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Central Ring Structure Identified in One of the World's Best-Preserved Impact

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Central Ring Structure Identified in One of the World's Best-Preserved Impact Central ring structure identi®ed in one of the world's best-preserved impact craters A.C. Gebhardt F. Niessen Alfred Wegener Institute for Polar and Marine Research, Columbusstraûe, 27568 Bremerhaven, Germany C. Kopsch Alfred Wegener Institute for Polar and Marine Research, Telegrafenberg A43, 14473 Potsdam, Germany ABSTRACT Seismic refraction and re¯ection data were acquired in 2000 and 2003 to study the morphology and sedimentary ®ll of the remote El'gygytgyn crater (Chukotka, northeast- ern Siberia; diameter 18 km). These data allow a ®rst insight into the deeper structure of this unique impact crater. Wide-angle data from sonobuoys reveal a ®ve-layer model: a water layer, two lacustrine sedimentary units that ®ll a bowl-shaped apparent crater mor- phology consisting of an upper layer of fallback breccia with P-wave velocities of ;3000 m/s, and a lower layer of brecciated bedrock (velocities .3600 m/s). The lowermost layer shows a distinct anticline structure that, by analogy with other terrestrial and lunar cra- ters of similar size, can be interpreted as a central ring structure. The El'gygytgyn crater exhibits a well-expressed morphology that is typical of craters formed in crystalline target rocks. Keywords: impact crater, El'gygytgyn, lakes, seismic refraction, seismic re¯ection. INTRODUCTION (Belyi, 1998; Gurov et al., 1979a, 1979b). The Grette, 2006; Nolan et al., 2002) formed in- The El'gygytgyn crater, located in the Rus- Anadyr Mountains are part of the Okhotsk- side the crater (the crater and crater lake are sian Arctic, is one of the world's best- Chukotka Volcanic Belt, composed of Late not concentric; see Fig. 1), and has become a preserved impact craters of its size (Dietz and Cretaceous calc-alkaline volcanic rocks (Be- major focus of multidisciplinary international McHone, 1976), and is the only known crater lyi, 1994; Layer, 2000; Layer et al., 2001). research (e.g., Nolan et al., 2002; Nowaczyk on Earth that was formed by an impact into The target rocks are mainly rhyodacite tuffs et al., 2002) because it could provide a 3.6 siliceous volcanic rocks (e.g., Gurov et al., and ignimbrites, with some rhyolites, andesite m.y. paleoclimatic record unique to the Arctic. 1979a, 1979b; Gurov and Koeberl, 2004); its tuffs, and basalts. A lake of ;12 km diameter When the impact formed the crater, the Arctic lake is one of only a few inside an impact and 170 m water depth (Nolan and Brigham- was forested, and ;1 m.y. passed before the crater (Lerman et al., 1995). The crater's age, 3.6 Ma (Layer, 2000), and its excellent pres- ervation provide unique information about im- pact mechanisms and shock metamorphism in siliceous volcanic rock. The lacustrine sedi- ments have never been glaciated (Glushkova and Smirnov, 2005; Minyuk et al., 2003), and so may contain a high-resolution paleoclimate record of the past 3.6 m.y. In a ®rst step to- ward continental deep drilling, a seismic re- fraction and re¯ection survey was carried out to obtain information on the structure of the crater and the geometry of its sedimentary ®ll (Niessen et al., 2006). Seismic refraction data form the basis of a ®ve-layer velocity-depth model that reveals the structural geometry of the impact crater, and multichannel re¯ection data provide information on its lacustrine sed- imentary ®ll. GEOLOGICAL BACKGROUND AND PREVIOUS WORK The 18-km-diameter El'gygytgyn crater is Figure 1. A: Geological overview of crater and its surroundings (modi®ed after Nowaczyk located in the remote Anadyr Mountains (Fig. et al., 2002). B: General overview of crater location. Note that crater and crater lake are not 1) in central Chukotka, northeastern Siberia concentric. q 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; March 2006; v. 34; no. 3; p. 145±148; doi: 10.1130/G22278.1; 4 ®gures. 