Journal of Petroleum Geology, 3, 3, pp. 279-302,1981 279

IMPACT CRATERS: IMPLICATIONS FOR BASEMENT HYDROCARBON PRODUCTION

Richard R. Donofrio*

The impact cratering process results in unique structures and extensive fracturing and brecciation of the target rock which can be conducive to hydrocarbon accumulations. Examination of Viewfield and Red Wing Creek oil pools in North America reveals that they may have been formed by meteoritic impact in Paleozoic sediments. Additional hydrocarbon traps have most likely been produced by impact but have not been recognized as such because geologists are generally not familiar with crater structures and shock-metamorphic effects in rocks. It is proposed that petroliferous basement impact craters also exist and that despite arguments to the contrary, at least one may have already been found. Further discoveries are severely limited because of conservative exploration procedures, which characteristically avoid penetrating crystalline basement. Core analysis from several large impact sites developed in crystalline rocks reveals that while permeability factors are marginal, the reservoir potential of these craters exceeds those of many of the largest known hydrocarbon accumulations. Preservation age studies of craters in conjunction with size frequency distribution curves implies that many will have been buried before erosional eradication. As with normally-fractured and brecciated basement areas, some will have accumulated hydrocarbons. In addition to classical source rocks flanking or overlying these potential reservoirs, recycled kerogen and the possibility of inorganic sources are also considered. A basement may afford a unique way of testing the inorganic hydrocarbon proposals.

Introduction

The revelations of the , Viking a n d The cratering phenomenon appears to be universal V o y a g e r space probes have shown the extent of in scope and is indicative of catastrophic forces cratering on bodies other than the moon. As the at work throughout the geologic record. earth’s atmosphere affords insignificant velocity Random in time and space, impacts create, retardation to relatively large incoming objects, it is in the order of seconds, features which are assumed that the earth’s pretransgressive basement largely independent of the regional geology. configuration was extensively cratered as well and, As the search for hydrocarbons continues to despite erosional elements, that many of these impact probe deeper than in the past, it becomes scars are preserved by overlying sediments. essential for geologists to recognize the

* Exploration and Production Dept., American Ultramar Ltd., 90 S. Bedford Road, Mt. Kisco, New York 10549.. USA. (Ultramar Company Ltd., London, UK is the parent company of American Ultramar Ltd.). Extracts from this paper were presented to the Petroleum Society of New York in September 1979, under the title: “Subsurface Impact Craters as Hydrocarbon Traps”. 280 Richard R. Donofrio

The Cratering Record Terrestrial cratering estimates have been made from impact probability rates of earth-crossing (Shoemaker, 1977), and from the occurrence of ancient probable impact sites of known age on the North American and East European cratons (Grieve and Dence, 1979). The data suggest that the production rate for craters 10km and larger is about (0.7 ± 0.35) X 10-l4sq km/yr whereas it approximates (0.35 ± 0.1) X 10- 1 4sq km/yr for 20km and larger diameter features. The size-frequency distribution, or mass distribution, of crater-forming follows a log-normal curve (Brown, 1960), which means that their population is composed of an increasing number of smaller bodies and a decreasing number of larger bodies. When combined with present meteoritic infall rates and extrapolated into the Precambrian, the calculations show that over 150,000 craters with diameters of 1km or larger will have been formed on the earth’s land surface during the past 3 billion years. Of this number, over 3,000 will have dimensions greater than 10km of which approximately 60 will dynamic forces from outside the earth that have exceed 100km in size. modified petroliferous basins. The accuracy of such cratering estimates was discussed by French (1968) who noted that impact Economic Significance probability rates for the past 2 billion years had The Baltysh depression in the Ukraine SSR has predicted 20 craters with diameters greater than 5km been recognized as a fossil crater and is well on the Canadian Shield. Of this predicted number, 15 known as a locality for oil shales (sapropelites), the i m pact structures had been identified by 1968. Additional reserves of which constitute about three billion tons craters have since been recognized, bringing the current (Yurk, et al., 1975). A number of other possible number to 20 (Grieve and Robertson, 1979). impact structures associated with mineral deposits in the Ukraine are being investigated. Cratering Mechanics In addition to the mineral resources of Sudbury, a Incoming extra-terrestrial bodies having masses Canadian probable impact crater containing over 75% greater than 108gm will retain their interplanetary of the Western World’s nickel deposits, at least two kinetic energy while traversing the atmosphere and commercial hydrocarbon-bearing structures in North strike the earth’s surface at velocities ranging from America are suspected as being of impact origin. 15 to 70km/sec (Wetherill, 1977). Following impact, a These are the Viewfield and Red Wing Creek catastrophic sequence of events is set in motion. These structures of the Williston Basin. Three additional events have been observed in laboratory ballistics curious subsurface features currently are under experiments by Gault et al. (1968) and show that upon investigation in two southern US basins. impact the kinetic energy of the projectile is The possibility of petroliferous impact craters in instantaneously transferred to the target surface in the the sedimentary column has important implications form of heat and intense shock waves, which for the most neglected of all potential reservoirs— produce local pressures in the orders of megabar c r y s t a l line basement. I propose that commercial range. As these shock waves move radially hydrocarbons exist in basement impact sites, and that outward they decrease in pressure and distribute some of these structures will prove to contain major the impact energy over a steadily-increasing volume r e s e r v e s . of target rock. The complex interaction Impact craters 281

of compression and rarefaction waves results in shock pressures, indicate the existence of excavation of material, which creates an enlarging temperatures in excess of 1,500°C, much too high embryonic cavity. This excavation ceases when the for normal geological activity. pressure drops below the yield strength of the target In addition to shock-metamorphic effects on the rocks. Slumping, isostatic adjustments, and erosion microscopic level, unusual conical fracture surfaces with resultant infilling then proceed to modify the called shatter cones are often noted within impact crater profile. sites. These are not to be confused with cone-in-cone, The term has been created which are concretionary structures formed mostly in to describe all changes in rocks and minerals resulting calcareous-type rocks. Shatter cones are usually not from the passage of transient, high-pressure shock limited by lithology and according to Milton (1977) waves. French (1968) has described and classified are produced within a shock pressure range extending these changes into three major groups. These are best from about 20 to perhaps 250kb. These are large-scale developed in quartzofeldspathic rocks, are detectable shock indicators whose proof of impact origin by in minute quantities from drill cores or cuttings, and direct association with meteoritic material has been can be preserved in rocks for up to 2 billion years: reported fromseveral craters ( Roddy and Davis, 1977). 1. High pressure effects are represented chiefly by the production of high-pressure polymorphs such as Crater Morphology , , and diamond, which can also Two basic classes of impact craters have been be produced under static high-pressure conditions recognized on the earth and other planetary bodies— (indicative of the upper mantle). During normal the simple and the complex variety. The term rock metamorphism, however, pressures are astrobleme (Dietz, 1961) is frequently used when usually lower than 10kb. referring to either type. 2. High strain-rate effects develop “planar features” Simple craters are characterized by a bowl-shaped in quartz and produce isotropic phases of quartz depression and a raised and overturned rim, a classical and feldspar (maskelynite). example of which is in Arizona (Fig. 2). 3. High-temperature effects are produced by shock This feature is developed in sedimentary rocks and is pressures so high that the resultant relaxation about 1.2km in diameter, 180m deep, and has a raised temperatures are hundreds of degrees above the rim of about 40m. normal melting points of the component minerals, Shoemaker (1960), and Shoemaker and Eggleton thus initiating reactions such as the melting of (1961) have defined a number of terms from their quartz to (fused silica glass) and the field-studies of Meteor Crater and other impact sites. decomposition of zircon to baddeleyite. Such These include: throwout, rock debris ejected along features, while they are not direct evidence of high ballistic trajectories; fallout, a relatively-thin veneer of 282 Richard R. Donofrio

