Impacts, Tillites, and the Breakup of Gondwanaland

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1993

Verne R. Oberbeck NASA Ames Research Center

John R. Marshall NASA Ames Research Center

Hans Aggarwal NASA Ames Research Center

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Oberbeck, Verne R.; Marshall, John R.; and Aggarwal, Hans, "Impacts, Tillites, and the Breakup of Gondwanaland" (1993). NASA Publications. 74. https://digitalcommons.unl.edu/nasapub/74

This Article is brought to you for free and open access by the National Aeronautics and Space Administration at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in NASA Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. ARTICLES Impacts, Tillites, and the Breakup of Gondwanaland'

Verne R. Oberbeck, JohnR. Marshall, and Hans Aggarwal2 NASA Ames ResearchCenter, Moffett Field CA 94035, USA

ABSTRACT Mathematicalanalysis demonstratesthat substantial impact craterdeposits should have been producedduring the last 2 Gy of Earth'shistory. Textures of impactdeposits are shown to resembletextures of tillitesand diamictites of Precambrianand youngerages. The calculatedthickness distribution for impact craterdeposits produced during 2 Gy is similarto that of tillitesand diamictites<2 Ga. We suggest,therefore, that some tillites/diamictitescould be ofimpact origin. Extensive tillite/diamictite deposits predated continental flood basalts on the interiorof Gondwa- naland. Significantly,other investigators have alreadyassociated impactcratering with flood basalt volcanismand continentalrifting. Thus, it is proposedthat the breakupof Gondwanalandcould have been initiatedby crustal fracturingfrom impacts.

Introduction Planetesimal impacts played an important role in pographic dichotomy of 3-4 km, massive volca- Earth's history (Chamberlin 1920), the growth of nism, insulating ejecta deposits, subsidence, and the planets (Safronov and Zvjagina 1969; Safronov reworking of volcanics leading to stable shield ar- 1972), the origin of Earth's moon (Hartmann and eas. This occurred in the first billion years of Davis 1975), and in forming the topographic di- Earth's history when the impact rate was highest. chotomy on Mars (Wilhelms and Squyres 1984). Although the existence of terrestrial cratering is Comet and asteroid impacts produced major for- well documented after3.9 Ga, not much attention mations on the surface of the moon; Short and has been given to crater deposits that may have Forman 1972 used the observed population of lunar survived in the rock record nor to the implications impact craters to determine that impacts produced of the existence of such deposits. In this paper, we an average megaregolith layer 2 km thick. Hart- present the first comprehensive analysis of the mann 1980 calculated that a much thicker mega- thickness distribution of expected crater deposits regolithwas produced very early in the moon's his- on Earth in the past 2 billion years (Gy), when the tory from craters no longer visible. Some of the crateringrate was constant, and afterwhich most major periods of lunar geologic history were de- of the now surviving sedimentary rocks were fined by the formation of large craters and basins. formed.Because our results suggest that extensive Many of the rock stratigraphic units within the crater deposits should have been produced, we time systems are the deposits of large craters and then consider the nature of impact deposits and basins (Wilhelms 1987; Oberbeck 1975). we compare them to tillites and diamictites here- Because the cratering rate on Earth was higher toforethought to be of glacial origin. We then con- than that on the moon (Maher and Stevenson sider the possibility that many tillites are of im- 1988), even more extensive impact deposits must pact origin and explore the implications of this have formed here. Impacts could even have played idea for the generation of continental flood basalts a dominant role in crustal evolution. Frey 1980 and and the breakup of Gondwanaland. Grieve 1980 argued that the formationof large impact basins played a key role in the formation of proto-continentsbefore 3.9 Ga by producing a to- Production of Impact Crater Ejecta Deposits during Geologic History 'Manuscript received March 31, 1992; accepted August 28, 1992. To assess the extent of sedimentary deposits pro- 2 Eloret Institute, Sunnyvale, CA 94087. duced by impact, we calculated the fractionof the

[TheJournal of Geology,1993, volume 101,p. 1-19] No copyrightclaimed for this article. It remainsin the public domain.0022-1376-001$1.00 4 OBERBECK, MARSHALL, AND AGGARWAL

Earth's surface covered by crater ejecta during the pact craterformed on Earth is given by McGetchin last 2 Gy. Grieve and Dence (1979) tabulated all et al. (1973) and Seebaugh (1977) as: identifiedimpact structuresfor the Phanerozoic by counting craters on the North American and East ( r(km)\~ European cratons and estimated the rate of forma- t(km)= ko \R(km)J (2) tion of terrestrialcraters. We averaged their crater production rates (listed in their table 3) for craters where k0 is a constant determined by total crater >20 km and obtained a lower bound crateringrate ejecta, a is the ejecta blanket thickness decay con- of 2 x 10-15 km-2 yr-1 which is appropriate for at stant = 3.5, r is the distance fromthe cratercenter, least the last 2 Gy. Crater production functions at which ejecta thickness is t and R is craterradius. obtained in the Basaltic Volcanism Study Project The volume of the ejecta is then given by: (1981), found that the number of craters produced largerthan D in kilometers was proportional to the -1.8 power of D and cratering rate was constant Ejecta Volume (km3) = 2l rO dr =27koF R a -2 after3 billion years ago (Ga). This production function has been adopted for terrestrialcrater produc- (3) tion by Maher and Stevenson (1988) and Oberbeck where, and Fogleman (1989, 1990). Thus, fromthe conservative estimate of the production of craters >20 __1 km (Grieve and Dence 1979) and this functional F=1 oa (4) relationship, we derive the general expression for production of the number of cratersper km2 per yr, and is R 10 R, N, larger than any observed diameter, D (apparent ejecta thickness integrated from to practically ejecta depos- diameter), after 2 Gy ago: within which all of the is ited. Craters are assumed to be spherical segments, so cratervolume is given by: N(D)[km2 . yr]- = kD[km]-18 (1) = R3C(3 C2) where k = 4.4 x 10 13.This rate equation is sup- CraterVolume (km3) -rr/6 + (5) ported by astronomical observations of present Earth-crossingasteroids and comets. For example, where R is apparent crater radius, C is the ratio Shoemaker (1983) made astronomical observations of the crater depth to crater radius. Setting ejecta of such asteroids and comets and converted the re- volume equal to crater volume and simplifying sults to equivalent crateringrates that agree within gives: the error bars with the rates determined from Grieve and Dence (1979) for the past 600 m.y. 2r koF R2 R3C (3+ C2) (6) From equation (1), and the surface area of Earth, a- 2 6 the expected number of impact craters that should formed over 2 Gy three >750 Sa - 2 RC(3 + C2) have a period is: km, - 12 F (7) six >500 km, 110 >100 km, 2003 >20 km, 6974 >10 km, and 24,283 >5 km. Although crater num- Substituting this value of k0 in equation (2) gives bers alone suggest that abundant ejecta deposits the change in the ejecta thickness, t, with distance should exist in the geologic record, it is possible to r froma crater of radius R: determine the percentage of the area of the Earth that would be covered by ejecta of thickness - a t(km) - 2 C(3 + C2) R+' given thickness over the last two Gy. t(km) F r 12 F r (8) Although Oberbeck (1975) and Oberbeck et al. (1975) have shown that crater deposits on the moon and Earth also include debris ejected from where r and R are in kilometers. This may be ex- pressed pre-existingrocks during emplacement of primary as: crater ejecta, we consider only the primary crater a - 2 C(3 + C2) ejecta here for a conservative estimate of the 12 Ft (9) amount of debris. We now derive a relationship for the fractional coverage of the Earth's surface, Af, or by ejecta blankets with thickness -t (km) produced by craters between Dmin and Dmax in 2 Gy. r a - 2 C(3 + C2) +1 (10) The thickness, t, of ejecta, surrounding an im- S 12 Ft Journalof Geology IMPACTS, TILLITES, AND GONDWANALAND 3