145 Figure 2. Sonobuoy refraction data. Left: W-E pro®le. Right: N-S pro®le. Two seismic record sections are given in upper part (A, B); picked phases are marked as Ps (sedimentary units) and Pg (breccia and bedrock). Observed and calculated traveltimes are shown in C and D. Picks as error bars indicate pick uncertainties, and modeled synthetic rays are lines. Five-layer seismic velocity model is shown in E and F. For locations of pro®les and sonobuoys, see Figure 1. ®rst major glaciation of the Northern Hemi- velocities for the lower layers (breccia and ture are ;50 m in the central part and ;80 m sphere (Brigham-Grette and Carter, 1992). To- bedrock). in the more distal part of the refraction data. day's lake level is ;500 m above sea level In 2000, a single 5 in3 (82 cm3) Bolt 600B (asl), and the highest elevations of the crater airgun was used as seismic source for the re- RESULTS AND INTERPRETATION rim reach 850±950 m asl; the crater rim forms fraction pro®les. The shot interval was 6 s, The ®nal ®ve-layer model (Fig. 2) includes the outer margin of the catchment area of the equating to an average of 8 m shot distance. a water layer and two upper sedimentary units lake. Two sonobuoys were used on two perpendic- of low seismic velocities (1550 m/s and 1650 ular pro®les across the center of the crater lake m/s); the upper unit (Ia) is 170 m thick, and SEISMIC DATA ACQUISITION AND (Fig. 1) and recorded the airgun-borne acous- the lower unit (Ib) is 190±250 m thick. These PROCESSING tic pulses by a single hydrophone under the units are underlain by a unit with a distinctly During two expeditions in 2000 and 2003, buoy. Signals were ampli®ed and transmitted higher seismic velocity of ;3000 m/s and a seismic refraction and multichannel re¯ection via radio to the platform (Niessen et al., thickness of 100±400 m (unit II). The lower- data were obtained using 2 sonobuoys and a 2006). Both sonobuoy pro®les were reversed. most unit (III) has a seismic velocity of 260-m-long streamer (Niessen et al., 2005). The seismic refraction data recorded by the .3600 m/s. Due to the remote location (Fig. 1), all geo- sonobuoys covered an offset range to 4.4 km The most striking feature revealed by the physical equipment, including the vessel (a 3 (E-W pro®le) and 3.3 km (N-S pro®le, Fig. 2) sonobuoy data is the clearly visible uplift 3 4 m aluminum platform), had to be trans- and were improved by standard processing structure in the lowermost layer of our model. ported to the lake by helicopter. For the mul- (bandpass ®ltering 3-5-50-60 Hz, AGC) to Its position in the two perpendicular pro®les tichannel survey, a 26 in3 (426 cm3) mini-GI identify different re¯ection and refraction shows that it is located in the southwestern gun (triggered in G-gun mode) at a pressure phases. Picking and modeling of the identi®ed part of the lake (Fig. 1), ;2 km away from of 110 bar was used. The shot interval was 10 phases (Pg and Ps, Fig. 2) were done using the crater center. Complex impact craters are s, resulting in ;12 m shot distance. A 14 the ``zplot'' and ``rayinvr'' program package characterized by a central uplift area consist- channel streamer with an initial offset of 130 (Zelt and Smith, 1992). A forward ray-tracing ing of a central topographic peak and/or m and a hydrophone spacing of 10 m was modeling technique was used to develop a ring(s) (Grieve and Pesonen, 1992). The crater used as a receiving array. Re¯ection data were model of layer thicknesses and velocities. The morphology is strongly related to its size and processed in a standard sequence including initial ®ve-layer model was used to calculate target rock (Grieve and Pesonen, 1992; Spu- bandpass ®ltering (70-90-240-300 Hz), veloc- synthetic seismic traveltimes. It was iterative- dis, 1993). Grieve and Pesonen (1992) report- ity analysis, CMP (common midpoint) stack- ly modi®ed until the calculated and observed ed a central uplift morphology succession ing, and predictive deconvolution. The ®nal traveltimes were concordant (Fig. 2B). Mod- from craters with a simple central peak (e.g., stacking was done using the refraction data eling uncertainties for the central ring struc- Steinheim crater, Flynn Creek crater, both with 146 GEOLOGY, March 2006 the central uplift structure and is 100 m thick on top of the central ring, increasing to 400 m thick in the surrounding annular basin. Com- parable layers have been found, for example, in the Bosumtwi crater (velocities of ;3200 m/s; Karp et al., 2002; Scholz et al., 2002) and in the NoÈrdlinger Ries crater (e.g., Engelhardt et al., 1994; WuÈnnemann and Morgan, 2004). Seismic velocities are much lower within the lacustrine layers, averaging 1550 m/s in the upper and 1650 m/s in the lower sedimen- tary unit. The lower sedimentary unit shows a slight drape on top of the central uplift, Figure 3. Re¯ection pro®le along W-E sonobuoy pro®le. Locations of pro®le and sonobuoys whereas the contact between the two sedimen- are given in Figure 1. Colors at sonobuoy positions correspond to Figure 2. MÐmultiples. tary units is almost ¯at. Re¯ection seismic data (Fig. 3) show a well-strati®ed 170-m- thick upper unit that is locally intercalated diameters of 3.8 km), to craters whose central ring; there is no evidence for a central peak with debris ¯ows, mainly at the lake margins.
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