SEMINARY (auditorium CAB) 11-05-2016. Ejecta layers and sedimentation at wet-target impact sites

By Jens Olof Ormö and Pierre-Etienne Martin, from Planetology and Habitability Department, CAB.

Day: June 10th, 2016
Hour: 12:00
Venue: Auditorio del Centro de Astrobiología (CSIC-INTA)
Instituto Nacional de Técnica Aeroespacial
Ctra de Ajalvir, km 4
28850 Torrejón de Ardoz, Madrid, Spain.

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Title: Ejecta layers and sedimentation at wet-target impact sites (Jens Olof Ormö)

Abstract: The geology of wet-target impact craters differ in some ways significantly from their counterparts formed in solid (i.e., "dry") targets, the extreme case being the impacts into shallow seas. The frequently layered target of often saturated sediments of different degree of consolidation covered by seawater affects both the cratering- ejection-, as well as the modification processes. The resulting crater may obtain a concentric morphology due both to deviations in the excavation flow as well as slumping and resurge erosion of target sediments. The slumping and water resurge generate a conspicuous crater infill sequence. Depending on the relative target water depth the resurge and potentially oscillating water movements (i.e., tsunami) can affect the sediment deposition to great distances beyond the crater rim. Illustrations will be given from the Tsenkher structure, Mongolia (fluidized ejecta), the Vakkejokk Breccia, Sweden (ballistic ejecta, resurge deposits) and the Lockne-Malingen doublet impact structure, Sweden, where relative differences in target water depth due to the simultaneous formation of two different-sized craters in the same target setting can be studied.

Title: Drill core logging within the crater ejecta layer of the concentric, marine-target Lockne crater, in Sweden (Pierre-Etienne Martin)

Abstract: The Lockne crater, situated in Jämtland, Sweden, was formed during the Upper Ordovician (around 458 Ma) in an epicontinental sea. It is seen nowadays as a concentric impact structure with an outer diameter of 13.5 km and with a deeper, nested 7.5 km wide crater in the basement. According to previous geological, geomorphological, geochemistry and geophysical data the impact occurred in a marine setting. This depositional environment helped preserve the crater in the seafloor, making it a perfect subject for studying the marine impact process.
The Lockne-8 drill core is situated in the ejecta of the nested crater in the basement. This ejecta emplacement caused secondary cratering processes of the floor of the outer crater. The drill core mainly comprises breccias composed of granitic, doleritic and volcanic and limestone clasts. The matrix in the granitic sections is in general dark and very hard, possibly fine-crushed material. In some places, an unidentified very fine grained material occurs, which resembles melt. In the polymictic breccia dominated by limestone clasts the matrix is mainly a brown-greenish mud. In the dolerite breccias the matrix is fine-grained and black, likely originating from the aluminum-shale part of the target succession. The distinction of these dark matrixes is open to debate on certain points of the core as there is a need for supplemental geochemical and petrological data, for instance regarding the potential melt. From this thorough lithological description, 6 different lithologies were distinguished and separated into two main categories: the crystalline ejecta (mainly monomictic although with some mixed zones, seen as polymictic breccias) and the material that was impacted by the ejecta (seen as a polymictic breccia, dominated by limestone clasts).
The core was also subjected to magnetic susceptibility measurements (220 with an average spacing of 15 cm) to find correlations with the different lithologies; the granitic portions providing a signal around 0 SI while doleritic sections can go up to 10-2 SI. In addition to those studies, the line-logging method (Ormö et al. 2007; 2009; Sturkell et al. 2013 and references therein) was applied revealing the variations in the clast distribution within the core. The data collected comprises clast size, depth and their associated lithology and correlates with the previous results (i.e. ballistic ejecta, resurge deposits, etc.).
Nevertheless, the question still remains whether the granitic, doleritic and volcanic clasts were brecciated during the primary or the secondary impact process.
This lab work is to be complemented by impact cratering experiments, with a "rubble-pile" projectile in order to simulate the cratering process that produced the Lockne crater, as well as a geophysical survey in July 2016 (resistivity measurements) of the inner crater ejecta in the vicinity of the core hole.


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