Seismic Characterization of Meteorite Impact Craters Michael J. Mazur*, Robert R. Stewart,The CREWES Project, and Hans-Henrik Westbroek, Shell Canada. Summary Nearly one quarter of all known terrestrial impact craters are associated with economic deposits of some kind whether they are mineral ores, hydrocarbons, evaporite minerals, or even fresh water. Imaged by seismic means, these craters often show characteristics that are diagnostic of crater morphology and impact mechanics. Several examples of impact structures from the Western Canadian Sedimentary Basin (WCSB) are examined in detail. Introduction It is a common belief that impact cratering played an integral role in the formation of the planets in the solar system. By accretion of material in the early solar nebula, the planets formed in an environment in which relative collision velocities were low but collision rates were high. Examining the surface of the moon, we see that, while the current rate for large impacts is only 10 -5 yr -1 (Wetherill and Shoemaker, 1982) bombardment of its early surface was a major geological process. From this, it can be concluded that it is highly unlikely that the Earth was left immune to impact events. More than 140 terrestrial impact craters have been discovered worldwide. About one-quarter of these have economic importance in terms of mineral and hydrocarbon deposits (Masaytis, 1989; Grieve, 1991). It is also generally accepted that meteorite impacts have occasionally resulted in mass extinctions documented in the fossil record. Most notably, the nearly 300 km diameter Chicxulub crater on the Yucatan Peninsula, Mexico is thought to be the result of an impact at the end of the Cretaceous that caused the extinction of the dinosaurs (Hildebrand et al., 1991). Terrestrial craters are characterized by two basic forms: simple and complex (Figures 1 and 2). Simple craters have diameters up to a diameter of about 2 km in sedimentary rocks and 4 km in crystalline rocks (Dence, 1972). These craters are formed by lower-energy events with a subsequently lower explosion component to the impact often resulting in pieces of the meteorite remaining intact (Krinov, 1963). They are characterized by a simple bowl-shaped profile, the bottom of which is filled with an allochthonous brecciated lens from the slumping of the transient crater walls (Pilkington and Grieve, 1992). For larger impacts, the meteorite is not slowed appreciably by the Earth’s atmosphere (Grieve, 1991). Figure 1. Schematic depiction of a simple crater The resulting impact involves such high shock pressures (50-100 Gpa), that the meteorite is largely vaporized in the explosion. The transient cavity floor i n this case rebounds from its initial downward displacement to form a central uplift region characterized by shock metamorphic effects. The rim of the crater often is terraced due to rim faults and the annular trough is characterized by allochthonous shocked materials and impact melts (Grieve, 1991; Melosh, 1989). This is the basic morphology of a complex crater. Figure 2. Schematic of complex crater Economic Potential Impact craters on Earth have been linked to economic deposits of various materials and in some cases, these deposits are world class (e.g. the Cu-Ni deposits at Sudbury, Ontario). While materials in the vicinity of impact craters have been exploited for many decades, only recently has an inventory been made on the revenues generated by this exploitation (Grieve and Masaytis, 1994). Of the more than 140 known terrestrial impact craters, about 35 (25%) have, at some time, been associated with economic deposits. Of these, 17 are being actively exploited. The current estimated annual revenues from these deposits is estimated at over $12 billion (Grieve and Masaytis, 1994). This estimate is based largely on North American deposits (annual revenues ≈ $5 billion) and the gold and uranium ores of the Vredefort structure in South Africa (annual revenues ≈ $7 billion), and does not include revenues generated from the extraction of building materials (e.g. cement and lime products at Ries, Germany ≈ $70 million per year) or from the generation of hydroelectric power (e.g. 4000 Gwh/a from the reservoir at Manicouagan, Quebec ≈ $200 million per year).
Transcript
Seismic Characterization of Meteorite Impact CratersMichael J. Mazur*, Robert R. Stewart,The CREWES Project, and Hans-Henrik Westbroek, Shell Canada.
