A rock magnetic profile through the ejecta flap of the Lockne impact crater (central Sweden) and implications for the impact excavation process

Abstract

The well-documented, well-preserved, and well-exposed Lockne crater is a reference crater for marine-target impact on Earth. To date, it has been subjected to 11 short core drillings and over 5000 outcrop descriptions, as well as several geophysical surveys. The rich data allows detailed analysis of the cratering and modification processes. A unique feature with Lockne is its pristine ejecta layer. The ejecta differs, however, from the typical text book example of a land target crater in that it, to great extent, is made up of relatively extensive, coherent ejecta flaps resting on a target surface with no structural rim uplift. However, little is known about the marine impact excavation process generating the flaps. Here, we provide a lithological description coupled with an analysis of the rock magnetic properties of the Lockne-9 core through the western, downrange ejecta flap. The 31.04m long drillcore shows ~23m of monomict (mafic) breccia overlying a ~5m thick mixed zone of breccia with fracture fill and matrix derived from Palaeozoic sedimentary target sequence (i.e. Lower Cambrian alum shale and conglomerate). The whole breccia package rests with sharp contact on the fractured, granitic basement. The conspicuous lithological and petrophysical differences between the overlying breccias and the basement suggest the former was transported as ejecta. The rock magnetic properties of the ejecta show a magnetic signal that must have existed before the impact event took place. Thus, during the cratering process the ejecta at the studied location was relocated en masse from the central part of the crater to form an ejecta flap, in contrast to the standard ballistic emplacement of individual particles in an ejecta curtain. Highlights:-Rock-magnetism reveals a high magnetisation zone of pre-impact nature -Petrophysical characterization and description of the Lockne-9 core (central Sweden) -Magnetic signal is dominated by magnetite/titanomagnetite -Comparisons of affected and non-affected samples suggest a quick flap formation Research Highlights INSIGHTS IN THE EJECTION PROCESS OF THE LOCKNE MARINE-TARGET IMPACT CRATER FROM ROCK MAGNETIC PROPERTIES IN THE LOCKNE-9 DRILL Abstract:The well-documented, well-preserved, and well-exposed Lockne crater is a reference crater for marine-target impact on Earth. To date, it has been subjected to 11 short core drillings and over 5000 outcrop descriptions, as well as several geophysical surveys. The rich data allows detailed analysis of the cratering and modification processes.A unique feature with Lockne is its pristine ejecta layer. The ejecta differs, however, from the typical text book example of a land target crater in that it, to great extent, is made up of relatively extensive, coherent ejecta flaps resting on a target surface with no structural rim uplift. However, little is known about the marine impact excavation process generating the flaps. Here, we provide a lithological description coupled with an analysis of the rock magnetic properties of the Lockne-9 c...

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

“…The other two cores mentioned here, LOC10 and LOC11, were retrieved for this study with the use of the same type of equipment as for LOC08 and LOC09. The core log descriptions presented here are in line with the aim of the study, that is, to determine the thickness and main lithological composition of the ejecta and do, thus, not provide all additional information that would be possible to obtain from the material (e.g., Lindgren et al, 2007;Melero-Asensio et al, 2014). The LOC08 core hole was located at the center of a large solitary ejecta body that occupies the outer reach of the Tandsbyn gap.…”

“…Closer to the crater, remnant mud and clay are impacted by the basement crater ejecta and blend in as matrix around the clasts in a transition zone between ejecta and basement (cf. LOC9 drill core, Melero-Asensio et al, 2014. For location, see Figure 1). The Cambrian clay squeezes up between major coherent ejecta masses ("mega blocks"), and/or slumps down into the basement crater (cf.…”

“…In two instances, we use drill cores (i.e., LOC08 and LOC09) that were retrieved already in 2004 as the subject of those drillings also was to investigate the thickness and properties of the crystalline ejecta deposits. Only LOC09, however, has so far been described in publications (i.e., Lindgren et al., 2007; Melero‐Asensio et al., 2014). The other two cores mentioned here, LOC10 and LOC11, were retrieved for this study with the use of the same type of equipment as for LOC08 and LOC09.…”

“…The crystalline target within the crater damage zone shows a span from being completely brecciated to only slightly fractured. The impact breccia in the crystalline rock, the Tandsbyn Breccia, forms instantly at impact and has even been observed to have been ejected as coherent large (tens of meters) bodies of breccia (e.g., Melero‐Asensio et al., 2014). Similar bodies of Tandsbyn Breccia, albeit much smaller, are also seen as part of the resurge breccia, thus creating a “breccia in breccia” (Lindström et al., 1996).…”

“…The height refers to the elevation of the surfaces at the site. The result of the LOC09 drilling is given inMelero-Asensio et al (2014). The drillings were performed by the company Drillcon Core AB stationed in Nora, Sweden.…”