar, samt petrografiska och petrologiska analyser av bergartsprover och redan existerande tunnsklip från tidigare studier av Mien. Detta ledde till en lokalisering av kraterstrukturens centrum och central uplift, fördelning av impaktsmälta samt karaktäriseringen av Ramsö som sannolika rester av kraterfyllnad. Detta arbetet beskriver två typer av litisk breccia, två typer av suevitisk breccia samt fem typer av impaktsmälta. Storskaliga särdrag, som exempelvis förkastingar och radiella sprickor i berggrunden, kunde inte identifieras på grund av den fortskridna erosionen av astroblemet samt den svårtillgängliga och sig i dåligt skick befinnande berggrunden.

Popular Abstract

In the forests of Småland, southern Sweden, around 50 km north of Karlshamn, lies a round lake, a few kilometres in diameter. The depression, in which this lake has formed, is the remnant of an impact crater that resulted after a meteorite hit the Earth around 120 million years ago. Findings of minerals that form only under the extreme temperature and pressure conditions of meteorite impacts in the rocks around Mien have proved this. However, time and the repeated glaciations that haunted Scandinavia for the past 2 million years have removed or buried most of what is left from the crater structure, as well as the rocks that formed during the event. Except for rocks found during a drilling campaign on the island of Ramsö in the middle of the lake, only loose glacial boulders of the so called impactites have been found in the area. That is, of course, a problem when trying to determine the actual location, the size and inner structure of the old crater. Research on this has mostly relied on geophysical measurements: the detection of magnetic and gravitational anomalies in the ground, as well as seismic measurements, which detect borders between rock types, sediments and weak zones, such as faults and fractured bedrock. In this master’s thesis I have tried to get a clearer picture of the impact structure by combining all the old research on Mien, dating back to the mid-nineteenth century when impact craters as a thing itself were still debated, with some of my own mapping and sample collection. The results include 1) determining the crater centre and the original central uplift, a central peak of bedrock common in larger impact craters, to coincide with the centre of the modern day Mien lake, 2) constraints on the distribution of impact melt, which forms close to the point of impact, since high temperatures and pressures are needed to melt the bedrock, and 3) the island Ramsö is probably a leftover piece of crater infill, meaning the rocks that form and fill up the crater cavity during and after the meteorite hits the ground. The thesis also includes the description of the different rock types found during my own and earlier sample collections at Mien. However, large-scale evidence of the event, such as radial fractures or faults around the rim of the crater, that could help with determining the size of the structure and thereby calculate the size of the meteorite, are still much of a mystery and probably hidden underneath all the thick layers of glacial sediments and the thriving vegetation.

• author: Schönherr, Lara
• supervisor: Sanna Alwmark
• organization: Department of Geology
• course: GEOR02 20251
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Impact structure, Mien, impactites, structural morphology, shock metamorphism
• publication/series: Dissertations in Geology at Lund University
• report number: 728
• language: English
• id: 9228167
• date added to LUP: 2026-05-27 09:49:43
• date last changed: 2026-05-27 09:49:43