Summary
Nearly one quarter of all known terrestrial impactcraters are associated with economic deposits of somekind whether they are mineral ores, hydrocarbons,evaporite minerals, or even fresh water. Imaged byseismic means, these craters often show characteristicsthat are diagnostic of crater morphology and impactmechanics. Several examples of impact structures fromthe Western Canadian Sedimentary Basin (WCSB) areexamined in detail.
Introduction
It is a common belief that impact cratering played anintegral role in the formation of the planets in the solarsystem. By accretion of material in the early solarnebula, the planets formed in an environment in whichrelative collision velocities were low but collisionrates were high. Examining the surface of the moon, we see that, whilethe current rate for large impacts is only 10-5 yr-1
(Wetherill and Shoemaker, 1982) bombardment of itsearly surface was a major geological process. From this,it can be concluded that it is highly unlikely that theEarth was left immune to impact events. More than 140 terrestrial impact craters have beendiscovered worldwide. About one-quarter of these haveeconomic importance in terms of mineral andhydrocarbon deposits (Masaytis, 1989; Grieve, 1991).It is also generally accepted that meteorite impacts haveoccasionally resulted in mass extinctions documentedin the fossil record. Most notably, the nearly 300 kmdiameter Chicxulub crater on the Yucatan Peninsula,Mexico is thought to be the result of an impact at theend of the Cretaceous that caused the extinction of thedinosaurs (Hildebrand et al., 1991). Terrestrial craters are characterized by two basicforms: simple and complex (Figures 1 and 2). Simplecraters have diameters up to a diameter of about 2 km insedimentary rocks and 4 km in crystalline rocks (Dence,1972). These craters are formed by lower-energy eventswith a subsequently lower explosion component to theimpact often resulting in pieces of the meteoriteremaining intact (Krinov, 1963). They are characterizedby a simple bowl-shaped profile, the bottom of which i sfilled with an allochthonous brecciated lens from theslumping of the transient crater walls (Pilkington andGrieve, 1992). For larger impacts, the meteorite is not slowedappreciably by the Earth’s atmosphere (Grieve, 1991).
Figure 1. Schematic depiction of a simple crater
The resulting impact involves such high shockpressures (50-100 Gpa), that the meteorite is largelyvaporized in the explosion. The transient cavity floor inthis case rebounds from its initial downwarddisplacement to form a central uplift region characterizedby shock metamorphic effects. The rim of the crater oftenis terraced due to rim faults and the annular trough i scharacterized by allochthonous shocked materials andimpact melts (Grieve, 1991; Melosh, 1989). This is thebasic morphology of a complex crater.
Figure 2. Schematic of complex crater
Economic Potential
Impact craters on Earth have been linked to economicdeposits of various materials and in some cases, thesedeposits are world class (e.g. the Cu-Ni deposits atSudbury, Ontario). While materials in the vicinity ofimpact craters have been exploited for many decades,only recently has an inventory been made on therevenues generated by this exploitation (Grieve andMasaytis, 1994). Of the more than 140 known terrestrialimpact craters, about 35 (25%) have, at some time, beenassociated with economic deposits. Of these, 17 arebeing actively exploited. The current estimated annualrevenues from these deposits is estimated at over $12billion (Grieve and Masaytis, 1994). This estimate i sbased largely on North American deposits (annualrevenues ≈ $5 billion) and the gold and uranium ores ofthe Vredefort structure in South Africa (annual revenues≈ $7 billion), and does not include revenues generatedfrom the extraction of building materials (e.g. cement andlime products at Ries, Germany ≈ $70 million per year)or from the generation of hydroelectric power (e.g. 4000Gwh/a from the reservoir at Manicouagan, Quebec ≈$200 million per year).