Note that within the area of an ejecta blanket at surface covered by ejecta of thickness, -t in the radius r from a crater of radius R, where the ejecta past Gy of earth history,we have: thickness is t, the ejecta thickness is -t because ejecta thickness decreases with distance fromthe crater rim (see equation 2). The incremental area Af= 12.44x 10-4f()[{O -2C(324 F+C)}3 of ejecta -t (per unit surface area) from craters of diameter D is the product of n(D) and the area around the crater within which the ejecta is t. X 2 + 0.(D 02a)/a - )(2 + 0.2)/a) - 5(D x - D0.2 Therefore, the fractional coverage per year of the surface (A) of ejecta of thickness t by all craters (14) is given by: Let us calculate the fractional area of Earth covered by ejecta with thickness -t produced in a 2 Gy period of time for all possible values of t. We ASurface area of earth (km2) (11)) yr must firstobtain the largest expected crater,Dmax, in 2 for eq. it is = xn(D)rr(r2 - R) dD Gy because it is needed (14), and Dmin necessary to know the upper limit of t at which equation (14) may be evaluated. The largest value where r is given by equation (10), R = D/2, Dmax of t is the rim ejecta thickness forthe largest crater, and Dminare the maximum and minimum diame- Dmax. We now use the rate of formation of craters ters of the craters, n(D) is the incremental crater on Earth and the Poisson probability distribution number density at diameter D, per unit crater di- to estimate the maximum size crater that should ameter, per km2 of the surface, per yr, given by be expected during the last 2 Gy. Maher and Stevenson (1988) by a function of form: Using the cratering rate equation for craters n(D)/[km3 yr] = dN/dD = 1.8k f(T)/D28.k is a equal or larger than a given diameter given by constant determined here from the conservative equation (1), on average, about three craters -750 estimate of the terrestrialcratering history of the km should have been formed in 2 Gy time inter- last several billion years, given in equation (1) as vals on an Earth-size body if it were repeatedly sub- 4.4 x 10-13, f(T) = 1 during the last 2 Gy (Maher jected to an impactor flux identical to Earth's im- and Stevenson 1988). The integral of the incremen- pact flux. This does not ensure that craters as large tal crater number density at D gives Equation (1). as 750 km actually formed on Earth in the last Substituting n(D), defined above in the text and 2 Gy. The probability of n impact craters of this r fromequation 10, and replacing R by D/2 in equa- diameter, given that the expected number, NP = tion 11 and simplifyinggives: 3 is given by the Poisson probability distribution:

1.8 Dmax P(n) = (NP)"/n! e-NTP (15) A = kf(T) D where NTP = 3. N, is the total of all craters >5 t 2C(3 + C) D2- .8ala_ D-o.8 dD km produced in Gy and P is the proportion of cra- L 24 Ft J ters >750 km. This probability distribution gives (12) the probability of occurrence of rare events (craters 750 km) when the proportion of such events of or, afterintegration: the total events (all other crater sizes) is very small (Snedecor and Cochran 1976). Using equation (15) for discrete values of n, we S1.8 a - 2 C(3 + C2)} find that the probability is 0.95 that n - 1 crater 24 Ft 4 >750 km should have formed on Earth in the last 2 Gy (there is 5% probability that 0 craters this x a (2 + .2)/ -_ )(2 + 0.2a)/a 2 + 0.2L - max min large actually formed). Repeating the calculation for500 km craters,we find that the probability for formation of one or more craters -500 km is -5(D x - Din) 0.9975. The probability for 0 craters 500 km ap- (13) proaches 0. Therefore, we can be certain that craters as large as 500 km formed in the last 2 Gy. Substituting k and multiplying by 2 x 109 yr to Dmax in equation (14) can be safely taken as 500 express A in terms of Af, the fraction of Earth's km and Dmi = 5 km. We next need a value of C 4 OBERBECK, MARSHALL, AND AGGARWAL

for equation (14) and it, together with R = Dmax/2 estimates of the area of Earth's surface covered by in equation (8) determines the largest value of t for ejecta because they use C = 0.4 and observed cra- which equation (14) should be evaluated. ter volume as an estimate of ejecta volume. Head et al. (1975) determined C = 0.02-0.4 for Both methods of calculation suggest that sub- large lunar impact craters. The smallest value was stantial crater deposits should have been produced obtained fromdepths of craters and basins filled by during geologic history. Deposits of small craters a variety of post-impact processes. The largest is formedon land could have been eroded. However, the ratio forfresh small craters; some authors also at many times in geologic history vast regions of believe it characterizes large fresh craters (Melosh the continents were covered by shallow seas which 1982; Orphal 1979). A lower bound estimate of would hardly reduce the amount of material area covered by ejecta -t results from use of C = ejected froma 5 km cratercompared to that ejected 0.02 in equation (14). This ejecta rim height is near fromsuch a crater on land. Craters >5 km formed the lower bound rim thickness estimated for an on the continental shelf or shallow inland seas impact crater of this size on the moon as found by would produce slurry waves several hundred me- Head et al. (1975). With C = 0.02 in equation (14), ters high. Wave action would spread the deposits the computed fraction of the Earth's surface cov- furtheraway fromthe impact site than on land and ered by ejecta with thickness -t in 2 Gy is calcu- material backwashed from the continents would lated and given in figure 1. We evaluate this rela- result in thicker deposits than those of terrestrial tionship up to 1.94 km, the maximum thickness craters.Deposits of even small craters that formed of ejecta fora 500 km craterwith this depth-radius in shallow water should have been preserved be- ratio (equation 8). Calculations suggest that at neath later sediments. least 10% of the Earth's surface could have been covered by ejecta deposits 2 Gy. -10 m in About Deposits of Impact Craters Formed on Land 2% of the Earth's surface would have been covered with ejecta -200 m in thickness. Maximum thick- Although the deposits of many of the ancient im- ness is 1.94 km. We obtained an alternate estimate pact craters on land are often eroded, the ejecta of that agrees with these results by using our value the Ries crater,Germany survives relatively intact. of c = 3.5 and the empirical relationship for Horz et al. (1977) drilled deposits surrounding this ejecta thickness around impact craters given by 26 km crater that formed on land and found that McGetchin et al. (1973). the cores contained mixtures of monomict or poly- As noted earlier, results of Orphal (1979) and mict clasts, and megaclasts in a matrix of fine- Melosh (1982) suggest that the ratio of crater depth grained material. Clasts were defined as those be- to crater radius of a fresh crater could be as large tween 1 cm and 1 m in size; these had random as 0.4. Using equation (14) and C = 0.4, in the last orientation within the fine-grainedmatrix. Mega- 2 Gy, craters covered 61% of the Earth with ejecta clasts measured into the tens of meters occurred -10 m, 10% with ejecta -200 m thickness, and within the Ries crater ejecta (Bunte Breccia) which 5.5% with ejecta 500 m. These are upper bound was itself as thick as 84 m. The deposit contained predominantlycrater ejecta in regions near the crater and predominantly local material (which had been torn up and incorporated with crater ejecta) Crater at great distances. Dislodged blocks as large as 200 depth/radius --C = 0.02 m were found. Contorted clast and matrix struc- - C- =0.1 tures indicated a very energetic emplacement ...... C=0.2 mechanism. The deposits are predominantly mas- -. C=0.4 sive although, on occasion, clasts show preferred orientation,and there are rare lineations in the matrix.Figure 2 shows a quarry section of Bunte Brec- cia. Much of the ejecta is very fine-grained,but there is an unsorted chaotic mixture of clasts of various sizes in the fine-grainedmatrix. Horz et al. (1977, 1983) found that Ries impact Thickness 2 t crater ejecta stripped local ground during emplace- Figure 1. Calculated fractionof Earth's surface covered ment to regions below the weathering zone. They by ejecta of thickness >t(km) for differentassumed val- described radial scouring and striations on pre- ues of crater depth-radius ratios, C. existing Jurassic limestones where ejecta im- Journalof Geology IMPACTS, TILLITES, AND GONDWANALAND 5

