The Brim

The brim (Fig. 4) is identified as a body of more or less strongly fractured, crystalline rock that includes monomictic breccia (Tandsbyn Breccia). Where it is preserved from erosion, it extends continuously outwards from the rim of the inner crater. At its outer margin the brim rises slightly above the adjacent terrain which consists of either the well-preserved basement rocks of the sub-Cambrian peneplain, or Cambrian and Ordovician sedimentary rocks that rest on the peneplain. In the west and north sectors, where this brim is widest, the width is about 2.5 km. In the southwest and south the brim forms outliers on the sub-Cambrian peneplain. At least some of these outliers were isolated from one another by erosion. The brim

Fig. 5. Structure of the flap of crystalline ejecta at Nordanbergsberget and underlying geological units. The section is about 15 m high. The symbols are the same as in Fig. 1, excepting the conglomerates. cgl: basal conglomerate of Cambrian. s-Cp: sub-Cambrian peneplain. The sketch shows an inverted succession of strongly fractured, crystalline basement, Cambrian basal conglomerate, and Cambrian black shale that rests on a normal succession of (downwards) Cambrian black shale, basal conglomerate, the sub-Cambrian peneplain, and intact crystalline basement.

is less well exposed in the eastern part of the crater, but available data indicate a width less than 1 km.

The topography of the brim has numerous irregularities with amplitudes from a few meters to tens of meters, and rises at the most about 50 m above the rest of the terrain that surrounds the inner crater (Sturkell and Lindström 2004). Thus it appears as a low platform on the larger scale. It does not conform to the concept of a raised rim, as described from different terrestrial craters and summarized by Melosh (1989). Much of the crystalline rocks of the brim is brecciated to various degree, but at least as much looks fairly well preserved in outcrop. However, no blocks of apparently well-preserved rock are demonstrably larger than about 100 m. The preservation of patches of resurge deposits and post-impact marine sediments on the brim, indicates that its upper surface has not been significantly lowered by erosion after the impact.

Our previous studies of the Lockne crater (Lindström et al. 1996) have dealt with major parts of the brim as if they consisted essentially of autochthonous basement granite that was crushed to various degree by the impact. This interpretation can be regarded as an attempt to apply the concept of structural, or stratigraphic, uplift (Melosh 1989). However, we are now convinced that the brim is not autochthonous but consists of ejecta from the inner crater, and that these ejecta are underlain by the essentially intact sub-Cambrian peneplain.

The only vertical outcrop section through the brim just outside the inner crater occurs at the Skanska quarry at Nordanbergsberget (National Grid Reference 699320/144900) (Lindström and Sturkell 1992, Lindström and

von Dalwigk 2002). This section (Fig. 5) exposes about 350 m of a sheet of fractured granite in a southeast-northwest direction. 150 m of it rests on Cambrian black shale in an identifiable state, including pyritized trace fossils (Simon 1987a). The basal Cambrian gravel deposit was found still in place under this shale. The gravel deposit was in its turn underlain by the granite of the sub-Cambrian peneplain that apparently extends throughout the quarry. It is a most significant feature of this section that the Cambrian basal gravel is present above as well as beneath the black shale. The upper gravel sheet has an overturned sedimentary contact against the granite that rests upon it (Lindström and Sturkell 1992, Lindström and von Dalwigk 2002). Thus, it is the very top of the basement that became overturned and ejected from the nearby outer part of the inner crater.

The section was described by Simon (1987b), who explained it as a possible instance of Caledonian overthrusting. This explanation appeared to be justified before the Lockne impact structure was identified. However, in order to be emplaced by overthrusting, the over 350X50 m2 large and almost 20 m thick, heavily segmented sheet of granite, with weathering crust and overlying basal conglomerate, would have had to be detached along a horizontal surface. Thereafter it must have rotated 180° about a horizontal axis. All of this must have happened without much rotation of the granite segments relative to one another or other significant change of shape. Whereas the shearing-off of horizontal basement slices occurs commonly at the base of major overthrust complexes, the practically nondestructive overturning of entire slices of this structure. shape, and size is mechanically and geometrically most unlikely to occur within an overthrust. Therefore we explain the inverted basement sheet in the Skanska quarry as overturned ejecta, which normally occur at impact craters.

Cambrian shale has been reported to underlie about 50 m of granitic rock belonging to the brim in water wells at Tand 5 km southwest of the Skanska quarry and 4.7 km west of the crater centre (Fig. 2) (Lindström and von Dalwigk 2002). Another occurrence of Cambrian shale relevant to the interpretation of the brim was reported from 1 km west of Tandsbyn by Thorslund (1940). This outcrop in the midst of the Tandsbyn gully is now covered by bush and a road, so that it cannot be verified. If the Tandsbyn gully was formed by resurge flow eroding the crushed granite of an essentially autochthonous brim, this would have been the last place where one would look for an outcrop of autochthonous Cambrian shale. Because this was the interpretation of Lindström et al. (1996), the Cambrian shale reported by Thorslund (1940) was neglected as questionable, probably for no good reason. If this outcrop was correctly identified by Thorslund, as

Fig. 6. North-south section through inlier of Cambrian black shale in the Tandsbyn gully west of Tandsbyn. Vertical and horizontal scales are in meters; the vertical scale is in meters above sea-level. The symbols are the same as in Fig. 1. s-Cp: sub-Cambrian peneplain. The sketch shows a sheet of crystalline Proterozoic ejecta than rests on the sub-Cambrian peneplain with patches of preserved Cambrian shale. The ejecta sheet is itself overlain by a thin cover of resurge sediment and secularly deposited, post-impact Dalby Limestone. The section roughly coincides with a section given by Thorslund (1940, Fig. 28).