Deposits of materials formed in or around impactcraters are divided among three categories: progenetic,syngenetic and epigenetic deposits (Masaytis, 1989).Progenetic deposits are those which originated byendogenic geological processes. In this case, the impacthas the effect of redistributing the deposit allowing it tobe more easily retrieved. Syngenetic deposits are thosewhich originate during or shortly after an impact event.These types of deposits are generally attributed to thedirect deposition of energy into the target rocks causingphase changes and melting. Epigenetic deposits areformed after the impact and are generally attributed tohydrothermal alteration, formation of enclosed basinswith isolated sedimentation, or the flow of fluids intostructural traps associated with the crater. It i shydrocarbon accumulations that are of this latter type. The impact craters at Boltysh (25 km wide, 88 Maold), Obolon (15 km wide, 215Ma old) andRotmistrovka (2.7 km wide, 140Ma old), all in theUkraine, contain oil shales equal to some 90 millionbarrels of unmatured oil. Boltysh alone contains 4.5billion metric tons of oil shale in a 400-500 m thickproductive sequence which lies over the trough andcentral uplift (Grieve and Masaytis, 1994). Evidently,these impact craters formed isolated basins in whichalgae activity thrived, providing the biogenic mass forthe development of the oil shales. The structural facies associated with impact cratersmakes them potential traps for migrating hydrocarbons.As an analog to the development of oil shales, impactcraters can result in the formation of source rocks as well(Castaño et al., 1995). Thus, hydrocarbon reservoirs ofthis nature do not necessarily have to develop intraditional basin-type regions. The Ames structure inOklahoma is by far the most prolific hydrocarbonproducer of all impact craters and an example of a craterproviding both the isolated basin in which the sourcerocks form as well as the structural traps in which thehydrocarbons accumulate. The simple Newporte craterin North Dakota is a similar case where source oilshales are localized in the crater. Total reserves at Amesare estimated at 50 million barrels of oil and some 20-60billion cubic feet of gas (Isaac and Stewart, 1993;Grieve and Masaytis, 1994; Kuykendall and Johnson,1995). While the first discovery came from karsted rimdolomites , the largest deposits are found in the granite-dolomite breccia of the central uplift and brecciatedgranite in the floor of the crater, the transient craterhaving excavated to basement. Over 100 wells havebeen drilled on the structure of which 52 produce oiland 1 produces gas. The 12 km diameter Avak structure on the north coastof Alaska is an interesting case in which the impact mayactually have disrupted hydrocarbon accumulationsthat already existed in the region (Kirschner et al.,1992). Nonetheless, gas fields still exist along theoutside of the rim with 37 billion cubic feet in gasreserves. The structural traps are formed by listric rimfaults which juxtapose lower Cretaceous shales against
Jurassic sands down dip. These and other structuresassociated with hydrocarbons are shown in Table 1.
Structure Diameter(km)
Age(MA)
HydrocarbonAccumulation
Ames, OK 14 450 • 50MMbbl oil• 20-60 BCFG• source rock
controlled bystructure
Red WingCreek, ND
9 200 • 40-70MMbbl oil• 100 BCFG• provided trap to
migratinghydrocarbons
Avak, Alaska 12 3-100 • 37 BCFG• provide d trap to
Calvin, Mich. ? ? • 600MMbbl oilSteen, AB. 22 95 • 600bbl per dayViewfield,
Sask.2.4 Triassic
Jurassic• 400bbl per day• 20MMbbl oil• formed trap to
migratinghydrocarbons
Tookoonooka,Australia
55 ? • forms shadowzone tomigratinghydrocarbons
Table 1. Structures associated with hydrocarbon accumulation.(Sources: Isaac and Stewart, 1993; Grieve and Masaytis, 1994;Hodge, 1994; Buthman, 1995).
