Hummeln - Hypervelocity Impact Crater

Alternate Names
Local Language
Coordinates 57° 22' 3" N; 16° 15' 20" E
Notes
  1. Located in Southern Sweden.
Country Sweden
Region Småland province
Date Confirmed 2015
Notes
  1. Confirmed based on the presence of PDFs in quartz (Alwmark et al., 2015).
Buried? No
Notes
  1. Part of structure has been infilled by Lake Hummeln (Alwmark et al., 2015).
Drilled? Yes
Notes
  1. One drill hole from eastern part of structure. Drilling was terminated at ~160 m depth due to economic reasons. Core showed Palaeozoic rocks occurred as a sedimentary breccia in places where the sequence was overturned (Alwmark et al., 2015).
Target Type Mixed
Notes
  1. Depression in Precambrian crystalline basement with Palaeozoic sedimentary rocks preserved in situ below Lake Hummeln. This is the only area in this part of Sweden that has any preserved Palaeozoic sedimentary rocks preserved since in all other locations these sedimentary rocks have been eroded away (Alwmark et al., 2015). (Lindström et al., 1999) perform a reconstruction that suggests the Precambrian basin was covered by not more than 0.13km of Paleozoic sediments, "of which the top 15m was well-lithified Lower Ordovician limestone that was underlain by thick, poorly consolidated and highly porous sand and clay of Cambrian age."
Sub-Type Limestone, Mudstone, Granite
Apparent Crater Diameter (km) 1.2 km
Age (Ma) ~465
Notes :
  1. A rough age of ~465 Ma is recommended based on stratigraphic constraints (Middle to Late Ordovician) (Grahn et al., 1996) (Alwmark et al., 2015).

Method :
  1. Stratigraphy
Impactor Type L chondrite
Notes
  1. Timing of this impact furthers the hypothesis of increased bombardment during the Ordovician related to the suggested 470 Ma L chondrite break-up event (Alwmark et al., 2015).

Advanced Data Fields

Notes

Erosion
5
  1. Only ~150 m of upper part of structure or all of elevated rim has been removed (Alwmark et al., 2015).
Final Rim Diameter
Unknown
Apparent Rim Diameter
1.2 km
  1. (Alwmark et al., 2015)
Rim Reliability Index
2
  1. Structure is a >160 m deep and 1.2 km wide depression. Diameter of 1.8 km is proposed for pre-erosion crater and also based on reconstructing target stratigraphy (Awlmark et al., 2015).
Crater Morphology
Simple
Central Uplift Diameter
km
Central Uplift Height
Unknown
Uplift Reliability Index
Structural Uplift
Unknown
Thickness of Seds
<0.130
Target Age
Precambrian Palaeozoic
Marine
No
Impactor Type
L chondrite
  1. Timing of this impact furthers the hypothesis of increased bombardment during the Ordovician related to the suggested 470 Ma L chondrite break-up event (Alwmark et al., 2015).
Other Shock Metamorphism
No
Shatter Cones
No
Planar Fractures
Yes
  1. Several quartz grains had one set of PFs parallel to c(0001) (Alwmark et al., 2015).
Planar Deformation Features
Yes
  1. Quartz grains with PDFs are found in larger clasts or free grains in the matrix of crystalline breccia samples. Up to 3 sets of PDFs were identified with U-stage microscope. With SEM, PDFs were identified as being decorated with vugs or tiny fluid inclusions. PDFs oriented parallel to c(0001), π{112}, [1122}, and m{1010}.
Diaplectic Glass
No
  1. Not detected (Alwmark et al., 2015).
Coesite
No
Stisovite
No
Crater Fill
  1. Clast-supported monomict breccia with a clastic matrix and a matrix-supported polymict breccia with large gravel to boulder-sized clasts found along the southern border of the lake shore as erratic boulders. Shocked quartz is found within the breccia samples (Alwmark et al., 2015). (Ferrière et al., 2016) report melt-bearing breccia (dikes along southern shore) and impact melt veins (in boulders on southern shore).
Proximal Ejecta
LB
Distal Ejecta
LB
Dykes
Volume of Melt
Depth of Melting

References

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J Ormö, E Sturkell, G Blomqvist, R Törnberg (1999) Mutually constrained geophysical data for the evaluation of a proposed impact structure: Lake Hummeln, Sweden, Tectonophysics 311(1-4), url, doi:10.1016/S0040-1951(99)00158-4

M Lindström, T Flodén, Y Grahn, S Hagenfeldt, J Ormö, E F F Sturkell, R Törnberg (1999) The Lower Palaeozoic of the probable impact crater of Hummeln, Sweden, GFF 121(3), url, doi:10.1080/11035899901213243

C Alwmark, L Ferrière, S Holm-Alwmark, J Ormö, H Leroux, E Sturkell (2015) Impact origin for the Hummeln structure (Sweden) and its link to the Ordovician disruption of the L chondrite parent body, Geology 43(4), p. 279-282, url, doi:10.1130/G36429.1