Lithostratigraphy of the impactite and bedrock section of ICDP drill core D1c from the El'gygytgyn impact crater, Russia

摘要

In 2008/2009, 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.