Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.436s&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 436
Other
1
Cretaceous-Tertiary Boundary, Impact Craters, Manson Crater, Petrography, Shock
Scientific paper
The Manson impact structure, largest (36 km) in the U.S., is inferred to have produced shocked materials found in the upper layer of some K-T Boundary deposits, mainly because its radiometric age (66 my) is compatible. Short, in 1966 [1], was first to show that Manson is an impact crater through casual analysis then of 22 samples from a 1953 drill hole (2-A). These samples have now been studied in detail, with these key results: (1) the lithology of clasts within 2-A is dominantly granitic; (2) most quartz is strongly shocked (many planar deformation features, PDFs) and shows a pervasive alteration (clay minerals?; iron stain); (3) a unique texture (single crystals broken into hundreds of small fragments [polycrystalline]) occurs in some heavily shocked quartz; and (4) feldspars display a wide range of shock features from multiple PDFs to incipient melting (internal flow) and extensive recrystallization. Table 1 summarizes the major shock features arranged in stages of progressive shock metamorphism for the three principal minerals: quartz, feldspars, and biotite. The predominant mode of PDF occurrence in quartz within leucogranitic clasts, and in most quartz fragments in matrix material is marked by light, orange-brown to grayish-brown in plane-transmitted light, and a deeper reddish-brown, with reduced birefringence, cross-polarized light. At high magnification, the alteration consists of tiny specks of unknown identity that often obscure but do not destroy the sets of PDFs. The effect under the microscope sometimes resembles the "texture" of toasted bread. This hallmark of Manson shocked quartz is rarely seen in shocked quartz from other impact structures (occasional in materials examined by NMS from West Hawk Lake and Steen River in Canada). Sharpton et al [2] describe similar quartz in their examination of Manson materials, stating the origin of this alteration to be due to in-crater postimpact hydrothermal alteration; if so, such a condition would not be diagnostic of shocked quartz grains in K-T deposits and is therefore not a criterion for relating these deposits to the Manson event. Single (larger) crystals of "toasted" quartz contain an average of 5.5 sets of PDFs whose principal crystallographic orientation is pi-1012 (omega-1013 is second most common). Much less frequent in clasts and matrix grains are untoasted but decorated PDFs in quartz, with omega predominant in the average 2.2 sets per grain. In some strongly shocked leucogranites, and in occasional matrix fragments, single crystals have been broken into numerous small (100 micrometers) interlocking quartz grains (toasted), containing an average of only 1.4 PDF sets, in which omega is prevalent. These sets do not cross individual micrograin boundaries and orientations vary between grains. This highly distinctive texture, which we interpret as shock-induced shattering of single crystals accompanied by rotations, may be unique to Manson: a similar texture has been described by Schreyer [3] in Vredefort Central Core granites, but in those quartzes the PDFs pass across grain boundaries. In highly shocked Manson quartz, recrystallization may completely remove PDFs and the toasted effect is absent. Manson feldspars show a range of PDFs, some resembling those in quartz, others arranged en echelon in alternating albite twins, others concentrated in deformation bands. Feldspars may partially isotropize or display internal flow banding in thetomorphic crystals or may be recrystallized. Biotite responds by intricate kinking progressing through nearly complete decomposition. Un-devitrified glass is rare in 2-A. In 1991-92, the U.S.G.S. drill-cored 12 holes to depths under 380 m along a zone from crater center to assumed rim. Hole M-1 lies about 4 km northeast of 2-A within the central peak (probably a ring). Materials in the upper 100 m or so are mainly shales and some carbonates that show indecisive shock effects except for occasional melting. Crystalline clasts below the sedimentary materials have proportionately less leucogranites and more dioritic and amphibolitic clasts. The variety and characteristics of shock effects in these rocks are often notably different from those in crystalline 2-A clasts. References: [1] Short N. M. (1966) J. Geol. Educ., 14, 149-166. [2] Sharpton V. L. et al (1990) GSA Spec. Paper 247, 349-357. [3] Schreyer, W. (1983) J. Petrol., 14, 26-37. Table 1, which appears here in the hard copy, shows stages of progressive metamorphism of 2-A Manson minerals.
Gold David P.
Short Nicholas M.
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