Excavation produces a bowl-shaped “tran-
sient” crater. During postimpact modification, the transient crater collapses because of gravity. The morphology of the resulting crater depends on target rock type and impactor size.8
Simple
structures retain their bowl shape and uplifted rim. On Earth simple impact structures are usu- ally small, with diameters up to 2 km [1.2 mi] in sedimentary rock and up to 4 km [2.4 mi] in crystalline rock. An example of a well-preserved simple structure in sedimentary rock is the Barringer crater, located in Arizona, USA (right). Beneath the apparent floor of the crater lies a layer of brecciated target material, which over- lies the fractured but autochthonous target rocks of the crater’s true floor. Analysis of shocked min- erals from the crater floor indicates that pres- sures reached approximately 25 GPa [3.6 million psi]. The walls of the final, collapsed, crater are shorter than in the original, transient, crater. Such postimpact modifications produce a final diameter slightly greater than that of the tran- sient cavity. Craters larger than a few kilometers in diame-
ter usually have complex morphologies character- ized by an uplifted central area. The central uplift may be a peak or, in the largest craters, a ring. The central high consists of shocked target rock that
3. For general references on the following discussion:
Melosh HJ: Impact Cratering: A Geologic Process. New York: Oxford University Press, 1989.
Koeberl C: “Impact Cratering: The Mineralogical and Geochemical Evidence,” in Johnson KS and Campbell JA (eds): Ames Structure in Northwest Oklahoma and Similar Features: Origin and Petroleum Production (1995 Symposium). Norman, Oklahoma: Oklahoma Geological Survey, Circular 100 (1997): 30–54.
Koeberl C: “Mineralogical and Geochemical Aspects of Impact Craters,” Mineralogical Magazine 66, no. 5 (October 2002): 745–768.
4. Koeberl (2002), reference 3.
5. Simonson BM and Glass BP: “Spherule Layers—Records of Ancient Impact,” Annual Review of Earth and Planetary Sciences 32 (May 2004): 329–361.
6. Smit J: “The Global Stratigraphy of the Cretaceous- Tertiary Boundary Impact Ejecta,” Annual Review of Earth and Planetary Sciences 27 (May 1999): 75–113.
7. Koeberl (2002), reference 3.
Koeberl C, Milkereit B, Overpeck JT, Scholz CA, Amoako PYO, Boamah D, Danuor S, Karp T, Kueck J, Hecky RD, King JW and Peck JA: “An International and Multidisciplinary Drilling Project into a Young Complex Impact Structure: The 2004 ICDP Bosumtwi Crater Drilling Project—An Overview,” Meteoritics & Planetary Science 42, no. 4/5 (2007): 483–511.
8. Grieve RAF: “Terrestrial Impact Structures: Basic Characteristics and Economic Significance with Emphasis on Hydrocarbon Production,” in Johnson KS and Campbell JA (eds): Ames Structure in Northwest Oklahoma and Similar Features: Origin and Petroleum Production (1995 Symposium). Norman, Oklahoma: Oklahoma Geological Survey, Circular 100 (1997): 3–16.
D
A Projectile Shock wave Target rock
B Ejecta
Transient crater Vapor
Melt
Rarefaction Shock wave
C Air-fall breccia Original plane Sediments
Mixed breccia
Melt Fracture
Shocked target rock
Dyke with impact melt Overturned flap
> Simple impact structure. During contact and compression (A) the asteroid hits the Earth’s surface and pushes target material downward. In the excavation stage (B), the transient crater forms. Following impact, the crater walls have collapsed slightly, and ejecta have fallen back into the crater (C). The Barringer crater, in Arizona (D), is an example of a simple impact crater. It has a diameter of 1.2 km [0.7 mi] and was formed 50,000 years ago. (Photograph courtesy of the Lunar and Planetary Institute.)
Oilfield Review Autumn 09 Impact Fig. 3
ORAUT09-Impact Fig. 3
Winter 2009/2010
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