Tehostekuva

Lumparn impact structure

Geographical setting

The Lumparn impact structure is situated in the main island of the Âland archipelago about 60 km southwest of the Finnish coast. Center coord.: 60°09'N Lat., 20°07'E Long.; NFRS: X 6690.0, Y 119.0; sheet 1012

The structure is represented by an almost land-locked, rhomb-shaped bay with side-lenghts of approximately 9-10 km and ~80 km2 in area. Only a few little marginal islands lying in the bay, making it a very conspiceous feature in the Âland archipelago. The deepest troughs of the bay (max. ~35 m) are located close to the eastern shore. The mean depth is about 20 m.

General geology

The primary rock type is 1.59 - 1.57 Ga old rapakivi granite widely exposed along the shore lines. Within the bay, close to its northern shore, an in situ deposit of Ordovician limestone has been preserved from glacial erosion. A small skerry of this limestone becomes visible only at low water. Sandstones and siltstones of Lower Cambrian and Lower Ordovician age occur as clastic dikes on the Âland island and its wider surrounding. In other areas of Lumparn bay the information was obtained from a number of drillings, including cores from the central part of the bay.

Petrology

Investigations of drill core material have shown, that below Quaternary deposits the bay is partly at least 50 m thick breccia unit consisting of crushed rapakivi granite breccia. Samples from this breccia reveal unequivocal PDFs in quartz (fig. 1), mosaicism in feldspar, melt fragments. Conically striated surfaces as well as breccia dikes and breccia patches have been found along the southwestern and western shore line of the bay and on the islet in the western periphery of the bay. Some star-like fracture sets, centered on perpedicular situated faults, were observed on the eastern shore line. As this features are not known from anywere else in the region, they may be also of impact origin.

PDFs in quartz grains PDFs in quartz grains from drill core material; horizontal edge is 0.6 mm (courtesy of M. Lehtinen, 1997)

Geophysics

Above Lumparn bay a a weak aeromagnetic signature (flight-altitude 150 m) is present.

Age of the impact

Stratigraphic relationships indicate that the impact event occured between about 1.2 (post-Jotnian) and 0.5 Ga (pre-Lower Cambrium).