1000

Fig. 6. North-south section through inlier of Cambrian black shale in the Tandsbyn gully west of Tandsbyn. Vertical and horizontal scales are in meters; the vertical scale is in meters above sea-level. The symbols are the same as in Fig. 1. s-Cp: sub-Cambrian peneplain. The sketch shows a sheet of crystalline Proterozoic ejecta than rests on the sub-Cambrian peneplain with patches of preserved Cambrian shale. The ejecta sheet is itself overlain by a thin cover of resurge sediment and secularly deposited, post-impact Dalby Limestone. The section roughly coincides with a section given by Thorslund (1940, Fig. 28).

we now believe, one can conclude that the bottom of the Tandsbyn gully is close to the level of the sub-Cambrian peneplain, with sporadic patches of preserved Cambrian shale, and that the brim on both sides of the gully consists of rootless ejecta that rest on the peneplain (Fig. 6).

An analogous outcrop of Cambrian shale in the Langmyren gully (Fig. 2; see below, chapter Resurge Gullies) is evidence that this gully, too, locally bottoms at the sub-Cambrian peneplain. An outcrop of fossiliferous Upper Cambrian shale at Haga (at approximately 698710/145350, east of Locknesjon), briefly referred to by Thorslund (1940), is evidence that this locality is outside the inner crater. Nearby outcrops of monomictic Tandsbyn Breccia, referred to in Thorslund's (1940) publication, but likewise nonexistent nowadays, are evidence of the ejected brim.

Cambrian shale with preserved basal conglomerate and a substratum of weathered Proterozoic dolerite occurs in an erosion window within the domain of the brim at Nordanbergsberget west of the Skanska quarry (National Grid coordinates 699305/144875). The locality is occupied by a water ditch in a golf course and is surrounded on all sides by crystalline ejecta. Other occurrences of Cambrian shale have been identified at outliers of the brim outside the southwest sector of the inner crater (Morttjarnen and 1 km E of Morttjarnen; Fig. 2).

Ar distances over 1.5 km outside the rim of the inner crater the substratum of the crystalline ejecta may include not only Cambrian shale, but Ordovician limestone as well. As a rule the Ordovician limestone is either strongly folded, or its bedding is disintegrated into small limestone nodules chaotically scattered in a marly matrix. Limestone affected in this way occurs for instance on both sides of the small road to Lofsasen 500-

Fig. 7. Matrix-supported Tandsbyn Breccia (monomictic; granitic protolith). 900 m southeast of Hallnaset. Photo Jens Ormo.

800 m E of Nyckelberg (Fig. 2). The segment of the crater on the east shore of Locknesjon has Upper Cambrian and Lower to Middle Ordovician rocks within a few hundred meters from the rim of the inner crater, and apparently overlain by ejected monomictic breccia from the crystalline basement.

In accordance with the interpretation of the brim as ejecta, monomictic breccias and strongly deformed crystalline rock are prominent in its composition. Close to the margin of the inner crater the crystalline rock of the brim may, however, be apparently well-preserved and continuous within about 100 m long segments, as for instance in the Skanska quarry at Nordanbergsberget and on the south side of the Tandsbyn gully southwest of Tandsbyn.

There are three principal varieties of monomictic crystalline breccia at Lockne: matrix-supported breccia, clast-supported breccia, and carbonaceous breccia. The matrix of the matrix-supported breccia (Fig. 7) is very hard and consists of particles near and below the size that can be

Fig. 8. Clast-supported, granitic Tandsbyn Breccia. 1,300 m southwest of Hallnaset. Photo Jens Ormo.

resolved in thin sections under the light microscope, i. e., the matrix tends to appear isotropic under crossed Nicols. Macroscopically this matrix is commonly dark grey and porcellaneous in fresh specimens and light brownish in weathered outcrops. Clasts are commonly smaller than 4 cm; they frequently have irregular outlines. These breccias show microscopic evidence of shock, such as deformed feldspar lattices and micas, but no quartz with PDF has been identified so far.

Clast-supported breccias (Fig. 8) mostly have angular clasts with a size variation from less than a millimeter to many centimeters. However, the smallest clasts do not dominate to the extent that they form a continuous, low-porosity matrix. These breccias represent crushing with little evidence of shock.

In the carbonaceous breccias the crystalline clasts vary in size from less than a millimeter to several centimeters. They are separated by dark, sooty or asphalt-like matrix with an abundance of curved and striated shear-planes. The planes and striations lack any obvious preferred orientation. These breccias disintegrate easily and do not appear in natural outcrops. They probably originated from the topmost level of the crystalline basement, because the area has no other known source of carbonaceous matter than the Cambrian black shale that overlies the basement in undisturbed successions.

Among evidence of shock deformation in the brim zone is small-scale disharmonic folding of the basement/Cambrian contact. This was observed in several places in the Skanska quarry and would not occur under normal tectonic stresses at the contact between a massive, competent unit (basement) and a very weak unit (Cambrian bituminous shale). Another observed evidence is splayed crinkling of foliated inclusions in basement rocks. Thirdly, movement parallel stretching and fluting of shock-weakened basement rock occurs in movement zones (Fig. 9). Despite much search, no convincing shatter cones have been found.

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