material formed by the free-fall of rock fragments as geologists become more familiar with cratering ejected at high angles of elevation; allogenic , mechanics and shock metamorphism. a breccia composed of mixed fragments of rocks derived from different areas in the crater: and Effects on Target Rock authigenic breccia, material in the crater which has In addition to the crater depression itself, the essentially been brecciated in place. extensive crushing, fracturing and brecciation ability At a crater diameter of approximately 4km in of hypervelocity impacts is evident from field crystalline rock and 2 to 3km in , a investigations. This exogenic fracturing mechanism is morphological change from simple to complex type distinct from normal terrestrial processes which occurs (Fig. 3), which results in a comparatively commonly fracture rocks by crustal movement, shallow crater having an uplifted central area or peak unloading, weathering and volume shrinkage. Tests and a slumped or depressed rim (Dence, 1972). In have shown that projectile impact or explosive field-studies of complex craters, Milton and Roddy detonation causes a compressive stress to traverse the (1972) note that central uplift commonly is 10-15% of rock and give rise to subsequent tensions. At locations the crater diameter. This results in a considerable in the rock where tensile stresses are high enough, displacement effect on the target rock. inherent flaws in the material become unstable and Generally, complex structures up to approximately begin to grow. As the cracks continue to extend, they 30km diameter will have distinct central peaks or begin to encounter one another and up. uplifts. Beyond this diameter, the central uplift may be As crack coalescence continues, chunks of material replaced or augmented by a concentric series of uplifts are isolated from the rock body and, if near a and depressions, giving the structure a multi-ring form surface, may be ejected at considerable velocity (Dence et al., 1977; Grieve and Robertson, 1979). (Curran et al., 1977). By convention. a suspected impact crater in which Experiments of this nature indicate that the harder meteoritic fragments have been found is classified as and less porous the rock, the greater will be the brittle proved. If only shock-metamorphic features are fracturing. During natural hypervelocity impact, detected, the crater is classified as probable. A n y shock-induced pressure and temperature effects in other criteria used in suggesting an impact origin such rocks of various composition must also be as structure, classify it as possible. The cumulative considered. Chao (1968) noted that these effects number of proved, probable and possible i m p a c t can induce volatilization, melting, craters on earth now stands at 145 (Grieve and o x i d a t ion/decomposition reactions and hydrothermal Robertson 1979 and personal communication). These alteration, the latter producing low-temperature include both surface and subsurface craters, many of minerals such as zeolites, chlorite, hematite and clay which have only recently been recognized. The (Dence, 1972). In some cases, ore minerals are number of suspected impact sites continues to increase mobilized as the result of the disturbance and Impact craters 283

may form vein or other deposits. Despite these were simultaneously formed in crystalline rock. As potentially-detrimental reservoir factors, the impact can be seen in Fig. 6, the limit of bedrock disturbance process has important implications for petroleum is not determined by the crater rims, but extends geology in that an impact can instantaneously create radially outward for two crater diameters. Frequently, porous and permeable rock from non-reservoir a similar distance will be manifested by a radial material, and also modify the structural configuration drainage pattern emanating from some craters, which of the target rock independently of the regional indicates the degree of control that craters can exert on geology. With large magnitude impacts the extent of the local topography. (“Local”, however, is an entirely these alterations is tremendous. relative and often misleading term.) Fig. 7 shows the At the Ries Basin in Germany, for example effect of a titanic impact on the “local” lunar surface. (Fig. 4), the hypervelocity impact of an estimated Cratering events of this magnitude in addition to 1km-diameter stony meteorite penetrated about 600m creating mountainous basin rings and mega-terraces, of sedimentary sequence and continued for another are believed to be responsible for magmatic up- 650m into crystalline basement. The ensuing welling. Terrestrial equivalents to the lunar Orientale explosion excavated between 124cu km and basin are not observable on the surface. Dence (1972) 200cu km of rock, and produced a complex ringed suggested that large-scale impacts may have occurred crater 22-23km in diameter. Seismic measurements while the early continental crust was assuming its have revealed that basement rock at the crater center present form and that impact events possibly played a has been brecciated and fractured down to about 6km major role in the initial stages of continental (Pohl et al., 1977). formation. The extensive vertical brecciation and fracturing Some of the larger probable surface impact sites at an impact site is frequently augmented by a radial are Manicouagan, (70km diameter), Popigai, and concentric interconnecting fracture pattern in the USSR (100km). Sudbury, Canada (140km) and target rock. This is best observed in small-scale Vredefort, S. Africa (also 140km diameter). In cratering experiments conducted by Curran et al., addition to thousands of smaller craters, there is no (1977), and can be seen in Fig. 5, which shows the reason to believe why craters of these larger typical pronounced effects of high-velocity impact on dimensions have not contributed to extensive slabs of Arkansas novaculite, a hard, dense, worldwide basement modification. polycrystalline quartzite. This pattern frequently The potential for structural and stratigraphic traps extends well beyond the basal excavated area. created by the impact process is readily evident in On a much larger scale, a similar example of this Figs 2 and 3. Under proper conditions, both simple- pattern occurs at Clearwater Lake, Quebec, where two and complex-type impact craters have a significant potential complex ringed craters (30km and 25km diameter) as hydrocarbon reservoirs. This potential can be 284 Richard R. Donofrio

appreciated by briefly examining several petroliferous Viewfield structures suspected as being of impact origin, two of The Viewfield oil pool was discovered in 1969 which are developed in sediments and one which is after a routine seismic survey indicated the presence proposed to be a crystalline basement astrobleme. All of a subtle circular-shaped anomaly. Subsequent well three are found in the North American Williston control has shown that the feature is composed of Basin, where “pancake” stratigraphy often three principal elements: facilitates the detection of many subsurface 1. A deep cavity cut into Mississippian structural anomalies. carbonates which has been filled by an Impact craters 285

