Abstract-The 3.6 Ma old and 18 km diameter El'gygytgyn impact structure in NE Siberia was drilled in 2008/09 by ICDP (International Continental Scientific Drilling Program). A 517 m long core hole (D1c) was drilled into the outer flank of the central uplift structure, with an overall core recovery of approximately 63%. Thereby, approximately 315 m lake sediments and approximately 202 m impactites were recovered. Here, we present a detailed petrographic and geochemical assessment of the impact breccia and bedrock sections in this core. 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 sequence of similar ignimbrite and several decimeters of mafic rocks. We interpret these units as rocks that are located close to their former, preimpact position, but have been somewhat rotated due to collapse of the central uplift (i.e., it represents parautochthonous basement). From about 390 to 328 mblf occurs a suevite package with an impact melt poor, clast-dominated matrix, and lithic and mineral clasts that cover the entire range of volcanic target rocks known from the El'gygytgyn region. All stages of shock metamorphism (unshocked to melted) were observed in clasts, and in microclasts of the matrix, of suevite from different depths. Immediately below this package, at the contact to the underlying bedrock, occurs a 1 m wide sheared zone within vitrophyric ignimbrite, which we consider the actual crater floor. The uppermost approximately 12 m, from 328-316 mblf depth, seem to comprise reworked suevite, consisting of a mixture of sediments and suevite with more and, on average, stronger shocked minerals than found in the main suevite unit. This includes a small component of glassy spherules and impact melt fragments. Toward the top of this unit, lake sediments progressively become the dominant material in this section. We assume that this unit contains a fallback component from the ejecta plume that was mixed with the first sediments of the postimpact crater lake, and possibly some rocks that slumped off the inner crater wall-similar to a thin layer at the base of the sediment section of borehole LB-5A recovered in Lake Bosumtwi (Ghana).

Abstract-In 2008, the International Continental Scientific Drilling Program (ICDP) obtained drill cores from the El'gygytgyn impact structure located on the Chukotka Peninsula (Russia). These cores provide the most complete geological section ever obtained from an impact structure in siliceous volcanic rock. The lithostratigraphy comprises a thick sequence of lacustrine sediments overlying impact breccias and deformed target rock. The interval from 316 m (below lake floor-blf) to the end of the core at 517 m depth can be subdivided into four lithological sequences. At 316 m depth, the first mesoscopic clasts of shocked target rock occur in lacustrine sediments. The growing abundance of target rock clasts with increasing depth and corresponding decrease of lacustrine sediment components indicate the extent of this transition zone to 328 m depth. It constitutes a zone of mixed reworked impact breccia and lacustrine sediments. Volcanic clasts in this reworked suevite section show all stages of shock metamorphism, up to melting. The underlying unit (328-390 m depth) represents a suevite package, a polymict impact breccia, with considerable evidence of shock deformation in a wide variety of volcanic clasts. This includes fragments with quartz that exhibit planar fractures and planar deformation features (PDF). In addition, at three depths, several centimeter-sized clasts with shatter cones were detected. Due to microanalytical identification of relatively rare, microscopic impact melt particles in the matrix of this breccia, this material can be confidently labeled a suevite. Also in this sequence, three unshocked, <1 m thick intersections of volcanic blocks occur at 333.83, 351.52, and 383.00 m depths. The upper bedrock unit begins at 390.74 m depth, has a thickness of 30.15 m, and represents a sequence of different volcanic rocks-an upper part with basaltic composition from 390.74 to 391.79 m depth overlying a lower, rhyodacitic part from 391.79 to 420.27 m depth. This (parautochthonous) basement unit is only very weakly affected by the impact: only one shocked quartz grain with two sets of PDF was recorded at 391.33 m depth. The lower bedrock unit (420.89-517.09 m depth [end of core]) is a brittly deformed, rather homogeneous welded ignimbrite that in part can be considered a cataclasite. The top three meters of this section are sheared, which could represent pre-impact tectonic deformation. A 54 cm thick injection of polymict impact breccia occurs at 471.42-471.96 m depth.

Shatter cones are the only distinct meso‐ to macroscopic recognition criterion for impact structures, yet not all is known about their formation. The Keurusselkä impact structure, Finland, is interesting in that it presents a multitude of well‐exposed shatter cones in medium‐ to coarse‐grained granitoids. The allegedly 27 km wide Keurusselkä impact structure was formed about 1150 Ma ago in rocks of the Central Finland Granitoid Complex. Special attention was paid in this work to possible relationships between shatter cones and local, as well as regionally occurring, fracture or joint systems. A possible shatter cone find outside the previously suggested edge of the structure could mean that the Keurusselkä impact structure is larger than previously thought. The spacing between joints/fractures from regional joint systems was influenced by the impact, but impact‐induced fractures strongly follow the regional joint orientation trends. There is a distinct relationship between shatter cones and joints: shatter cones occur on and against joint surfaces of varied orientations and belonging to the regional orientation trends. Planar fractures (PF) and planar deformation features (PDF) were found in three shatter cone samples from the central‐most part of the impact structure, whereas other country rock samples from the same level of exposure but further from the assumed center lack shock deformation features. PDF occurrence is enhanced within 5 mm of shatter cone surfaces, which is interpreted to suggest that shock wave reverberation at preimpact joints could be responsible for this local enhancement of shock deformation. Some shatter cone surfaces are coated with a quasi‐opaque material which is also found in conspicuous veinlets that branch off from shatter cone surfaces and resemble pseudotachylitic breccia veins. The vein‐filling is composed of two mineral phases, one of which could be identified as a montmorillonitic phyllosilicate. The second phase could not be identified yet. The original composition of the fill could not be determined. Further work is required on this material. Observed joints and fractures were discussed against findings from Barringer impact crater. They show that impact‐induced joints in the basement rock do not follow impact‐specific orientations (such as radial, conical, or concentric).

Abstract-El'gygytgyn is a 3.6 Ma, 18 km diameter, impact crater formed in an approximately 88 Ma old volcanic target in Northeast Siberia. The structure has been the subject of a recent ICDP drilling project. In parallel to those efforts, a Russian-German expedition was undertaken in summer 2011 to investigate the permafrost soil, lake terraces, and the volcanic rocks of the southern and eastern crater rim. This provided the unique opportunity for mapping and sampling of the volcanic target rocks around a large part of this complex impact structure. Samples from 43 outcrops were collected and analyzed petrographically and geochemically. The results 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 our rock suites are compared with the lithologies of the 2009 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 on surface. The suevite sequence exposed in the ICDP drill core is a mixture of all observed target rocks at their respective proportions in the area. In contrast to previous studies, the calculated average target composition of El'gygytgyn takes the contribution of the basic target rocks into consideration: mafic and intermediate rocks approximately 7.5%, and felsic rocks approximately 92.5%.