Structures at the microscopic scale typically
occur at higher pressures. At pressures between 5 and 45 GPa [725,000 to 6,500,000 psi] some mineral grains develop microstructures called planar deformation features (PDFs).12
These fea-
tures are sets of closely spaced parallel lamellae (left). Individual grains may contain multiple groups of PDFs at different angles. Analysis of the orientations helps quantify the stress levels attained during impact. At pressures greater than about 30 GPa, some
300 µm 300 µm
300 µm
mineral crystals transform into an amorphous structure called diaplectic glass. This glass forms without melting. Such a solid-state transforma- tion can preserve the grain shape and some of the grain’s original defects, distinguishing the product from melt glass. At pressures above 50 GPa [7.3 million psi],
50 µm 50 µm
50 µm
silica and other mineral grains melt, and above 60 GPa [8.7 million psi] rocks undergo bulk melt- ing and form impact melts. If they cool quickly, these molten materials form impact glass, and if they cool slowly, they become fine-grained impact- melt rocks. With time, shock-generated glass recrystallizes, or devitrifies, explaining why no glass remains in older impact structures. As of 2010, more than 175 impact structures
that exhibit shock-metamorphic effects have been identified on the Earth (next page, top).13 These circular features range in diameter from 15 m [49 ft] at the Haviland crater, in Kansas, USA, to 300 km [190 mi] at the Vredefort crater, in the Witwatersrand basin of South Africa. More than 30 of these structures have some form of potentially economic deposits, including oil and gas, precious metals and diamonds.14 Three types of resources can result from impact:
• Progenetic deposits originate before impact; examples are the uranium deposits of the Carswell structure, in Saskatchewan, Canada, which were lifted up during crater formation, and the gold and uranium deposits of the Witwatersrand basin, which were buried and preserved in the Vredefort crater.
50 µm 50 µm
50 µm
> Grain deformation. At impact pressures, some mineral grains develop planar deformation features (PDFs), which are closely spaced parallel lamellae that penetrate the whole grain. A shocked quartz grain assemblage from the Woodleigh impact structure, in Australia (top), displays multiple orientations of PDFs (from Reimold et al, reference 12). A single shocked quartz grain from an impact structure near Manson, Iowa, USA, exhibits two main PDF orientations (middle). The colors result from interference with the microscope’s light, caused by the grain thickness. An unshocked quartz grain (bottom) shows some features that inexperienced observers might confuse with PDFs. The lines are traces of subplanar fluid-inclusion trails that are the result of alteration caused by low-grade tectonic deformation. They are not straight, planar or closely spaced. (Photographs courtesy of Christian Koeberl.)
Oilfield Review Autumn 09 Impact Fig. 7
Oilfield Review Autumn 09 Impact Fig. 7
Oilfield Review Autumn 09 Impact Fig. 7
ORAUT09-Impact Fig. 7 ORAUT09-Impact Fig. 7
ORAUT09-Impact Fig. 7 20 Oilfield Review
• Syngenetic deposits originate during or because of impact; examples are diamonds found in impact melts in several craters in Germany, Ukraine and Russia, and copper- nickel and platinum-group elements in sul- fides, such as those in the prolific mines of the Sudbury structure, in Ontario, Canada.
• Epigenetic deposits result from postimpact processes; examples include hydrocarbons and hydrothermal deposits. The remainder of this article focuses on hydrocarbon resources.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64