area; the underlying Stoughton beds also are oil productive in certain areas. The rim facies isopach reveals an irregular lobe- like pattern radiating outward from the central cavity, with highest isopach values generally found midway between the inner and outer 0-foot contour intervals. It is interpreted to slump towards the central cavity, and in the 13-33 and 14-20 we l ls, lies between the same red beds filling the central depression near the 15-29 location (Fig. 8). Pay thickness in the rim ranges from l2-170ft (3.6- 52m) with some producing up to 400 brl/day. Core analysis of the breccia from these wells has shown an average porosity of about 14% and an average permeability of about 400 md. The rim facies is stratigraphically trapped by anhydrite and siltstone. Adjacent to, and in some areas underlying. the rim facies is the oil-bearing Mississippian Griffin carbonate. Oil accumulation in these beds is largely unrelated to the Viewfield structure and the trapping anomalously-thick Jurassic Watrous Red Bed mechanism is due to erosional truncation of the section (Fig. 8). Griffin beds up-dip to the north. These beds are 2. An oil-bearing Mississippian carbonate breccia secondary objectives, with pay sections ranging from which forms a rim facies around the central cavity, 15-55ft (4.6-16.7m). and is located at the Watrous Red Beds time- Original oil-in-place of the structure exceeds stratigraphic level. (Fig. 9). 75MM brl of which about 20MM brl are 3. Mississippian beds which surround the cavity and recoverable. No wells have penetrated the rim facies. Oil-bearing Griffin beds are located at Mississippian interval below the central the Mississippian unconformity over most of the d e p r e s s i o n. The extent of cavitation shown has 286 Richard R. Donofrio

been estimated by applying Pike’s (1977) equations o v e r t u rning of the target rock which, if brittle, can be derived from studies of explosion and meteorite fractured, but not usually brecciated to the degree shown in craters. Crater depth will vary depending on the the Viewfield cores. If left uncovered, rim rocks will be original diameter selected. eroded in a similar manner as any hill or elevated structure, One of the unpublished endogenic theories for the however, and can become breccia or talus deposits. evolvement and subsequent trapping mechanism of oil Sawatzky (1972) proposed an impact origin for for Viewfield was developed by W. H. (personal Viewfield and showed that the dimensions could be communication). He proposed a normal Cambrian- explained by employing empirically-derived through-Mississippian deposition followed by explosion-crater equations which indicate that almost localized upward movement during the period of two-thirds of the original rim has been eroded. The Mississippian erosion. This uplifted area subsequently calculated depth of the structure is similar to the was eroded and limestone talus accumulated around profile in Fig. 8. He also presented seismic isochron the base of the eroding hill. Downward tectonic and well-control data which precluded any possibility movement during Jurassic Red Bed deposition then of explaining the entire Viewfield anomaly by means resulted in a relatively-thick Red Bed section in the of multi-stage solution and collapse of the underlying central depressed area and thin or absent Red Beds Middle Devonian salt horizon. Post-impact solution over the talus deposits (rim facies). Final downward and collapse did, however, appear to have contributed movement of the structure’s core in Late Jurassic- to the present-day configuration. Early Cretaceous times resulted in an increased After examining cores, it is evident that the thickness in these beds. lithology of Viewfield is not conducive to the At the present time, while there are examples of formation of microscopic shock-metamorphic steep-sided, localized highs in SW Saskatchewan, features. Carbonates, however, do afford a good there are none involving the small-scale uplift and medium for development, but when subsequent collapse proposed for Viewfield. This found in simple craters, their location is limited to the feature appears to have formed independently of the crater floor area. As previously noted, no wells have regional tectonics and has the characteristics of a reached this depth in the central cavity. simple-type crater. The breccia, or talus, which forms The presumed lack of meteorite fragments at the rim facies, however, may have been created Viewfield could be a function of crater size rather following the . Crater rims are formed by than the Jurassic/Triassic impact age. If

288 Richard R. Donofrio

fragments originally existed in the breccia at Viewfield. This category will remain until shock- V i e wfield, they would probably remain because of metamorphic features are detected. preservation by later sedimentation. However, discrete meteoritic fragments have not been found in craters larger Red Wing Creek than Meteor Crater presumably because the initial energy at Exploration of the Red Wing Creek structure was larger craters vaporizes or melts the meteorite. According to begun in 1965 after seismic coverage had revealed a Grieve and Robertson (1979), only 13 proven i m p a c t pronounced anomaly. The first well, 22X-28 initially craters have been found, all of which are Recent in ag e was abandoned as dry, and was followed three years and none of which islarger than 1.2km in diameter. later by the 13X-15 well, also dry (Fig. 10). A Although the Viewfield structure does not appear significant difference in the structural elevations and adequately to be explained by normal endogenic thicknesses of several beds was noted, tectonics, the process of elimination is insufficient to but no further drilling took place until the leases were warrant impact classification other than possible t o relinquished and acquired by another operator Impact craters 289

about four years later. moderate- to steep-dipping beds, and overturned The 22-27 well in this new exploration program beds. Based on log correlations, the structural was located about 2km east of the 22X-28 well. The pattern is interpreted as having a 1.6km diameter primary objective was rumoured to be a small inner zone of intense deformation and uplift, away structure believed to be present beneath an from which deformation decreases in both anomalous seismic zone interpreted as an area of horizontal and downward directions. Within this intensive deformation. En route to the Ordovician, area of maximum deformation which has created a the well found an 823m (2,700ft) oil column in the mega-breccia, Mississippian carbonates have been Mississippian, 487m (1,600ft) of which was net pay. thrust as much as 915m above regional subsurface An initial production test in the brecciated carbonate elevation. The main productive area of the flowed 750 brl/day of 41.8°API oil. The Ordovician structure is confined to this mega-breccia. The structure turned out to be a velocity anomaly. original 22X-28 “dry well”, however, is an Additional drilling revealed the unique structure exception; it was later recompleted as an oil well of Red Wing Creek (Fig. 11) and resulted in numerous by another operator. theories of origin, which included everything from a 2. The ring depression is a syncline or graben which fault breccia to a pregnant bioherm. After reviewing surrounds the central uplift. It is approximately geological and geophysical data, Brenan et al., (1975), 1.6km wide and is bounded by deformed rocks proposed that the structure was of meteoritic origin. of the outer rim. The principal deformation They noted that the 10km diameter structure could be evident in the ring depression is normal divided into three main provinces: a central uplift faulting, with fault blocks having move d surrounded by a ring depression which in turn is surrounded inward towards the central uplift as well as by an outer rim. These are summarized below: downward. Displacements in the upper horizons 1. The central uplift is about 6.5km in diameter and range from 175 to 115m below expected regional consists of a chaotic arrangement of thrust faults, elevations, and as much as 975m below 290 Richard R. Donofrio

