[Reimold] Geological study of the crater rim and the ejecta blanket of the El´gygytgyn impact structure, Siberia
German Title: Geological study of the crater rim and the ejecta blanket of the El´gygytgyn impact structure, Siberia
Current Status: completed
Main Applicant:Prof. Dr. Uwe Reimold
The El´gygytgyn impact structure in NE Siberia was the target of an ICDP drilling in 2008/09. The 517 m long core hole D1c was drilled against the flank of the central uplift structure, and recovered ~316 m lake sediments and ~201 m impactites. The impactite part of this core was in the focus of this study, and had been curated at Museum für Naturkunde Berlin (MfN) including the development of a core lithostratigraphy, the arrangement of the international sampling party, and the distribution of samples. In 2011 an international expedition with the participation of the MfN investigated the El´gygytgyn impact structure, and enabled the mapping and collecting of surface target rocks. For this project samples from the drill core and the surface were investigated by optical microscopy, SEM, RAMAN spectroscopy, electron microprobe analysis (EMPA) and XRF at MfN, and in cooperation with our partners in Vienna (INAA) and Cardiff (LA-ICP-MS) for the determination of REE´s and PGE´s. The 97 m long lower bedrock unit (517-420 m below lake floor [blf]) consists of an ignimbrite. In the overlying upper bedrock unit (420–390 mblf), the core recovered a similar ignimbrite sequence, and several decimeters of transformed mafic rocks with an extreme high siderophile element content possibly due to alteration. We interpret both units as rocks that are located close to their former, pre-impact position, but have been somewhat rotated due to collapse of the central uplift (i.e., parautochthonous basement). From ~390 to 328 mblf occurs a suevite package with an impact melt-poor, clastics-dominated matrix, and lithic and mineral clasts that cover the entire range of target rocks known from the region. Chemically the suevite is a mixture of all known target rocks dominated by the felsic volcanics that are predominant in the crater vicinity. All stages of shock metamorphism between unshocked and melted were observed in clasts and matrix of suevite from different depths. Basic investigations for the distinguishing impact vs. volcanic melts had been carried out, but will need a more detailed study in the future. Immediately below the suevite, at the contact to the underlying bedrock, occurs a 1 m wide shear zone, which we consider as the actual crater floor. The uppermost ~12 m, from 328-316 mblf depth, seems to comprise reworked suevite, consisting of a mixture of lacustrine sediments and suevite with more and, on average, stronger shocked minerals than found in the suevite unit. This includes a small component of glassy spherules and impact melt fragments. In contrast to the suevite, the reworked suevite contains based on chemical studies a slightly higher proportion of mafic target components. The results of the 2011 expedition were combined with earlier mapping outcomes to create a new geological map of this impact structure and its immediate environs at the scale of 1:50,000. Compositions of the sampled rock suites are compared with the lithologies of the ICDP drill core. The ignimbrite described as lower bedrock in the ICDP drill core shows petrographically and chemically strong similarities to the rhyolitic and rhyodacitic ignimbrites observed predominantly in the crater vicinity at the surface. The preimpact geological setting looks different to the recent rocks at the crater area, and a higher proportion of mafic and intermediary rocks seems plausible. The suevite is lacking a discernable meteoritic component based on PGE abundances. In contrast, the reworked suevite contains the highest concentrations of Os, Ir, Ru, and Rh compared to other impactites. This is to a lesser extent the result of admixture of a mafic component, but more likely the signature of a probably chondritic meteoritic component. Nevertheless, the possible contribution of the altered mafic blocks of the ICDP drill core with their high siderophile element and PGE concentrations to the composition of the impactites is still only poorly constrained, which makes it difficult to unambiguously confirm the presence and type of a meteoritic component.