Interpretations
Several enigmatic circular structures have beenobserved on seismic data obtained within the WCSB(Sawatzky, 1976; Isaac and Stewart, 1993) and otherparts of Canada (Scott and Hajnal, 1988; Jansa et al.,1989). The Purple Springs structure, The White Valleystructure, and the James River structure, all located insouthern Alberta, have been interpreted in detail. The James River structure (Figure 3), located insouthwestern Alberta, is consistent with complex cratermorphology. The top of this structure is buried at adepth of nearly 4500m and is truncated by an erosionalunconformity marking the top of the Cambrian. Thiscrater has a diameter of nearly 5 km and is marked by anannular moat and a central uplift 2.4 km in diameter. Ofexploration interest are the terraces observed along thecrater walls. These terraces result in large blocks ofcompetent rock being displaced forming structural traps. The White Valley structure (Figure 4) insouthwestern Saskatchewan is an unusual circular
anomaly evident on four 2-D seismic lines. Thisstructure has many of the morphological characteristicsof a complex impact crater (Westbroek et al., 1996). Thestructure is observed to have a diameter of about 6 kmand is seen to have an annular trough and a raisedcentral uplift. Also observed is an apparent asymmetryof the appearance of the Milk River formation across thecentral uplift possibly yielding important informationabout the dynamics of crater formation. Located in south-central Alberta, the Purple Springsstructure (Figure 5) is approximately 3 km in diameter.The structure is imaged well on seismic data and showsthe basic bowl-shaped basin characteristics expected ofa simple impact crater.
Summary
A thorough examination of the seismic characteristicsof impact craters has been undertaken. The structuresdiscussed here exhibit the classic characteristics of bothsimple and complex crater morphologies. Byunderstanding these characteristics, one can hope tobetter understand both the structural characteristics andthe formation mechanics of terrestrial impact craters.
References
Castaño, J.R., Clement, J.H. and Sharpton, V.L., 1995,Source rock potential of impact craters: Ames structureand similar features, Expanded Abstracts, 5.
Dence, M.R., 1965, The extraterrestrial origin ofCanadian craters: NY Acad. Sci. Ann, 123, 941-969.
Grieve, R.A.F., 1991, Terrestrial Impact: The record inthe rocks: Meteoritics, 26, 175-194.
Grieve, R.A.F. and Masaytis, V.L., 1994, The economicpotential of terrestrial impact craters: Internat. Geol.Rev., 36, 105-151.
Isaac, J.H. and Stewart, R.R., 1993, 3-D seismicexpression of a cryptoexplosion structure: Can. Jour.Expl. Geophys. Res., 90, 11930-11942.
Jansa, L.F., et al., 1989, Montagnais: a submarine impactstructure on the Scotian Shelf, eastern Canada: Bull.Geol. Soc. Am., 101, 450-463.
Kirschner, C.E., Grantz, A., and Mullen, M.W., 1992,Impact origin of the Avak structure, Arctic Alaska, andgenesis of the Barrow gas field: Amer. Assoc. Petrol.Geol. Bull., 76, 651-679.
Masaytis, V.L., 1989, The economic geology of impactcraters: Internat. Geol. Rev., 31, 922-933.
Melosh, H.J., 1989, Impact cratering: a geologicprocess: Oxford Univ. Press, Inc.
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Sawatzky, H.B., 1976, Two probable late Cretaceousastroblemes in western Canada - Eagle Butte, Albertaand Dumas, Saskatchewan: Geophysics, 41, 1261-1271.
Scott, D. and Hajnal, Z., 1988, Seismic signature of theHaughton structure: Meteoritics, 23, 239-247.
Westbroek, H.-H., Stewart, R.R. and Lawton, D.C.,1996, Seismic description of subsurface meteorite impactcraters: 58th Ann. Con. Tech. Ex., Eur. Ass. Geo. Eng.,Expanded Abstracts, 2, A016.
Wetherill, G.W. and Shoemaker, E.M., 1982, Collisionof astronomically observable bodies with the Earth: inSilver, L.T., and Schultz, P.H., Eds., Geologicalimplications of impacts of large asteroids and comets onthe Earth: Geol. Soc. Am., Special Paper, 190, 1-13.
Figure 3. Radial seismic section from James River 3-D dataset.
Figure 4. White Valley line WV-017.
Figure 5. Migrated section over the Purple Springs structure.