equivalent formations in the central uplift. high-pressure, high-strain-rate, high-temperature rock 3. The outer rim surrounds the ring depression phases and shatter cones are usually explained by and is composed of mildly-deformed rocks. long-term, low-pressure, conditions or unknown Formations in this area are 90-185m endogenic processes. Were it not for the tons of structurally higher than their equivalents in meteoritic fragments at Meteor Crater, the fault-line the ring depression. A discontinuous narrow argument could also be proposed. Bucher initially anticline with 45-60m of closure parallels the believed Meteor Crater to be of endogenic origin, and boundary between the rim and ring considered the meteoritic débris to be unrelated to the depression. Seismic investigation has shown depression. It did not take long for one of the founders that it has a width of less than 1.6km at its of astrogeology, Gene Shoemaker, to convince him widest location. otherwise. The Red Wing Creek structure can be classified as Relative to the petroleum industry, it makes little a p r o b a b l e , since the requisite shock difference as to the exogenic or endogenic origin of metamorphic features have been detected in drill cores certain craters. The important consideration is to and cuttings. These features include distinct shatter recognize that these features are unique hydrocarbon cones and radiating concussion fractures in quartz traps having considerable economic potential. grains. French et al. (1974) and Kieffer (1971) discuss Therefore, while sufficient evidence for an impact the occurrence of the latter at meteorite impact sites origin is present at Red Wing Creek, the proposal by and interpret the fracture pattern as the result of grain Bridges (1978), or similar lineation rebuttals for other motion and contact during the passage of a shock craters, should not be discarded. In areas where strike- wave. The impact age at Red Wing Creek is slip faults are believed to intersect, the explorationist Jurassic/Triassic, and the size of the crater precludes should consider the possible presence of “pseudo- survival of meteoritic fragments, thus preventing impact features”, recognize the hydrocarbon potential a proved impact crater designation. of such structures, and plan an exploration program Calculations show that the central uplift accordingly. megabreccia has trapped in excess of 130MM brl of oil, virtually all of which is confined to a 1.6km Hypothetical Model diameter area. Recoverable reserve estimates range At Viewfield and Red Wing Creek, the suspected from 40-70MM brl. Additional pay zones are possible impacts occurred in the sedimentary column and in this type of crater because of secondary structures resulted in normal, though relatively large, oil resulting from vertical and horizontal deformation of accumulations. A logical extension of the impact the main producing interval. An alternative formation process is to apply it to crystalline basement and for mechanism has been proposed by Bridges (1978), the present, assume the same classical criteria who noted the proximity of supposedly intersecting espoused by the organic hydrocarbon theory. Precambrian strike-slip faults to the central uplift area, According to Landes et al., (1960), source rock, and suggested that Red Wing Creek is a c o n c e n t r i c l i n e reservoir rock, seal and trap are the four essentials for of structural origin. He defined the term as a small any oil pool. The only major difference between structural uplift composed of concentric elements in basement rock and overlying sedimentary rock oil which the dip is inclined either toward or away from a deposits is that in the former case the original oil- common center. These features are proposed as yielding formation (source rock) cannot underlie the developing slowly over a period of millions of years at reservoir. Normal basement-rock accumulations the intersection of strike-slip faults. The presence of obtain their oil from one of three possible sources: (1) shatter cones, shattered quartz grains and breccia at overlying organic rock from which the oil was Red Wing Creek accordingly would not be due to expelled downward during compaction, (2) lateral, shock, but attributed to fault movements. off-the-basement but topographically-lower, organic This proposal is similar to the lineation argument rock from which the oil was squeezed into an which has been used to favor an endogenic origin for underlying carrier bed through which it migrated certain craters e.g., the Ries and Steinhem Basins updip into the basement rock, and (3) lower-lateral (Bucher, 1963). In this argument, the occurrence of reservoirs, from which earlier trapped oil was Impact craters 291

spilled due to tilting, or to overfilling. The hypothetical complex impact crater used in With the basic rules of organic origin, migration the analogy has fractured and elevated the and accumulation in mind, a comparison is now made Precambrian prior to the deposition of the Paleozoic between a petroliferous basement section of the sequence. Provided the central uplift was moderately Central Kansas Uplift and a complex impact crater preserved, the proposed Cambro-Ordovician/ having similar dimensions (Fig. 12). The intention is Pennsylvanian source beds could have been to illustrate what might have occurred had an impact contiguous, with fractured and brecciated crystalline feature been present. rock having much greater horizontal and vertical Tectonically, the Central Kansas Uplift is a broad extent than that produced by endogenic processes. A arch which trends NW. It has a Precambrian smaller, simple-type crater, could have had a crystalline core and is flanked by upturned and preserved elevated rim structure flanked or overlain bevelled Cambro-Ordovician, Ordovician, and by source beds as well. Depending on the depositional Mississippian sediments. Pennsylvanian sediments environment, elevated basement areas resulting from both abut against buried Precambrian hills and are impact structures can also be conducive to re e f a l draped over them in gentle anticlinal folds. The oil formation as well as affecting the configuration o f occurs in fractured Precambrian quartzites on the overlying and adjacent sedimentation. summits of these buried hills and is believed to have C o n s idering the extent of source rocks overlying or in migrated into the quartzite from either the flanking proximity to basement in many petroleum basins, conditions Cambro-Ordovician beds or from the overlying similar to the presented hypothetical model are probably Pennsylvanian rocks (Landes et al., 1960). not uncommon. The reader is requested to 292 Richard R. Donofrio

turn to Fig. 1 and imagine a marine depositional 13.70 md respectively. The Lake St Martin sample is sequence over the photographed region. It should be pulverised basement rock and also showed extensive evident that there is nothing exotic or unusual about mineral alterations. reservoirs of this nature, as the impact process is The sampled rock from the Ries crater originates simply an exogenic means of modifying crystalline in a homogeneous layer up to 400m thick and is basement. According to the biogenic theory, the termed a . Pohl et al., (1977) discuss the presence of hydrocarbons will depend on the same occurrence of this and other at the Ries. It can rules that apply to normal petroliferous basement be described as a depositional polymict breccia of areas. predominantly basement material, containing glassy inclusions which give the appearance of volcanic tuff- Core Analysis breccia but can be distinguished in thin-section by the With the assistance of NASA and the Canadian presence of distinct shock-metamorphic effects, some Department of Energy, Mines and Resources, a of which (Stoffler, 1971) indicate overpressures number of diamond drill cores were obtained from approaching 1,000 kb and temperatures (Chao, 1968) four currently-exposed basement impact craters, and exceeding 1,500°C. subjected to routine core analysis. Cores were not The prime reason for the marginal permeabilities available from any of the subcrater fracture zones nor in Table 1 is most likely the prolonged cooling of the from buried craters developed in crystalline rock; breccias to ambient temperature following impact. pressure drawdown and related material balance data Pohl (1977) calculated that one particular 200m thick also had not been taken. These few breccia samples suevite layer at the Ries took about 2,000 years to cool are therefore not intended to be representative of from 600° to 100°C. The mineral alterations with basement astroblemes, but are suggestive of impact- subsequent permeability reductions are understandable generated temperature/pressure effects and unprotected given these or similar conditions. cr a ter age on reservoir properties (Table 1 ) . The reservoir potential of the larger partially- Hydrothermal alteration and decomposition preserved astroblemes in Table 1 exceeds many of the most products along with melt rock were noted in the Brent pr olific conventional petroleum accumulations known. cores, and are mostly responsible for the low Even with the relatively small-diameter , permeability factors. Similar alterations were noted in the calculated reservoir volume of almost cores from the eroded crater rim base and central 2MM acre-feet reveals the uniqueness of these uplift at Deep Bay. A core from the breccia lens reservoirs. Basement astroblemes of various encompassing the central uplift however, showed the dimensions invariably exist in the subsurface highest porosity and permeability at 21.4% and and have succeeded in accumulating hydro- Impact craters 293

