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Emplacement of Cretaceous-Tertiary Boundary Shocked Quartz from Chicxulub Crater Walter Alvarez,* Philippe Claeys,T Susan W

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Emplacement of Cretaceous-Tertiary Boundary Shocked Quartz from Chicxulub Crater Walter Alvarez,* Philippe Claeys,T Susan W ARTICLES Emplacement of Cretaceous-Tertiary Boundary Shocked Quartz from Chicxulub Crater Walter Alvarez,* Philippe Claeys,t Susan W. Kieffer Observations on shocked quartz in Cretaceous-Tertiary (K-T) boundary sediments get rock and with the crystallization age of compellingly tied to Chicxulub crater raise three problems. First, in North America zircons from Chicxulub melt rock (17). It shocked quartz occurs above the main K-T ejecta layer. Second, shocked quartz is now seems clear that both layers were pro- more abundant west than east of Chicxulub. Third, shocked quartz reached distances duced by the Chicxulub impact and that requiring initial velocities up to 8 kilometers per second, corresponding to shock the shocked quartz and other minerals orig- pressures that would produce melt, not the moderate-pressure shock lamellae ob- inated from the basement granite. Iridium served. Shock devolatilization and the expansion of carbon dioxide and water from and quartz are believed to come from dif- impacted wet carbonate, producing a warm, accelerating fireball after the initial hot ferent sources: vaporized meteorite and un- fireball of silicate vapor, may explain all three problems. melted basement rock, respectively. Prob- lem 1 is thus, how did the shocked quartz and the iridium end up together in a sepa- rate layer, above the layer ofkaolinitic clay? In wells and outcrops of uninterrupted ma- impact crater (12) dating precisely from the The geographic distribution of K-T rine sedimentary rocks outside of North K-T boundary at 65.0 Ma (13) and sur- shocked quartz is not fully known, but it America, the K-T boundary is marked by a rounded out to -4000-km radius by proxi- appears to be much more abundant and single 1- to 10-mm clay layer often contain- mal ejecta at the biostratigraphic K-T slightly coarser grained at longitudes west of ing anomalous iridium and altered impact boundary (5, 7, 8) has strongly confirmed Chicxulub. Many sites in the western inte- spherules, widely interpreted as evidence for the general validity of the impact theory for rior of North America are rich in coarse the impact of a large comet or asteroid (1) the K-T mass extinction. Problems of detail shocked quartz grains (up to 0.64 mm) (2- at the time of the K-T mass extinction of remain, however, including three problems 5). In addition, shocked quartz grains are on July 27, 2009 organisms 65 million years ago (Ma). A concerning the shocked quartz grains: (i) abundant and fairly coarse in all seven drill more complicated K-T boundary stratigra- their vertical distribution in the double K-T holes on the Pacific plate in which the K-T phy occurs in nonmarine sediments from layer of North America and their admixture boundary has been found (18, 19). This New Mexico, United States, to Alberta, with the iridium, (ii) the asymmetry of their presents a sharp contrast with sites in Eu- Canada, where the boundary interval be- geographic distribution about the Chicxu- rope, Africa, and Asia, where shocked quartz gins with a bed of kaolinitic clay, which lub crater, and (iii) their occurrence at great has been difficult to find (3, 4, 5, 20). sometimes containes goyazite spherules and distances from the crater. Problem 2 is thus, why is there an asymmetry is typically about 1 cm thick, overlain by a In North America, shocked quartz grains in the abundance of shocked quartz? layer 1 mm to a few millimeters thick that is are virtually absent from the kaolinitic clay K-T shocked quartz grains are found as www.sciencemag.org rich in shocked mineral grains, particularly layer, except where apparently carried far as 10,000 km from Chicxulub crater, at quartz (2, 3). The kaolinitic clay and downward by biogenic disturbance. The Ocean Drilling Project site 596 in the goyazite spherules are probably alteration clay layer and the quartz-bearing layer are southwest Pacific (18, 19). Ballistic trans- products of glassy ejecta (4-6) like that still sharply separated, and carbonized remnants port from Chicxulub to sites in the south- preserved in a few sites around the Gulf of of vegetation in the lower layer seem not to west Pacific requires a launch velocity of 7 Mexico (5, 7, 8). Shocked quartz is formed be present in the overlying quartz layer. to 8 km/s, corresponding to shock pressures under pressures of a few tens of This observation led to the no- that would anneal or melt the de- dynamic previously quartz, Downloaded from gigapascals, depending on the target com- tion that the layers were produced by two stroying the deformation lamellae that pro- position. An iridium anomaly is found in impact events at least one growing season vide the evidence