Geology

The rocks of the Ävike Bay region have been mapped and described by Lundquist et al. (1990). Gneisses of meta-sedimentary rocks predominate, with ages older than 1.9 Ga. The gneiss is intruded by small local massifs of granite of 1.8 Ga age (Claesson 1987). Gneisses are predominant over the local granite bodies. These Svecofennian rocks are intruded by a large north-south striking and eastwards dipping Mesoproterozoic dolerite sill of

Fig. 3. Interpretation of fracture patterns based on the detailed standard topographic map in scale 1:50 000 with 5 m contour interval.

so called Äsby type. These dolerite intrusions are widespread in central Fennoscandia and are characterized by plagioclase, augit and olivine as their major mineral constituents, and a coarse grained ophitic texture (Lundquist 1979). Dolorites intrude the Jotnian sandstone and have been given an age of 1.2 Ga (Welin 1979). Boulders (Fig. 4a) of dolerite have been recovered from shallow water areas within the bay at locality 3 (Fig. 2), indicating that the sill extends several km into the bay.

The circular Ävike Bay structure clearly cuts the dolerite sill. Along the shore, at locality 1 (Fig. 2), an enigmatic quartzite breccia occurs (Fig 4b). The stage of metamorphism of this quartzite is much lower than that of the migmatized Svecofennian gneiss and granite complex. The quartzite is assumed to be of Jotnian (Riphean) age due to its similarity with known occurrences around the Bothnian Sea and the Bothnian Bay. The closest location of outcrops of Jotnian sandstone at the coast is ca. 50 km north of Ävike Bay (Lundquist et al. 1990). The brecciation pattern of the quartzite is identical with patterns observed in ejected clasts of sedimentary target rocks in the Ries and Steinheim impact structures (Stöffler and Ostertag 1983), in terms of the variation in clast size, rotation of clasts and a matrix of very fine grained fragments. At low waterstand in 2000 and 2001, the breccia was observed to be part of a larger occurrence of polymict breccia with clasts of quartzite and local crystalline rocks, cemented with fine grained calcitic breccia of mineral fragments (Fig. 4c).

Summarizing the results of field observations and thin section studies from 2000 to 2003, the characteristics of different types of breccias at the shore of Ävike Bay are as follows: (1) Breccia dikes are derived from Svecofennian rocks and distributed inside Svecofennian basement fracture zones. The macroclasts and brecciated micromatrix of the dykes are cemented with carbonate. (2) Meter-sized blocks of so called "enigmatic" brecciated quartzite are also distributed within fracture zones. (3) The regolith of the polymict breccia is composed of both Svecofennian and Jotnian (?) clasts, scattered as erratic cobbles and boulders along the sea shore near the zone of brecciated bedrock. These erratics are derived from within the bay, where further outcrops could be observed at low waterstand. A Jotnian age of the low-metamorphic quartzite is suggested based on the occurrence of Jotnian sediments covering the Svecofennian basement of the Sea and Gulf of Bothnia, including the area just east of Ävike Bay. The Jotnian sediments were eroded from the western coast of the gulf as late as in Phanerozoic time (Puura and Plado, this volume). (4) Small dikes of alnöitic intrusives occur scattered in the region and have ages from 210 to 600 Ma (Brückner and Rex 1980).

Fig. 4. Pictures of the deformed rocks. For locations see Fig. 2.

a. Boulder of brecciated dolerite recovered from shallow water. The scale has cm divisions.

b. Outcrop of quartzite breccia. The compass is 6 cm wide.

d. Strongly fractured gneiss with shatter cone like lineations.

Fig. 4. Pictures of the deformed rocks. For locations see Fig. 2.

a. Boulder of brecciated dolerite recovered from shallow water. The scale has cm divisions.

b. Outcrop of quartzite breccia. The compass is 6 cm wide.

c. Polymict breccia with fine grained breccia matrix containing calcite healed fractures, deformed mica, melted quartz grains and recrystallised rims of mineral grains. The scale divisions are 1 cm wide.

d. Strongly fractured gneiss with shatter cone like lineations.

Fig. 5. Planar features in mineral grains from polymict breccia at locality 1; for location see Fig. 2.

a. Sheared quartz fragments with fluid inclusion trails in polymict breccia from thin section Av-5. The length of the largest quartz fragment is 1.4 mm. Oblique polarizers at ca. 30o angle.

b. Interior (SW marginal part) of the quartz grain from Fig. 5a with two systems of microdeformation features. The spacing is 3-8 ^m. Oblique polarizers. The width of the picture is 0.1 mm.

c. Clast of Jotnian (?) sandstone in a carbonate dominated breccia matrix. The quartz grain with microdeformation features is in the center of the image. Oblique polarizers. The width of the picture is 0.5 mm.

d. The quartz grain with microdeformation features seen in the center of Fig. 5c with one system with spacing of 7 ^m. The width of the grain is 0.28 mm.

Fig. 6. Distribution of microdeformation orientations in quartz from polymict breccia. The histogram shows the angle of observed planes with the c-axis. The class width is 5o.

Along the shore of Avike Bay, numerous boulders of brecciated alnoitic material (beforsite etc.) have been observed (Lundquist et al. 1990). Also a few boulders of unfractured, orthoceratitic, brown limestone of Ordovician age were recovered from the shore of the bay and from shallow water within the bay, at locality 3 (Fig. 2). Ordovician limestone is interpreted to occur in a large basin just east of the distinct offshore escarpment marking the west coast of the Bothnian Sea (Ahlberg 1986). This escarpment is located right at the eastern margin of the circular bay and truncates a sector of the structure. To the NW, i.e. in the direction from which the ice flowed, the nearest preserved occurrence of this limestone is in Jamtland, ca. 150 km from the bay.

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