carbons in these massive breccia lenses and central age and erosional levels of 70 probable impact uplifts, as well as in the rim rocks of smaller simple- structures. For a I km diameter crater, the maximum type craters. exposed preservation age is about 30my,, whereas a Extensive open fractures, however, would not be 20km diameter crater will survive for about 600my. necessary to enhance the reservoir properties of any Referring to Table 1, it will be noted that the diameter of fractured or brecciated reservoir. According to Daniel Brent is 3.8km, which corresponds to a maximum exposed (1954) and Gorham et al., (1979) depending on preservation age of about 125my. The crater age, pressure, oil gravity and attitude of the fracture plane, however, is 450±30my, indicating that the presently- a single Imm wide fracture intersecting a well bore exposed feature was covered for a lengthy period. can provide permeability sufficient to produce With simple craters, the rim will be eroded the between 7,000 and 10,000 brl/day of oil. fastest followed by gradual downcutting of the breccia lens which eventually will expose the crater Crater Preservation Age substructure (Fig 14). At this erosional level, the In addition to pressure and temperature effects impact feature will have lost its identity. As complex occurring in the target rock, the exposure time of an craters have subdued rims and pronounced central impact crater to weathering elements is an essential peaks, the latter will be eroded the faster. No reservoir consideration. Exposure affects not only the published estimates have been made for the porosity and permeability of the host rock but, in preservation age range of subcrater fracture zones. At tectonically-stable areas, is the prime agent this erosional level it would be comparable to a determining the degree of post-impact structural peneplain and the preservation age would depend on modification. the depth and extent of fractures. The subcrater Fig. 13 shows a plot of maximum preservation age fracture zones of large craters such as the Ries would against crater diameter. The slope corresponds to a probably take billions of years to eliminate. preservation index of 0.03, which was derived by It will be noted in Fig. 14, that elimination Grieve and Robertson (1979) in a study comparing the of the rim at “B” level, still leaves the possibility 294 Richard R. Donofrio

of stratigraphic trapping in the breccia lens area Newporte contiguous with the crater wall. Even downcutting The Newporte structure (Des-Lacs Field) in into the crater substructure (D) may not totally Renville County, North Dakota, probably eliminate the reservoir potential of either simple-or originated by hypervelocity impact, and could complex-type craters. Such syncline f e a t u r e s r e p r e s ent the first discovery of a petroliferous basement developed in basin areas characterized by astrobleme. A conceptual cross-section of the 3.2km diameter overpressured conditions which frequently favor ci r cular depressed structure is shown in Fig. 16. downward hydrocarbon migration and tight cap rocks, According to Clement and Mayhew (1979), should be considered prime drilling objectives as geophysical evidence indicated that the structural much as craters having preserved rims or central feature involved all early Paleozoic sediments and uplifts at higher structural elevations. probably the Precambrian basement as well. The Ideal crater preservation lends itself to the term Larson 23X-9 wildcat was subsequently drilled to intact impact (Fig. 15). Factors favoring this basement with primary objectives in the condition would consist of a fresh astrobleme covered Ordovician. No encouraging recoveries of by a relatively rapid-moving transgression. For hydrocarbons were found in this interval, but the craters of increasing diameter, the exposure time C a m b r o - O r d o v i c i a n Deadwood sandstones, can be extended without severely diminishing resting unconformably upon Precambrian schist, the integrity of the crater profile. Actively- rising continental areas are the least desirable for preservation purposes, whereas rapidly-subsiding basins favor the intact impact. The previously-discussed Viewfield and Red Wing Creek structures both exhibit moderate erosional profiles developed in sedimentary rocks. An example of a similar erosional profile developed in crystalline rocks and buried by a marine transgression is now given. Impact craters 295 flowed 233 brl of 30° gravity crude at an estimated diameter, an original apparent depth of 594m (1,950ft) rate of 166 brl/hr. The well was drilled about 130ft is obtained. A well positioned above the Newporte (40m) into basement with no further hydrocarbon shows. depression would have to exceed 3,350m (11,000ft) in Another well, Mott 14-34, positioned on the other side of depth to encounter the top of the allogenic breccia the structure, penetrated a “highly-fractured, vuggy and where shocked features, if present, could be found. weathered Precambrian gneiss-schist sequence” nearly Another possibility is the presence of 350ft (106m) structurally high to Precambrian rocks in the p s e u d o t a c h y l i t e s , an injection breccia containing Larson 23X-9 well. The Mott well was cored and drill-stem shocked material which is emplaced in fractures tested a flow of 202 brl of oil during an open period of 190 created during an impact event. This particular feature minutes. It was subsequently completed, pumping 40 has been found in the rim area of at least two larger brl/day of oil, 380 brl/day of water and 10 million cu astroblemes and could be present in some of the ft/day of gas. Newporte cores. A thorough examination of available Core analysis of the Precambrian interval 9,094- basement cores will be undertaken to clarify the origin 9,099ft (2,771-2,773m) shows an average porosity of of this structure. If an impact origin can be 4.7% and an average permeability of 0.48 md. Fluid established, it will indicate that petroliferous basement production rates are incompatible with the low matrix astroblemes are more than mere speculation. values indicating that some of the fractures are open For the present, however, petroliferous impact and interconnecting. craters in crystalline rock have not been reported. This Clement and Mayhew (1979) favor “localized late does not necessarily mean that none has been found, Precambrian-early Paleozoic differential vertical- but rather that none has been recognized. As the vast basement faulting” and not an impact origin for majority of geologists are unfamiliar with Newporte. This proposed endogenic mechanism is astrogeology, especially with regard to shock similar to one of the alternatives suggested for metamorphism, an accidental “bulls-eye” into a Viewfield, and necessitates an extremely localized subsurface impact crater would usually see the feature circular uplift and subsidence. In both cases, the attributed to some normal endogenic process. structures are not related to volcanics nor to the Encounters with “fault breccia”, “cone-incone”, regional tectonics, and have the profiles of simple- “granite wash” and curious “volcanic” or “pyroclastic type impact craters. debris” are frequently noted in drilling reports and Detection of shock-metamorphic features in this core analyses which, in view of terrestrial cratering structure would clarify its origin. The proposed estimates, raises the question as to just how accurately target rock at Newporte is Precambrian gneiss- the lithology is being interpreted. An example of this schist which is conducive to a variety of confusion is illustrated by core analysis of an unusual microscopic shock effects. Like shatter cones, basement feature in Alberta, Canada. which can also develop in crystalline rock, these shock effects are not found in rim rocks, but The Steen River Structure generally are restricted to the breccia lens or The Steen River geophysical anomaly in NW authigenic breccia of the crater floor. At present, no Alberta was drilled and cored in 1963. This roughly- wells have penetrated this area. circular 22km diameter feature lies about 183m Pike (1977) derived equations from experimental beneath the surface, where the basement is elevated explosion and meteorite impact craters which can be about 760m above the regional Precambrian level. The used to estimate the apparent depth, if the original surrounding area exhibits no other pronounced diameter is known: subsurface or surface features. Cores were cut at 1.010 Ri = 0.196 Dr various intervals and sent to a leading Canadian where Ri = depth (km) laboratory for analysis. A copy of the original Detail Dr= diameter (km) Core Study was recently obtained, part of which As evident in the core description, the Newporte appears in Fig. 17. structure has been eroded, and consequently the It will be noted in the description of the present 3.2km diameter is larger than the original selected interval that the interpretation is “in dimensions. If 3.0km is used as the uneroded volcanic”, with mineral alterations attributed 296 Richard R. Donofrio