Figure 2. Ries impact crater ejecta deposit (Bunte Breccia) resting on pre-impact limestone surface. Note the clasts and megaclasts within the fine-grainedmatrix and the chaotic texture of the deposit. pacted, eroded the ground, and moved along the distances from the crater rim, ever increasing surface. Pohl et al. (1977) described Bunte Breccia amounts of local ground are torn up by the ejecta. deposits as thick as 200 m in places where pre- Excavated material is added and mixed with the crater depressions trapped the ground-hugging primarycrater ejecta. This complex mixture of cra- flow; in places they produced polished and striated ter ejecta and local ground moves away from the surfaces. impact site in a ground-huggingflow behind the Horz et al. (1977, 1983) explained the complex ejecta curtain. It produces a chaotic mixture of Bunte Breccia mixtures of crater ejecta and local clasts of rock in a fine-grainedmatrix resting on ground using the ballistic erosion and sedimenta- deformed,striated, and eroded substrates. tion model of Oberbeck (1975). According to this model (figure3), material is ejected fromcraters in Deposits of Impact Craters Formed in Water ballistic trajectories. The first material ejected fromnearest the impact point has the highest ve- Deposits of even small impact craters should have locity; it is found at the top of an expanding conical survived in the rock record if they were pro- ejecta curtain and it is deposited at the greatest duced in shallow inland continental seas or shelf distance. The lowest velocity material is ejected areas. Throughout geologic history, much of the last from the crater from points near the final cra- continental regions were flooded by shallow seas; ter rim, and it is also deposited nearest the crater these would not have substantially decreased the rim. For large craters, much of the ejecta impacts amount of ejecta from the sea floor even forcraters with high enough velocity that it excavates pre- as small as 5 km. Such craters would have been existing ground. Thus, at progressively greater over 1 km deep and the shallow water would only 6 OBERBECK, MARSHALL, AND AGGARWAL

fine

Figure 3. Schematic of calculated positions of ejecta curtain with respect to ejecta in ballistic trajectoryfor the lunar crater Copernicus, from Oberbeck (1975). Note that material from the crater is ejected in ballistic trajectories and material with the highest launch velocities is transportedfarthest. When ejecta strikes the ground, secondary crateringcauses ejecta ground mixing, material moves in a ground huggingflow, and a chaotic mixture of clasts and fine grained matrix material is produced.

have been a small part of the ejecta. Material from fine spherules in a sandy marl, and coarse spherule- the deep ocean floor would only have been ejected rich crater ejecta. These textures were interpreted fromcraters >15 km. to have formed from a complex series of events It is largely due to the search forthe impact cra- including deposition of crater materials followed ter that occurred at the end of the Cretaceous pe- by reworking from disruptive impact-generated riod that we can now anticipate the wide variety tsunamis. of lithologies of impact deposits formed in water. Bourgeois et al. (1988) described another K/T Smit and Romein (1985) describe deep sea cores boundary turbidity deposit in Texas about 1 m spanning the K/T boundary and show that in 15 thick (figure4). Here, material exhibiting an irid- cores world-wide, there is only a thin layer of mi- ium anomaly (diagnostic of extraterrestrialmate- crotectites (melted ejecta). Ahrens and O'Keefe rial) overlies a coarse-grained sandstone with large (1983) performed a theoretical analysis that clasts of mudstone and reworked carbonate nod- showed that a minimum threshold energy is re- ules with an erosional base with up to 70 cm of quired to erode deep ocean sediments and produce relief; the deposit grades upward to very fine- widespread turbidity deposits. Thus, only micro- grained sandstone. Slump structures and parallel tectites occur in most deep sea cores. However, a and wave ripple laminations are found above the series of thicker ejecta and turbiditydeposits along graded base of the deposit. The authors concluded with altered microtectites exist closer to the K/T that the deposit resulted from a 50 to 100 m high impact. These and similar deposits indicate the tsunami wave produced by impact of a bolide; the nature of water-laid ejecta deposits. Florentin et al. mudstones eroded by the wave existed in water (1991) described a deposit about 72 cm thick in about 75 m deep, and the sandstones were trans- Beloc, Haiti some 1100 miles from the suspected ported from inner shelf facies. Multiple sediment source crater Chicxulub on the north coast of Yu- beds indicated that multiple waves could have catan. From the top down, it consists of a thin clay been responsible. The graded sandstone was depos- layer, a laminated sandy marl containing small ited in about one day, whereas the mudstone above lenses of coarse clay spherules (altered microtec- it was deposited within a few weeks of the event. tites), a white chalk lens, an upward-fininggraded Slump structureswere also produced when the im- sandy marl, a coarse spherule-bearing marl lens, pact caused instabilities in shelf sediments. Impact Journalof Geology IMPACTS, TILLITES, AND GONDWANALAND 7

