The size and abundance of shocked quartz in Cretaceous-Tertiary boundary sediments from the Pacific Basin

Abstract

Quantitative analyses of the size distribution and abundance of shocked quartz (quartz with planar deformation features [PDFs]) were performed on CretaceousTertiary (K/T) boundary sediments from seven localites in thePacific basin: LL44-GPC3; DSDP (Deep Sea Drilling Project) Sites 465, 576, 577, and 596; and ODP (Ocean Drilling Project) Sites 803 and 886. Quartz with PDFs was found at each site.On average, 63% of the quartz grains in Pacific K/T sediments with diameters >30 µm have PDFs. The fluences of quartz with PDFs are relatively uniform across the Pacific; estimates for the abundances of shocked grains >30 µm in diameter range from ~1,000 to 2,000 grains/cm 2 . One exception is ODP site 886, where we estimate a fluence of 13,800 grains/cm 2 . Ten PDF-bearing quartz grains larger than 200 µm were recovered, including one with a maximum diameter of 320 µm. These largest grains were all from the sites nearest North America: DSDP 465, ODP 886 and LL44-GPC3. This may reflect proximity of these sites to Chicxulub.Comparison to other globally distributed sites is difficult. We conclude that shocked quartz in the Pacific is on average significantly smaller and about an order of magnitude less abundant (in mass/cm 2 ) than that deposited in North America. Maximum grain sizes of shocked quartz in the North Pacific appear to be larger than at sites in Europe, although these sites were similar distances from Chicxulub at 66 Ma.

Analyses of shocked quartz at the global K-P boundary indicate an origin from a single, high-angle, oblique impact at Chicxulub

“…3a), are found in all sites sampled except for Poty Quarry (Tables 2-3), in accordance with previous reports on K-P sites from North America [5][6][7], the Pacific plate [31,32] and Europe [8]. We have identified quartz characterized by planar fractures, PDF and shock mosaicism, which have been shown to be indicative of increasing degrees of shock pressure [39].…”

“…Even if our estimates of nos/cm 2 for the Pacific are slightly underestimated, they still appear to contradict previous reports that shocked quartz is relatively abundant in the Pacific [31,32 ], that led to publications on the preferential westward distribution of the Chicxulub ejecta [18,30,31].…”

“…Previous studies of the K-P ejecta have suggested some intriguing asymmetries in the distal ejecta pattern [18,30,31], with shocked quartz being more abundant in sites to the west of Chicxulub [30,31] However, these observations are based upon data acquired by a number of groups, who have adopted different sampling and/or analytical procedures. Detailed mineralogical studies that report the absolute abundance and grain size of shocked quartz are rare [31,32,33], and some sites are better documented than others. Hence, it is difficult to make a quantitative assessment of asymmetries in the distribution of the distal ejecta using the existing data alone.…”

“…Where detailed sampling has been performed across several 10s of centimeters above and below the K-P iridium peak, for example site 576B ( Table 2 in Bostwick and Kyte [31]), the shocked quartz is focused within a relatively thin interval of > 3 and < 7 cm. This contrasts with the iridium anomaly at K-P sites (in the Pacific and elsewhere), which continues to be elevated outside the shockedquartz bearing intervals for up to a few 10s of cm, and is thought to be mobilized through diffusion [44].…”

“…Quartz grains were extracted after sequential leaching of fossil, carbonate and clay mineral components (as described in Bostwick and Kyte [31]) and dry sieving of the remaining mineral separate. Analyses were carried out on grains extracted from 50 g bulk samples and then mounted into smear slides with ultra violet cement, following the methodology of Montanari and Koeberl [33].…”