to igneous activity. Descriptions such as this appear Undoubtedly many explorationists have unknowingly throughout the report. The lower part of Fig. 17 shows penetrated impact features and, not finding another interpretation, in this case by a geologist commercial hydrocarbons on the first or second familiar with shock metamorphism. It was concluded attempt have given up, only to have another operator move in by Winzer (1972) that these rocks show all the main and make the discovery. If the petroleum geologist encounters petrographic features found in terrestrial meteorite and is unable to recognize an impact crater, he cannot realize impact craters, with the exception of coesite, and that that the well is into a structure which is independent of the the absence of a terrestrial process capable of regional geology and possesses a unique geometry of producing maximum shock pressures of 400-500 kb its own. Housed within that unique geometry are required for the noted shock-metamorphic effects, potential pay zones of magnitudes which tend to provides convincing evidence for an impact origin. dwarf classical geological features. Winzer’s work confirmed the earlier studies by Recognition, however, constitutes only part of the Carrigy (1968) who first proposed that the shock problem. With basement impacts it would not be effects in Steen River cores were similar to those from surprising if only an insignificant number of impact structures. petroliferous astroblemes were ever found. The Having recently examined a section of the same possibility that this could occur is directly related to core, it is evident that the deformation is not due to or the continued ignorance of crystalline rock associated with true volcanism, but is the product of hydrocarbon potential, something which I have termed high-pressure shock waves in Precambrian crystalline the basement syndrome. rocks. The core location corresponds to the central uplift area in complex-type impact craters. One can The Basement Syndrome only question how frequently errors of this nature are It is the overwhelming opinion of explorationists made during drilling operations into sedimentary and that basement rock is essentially “non-productive”. crystalline rock. Calculations show that crystalline basement currently On the basis of probability, considering terrestrial supplies less than 1% of total world oil production, cratering rates and the 3.3 million wells drilled into which would appear to justify stopping the drill bit the earth’s crust, it is with certainty that unrecognized short of entering it. Such justification however, is impact features have been encountered and that many based on totally inadequate statistical information, of these contain commercial hydrocarbons. and represents poor geological reasoning. The Impact craters 297

explanation for the “non-productive” reputation eliminated from consideration in Table 2 basin areas of basement may not lie in the nature of and other regions throughout the world. the rock, but results from the routine practice of not drilling into it. Detection Methods Table 2 shows the drilling compilation of various Seismic US basins used for this study. It is immediately Sawatzky (1977) gave the seismic profiles of evident that basement in these particular areas several suspected astroblemes developed in the remains virtually unexplored; probably in no instances sedimentary column. Outside of Newporte (Clement was it the primary objective of the well in question, and Mayhew, 1979), high-resolution seismic data of and of the 107,489 cumulative wells drilled as of suspected buried basement craters either is not November 1979, only 485 (0.45%) penetrated available or has not been recognized. basement. Of these 485 wells, 8 ran production tests in A method of circumventing this problem is to use basement, which resulted in two completions. processed seismic-section models, as these can reveal Twenty years ago, Landes et al.( 1 9 6 0 ) the expected signals from subsurface basement documented a number of commercial basement astroblemes at various erosional levels, overlain by oilfields (practically all of which were accidental differing sedimentary sequences. An example of this discoveries), and attempted to alert geologists to the is shown in Fig. 18. The parameters used for this hydrocarbon potential of weathered and fractured particular seismic model were taken from Brent crater, crystalline rock. Table 2 is evidence that this advice and include the crater dimensions and basement rock was not heeded in the slightest. Further evidence is densities in the breccia lens and surrounding country found in estimates of total world recoverable reserves rock. The erosional profile of Brent parallels “B” in (Grossling, 1976) which, exclusive of a few known Fig. 14.Bu r i a ld e p t h was programmed to about 12,500ft, petroliferous basement areas such as Venezuela and with a normal marine sequence covering the feature. Algeria, only consider the volume of rock above All seismic multiples are included in the model. basement as having any hydrocarbon possibility. The low-velocity pullup will be noted towards the Basement production and reserves estimates center of the breccia lens. Negative deflections at the will continue to be insignificant for as long as top of the unconformity are due to the brecciated zone the practice of avoiding crystalline rock continues. beneath higher velocity shales. Of interest to anyone The basement syndrome continues to dominate familiar with seismic sections is that similar basement exploratory drilling, regardless of the basement signals are occasionally observed in routine seismic depth. In addition to normal porous endogenic surveys, suggesting the possible presence of impact basement structures, one can only speculate about cr a t e r sa n d consequently of potential reservoir material. the number of pronounced and subtle basement As previously noted, a basement syncline profile of features indicative of impact craters which were this nature should be a prime drilling objective. 298 Richard R. Donofrio

Magnetic mechanism is that of Shock-lnduced Polarization, In addition to a thorough review of existing which is believed to be a phenomenon arising from seismic records, emphasis should be directed towards impact in crystalline rock containing piezoelectric. the utilization of magnetic detection methods. While dielectric, and ferromagnetic materials. According to magnetic surveys are of little use in detecting impact Ivanov et al., (1977), Shock-lnduced Polarization can craters in the sedimentary column, such surveys be conSidered as an electrical current flowing through eventually should prove feasible in detecting impacts the shock-wave front. As the shock wave passes in crystalline basement. through a medium containing shock-polarized Enhanced natural remanent magnetization (NRM) materials ( e.g. quartz), the induced electromagnetic of in comparison with the NRM of field could be fixed by the ferromagnetic components constituent rocks has been noted by Pohl et al. (1977). (e.g. magnetite). Previous experiments by They believe this to be characteristic of suevitic Mineev et al., (1968) confirmed that shock- impact breccias on earth, and propose that it is mainly compression of polycrystalline materials induces due to the generation of magnetite by shock polarization in the direction normal to the shock-wave metamorphism and/or subsequent thermal front. The net effect of this could be an enhanced metamorphism. Expounding on this, if the target rock remanent magnetization, possibly in some instances contains hematite and experiences a shock pulse of comparable in intensity to the hematite-magnetite between 400-500 kb, a transition from hematite to transition. Intensive shock followed by rapid cooling magnetite will result. Slow cooling of the rock would appear to be necessary to “lock-in” the material from above the point of magnetite polarization effect. Ivanov et al., (1977) believe that (580°C) to ambient temperature will result in domain Shock-lnduced Polarization is a possible cause of alignment with the earth’s magnetic field. The magnetic field anomalies detected near certain impact combined effect of shock and temperature thus results craters by the Soviet lunar probe Lunokhod 2. T h i s in a magnetization higher than the surrounding mechanism could also be the explanation for the unusually- country rock, which usually is detectable by high remanent magnetization values of particular luna r conventional magnetic instruments. samples discussed by Fuller (1974), as well as for Another more enigmatic NRM-enhancing certain and (Donofrio, 1977). Impact craters 299