fining grains covered by reworked calcareous oozes. Equation 8 and the results of Hildebrand (1991) suggest that even thicker undiscovered deposits should exist in regions even closer to the impact site. The textures of deposits closest to the crater can be deduced from water-lain ejecta oc- curringelsewhere at differenttimes. For example, extensive 10-60 m Eocene marine beds of mixtites were described at the coastal plain of southeastern Virginia and attributed to impact by Poag et al. (1991). This bed consists of an upward-finding graded bed containing angular clast of pebbles and boulders up to 1 m in diameter within a glauco- nitic, clayey, sand matrix. The deposit occupies an area of 14,000 km2 and is equivalent stratigraph- ically to impact ejecta located elsewhere. Dropstones also exist in water-lain impact 0 .5 1.0 ejecta. Gostin et al. (1986) report a 1575 Ma soli- 30 Ir(PPb) tary layer of shattered volcanic rock fragmentsin 20 the Adelaide Geosyncline, South Australia, that cm exhibited shock lamellae and shatter cones charac- S10 teristic of impact ejecta. A distant source crater known as Lake Acraman has been located in the 0 Gawler Range of Volcanics 300 km west of the Adelaide Geosyncline (Williams 1986). The ejecta deposit was traced over a distance of 260 km in 600 Ma Precambrian marine shales. Fragments in this deposit, up to 30 cm, formed classic dropstone structures when they fell in the ocean on preexisting marine sediments. Figure 5 is a sketch of the Gowler range sediment section adapted from Figure4. K/T impact depositnear Brazos River, Texas, the section portrayedin Gostin et al. (1986). Layer in reproducedafter the stratigraphicsection diagram A is poorly sorted angular sand with blocks of Bourgeois et al. (1988). (a) Sand with shell debris; shocked volcanic material up to 30 cm ejected (b) Sand; (c) Mudstone; (d) Wave ripplelaminated ma- from the crater form struc- terial;and (e) Parallel laminatedsediments. impact to dropstone tures in ocean-bottom sediments. Layer B is a thin green drape of sand that was put into suspension of a 10 km bolide in the deep ocean as far away as by the impact. Layer C is a thin, graded layer of 5000 km, or a much smaller one nearby, could sandy impact debris that took longer to settle out. have produced the conditions required to explain Layers D and E exhibit multiple grain sizes and this deposit. crossbedding and were deposited at undetermined Hildebrand (1991) suggested that the thickness times after the impact by turbidity currents and of distal K/T ejecta deposits may be used with de- storms. Textures of these types should be present cay laws similar to equation (8) to predict the in the sea areas in regions distant from rims of thickness of proximal 100 m thick craterejecta de- impact craters. posits (intercalated with pelagic sediments) about Water-Formed,Large Impact Crater Model. In 100 km from Chicxulub. Vickery et al. (1992) have figure 6, an impact is depicted in a shallow sea. shown that the thickness of deposits at various dis- Ahrens and O'Keefe (1983) concluded that a kinm- tances from Chicxulub agree with those predicted scale wave could have resulted from an impact in by the McGetchin relationship that we used to ob- the ocean. McKinnon (1982) suggested that, be- tain area thickness distributions for impact ejecta cause the shock-wave velocity in the crust exceeds duringthe past 2 Gy. Alvarez et al. (1991) described that in water, material from the ocean floorwould a proximal incomplete 45 m thick section of peb- have been mixed with ocean water during crater bly mudstone overlain by 4 m of current-bedded formation.The crateringevent should have ejected sandstone containing glassy ejecta with upward a mud slurry; material in the crater rim wave 8 OBERBECK, MARSHALL, AND AGGARWAL

Figure 7 depicts sedimentary sequences generated by the events in figure 6. The dashed line is the original undisturbed surface level. Section (1) representsdeep sea deposits (not depicted in figure 6). In distal regions, the impact is represented by dispersed dropstones (d). The overlying calcareous horizons were produced by tsunamis that reworked and redistributed seafloor calcareous oozes. At verydistal regions, only a very thin (cm-scale) layer of microtectites exists. In regions close to the impact (sections 2 and 3) crudely graded ejecta deposits have several upper water-sorted beds separated 10cm by unconformities. Lower layers have megaclasts (m). Uppermost layers are finely-laminatedargilla- Figure5. Sectionof a Precambriancrater ejecta blanket adapted from a stratigraphicsection in Gostin et al. ceous materials (1) produced by turbiditycurrents (1986). (a) Basal layerof poorlysorted angular sand and that continue for many years after the impact as sparseblocks up to 30 cm of dacite to rhyoliteproduced the slopes regain equilibrium. Section 4 depicts a fromvertical fall of ballisticejecta. (b) Thin greendrape chaotic mixture of ejecta, reworked sediments, ofsand resettledfrom sediments put into suspensionby massive slump deposits, and turbidity-current impactgenerated seismic waves. (c) Thin gradedlayer of scour and fill. Section 5 is also a chaotic deposit sand fragments.Impact debrisfiner than in a, b taking produced from current reversals at the site (on- longerto settle. (d), (e) Lenticularlayers of sand up to shore tsunamis and off-shorebackwash). In section 40 cm thick showingmultiple grain size and in layere 6, the absence of water cushioning has allowed after crossbeddingdeposited at undeterminedtime im- high-velocity ejecta to plough and brecciate the pact by turbiditycurrents or storms. substrate beneath reworked blanket material. At all locations, host rock is mixed with ejecta. At sites 2 and 3, sediment overburden weight was would be a mixture of water and sediment. In 61, applied non-uniformlyto the sediments, and ejecta during the formation of the transient crater, the was squeezed into host rocks. At site 4, the mixing raised crater rim is a "slurry" wave. Before collapse also results from incorporation of slumped sub- of the rim, the supersonic ballistic ejecta curtain strate blocks (b). At site 5, mixing results from spreads some distance from the impact site. Note high-velocity penetration of ejecta into soft sedi- that the ejecta from the curtain that settles ments that are insufficientlyprotected by the shal- through the water has been sorted both aerody- low water. At site 6, high-velocity impact into a namically and hydrodynamically before coming to more rigid substrate produces a more highly brec- rest on the sea floor where graded beds result. ciated and splintered substrate (i; incorporatedma- In figure 62, collapse of the transient slurryrim terial). gives rise to a tsunami that is sufficientlyerosive Impact in water generates sedimentary facies at the sea bed to cause reworking of some of the that are (1) laterally and vertically extensive, ejecta deposits as well as sediments not overlain (2) very complex, and (3) highly variable from one by ejecta. Collapse of the transient crater causes a location to another. They contain clasts of rock in central water spout to formfrom the inrushing sea; a fine-grainedmatrix. Clasts are largest at the base such water spouts have been confirmed by experi- of the formations. These typically grade upward ment (Engel 1961; Gault and Sonett 1982). Col- to fine-grainedmaterial that displays laminations lapse of the water spout generates a second tsu- reflectingturbidity flows. Dropstone features oc- nami; several waves might be expected before the cur in water-lain impact deposits when large clasts disturbance to the sea finally damps out. of rock fall into the ocean and deform soft marine In figure63,4,5 and 6, the tsunamis are shown strik- sediments. Deposits can contain both sorted and ing a proximal deltaic region that suffersmassive unsorted massive graded and layered beds, erratics, disruption; large volumes of sediment slump down and cross-bedding. The type and character of such the foreset region, accompanied by numerous tur- deposits depends upon their distance fromthe im- bidity flows. Events in the deltaic/littoralzone are pact site. The thickest, most massive deposits with additionally complicated by the backwash of water the largest clasts exist nearest the impact site. Fur- and terrestrial sediments from the return flow of ther away, thinner deposits become more lineated the tsunamis that swept inland. and bettersorted; they can contain reworked mate- Journalof Geology IMPACTS, TILLITES, AND GONDWANALAND 3

Figure6. Hypotheticalsequence of eventsproduced by impact of a mantle-penetratingbolide striking a shallow sea. Sequence is described in text.Sequence 1 to 7 would occur overa periodof several hours. Verti- cal scale is exaggerated.

