Björkö, a possible Middle Proterozoic impact structure west of Stockholm, Sweden

Abstract

The Björkö structure in the Björkfjärden Bay area of Lake Mälaren is one of more than 20 possible impact structures in Sweden. Rasta on Ekerön Island forms the eastern part and the islands of Pingst and Midsommar the southwestern part of this 8 km wide probable impact, which seems to be thoroughly altered by erosion. Despite alteration by erosion and sediment infilling, we tentatively place the location of the central uplift of the impact at Ingaberg on Björkö Island which is surrounded by a depression in crystalline bedrock. Along parts of the outer rim of the impact we find possible remnants of a wall structure.

Nine published papers about the Björkfjärden Bay area are reviewed and the numerous observations that, in our opinion, support the impact hypothesis are emphasized. New results in favour of an impact origin of the structure are also presented.

Chemical analyses of the claystone which forms the basal part of the Björkö structure infilling show that it mainly consists of a mica-type mineral. K-Ar datings of the claystone have given Middle Proterozoic ‘Jotnian’ ages of 1203 and 1216 Ma. It is interpreted here as a possible layer of impact melt. Planar elements in quartz occur in sandstone from Midsommar Island.

The surface relief and internal geometry of the Björkö structure are demonstrated by seismic reflection and refraction soundings, which show that the bedrock of Björkfjärden Bay consists almost entirely of sandstone reaching a thickness of some 250 m in its southwestern parts. The solid rock is covered by a fairly even layer of glacial and postglacial deposits 20 to 40 m in thickness. The lake floor is largely depositional with the Recent sediments cut by erosional channels.

Key Words:

• Björkfjärden
• Mälaren
• eastern Sweden
• Jotnian
• Middle Proterozoic
• impact structure
• claystone
• sandstone
• echosounding
• mud penetrator sounding
• seismic reflection profiling
• seismic refraction sounding