for shock (21). Problem 3 samples taken from the shocked quartz layer apart; the vegetation traces in the lower is thus, how did these shocked quartz grains (9) and is attributed to the vaporization of layer were interpreted as roots of plants that achieve the velocities needed to reach dis- the impacting meteorite because of pres- grew before the overlying layer of ejecta was tal sites without melting? sures of many hundreds of gigapascals deposited (14). The lower layer was attrib- As shown below, the first two problems caused by a high-velocity impact. Some uted to the Chicxulub impact and the upper can be resolved if the shocked quartz trav- authors refer to the lower and upper layers, layer to Manson crater in Iowa (14) until eled on steep, high-velocity ballistic trajec- respectively, as the "melt-ejecta" and "fire- the following isotopic and age evidence tories. However, this requirement cannot be ball" layers (10). eliminated Manson as a K-T candidate cra- easily satisfied within the known constraints Recognition of the Chicxulub structure ter: (i) Sr, 0, and Nd isotopic measure- of impact cratering in dry silicate rocks: (11) in the Yucatan subsurface as a giant ments showed the K-T impact glass to be shocked quartz features are produced at pres- indistinguishable from Chicxulub melt sures of a few tens of gigapascals, and the W. Alvarez and P. Claeys are in the Department of Geol- rocks but very different from Manson melt particle velocities obtained are less than 4 ogy and Geophysics, University of California, Berkeley, rocks (15). (ii) The Manson impact was km/s on release from these pressures. Particle CA 94720-4767, USA, and Osservatorio Geologico di to shock Coldigioco, 62020 Frontale di Apiro (MC), Italy. S. W. dated as being earlier than the K-T bound- velocities of 7 to 8 km/s correspond Kieffer is in the Department of Geological Sciences, Uni- ary, at 73.8 + 0.3 Ma (16). (iii) Shocked pressures of 50 to 100 GPa, and the product versity of British Columbia, Vancouver, BC V6T 1Z4, zircons from the shocked have is melt, not moderately shocked quartz Canada. quartz layer crystallization ages much younger than the grains. To explain this contradiction, we *To whom correspondence should be addressed. basement rock at Manson but that of CO2 and tPresent address: Institut for Mineralogie, Museum fur compatible propose expansion H20 Naturkunde der Humboldt-Universitat zu Berlin, Invali- with the granitic Pan African basement vapor released from volatile target rocks denstraBe 43, D-10115 Berlin, Germany. thought to characterize the Chicxulub tar- would accelerate the quartz grains from sub- 930 SCIENCE · VOL. 269 · 18 AUGUST 1995 Itllll.IOI(IPB.PWI.PPslP1IIPB.PII. jacent moderately shocked granite to high site once they leave the atmosphere. from Chicxulub on trajectories steeper velocity. This process provides a mechanism Earth's rotation has two interesting ef- than about 65°. to address all three problems described. fects on these orbits. The first effect is that Developing this concept, we propose The present study is relevant not only to the semimajor axis of the elliptical orbit that the double layer of ejecta in the the specific case of the K-T boundary but depends only on the launch velocity in the western interior of North America reflects also to the understanding of impact process- inertial reference frame (25); to find this two different launch mechanisms during es in general. Large impact craters are com- velocity, the eastward rotational velocity of the cratering event. It has been proposed mon on rocky bodies in the solar system Earth (0.463 km/s at the equator) is added that the clay in the lower layer was formed with the exception of Earth, where craters to the target-frame launch velocity of east- by the alteration of glassy ejecta launched are relatively rare because they are erased by bound particles, but subtracted from that of as part of the ejecta curtain (4-6). The rapid geological processes. Comparison of westbound particles. This has little effect on ejecta curtain observed in hypervelocity lunar, martian, and venusian crater mor- the semimajor axis of slow particles, but at impact experiments is an outward-expand- phologies, as well as field and theoretical launch velocities from about 8 or 9 km/s up ing, downward-pointing cone inclined studies of terrestrial craters (22) suggest that to escape velocity (11.2 km/s), eastbound -45° to the horizontal (26) and repre- volatiles in the target body may significant- particles go higher and stay up longer than sents the coherent front of solid and melt ly influence the impact processes and prod- the corresponding westbound ones. The ejecta particles on independent trajecto- ucts. The Chicxulub crater is buried, so it is second effect is that, because Earth rotates ries, launched at elevation angles .45° inaccessible but uncommonly well pre- beneath in-flight ejecta, the reimpact site is [figure 17 in (27); figure 6.4 in (28)].
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