2 3 4 5 6 Position of sections

rial including calcareous deposits. At still greater lineations or other ordered structures exist. This distances, dropstones deform soft pre-existingma- type occurs in terrestrialenvironments and often rine beds. Microtectites are also found, produced creates grooved and striated deformations on the from the cooling vapor cloud containing material underlying formations. These deposits are inter- that experienced the greatest shock heating. preted as having been deposited directlyat the base of a grounded glacier. An example is the Jequitai facies, Minas Gerais, Brazil, which contains clasts up to 25 m intermixed with fine-grainedmaterial Description of Deposits Tillite and Diamictite (Gravenor and Monteiro 1983). Precambrian and younger tillites and diamictites The second class is similar to the firstbut, addi- have textural characteristics similar to those of im- tionally, it shows faint layers and laminations at pact crater deposits and predicted by our impact the top of the formations and wisps of sand and model. An excellent summary of tillites believed contorted structures. These textures are inter- to be indurated glacial tills is given by Gravenor preted to have been produced by melting at the et al. (1984), who divided them into seven different base of active floating or partially floating ice as lithologic classes. The first till-tillite class con- sediments were deposited from a slurry near the tains a polymodal size distribution of clasts, and grounding line. An example is the Cacaratiba fa- the supporting fine-grained matrix can contain cies, gradational to the Jaquita facies just de- sheared lenses of silt, sand, and conglomerate. No scribed. Other classic examples are in the Parana 10 OBERBECK, MARSHALL, AND AGGARWAL

100

1 2 3 4 5 6 Figure7. Sedimentaryrock sequences resultingfrom impact in shallow sea. Sections correspondto locations in Figure6 and are describedin the text.Sediment types: E = ejecta material:Eh = water-deposited;Ea = subaerially- deposited; Er = reworked. D = disturbed substrate material: Dd = dropstone disturbance; Dc = compressional disturbance; Db = brecciated. M = mobilized material: Mt = turbidity; Ms = slump; Mb = backwash; Mr = reworked. d = dropstone. m = megaclasts. i = injected host material. 1 = laminations, b = slump blocks, c = cross bedding,f = terrestrialflora deposits. basin in South America and in the Karoo basin of formations have graded bedding. These are inter- South Africa. preted as subaquatic slurry flows but may have a The third type is a massive layered bed con- large amount of sediment from suspension flows taining matrix-supported clasts that can grade to associated for example with turbidityflows. Drop- predominantly clast assemblages with a wide vari- stones fromclasts falling into soft preexisting sedi- ation of lithology. Clasts can be oriented or ran- ments are also present in this class. dom, and beds may be separated or superposed by The fifthtype is a chaotic melange of boulders rythmites,mudstones, or siltstones, and by slump of diamictite sandstone set in a variable matrix of structures. This type of tillite is interpretedas re- massive to laminated diamictites. They are inter- distributed subaquatic glacial material, some of preted as formingby slumping of masses of glacial which can arise from slumping. They are believed debris on the bottom of the outer shelf where to have resulted from debris flowing into paragla- slump planes pass through preglacial sediments. cial lakes or in shelf environments in the sea. The sixth type of till-tillite consists of conglom- The fourth type of till-tillite is partially com- erate, sand, silt, and clay that may contain frag- prised of laminated sorted sediments of coarse ments of diamictite or other glacigenic sediments. sand, silt and clay, where laminates occur at the These are interpretedto have formed in shelf envi- top of formations and vary in thickness from less ronments fromglacial meltwater where coarse sed- than 1 cm to tens of centimeters. The base of these iments were deposited from a retreatingice front. Journalof Geology IMPACTS, TILLITES, AND GONDWANALAND 11

