Background

The circular shape of the Avike Bay, centered at 62.5° N and 17.8° E, (Fig. 1), first drew attention of scientists during detailed mapping of the nearby Alno intrusive complex in the 1960s. A study of the dikes typical for the Alno complex showed that several dykes extended to the shores of the Avike Bay. Soderstrom (1966), therefore, suggested that the bay was a caldera structure related to the Alno complex. The find of an enigmatic quartzite breccia at the SW shore of the bay was reported by Lundquist et al. (1990), who regarded it as a tectonic breccia related to an unknown fault zone. Boulders of alnoitic and carbonatitic breccias containing clasts of similar quartzite and also a fragment of sovite have been reported from the shore of the bay and were considered to emanate from within the bay. The composition of this sovite is, however, different from that of other parts of the Alno region (Lundquist et al. 1990).

Recent studies have determined the physical properties, especially the electric resistivity, of rocks from this area. The enigmatic breccia has been studied in more detail. At several shallow areas in the bay, boulders of Paleozoic limestone and brecciated dolerite were recovered from the seafloor.

An aeromagnetic survey of the bay region was performed by the Boliden Mining Co.; later the region was re-measured by the Geological Survey of Sweden as part of a regional geophysical mapping program. Gravity measurements are so far restricted to land areas. Rock physical properties have been measured both in-situ and on rock samples to facilitate modeling of the magnetic anomalies observed in the aeromagnetic data and potential gravity anomalies.

The coastal region around Avike Bay is presently emerging from the Bothnian Sea at a rate of almost 8 mm a-1 (Sveriges National Atlas 1994). Wave erosion has removed un-consolidated Quaternary glacial sediments from the exposed parts of all near coastal hills. Due to extensive erosion in connection with glaciations, also the weathered top part of the bedrock has been removed; the rock outcrops are therefore extremely smooth and clean. The predominant ice flow direction is 130° (towards SE) as evidenced by glacial striae on rock outcrops on several small and thus the boulder transport is in a similar direction. The ice flow divide was located near the present western coast of the Bothnian Sea, but it is not known with sufficient precision to facilitate knowledge of the source region for erratic boulders within the bay.

Fig. 2. Simplified elevation and bathymetric map, showing the general morphology of the Avike Bay structure. The gray level intervals are 25 m and the present shore line has been marked together with a few elevations and depths, respectively. The structure is ca. 150 m deep with a ca. 40 m high and ca. 1 km wide submarine central mound. Sampling was made in the marked regions (Density and magnetic properties within A and B, in-situ electric resistivity measurements within C, D, and at scattered localities marked with +). Localities 1, 2, and 3 are mentioned in the text.

Fig. 2. Simplified elevation and bathymetric map, showing the general morphology of the Avike Bay structure. The gray level intervals are 25 m and the present shore line has been marked together with a few elevations and depths, respectively. The structure is ca. 150 m deep with a ca. 40 m high and ca. 1 km wide submarine central mound. Sampling was made in the marked regions (Density and magnetic properties within A and B, in-situ electric resistivity measurements within C, D, and at scattered localities marked with +). Localities 1, 2, and 3 are mentioned in the text.

The Ävike Bay structure was first suggested as an impact structure by Henkel and Lilljequist (2001), based on the extraordinarily circular topography and the presence of polymict breccia.

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