Large Masses of Crystalline Ejecta

Some of the most spectacular outcrops of Tandsbyn Breccia occur outside the brim at distances of 6-9 km from the centre of the crater. The Tandsbyn Breccia has resisted erosion better than the surrounding lithologies and commonly forms hillocks that are several meters high. Thorslund (1940) thought that these local rises of monomictic crystalline breccia are rooted in the basement vertically beneath them. He developed a widely accepted model of a local granitic archipelago in the Ordovician sea (Thorslund 1940).

Table 1. Major masses of crystalline ejecta. Coordinates Swedish National Grid (modified UTM), Names used in maps and text, V volume in estimated 10 m3, Distance given in km from crater centre, Direction to centre of ejecta mass from crater centre (east is 90°, south 180°, west 270°).

Coordinates

Name

V

Distance

Direction

698425/144525

Berget

20

6.3

228°

698458/144455

Kiocksâsen 1

50

6.5

232°

698525/1 44445

20

6.3

239°

698454/144430

Kiocksâsen 2

10

6.8

233°

698740/144360

Ynntjärnen S

2,000

6.4

258°

698785/144390

50

6.0

263°

698770/144405

10

5.9

260°

698800/144350

Ynntjärnen W

50

6.5

265°

698805/1 44330

50

6.7

265°

698700/144150

Sved

50

8.6

259°

698630/144095

Karsätjärnen

10

9.2

256°

699335/144550

Nyckeiberg

400

6.3

320°

The volumes given in Table 1 are low estimates based on observed horizontal dimensions and average thicknesses of 10 m. At Nyckelberg the estimate is based on the assumption that a few discrete bodies lying close to one another were continuous before becoming isolated from one another by erosion following upon minor folding. The distances are given from the approximate geometric centre of the inner crater to the centres of the ejected bodies.

Mercury Geological Features
Fig. 12. Loftarstone resting with sharp contact on Lockne Breccia. 1 km southwest of Hallnaset. Swiss army knife for scale. Photo Erik Sturkell.

The listed bodies spread within a sector of about 90°, with the bisectrix at about 270° and the arithmetic mean at 257°. Karsatjarnen (Fig. 2) is exceptional for the over 9 km distance of ejection, but also for its composition. It is the only isolated ejecta body that is carbonaceous. These two circumstances indicate that it was ejected from a position that was near the impact centre as well as close to the boundary between the crystalline basement and the Cambrian shale. The size and position of Ynntjarnen South (Fig. 2) suggest that it can be regarded as an outlier of the brim, although its separation from the brim existed already in the resurge phase, on the evidence of resurge deposition between it and the brim.

Mitochondrien Elektronenmikroskop
Fig. 13. Lockne Breccia transitional upwards to loftarstone. 1 km southwest of Hallnaset. Swiss army knife for scale. Photo Erik Sturkell.

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