The seventh type consists of mudstone and mix- Laminations, dropstones, and graded basal beds tures of mud and silt containing coarser clasts as of Gravenor's class 3 and 4 tillites, believed to have large as boulders. These are interpreted to have resulted from turbidity flows initiated by glaciers been derived in basin or shelf environments from and the release of boulders fromice rafts,are simi- plumes of suspended material from meltwaters, lar to the laminations, dropstones, and graded basal heads of debris and other flows, sediments from beds produced by impact-generated turbidityflows winnowing by bottom currents, and clasts from and ejecta dropped in the ocean from ejecta cur- floating ice. tains. The laminations, graded beds, and slump Like Gravenor et al. (1984), Hambrey and Har- texturesfound in classes 3 and 4 till-tillitesare the land (1981) also define tillites as rocks of glacigenic same textures as those found in the K/T impact origin, and they define diamictites as non-sorted, deposits at Brazos river by Bourgeois et al. (1988) non-calcareous deposits composed of bimodal or and at other K/T boundary locations discussed polymodal clasts in a muddy matrix. They are es- above (also predicted by our model for impact in sentially identical to those deposits known as water). Dropstones in class 4 tillites are the same mixtites, a term used to describe non-sorted or as those in the Precambrian ejecta deposit de- illsorted clastic sediments bearing megaclasts scribed by Gostin et al. (1986). The slump, tur- without regard to composition or origin. bidity, and redistribution textures found in tillite classes 5-7 are those expected in our idealized sediment sections for water impact deposits of Are Some Ancient Tillites and Diamictites figure7. of Impact Origin? Hambrey and Harland (1981) identifytillite de- A glacial origin for tillites has been questioned by posits based upon purportedly unique glacial fea- previous investigators. For example, Schermerhorn tures: abraded bedrock surfaces with striations, (1974, 1977) associated some tillite deposits with grooves, polished pavements, nailhead striations tectonic activity and hypothesized that tillites de- and chattermarks, stone-rich beds, dropstones in posited at sea level during the hot Precambrian laminated sediments, finely grained stratification, time were of non-glacial origin because they were striated pebbles and great thickness and extent of interbedded with warm water sediments. In his unsorted deposits with a wide range of grain sizes opinion, only exceptionally large overspilling gla- and association of tillites with resedimented mate- ciers may have descended to sea level to deposit rial. They note that striated surfaces could be char- tills or aquatills. One deposit in Gorky, USSR, dis- acteristic of mass flow processes other than glacia- cussed by Chumakov (1981) was originally de- tion but glacial striations tend to form in two or scribed by others as a tillite but has now been rede- three intersecting sets which they believe are scribed as a deposit of an impact crater! But this unique to the glacial process. Similarly, they point occurred only afterextensive drillingrevealed that out that nailhead striations or chattermarks are an impact crater existed beneath the center of the unique to glaciation. deposit. Other tillites of impact origin might have Chao (1976) reported that nailhead striations been similarly mistaken as glacigenic. Tillites have formed on clasts in deposits of the Ries crater. In been identified as glacial because they are charac- a review of tillites and diamictites in South terized by dropstones, graded bedding, crude varv- America and Africa described by Amos and Ga- ing or laminations, and chaotic mixtures of rock mundi (1981) and Rocha-Campos and Dos Santos clasts, some of which are striated, in fine-grained (1981) and others in Hambrey and Harland (1981), matrices. Striations are present on substrates be- we find that the main criteria used to label a de- neath some deposits. But these textures also exist posit a tillite rather than a diamictite, is the pres- in impact deposits. ence of dropstones, simple striations,boulder pave- Class 1 and 2 tillites of Gravenor et al. (1984) ments, rythmites,and varve like sediment layers. resemble the Ries impact deposit. Both have very As we have shown, thick, stone-rich beds, drop- large assorted blocks of rock mixed chaotically stones, laminations, rythmites and striations on into a fine-grained matrix. Textural similarity be- underlying formations can be produced by crater tween Ries ejecta and glacial deposits is empha- ejecta deposition. While we know of no instances sized by the observation of Chao (1976) that be- of chattermarks beneath known impact deposits fore the existence of impacts craters was widely we believe they could formin this environment (as known, investigators had noticed striations on peb- could nailhead striations). In addition, our sample bles within the Ries ejecta when attemptingto dis- of deposits that have been identified as impact de- tinguish the deposit from a glacial moraine. posits is very limited and furthersamples may pro- 12 OBERBECK, MARSHALL, AND AGGARWAL duce deposits with chattermarks, especially if Fractional Area of Earth with Ejecta -t some of the deposits now described as tillites or Fractional Area of Earth with Ejecta 100 m diamictites are proven to be impact ejecta! Similar processes acting during emplacement of SIa - 2C(3 + C2)] a impact and glacial deposits explain the similarity m 2 i4 Ft 2 + 0.2a in textures of both types of material. Both impact S(D(2o0.2a)/a D o(2+0.2a)/l)_ C5(Dn0.2 _ D0.2 cratering and ice erosion excavate a chaotic ill- - - Dmax -- Drin sorted assemblage of debris. Both grounded glaciers and crater ejecta curtains erode substrates during deposit emplacement. The erosional interface is a [{[a-2 C(3 + C2)]2 a mechanically coupled solid/solid interface,unlike 2.4 F 2 + 0.2a the typical depositional environment for other fluid (loose) boundary interfaces. Both impact cra- x (Dx0.2)/a - D(2n0.2)/) - 5(D0.2 - D0) ters and glaciers transportexotic material great distances and mix and move exotic material and ero- (16) sional debris along the ground and simultaneously This relative thickness distribution can be com- scout and striate the substrate (and clasts) during pared to that for tillites and diamictites produced deposit emplacement. Both glaciers and impact over the same time span. From the worldwide ejecta curtains drop debris in water and form compilation of the tillites and diamictites younger graded beds and dropstones. Finally, both glaciers than 2 Ga provided by Hambrey and Harland and impacts destabilize shelf slopes and produce (1981), we measured the thickness of 174 deposits turbiditysediment flows and structures. younger than 2 Ga between 10 m and 3300 m The idea that such disparate processes as impact thick. The number having an indicated thickness and glaciation might produce similar deposits is is plotted in figure 8. The actual production of de- supported by the conclusions of Weller (1960). He posits must follow the dashed line. The lower fre- origins of ancient notes that the many of the till- quencies for deposits <100 m thick must result ites are questionable because differentprocesses from erosion and lack of recording such very thin could produce the textures observed in tillites; deposits. The maximum tillite/diamictite thick- without corroborating geomorphic features, it is nesses we observed exceeded 3000 m. These values difficultto determine the origin of a till-like de- agree remarkably well with the maximum thick- posit based solely on textural information. How- ness of deposits of impact craters approximating ever, other criteria can reveal the origin of such the largestproduced in 2 Ga (equation 8). We replot deposits. While no mathematical predictions of the the data of figure 8 in figure 9 as the ratio of the thickness of glacial deposits that might be pro- number of deposits at to the total number of deduced in a given span of time can be made, such posits -100 m. This gives the relative thickness predictions can be for deposits. made impact crater distribution of tillite/diamictite deposits younger The relative thickness distributionof impact de- than 2 Ga. Figure 9 compares the calculated rela- posits that we would expect to encounter in the tive thickness distributions for impact deposits us- past 2 Gy of the geologic record can be calculated ing differentvalues of C in equation 16 with the fromour impact model. The relative thickness distribution is the number of deposits produced of thickness -t divided by the total number of deposits of all thicknesses. The number of impact deposits with thickness -t expected in 2 Gy of Earth's geological record is proportional to the fractional area of Earth's surface covered by ejecta with thickness -t in 2 Gy. We have already derived the fraction of Earth's surface covered in 2 Gy by impact ejecta with thickness _t (eq. 14). Therefore, the relative thickness distribution for impact deposits 0 100 500 1000 2000 3000 4000 that should be encountered in the geologic record Thickness,t (m) is given by the ratio of the fraction of Earth's sur- Figure 8. Number of tillite and diamictite deposits face covered by deposits -t and the fractionof the with indicatedthickness as sampled fromstratigraphic surface covered by all deposits 100 meters: sectionsin Hambreyand Harland (1981). Tournalof Geology IMPACTS, TILLITES, AND GONDWANALAND 13

nental drift find another non-glacial explanation for tillites, the reality of the new tectonics (seafloor spreading and mobile plates) should be re- garded as speculative. Since continental drift is now widely accepted, Myerhoff and Teichert's ideas regardingthe absence of glaciation on the interior of Gondwanaland may merit furtherconsid- eration within the context of an impact origin for some tillites. There are additional difficulties with a glacial origin for all tillites. Temperature profiles pre- sented in figure 70 of Salop (1983) indicate very high global temperaturesat the time of tillite depo- 0 1 2 3 4 sition. Salop notes that glaciation would have to Deposit thickness 2 t (km) have taken place during times when climate was very hot because tillites commonly have direct Figure 9. Comparison of the relative thickness distribu- contact with hot-climate sediments. Moreover, tion of tillites/diamictites with the relative thickness glaciation must distributions of impact crater deposits fromcraters with have taken place irrespective of depth/radiusratios C = 0.1-0.4 formed during the last geographic latitude. Such sudden and short periods 2 Gy. of glaciation just before and immediately aftervery warm periods, and the lack of terrestrialmecha- nisms to explain this phenomenon, prompted Sa- thickness distribution for tillites and diamictites. lop to suggest that catastrophic supernovae caused There is good agreement between the curve fortill- glaciation. Asteroid and comet impacts are also ites/diamictites and those for impact deposits of catastrophic extraterrestrial events that can ex- craters with C = 0.1. We assume here that the plain tillites and diamictites. In addition, Albritton curve for tillites/diamictites is reflective of the (1989) has pointed out that glaciation (inferredby production curve. Deposition of most tillites/dia- tillites at the Ordivician-Silurian boundary) con- mictites in marine environments may have fa- flicts with a period of greenhouse activity inferred vored preservation of deposits 100 m. at this time fromthe supercycle concept forglobal The lithological similarity of impact and glacial temperatures (Fischer 1984). deposits and similar deposit emplacement charac- An impact origin for some tillites could also ex- teristics, along with our calculation of the range plain the coincidence of major periods of biologic and form of the thickness distribution of tillites/ extinctions with periods of glaciation as inferred diamictites from our impact model, suggests that fromtillite emplacement as noted in figures70 and many tillites/diamictites could be impact deposits. 71 of Salop (1983). Salop notes that glaciations But, are there any compelling geologic and climato- could not be the cause of such biological crises and logic reasons to seek an alternate to a glacial expla- proposed supernovae outbursts to explain both bio- nation forsome of the ancient tillites? The concept logic extinctions and glaciations. However, the hy- of ancient glaciation has met with certain difficul- pothesis that dinosaurs may have become extinct ties. The discovery of tillites of supposed glacial as a result of impact (Alvarez et al. 1980) suggests originin tropical regions such as India was initially that early extinctions, as well as the tillites/dia- a puzzle until it was proposed that continental mictites, may be explained by large impact events. driftmoved India to warmer regions aftersupposed A glacial origin for some tillites can also be glaciation within Gondwanaland while it was at questioned because some of their characteristics high southern latitudes. However, Myerhoffand are difficultto assign to a glacial origin. For exam- Teichert (1971 a, 1971b) argued that glacial tills ple, Pleistocene glaciation was extensive, yet Pleis- could not have formed in the interior of Gondwa- tocene tills do not exceed 300 m thickness (Weller naland because a nearby ocean was required for a 1960). This is far less than the thickest tillite de- source of atmospheric water to form continental posits of thousands of meters compiled by Ham- glaciers. In their view, abundant atmospheric wa- brey and Harland (1981). We also note that glacial ter fromdistant oceans would not have been avail- motion (and by inference, corresponding deposi- able on the interior of the vast supercontinent. tion) is divergentfrom an erosive center-not con- They pointed out that until the advocates of conti- vergent. To obtain deposit thicknesses 3000 m 14 OBERBECK, MARSHALL, AND AGGARWAL implies erosion at a glacial center much in excess Ries impact crater; even larger ones are observed of 3000 m. This has never been demonstrated. near the rims of lunar craters (Moore 1972). Clear cut, sharply defined varves in known gla- Despite the extensive amount of excellent field cial deposits are mimicked in some tillites only by work that has produced observations compatible crude laminations. Tillites can contain individual with ancient glaciation, we have considered the rock clasts hundreds of meters in size. Glacial de- possibility of impact origin for some of the tillites posits of continental ice sheets do not contain such and diamictites because of our calculations and ob- large blocks of rock because they are not powerful servations, and because of the geological and geo- erosive agents. Advancing ice sheets simply collect physical implication of our hypothesis. the debris and deposit it after a short distance (Whillans 1978). Continental ice sheets only re- Impacts and the Breakup of Gondwanaland move irregularitiesin the pre-existingsurface and, The locations of post-Carboniferous tillites and in general, glide along the surface without fractur- diamictites between 280/345 Ma and 65 Ma are re- ing substantial amounts of bedrock. Impact crater- drawn fromHambrey and Harland (1981) in figure ing crushes the rock within the crater before 10. The uncertainty of the lower bound in age of ejecting it. Maximum block size is limited only by the tillites/diamictites is the duration of one geo- the scale of rock jointing in the formations. Blocks logic period. Many of the tillites shown are those of 200 m were produced during formation of the forwhich there has been the most extensive field

