The Proximal Ejecta Around the Marine‐Target Lockne Impact Structure, Sweden

Abstract

Except for some impact structures on low-gravity cosmic bodies, essentially all fresh impact structures are surrounded by ejecta from the crater interior (Melosh, 1989, p. 89). The geological activity on Earth does not favor the preservation of ejecta deposits. The preservation varies considerably. The best-preserved exposed ejecta deposits according to Osinski et al. (2011) are the Mauritanian Tenoumer structure (1.9 km diameter, 0.0214 Ma) and Pingualuit (New Quebec) in Canada (3.4 km diameter, 1.4 Ma). In Table 1 in Osinski et al. (2011), nine additional structures with preserved ejecta deposits are listed. Several other structures are completely buried and can only be observed with drilling and for these, the precise extent of the ejecta deposits is difficult to estimate. Also, there are ejecta deposits without a known impact structure. In this aspect, the Lockne structure offers a unique opportunity to study and explore the proximal ejecta (the ejecta blanket) around a marine target impact. The preservation of the crater and its proximal ejecta is favored by a combination of a relatively young age and large magnitude of the event (making a large impact crater), as well as that it occurred

Effect of Target Layering in Gravity‐Dominated Cratering in Nature, Experiments, and Numerical Simulations

“…The outer crater in the sedimentary strata formed by a shallow excavation flow, as illustrated in the numerical simulations by Lindström, Shuvalov, and Ivanov (2005) and Shuvalov et al. (2005), and was only slightly affected by erosion from the subsequent collapse of the water cavity (e.g., Sturkell et al., 2023). There was no crater modification collapse of the surrounding sedimentary strata other than some slumping of the Cambrian mud (Sturkell et al., 2013).…”

“…There was also at least 500 m of seawater, possibly as much as 700 m, at the target site, which formed a water cavity of similar diameter as the preserved outer crater (e.g., Lindström, Shuvalov, & Ivanov, 2005;Shuvalov et al, 2005). The outer crater in the sedimentary strata formed by a shallow excavation flow, as illustrated in the numerical simulations by Lindström, Shuvalov, and Ivanov (2005) and Shuvalov et al (2005), and was only slightly affected by erosion from the subsequent collapse of the water cavity (e.g., Sturkell et al, 2023). There was no crater modification collapse of the surrounding sedimentary strata other than some slumping of the Cambrian mud (Sturkell et al, 2013).…”

“…There was no crater modification collapse of the surrounding sedimentary strata other than some slumping of the Cambrian mud (Sturkell et al., 2013). The geological evidence for this is the primary position of the basement (nested) crater ejecta covering the floor of the outer crater before the deposition of resurge sediments from the collapsing water cavity (e.g., Sturkell et al., 2023). The diameter and layer thickness errors for Lockne in Table 4 are estimated from the resolution of the geological and geophysical data from the literature ( Lindström, Ormö, et al., 2005; Ormö et al., 2013).…”

“…(2013) note how concentric craters in layered targets form either during crater excavation , as described in the aforementioned studies, by variations in the subsurface excavation flow depending on target material properties (e.g., the 13 km Lockne Crater, Sweden, e.g. Lindström, Ormö, et al., 2005; Shuvalov et al., 2005; Sturkell et al., 2023), or during crater modification due to more extensive collapse of a weak upper layer (e.g., the 85 km Chesapeake Bay Impact Structure, CBIS, in the USA, e.g., Collins & Wünnemann, 2005; Poag et al., 2004). Both types have also been suggested for some Martian craters (Horton et al., 2006; Ormö et al., 2013).…”