Gravity More recently, and Soter (1980), while The breccia lens of an impact crater consists of agreeing that much of the earth’s hydrocarbon supply shattered rock, usually of lower density than the is of biogenic origin, also noted the correlation encompassing country rock from which it is derived. between major hydrocarbon-producing regions and Normally, if a density deficiency exists, it results in a zones of present or past seismic activity. They state negative gravity anomaly. Dence (1972) noted that that many of the known hydrocarbon reservoirs, sedimentary fill may enhance the negative reading, including those of Alaska, Texas, the Caribbean, which most clearly is developed in craters of moderate Mexico, Venezuela, the Persian Gulf, the Urals, size. Large complex craters, however, may have an Siberia and SE Asia, lie on deformation belts, obscured negative anomaly due to a central uplift of suggesting that abiogenic hydrocarbons are rising denser rocks, erosion, or regional gravity variations from deep within the earth along fissures in the crust. (Sweeney, 1978). Gold (1979) contended that if primeval methane Gravity studies of the Ries by Kahle (1969) show had been the chief source of carbon, then the amount a striking, concentric, residual anomaly necessary to produce all the carbon in the sediments pattern, having a value of about 0 at the crater would be the equivalent of 20 million years of present periphery and progressively reaching about—18 mgal day fuel consumption. His evidence for abiogenic at the center. Similar patterns appear to be typical of methane is based partly on earthquake outgassing well-preserved impact sites. Additional gravity phenomena, and lends support to Robinson (1966) investigations can be found in Innes(1961), whose geochemical data suggested both an organic Popelar(1972), Sweeney (1978), and Barlow (1979). and inorganic origin for terrestrial hydrocarbons. The possibility of a dual origin for hydrocarbons has Implications of Inorganic Hydrocarbons to significant implications for impact craters in basement Basement Astroblemes rock. In addition to the basement syndrome, t h e The fracture pattern emanating from an orthodox approach to exploration based on the astrobleme provides an intricate network of migration assumption that all hydrocarbons originate from channels into the breccia lens. The larger the impact biogenic sources may need to be rectified. This is not event, the deeper this fracture network. An impacting to advocate an inorganic origin for the earth’s body creating a 100km diameter crater or larger, for hydrocarbon resources, but rather to consider the example, is capable of fracturing basement down to a possibility, however remote, that some hydrocarbon depth in excess of 30km, thus literally “probing” the accumulations may have originated in part or entirely upper mantle. A basement astrobleme, therefore, from abiogenic sources. (regardless of size) represents the only k n o w n Inorganic hydrocarbon proposals speculate that potential reservoir in “proximity” to lower-crustal or deep disjunctive faults extending into the lower crust upper-mantle activity. It is deep within this area that or upper mantle are the migration routes to reservoirs abiogenic hydrocarbons have been proposed to be in the upper crust. Porfir’ev (1974), for example, synthesized by some, as yet inadequately explained, noted many instances of oil discoveries in zones process. If migration is occurring along deep- associated with deep basement faults. He cited disjunctive faults, then the presence of a basement examples of deep faults bordering platform grabens, impact crater in proximity to one of these proposed such as the Limagne graben in France, the Baikal and hydrocarbon conduits may afford an opportunity to Barguzin grabens in Siberia, the Rhine graben in West test the abiogenic theory. Such a test is feasible with Germany, the Suez in Egypt, the Dead Sea in Jordan, current drilling technology, and need not be limited to the Reconcavo in Brazil, and the Fusin in China. petroliferous areas. Major oil accumulations attributed to deep faults resulting from the formation of mountain foredeeps or Recycled Kerogen depressions were also noted. Among others, these If one prefers to adhere to organic concepts, included the pre-Urals, pre-Caucasus, western Canada, then an additional method of accumulating Iran and intermontane depressions such as California, hydrocarbons is suggested. In regions where western Tukmenia and Fergana. crystalline basement is not directly overlain or 300 Richard R. Donofrio flanked by classical source rocks, the possibility will have been formed on the earth’s land mass during remains that impact structures could accumulate the last billion years. The maximum exposure age of hydrocarbons without the necessity of being craters this size and larger is sufficient to assure that contiguous with fine-grained source beds. This could someone will have been buried before eradication. arise through r e c y c l e d k e r o g e n which was briefly The lower preservation age limit of small-dimension mentioned by Sanders et al. (1976), and is further craters is offset by their higher population numbers in expounded below. any given time interval, thus also assuring that some Sanders (personal communication) suggests that will survive as well. Crater preservation is more than the recycling of kerogen is a possible mechanism for likely at both ends of the dimension scale. transferring organic matter to, and for generating petroleum within, terrigenous sandstone reservoirs Conclusions without primary migration. This hypothesis requires The cratering process is fundamental to all the two (or more) cycles of sedimentation and subsidence planetary bodies so far examined and was one of the that are separated by an episode of uplift and erosion. dominant forces which sculptured the earth’s pre- In the first cycle, the kerogen forms at suitable depth transgressive basement configuration. This exogenic from “raw” organic matter in fine-grained source beds mechanism, in conjunction with normal terrestrial as is presently thought. If these source beds are later processes, assures the presence of many areas of uplifted and eroded, the kerogen particles are free to fractured and brecciated crystalline rock which are be transported at the surface and can accumulate in conducive to hydrocarbon accumulations. An coarse-grained sediments. It is assumed that much of unknown number (possibly in the thousands) of the recycled kerogen can survive weathering, as these astroblemes have been buried at various stages of sediments subsequently are buried, the recycled erosion some of which, like Newporte, were in the kerogen under optimum thermal conditions, can be right place at the right time. Had an even larger converted to oil or gas. magnitude impact occurred at this location, petroleum The implication of Sanders’ recycled kerogen accumulations of hundreds of millions of barrels could hypothesis is that porous rock can serve as both source have resulted. and bed reservoir. This possibility suggests that a Geologists should become familiar with shock basement impact crater could be far removed from metamorphic effects, and be aware of the structural carrier beds, thus negating primary migration and still parameters associated with astroblemes. Suspicious have petroliferous potential if flanked or overlain by gravity, seismic and magnetic anomalies in basement coarse-grained sediments containing kerogen. The should be penetrated and tested where drilling depth paleo-drainage system into a basin could determine if permits. These anomalies include elevated areas any source of kerogen was available. of basement as well as synclines, especially in The point being made in considering both basins having overpressured conditions. Structural inorganic and organic proposals is that if geological or and reservoir considerations should take precedence geophysical evidence suggests the presence of a over classical source rock arguments, and the subsurface impact crater in crystalline (or possibility of unconventional hydrocarbon generation sedimentary) rock, it should be drilled regardless of should be left open. This includes abiogenic synthesis the presence, absence, or position of source beds. as well as alternate biogenic mechanisms such as Unquestionably, with some deep-basement impacts, recycled kerogen, which eliminates the necessity of the capital expenditures will be considerable, but the primary migration from fine-grained carrier beds. possible rewards can be enormous. Based on Both of these proposals run contrary to our basic average porosity values in Table 1, and concepts in petroleum geology. considering the extent of subcrater fracture Detection of astroblemes by geophysical or deformation, the reservoir potential of all geological methods means that fractured fractured and brecciated rock in one 20km reservoirs have been located. In basement diameter crater exceeds the total recoverable rock, these features represent the closest reserves of the Middle East. Probability known potential reservoirs to proposed upper- predicts that over 300 craters of this size and larger mantle abiogenic activity. The implications Impact craters 301