Figure 10. Comparison of the locations of flood basalts and tillites/diamictites.

a) Tillites 280345 Ma -? 65 Ma

b) Flood basalts 250 Ma -+ 65 Ma Toumalof Geology IMPACTS, TILLITES, AND GONDWANALAND 15 work suggesting a glacial origin. However, our re- Alt et al. (1988) specified terrestriallava plateaus sults compel consideration of the consequences if as the sites of impact craters >100 km that col- some of these deposits are of impact origin. The lapsed and were buried by the basalt flows whose locations of post-Carboniferous flood basalts are eruptions they triggered.They suggested that cra- also shown in figure 10. These include three of the ter formation started hotspots by initiating pres- four major continental flood basalt plateaus as de- sure-relief melting and persistent low pressure scribed in Basaltic Volcanism Study Project (1981). cells that produced mantle plumes and associated They are the Parana Basin (119-149 Ma), the Karoo flood basalts. They concluded that large impact Province (166-206 Ma), and the Siberian Platform craters also produced continental riftingand oce- (216-248 Ma). The Lake Superior Basin basalts anic spreading ridges. They identified the Colum- (1100-1120 Ma) are not shown. The tillite/basalt bia Lava Plateau with its associated continental geographical and temporal relationships shown in riftingin the northern Basin and Range and Yel- figure 10 suggest the possibility that the flood ba- lowstone hotspot track along the Snake River salts and some of the tillites are related in origin. Plain, and the Deccan Plateau with the Chagos- During Carboniferous and Permian time, extensive Laccadive hotspot track and the Carlsberg oceanic tillite/diamictitedeposits were formedon what are ridge. The authors observed that these features now South America, South Africa, Antarctica, In- form abruptly within plates without apparent tec- dia, and Australia. This was followed by the most tonic cause. Frey (1980) also proposed that impact extensive eruption of flood basalts in geologic his- bombardment before 3.9 Ga influenced initiation tory, and then by breakup of the supercontinent. of plate tectonics when craters 1000 km and larger Flood basalts postdate tillites/diamictites and oc- fracturedthe lithosphere and facilitated formation cur in the same regions of the continents. of microplates. Thus, the distribution of tillites Richards et al. (1989) believe that the heads of and flood basalts at the interiorof Gondwanaland, mantle plumes initiated terrestrialflood basalt vol- the association of impacts and flood basalt volca- canism. White and McKenzie (1989) suggest that nism (as referenced above) and the association of basalt eruptions occur at continental margins as a crustal riftingwith impact cratering(Alt et al. and previously thinned region of crust is rifted as it Frey's suggestion), leads us to explore the possibil- driftsover a mantle plume. Impacts may also have ity that continental crustal fracturingassociated fracturedand thinned the crust and triggeredflood with impact cratering may have played a key role basalt eruptions. Green (1972) proposed that the in initiating the breakup of Gondwanaland. lower portions of the volcanic sequences of some Continental crustal plates are rigid and of high Archean greenstone belts represent 60-80% melt- strength; some mechanism is required to initiate ing of mantle sources as a result of catastrophic continental breakup. The most recent view is that major impacts on the surface of primitive Earth; high-temperaturemantle plumes were either pas- a similar process to that for lunar maria. Melting sively or actively involved. The passive model results from pressure relief as impacts excavated has been developed and reviewed by White and and uplifted material from the mantle. Wilhelms McKenzie (1989). They argue that magmatically 1987 proposed that flood basalts on the lunar maria active continental margins, characterized by erup- may have been erupted from magma chambers tion of flood basalts onto the adjacent continents, after impacts fractured the lunar crust and pro- are explained by crustal riftingabove a thermal vided escape conduits. Frey (1980) and Grieve anomaly in the underlying mantle when a previ- (1980) both suggested that formationof impact ba- ously thinned and stretched region of the crust sins larger than 200-1000 km that formed before driftsover a mantle plume. They suggest that man- 3.9 Ga played an important role in the formation tle plumes alone can sometimes riftthe previously of the proto-crust of the Earth. These impacts thinned crust and initiate continental breakup. would have generated extensive basaltic volca- The mantle diapirs actually initiate crustal fractur- nism. Grieve proposed that impacts initiated for- ing immediately above the plume in the active mation of mantle plumes. Alt et al. (1988) sug- model of crustal rifting.For example, Bahat and gested that terrestrialflood basalts might also have Rabinovitch (1983) proposed that crustal fracture formedas a result of deep excavation following for- initiation as well as subsequent propagation and mation of impact craters of the order of 100 km bifurcation along the Dead Sea rift are best ex- after the Cretaceous period. Thus, eruptions of plained by the "active" mechanism. However, Hill flood basalts in figure 10 may also have been trig- (1991) argues that mantle plumes alone would not gered by impacts that emplaced some of the tillites have been able to provide sufficientstress to initi- and diamictites. ate breakup. He believes that plumes lead only to 16 OBERBECK, MARSHALL, AND AGGARWAL

