Resurge Gullies

The brim is crosscut by radial channels (Fig. 2 and 4), at the bottom of which there are Locknee Breccia and loftarstone, deposited by the resurge from the collapsing water crater. The Nyckelberg gully to the north is neither deep nor particularly extensive. The Tandsbyn gully to the west is the best preserved and exposed. The Bergböle-Loke gully to the south is large, but its western margin is distorted tectonically. Its eastern margin is missing owing to erosion in late geologic time and, probably, tectonics as well. Roughly at mid-distance between the Tandsbyn gully and the Bergböle-Loke gully there is a less conspicuous gully that has come under observation only recently; it can be called the Langmyren gully.

Lindström et al. (1996) suggested the presence of a further similar structure, which they named the Rossbol gully, in the northeastern sector of the crater. The existence of this structure was inferred from two shallow core drillings. However, as will be discussed below, the informations from these drill-holes can be interpreted differently in the light of recent work. The Rossbol gully is therefore questionable but will be discussed at the end of this chapter.

The gullies are tens of meters deep. They either cut through to the base of the brim, in which case the sub-Cambrian peneplain forms their bottom, or they are not quite as deep and bottom in the rocks of the brim. The latter case is represented by the Nyckelberg gully. The Langmyren gully is eroded to the sub-Cambrian peneplain in one place (National Grid coordinates 698550/144690), where Cambrian shale is also exposed. Its

Fig. 9. Fluting in basal part of sheet of crystalline ejecta. North shore of Mörttjärnen. The height of the outcrop is about 1.5 m. Photo Jens Ormö

lower reach crosses ejected crystalline rock that subsided during the crater modification.

The gullies widen away from the inner crater. The Tandsbyn gully is about 1 km wide near the rim of the inner crater but widens considerably and branches to the northwest, west, and southwest about 1.5 km west of

Fig. 10. Ynntjarnen Breccia. Brunflo Church, lawn of the congregation localities. The width of the pictured surface is about 35 cm. Photo Erik Sturkell.

Tandsbyn. Its bottom is close to the sub-Cambrian peneplain and contains Cambrian shale (Fig. 6).

The existence of the gullies immediately after cratering is proven by the resurge deposits inside them. However, it is debatable, how much they owe their existence to erosion by resurge. Von Dalwigk and Ormo (2001) demonstrated that such erosion was active. The study of Shuvalov and von Dalwigk (2002) indicates that no other agent than erosion could have completed the formation of the gullies, but that it must be taken into account that the amount of erosion might be small in view of the enormous mass of water in motion. Furthermore, the preservation of soft Cambrian shale at the bottom of gullies shows that such erosion could not have cut deeply, if at all, into the autochthonous basement.

We argue that an ejected blanket of inhomogeneously disaggregated rock will develop thickness variations in the tangential direction, that is, normal to the ejection. Attenuation in this direction will appear as radially orientated tears that tend to widen outwards. If the mechanism is real, it implies that incipient, though perhaps not continuous, furrows were already there when the resurge current touched the surface of the brim. Subsequent erosion by the resurge extended these pre-existing furrows in the radial direction.

Although bedrock outcrops are much more abundant in the Lockne area than in most other areas in mainland Sweden, there is not enough of them to make it an easy task to follow the gullies beyond the brim, if they do at all exist there. The velocity of the culminating resurge in the identified gullies was demonstrably enough to transport cobbles, or coarser (Ormö and Miyamoto 2002). It was even greater than that according to Shuvalov et al. (this volume). For instance, Lockne Breccia in the Bergböle-Loke gully contains a mixture of slabs from diverse levels of the Lower-Middle Ordovician orthoceratite limestone, many slabs being over 5 m3 (Lindström et al. 1983). Some exceptionally large clasts may, however, occur near the sites they occupied when the water cavity was excavated (Shuvalov et al., this volume). This may be the case with an inverted slab of limestone of at least 2,000 m3 that occurs close to Grubban (Fig. 2; loc 20E, Lindström and von Dalwigk 2002)

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