that these reservoirs could accumulate BROWN, H., 1960. The density and mass distribution of abiogenic hydrocarbons from the base of the structure meteoritic bodies in the neighborhood of the earth’s upwards are profound and need to be tested. orbit, Journ. Geophys. Res., 65, 1679-1683. BUCHER, W. H., 1963. structures caused from without or from within the earth? (“astroblemes” or Acknowledgements “geoblemes”?), Amer. Journ. Sci., 261, 597-649. I thank American Ultramar Ltd. for making this CARRIGY, M., 1968. Evidence of shock metamorphism in publication possible. Mary Lawler, Linda Camilli and rocks from the Steen River Structure, Alberta. In: Shock Metamorphism of Natural Materials, B. M. French and Janet Ashbahian assisted in preparing the manuscript, N. M. Short [Eds.], 367-378, Mono Book Corp., Baltimore. and Jim Esposito drafted the illustrations. CHAO, E. C. T., 1968. Pressure and temperature histories of The inspiration for this paper had its foundation in impact metamorphosed rocks—based on petrographic meteorite and research under the guidance of observations. I n : Shock Metamorphism of Natural Materials, B. M. French and N. M. Short [Eds.], Robert L. Dubois at the University of Oklahoma, and 135-158, Mono Book Corp., Baltimore. through the assistance of Virgil E. Barnes at the CLEMENT, J. H. and MAYHEW, T. E., 1979. Newporte University of Texas. The author considers himself discovery opens new pay, Oil and Gas Journ., 7 7 , very fortunate to have had the opportunity of working 165-172. CURRAN, D. R., SHOCKLEY, D. A., SEAMAN, L. and with these individuals. The encouragement and advice AUSTIN, M., 1977. Mechanisms and models of of Frederic A. Bush, retired president of Sinclair cratering in earth media. I n : Impact and Explosion International Oil Co., has been invaluable to me ever Cratering, D. J. Roddy, R. O. Pepin and R. B. Merrill since excerpts of this paper were first presented to the [Eds.], 1057-1087. Pergamon, Elmsford, NY. DANIEL, E. J., 1954. Fractured reservoirs of the Middle Petroleum Exploration Society of New York. His East, Bull. Amer. Assoc. Petrol. Geol., 38, 774-815. consideration is deeply appreciated. DENCE, M. R., 1972. The nature and significance of Also thanked are P. Blyth Robertson and Michael terrestrial impact structures, 24th Int. Geol. Congr., R. Dence at the Canadian Department of Energy, Montreal, 15, 77-89. DENCE, M. R., GRIEVE, R. A. F. and ROBERTSON, P. B., Mines and Resources in Ottawa, for reviewing the 1977. Terrestrial impact structures: Principal manuscript. Their helpful comments, in addition to characteristics and energy considerations. In: Impact and those of Ben Peterson and Fred Hörz, the latter at Explosion Cratering, D. J. Roddy, R. O. Pepin and R. B. NASA, have made this paper a worthwhile effort. Merrill [Eds.], 247-275. Pergamon, Elmsford, NY. DIETZ, R. S., 1961. Astroblemes, Sci. Amer. 2 0 5 , Beven M. French at NASA Headquarters was always 50-58. available when assistance was needed. DONOFRIO, R. R., 1977. The magnetic environment of In addition to M. R. Dence, P. B. Robertson and tektites, Ph.D. Dissertation, Univ. Oklahoma, 183 pp. F. Hörz, cores were also supplied by H. R. McCabe FRENCH, B. M., 1968. Shock metamorphism as a geological process. In: Shock Metamorphism of Natural and Golden Eagle Oil & Gas Co., Calgary. Charles Materials, B. M. French and N. M. Short [Eds.], at Core Labs in Houston gave the samples 1-17, Mono Book Corp., Baltimore. extra attention, and Nancy Evans at Jet Propulsion FRENCH, B. M., UNDERWOOD, J. R. and FISK, E. P., Laboratory was responsible for selecting the V i k i n g 1974. Shock metamorphic features in two meteorite impact structures, southeastern Libya, Geol. Soc. Amer. photograph. Bull., 85, 1425-1428. FULLER, M., 1974. Lunar magnetism, Rev. Geophys. Space Phys., 12, 23-70. GAULT, D. E., QUAIDE, W. L. and OBERBECK. V. R., REFERENCES 1968. Impact cratering mechanics and structures. I n : Shock Metamorphism of Natural Materials, B. M. BARLOW, B. C., 1979. Gravity investigations of the Gosses French and N. M. Short [Eds.], 87-99, Mono Book Bluff , central Australia, BMR Journ. Corp., Baltimore. Austral. Geol. Geophys., 4, 323-339. GOLD, T., 1979. Terrestrial sources of carbon and BRENAN, R. L., PETERSON, B. L. and , H. J., earthquake outgassing, Journ. Petrol. Geol., 1, 3-19. 1975. The origin of Red Wing Creek structure: GOLD, T. and SOTER, S., 1980. The deep-earth-gas McKenzie County, North Dakota, Wyoming Geol. Assoc. hypothesis, Sci. Amer., 242, 154-161. Earth Sci. Bull., 8, 1-41. GORHAM, F. D., WOODWARD, L. A., CALLENDER, BRIDGES, L. W. D., 1978. Red Wing Creek field, North J. F. and GREER, A. R., 1979. Fractures in Cretaceous Dakota: a concentricline of structural origin. Williston rocks from selected areas of San Juan Basin, New Basin Symposium, Montana Geol. Soc. Billings, Mexico—exploration implications, Amer. Assoc. Petrol. Montana, 315-322. Geol. Bull., 63, 598-607. 302 Richard R. Donofrio

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