local reorganization of plate-scale motions, only that may have been involved in Gondwanaland enhance propagation of an existing ridge system, or breakup. Richards et al. (1989) consider that the provide only sufficientextra force to drive a weak Karoo phase of basaltic volcanism in South Africa plate-scale system from slow spreading throughto in the Early Jurassic period is associated with the continental rifting. Marion-Prince Edward hot spot. This hot spot may The passive and active models for riftingand have been the site of an impact in Permian time basaltic eruptions apparently require thermal in- when it produced some of the tillites (including stabilities at the core-mantle boundary to generate Karoo tillites) now found displaced from the flood plumes that reach all the way to the surface. How- basalts by relatively small distances equal to the ever, the concept of mantle plumes actually origi- driftof Gondwanaland between Permian and Juras- nated from the observation that progressive basal- sic time. Fractures formed in the upper mantle tic eruption from one spot in the upper mantle by that impact could have triggered eruption of produced chains of basaltic islands as the crust the Karoo flood basalts in the Jurassic. Fractures driftedover it. Pressure relief that produces basal- formed in the crust near the original impact site tic eruptions froman upper mantle site may result could also have assisted in continental breakup as fromimpacts that easily penetrate the thin oceanic the crust was subjected to lateral tensile stresses crust. We suggest that larger craters formingon the above the upper mantle plume. Relatively little de- continents could also have initiated riftingas well lay between impacts and ensuing flood basalt erup- as basalt eruptions. tions results in a good geographic correlation be- Impact craters are capable of removing entire tween tillites/diamictites and flood basalts. In sections of the Earth's crust and can fracturethe other cases, if there was a long delay between im- surrounding crust and subjacent upper mantle. pacts and eruptions, the stationary mantle hot spot Jones (1987) combined theoretical calculations would erupt basalts geographically displaced from with knowledge of the dynamic tensile behavior of the tillite/diamictite deposits being rafted on the rock and concluded that the propagating shock moving continental crust. The longer the delay, wave associated with formation of impact craters the greater the displacement. This could explain larger than 24 km would fracture the crust down the lack of flood basalts for some regions con- to the Moho. He also concluded that such faults taining tillites or diamictites in figure 10. might be observable. Our calculations indicate that Evidence of impact deformation associated with there would have been thousands of craters of this Gondwanaland breakup could be the South African size formed during the last 2 Gy. In particular, pa- Cape fold belt, adjacent to the Karoo basin, to- leomaps of the continents show that a superconti- getherwith the major riftingdeformation affecting nent, in one configuration or another, existed as central Africa during the Late Permian. Daly et al. long ago as 1.2 Ga (Morel and Irving 1978). (1991) noted that the riftingforce is uncertain but We suggest that prolonged impact crateringpre- could be a collision of Gondwanaland and another ceding breakup of Gondwanaland (indicated by continent. However, it may be difficultto produce Permo-Carboniferous tillites in South Africa, such major deformations at the interiors of collid- South America, India, and Antarctica) could have ing supercontinents. We suggest that the Cape fold extensively fractured the lithosphere and would belt and associated basins be investigated as possi- have facilitated the final continental fragmenta- ble remnants of large impact basins left afterconti- tion. Note that equation (1) shows that roughly 12 nental breakup. impact craters >100 km would have been formed It is noteworthy that King and Zietz (1971) also on the continents in the billion years before the have described midcontinent riftingfrom Minne- breakup; some of these might have initiated upper sota and Wisconsin through Iowa and Nebraska as- mantle plumes. The crustal fracturingfrom these sociated with the belt of Precambrian Keweenawan impacts, as well as that from many more smaller flood basalts. Again, the tillites predate the basalts. ones, would have produced an extensive network However, in this North American example, the of fractureweaknesses permeating the lithosphere. Huronian tillites predate the Keweenawan basalt These fractures would have propagated vertically by hundreds of millions of years. Perhaps the very and radially as the lithosphere was stressed by stable craton in this area prevented continental doming above magma plumes. Tillites could repre- breakup and delayed and minimized flood basalt sent deposits of the largest of the impact craters eruptions more than usual. formed during the last few hundred million years before continental breakup. Discussion Consider one possible relation between a hot We have considered the possibility of an impact spot, tillite/diamictite deposit, and flood basalt origin for tillites and diamictites because textures Journalof Geology IMPACTS, TILLITES, AND GONDWANALAND 17

of tillite/diamictite and impact deposits are simi- on the basis of shock lamellae. Detailed searches lar, and because tillite/diamictite and impact de- of the ancient tillites/diamictites should be un- posits have similar thickness distributions.On the dertaken. Horz et al. (1983) showed that such other hand, all of the existing field work implies a shocked samples occur in igneous and metamor- glacial origin for tillites. Our model suggests that phic rock clast making up <1% of the Ries ejecta. impact deposits in excess of 3 km thickness should However, shock lamellae in some ancient tillites have been produced during the past 2 Gy, and we could have been annealed by diagenesis and meta- observe tillites/diamictites up to and including morphism. this thickness. However, none of the tillite or dia- The idea that complex and thick successions of mictite deposits of this thickness have yet been tillites could be formed in a matter of hours to attributedto impact. Even though there is no way years seems absurd on first reflection. However, of knowing if glacial deposits 10 times thickerthan catastrophes are as much a part of the natural order Pleistocene deposits and with the same appearance as are events associated with quiescent periods. as impact deposits could have been produced in The solar system bears witness to the catastrophic ancient times, and even though there is no way of formationof massive planetary deposits of crushed predictingthe total extent of glacial deposits in the impact regolith, volcanic constructs, and gravity- past, it is widely assumed that even the thickest induced deposition by slumping, avalanching, of the tillites are of glacial origin. If none of the flooding,and violent turbiditycurrents. If many of tillite/diamictite deposits are of impact origin, the tillites are indeed impact deposits, impacting then we are left with a disturbingproblem: How do asteroids and comets may have been much more ancient glacial deposits become preserved, while important to geologic processes than is presently expected impact crater deposits equal to the thick- understood. est of the ancient tillites (and with the same ap- While it is currently assumed that flood basalt pearance as tillites) become removed without a eruptions and rifting were produced by mantle trace? This problem is compounded by the fact plumes that formed by instabilities associated that most glacial deposits should have formed on with heat released non-uniformlyfrom the Earth's land whereas most impact deposits should have core, we support earlier views that impacts may formed in water and should thereforehave had a have initiated continental riftingand basaltic vol- greaterchance for preservation. canism. Our results suggest that impacts may have If some of the tillites/diamictites are of impact been constantly at work in shaping the continents; origin, there would have been fewer early periods they could have determined the way in which con- of glaciation, the puzzle of association of extinc- tinents broke up and then drifted.Impact craters tions with glaciations would be solved, and there caused fractures, removed crustal material, re- would be fewer difficulties associated with sudden duced overburden pressure, initiated basaltic vol- glaciation during the time of globally hot climates. canism, and enhanced continental riftingwhich It may be possible, after considerable work, finally permitted the continents to separate. Impacts of to prove the origin of each tillite/diamictite de- asteroids and comets may have initiated breakup posit. Until all tillites/diamictites are examined for of Gondwanaland and other continental land shock-damaged materials, impact spherules, irid- masses. ium concentrations, and other impact signatures, impact and glacial origins viable both remain ex- ACKNOWLEDGMENT planations for any particular deposit. As Gostin et al. (1986) have noted, impact dropstones in ma- The authors wish to thank D. Alt for his helpful rine deposits of Precambrian age were identified review of the manuscript.

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