Ynntjrnen Breccia

Since the publication of the paper of Lindström et al. (1983) those Middle Ordovician (lower Caradoc) rocks occurring in the Lockne area that contain isolated bodies of reorientated or physically disintegrated limestone, have collectively been called Lockne Breccia. The original assumption was that all of these rocks have a shared origin as debris flows. The idea of a common origin remained even after it was realized that debris flows, properly defined, had not been active. The discussion of von Dalwigk and Ormö (2001) opened the door to doubt, and by now it has become clear that we are dealing with two discrete units of the impact stratigraphy, with slightly different lithologies, discrete timing, and important differences as regards the mode of origin.

The whole Lockne Breccia in the earlier sense was regarded as a product of the resurge flow (Lindström and Sturkell 1992). Frequently occurring transitions to the immediately overlying, arenitic loftarstone

Fig. 11. Ynntjarnen area. The mapped area is located on Fig. 1. 1:Crystalline basement. 2: Cambrian black shale. 3: Lower and Middle Ordovician, mainly limestone. 4: Ynntjarnen Breccia. 5: Sheet of crystalline ejecta. 6: Lockne Breccia (resurge deposit). 7: Loftarstone (resurge deposit). 8: Dalby Limestone (post-impact, secular deposit). 9: Lake Ynntjarnen (at the middle of the map's north margin). 10: Meters above sea-level. 11: Railroad. 12: Thrust fault.

Fig. 11. Ynntjarnen area. The mapped area is located on Fig. 1. 1:Crystalline basement. 2: Cambrian black shale. 3: Lower and Middle Ordovician, mainly limestone. 4: Ynntjarnen Breccia. 5: Sheet of crystalline ejecta. 6: Lockne Breccia (resurge deposit). 7: Loftarstone (resurge deposit). 8: Dalby Limestone (post-impact, secular deposit). 9: Lake Ynntjarnen (at the middle of the map's north margin). 10: Meters above sea-level. 11: Railroad. 12: Thrust fault.

were observed, and the occasionally abundant occurrence of crystalline ejecta was regarded as a characteristic feature. The failure to find such ejecta in many outcrops was simply taken for scarce occurrence, but now we are convinced that these outcrops consist exclusively of fragments of local limestone and calcareous-argillaceous ("marly") matrix.

Renewed mapping and improved outcrop conditions have corroborated the conclusion that the Lockne Breccia is a polymictic sedimentary breccia with crystalline ejecta clasts. It is a widely distributed lithology, occasionally transitional upwards to the loftarstone, and should keep its name. We have also become convinced that there is an earlier formed monomictic limestone breccia which may consist of limestone disaggregated into nodules that are chaotically dispersed in a calcareous-argillaceous matrix (Fig. 10). This early-formed breccia will be referred to under the new name Ynntjarnen Breccia (Fig. 4) (although the need for this kind of local names can indeed be discussed, we consider them useful because they can apply regardless of possible fluctuations of interpretation).

The Ynntjarnen Breccia has its name from the area within less than 1.5 km west, south, and east of the small lake Ynntjarnen (Ohntjarn in Thorslund 1940) (Fig. 11). The area is characterized by a succession of southeast-vergent thrust faults with a period of 40-400 m and throws of the order of 50 m. This instance of Caledonian deformation has led to an estimated 300 m of horizontal shortening, but more important, to a repetition of stratigraphic units that makes it relatively easy to unravel the stratigraphy. The lowermost outcropping unit is well-preserved beds of Lower to Middle Ordovician orthoceratite limestone that form exposed successions of at least 10 m thickness. At the top of this unit there is Ynntjarnen Breccia consisting of disaggregated marly-nodular limestone. In the eastern outcrops this breccia is but a few meters thick at the most, but the western outcrops, mainly along the railway, have thicknesses of over 10 m (Fig. 11).

The orthoceratite limestone and the Ynntjarnen Breccia are overlain in places by bodies of ejected Tandsbyn Breccia with volumes of the order of 10,000 m3. The next higher unit is water transported Lockne Breccia that includes a variable content of crystalline clasts of gravel to cobble size. The Lockne Breccia rests unconformably on either orthoceratite limestone, Ynntjarnen Breccia, or Tandsbyn Breccia.

We interpret the Ynntjarnen Breccia as derived from the top layers of the orthoceratite limestone. These layers took the rap from, first, cratering in the overlying water body, thereafter, the impact of crystalline ejecta, and, finally, the resurge with the full force of its initial phase. Its components were strongly stirred, but not carried by water. Its principal occurrence is in the west and northwest sectors of the area adjacent to the crater. A large area on both sides of the road between Nyckelberg and Lofsasen, as well as northwest of Nordanbergsberget, has Ynntjarnen

Breccia occurring on top of widely distributed orthoceratite limestone. This area was earlier mapped as Lockne Breccia.

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